COSMOS: A SKETCH A PHYSICAL DESCRIPTION OF THE UNIVERSE. ALEXANDER VON HUMBOLDT. TKANSLATED FBOM THE GEHMAN, BY E. C. OTTE. Nataraa vero rerum vis atquo majestas in omnibus momentis fide caret, si quis modo partes ejus ac non totam complectatur animo. Plin., Hist. Nat., lib. vil, c. L VOL. III. NEW YORK: HARPER & BROTHERS, PUBLISHERS, 329 & 331 PEARL STREET, FRANKLIN SQUARE. 1858. CONTENTS OF VOL. III. INTRODUCTION. nv Historical Review of the attempts made with the object of considering the Phenomena of the Universe as a Unity of Nature 6-25 SPECIAL RESULTS OF OBSERVATIONS IN THE DOMAIN OF COSMICAL PHENOMENA A. URANOLOGICAL PORTION of the physical description of the world. a. ASTROGNOSY 26-28 I. The realms of space, and conjectures regarding that which appears to occupy the space intervening between the heavenly bodies 29-41 II. Natural and telescopic vision, 41-73 ; Scintillation of the stars, 73-83 ; Velocity of light, 83-89 ; Results of pho- tometry, 89-102 41-102 III. Number, distribution, and color of the fixed stars, 103- 139; Stellar masses (stellar swarms), 139-143; The Milky Way interspersed with a few nebulous spots, 143-151 103-151 IV. New stars, and stars that have vanished, 151-160 ; Va- riable stars, whose recurring periods have been determ- ined, 160-177; Variations in the intensity of the light of stars whose periodicity is as yet uninvestigated, 177- 182 151-182 V. Proper motion of the fixed stars, 182-185 ; Problemat- ical existence of dark cosmical bodies, 185-188 ; Par- allax measured distances of some of the fixed stars, 188-194; Doubts as to the assumption of a central body for the whole sidereal heavens, 194-199 182-199 VI. Multiple, or double stars Their number and reciprocal distances. Period of revolution of two stars round a common center of gravity 199-21.* IV CONTENTS. TABLES. Fag. Photometric Tables of Stars 100-102 Clusters of Stars 141-143 New Stars 155-160 Variable Stars 172-177 Parallaxes 193 Elements of Orbits of double Stars ... 213 SPECIAL RESULTS OF OBSERVATION IN THE DOMAIN OF COSMICAL PHENOMENA. INTRODUCTION. IN accordance with the object I have proposed to myself, and which, as far as my own powers and the present stata of science permit, I have regarded as not unattainable, I have, in the preceding volumes of Cosmos, considered Nature in a two-fold point of view. In the first place, I have en- deavored to present her in the pure objectiveness of external phenomena ; and, secondly, as the reflection of the image im- pressed by the senses upon the inner man, that is, upon his ideas and feelings. The external world of phenomena has been delineated un- der the scientific form of a general picture of nature in her two great spheres, the uranological and the telluric or ter- restrial. This delineation begins with the stars, which glim- mer amid nebulae in the remotest realms of space, and, pass- ing from our planetary system to the vegetable covering of the earth, descends to the minutest organisms which float in the atmosphere, and are invisible to the naked eye. In order to give due prominence to the consideration of the existence of one common bond encircling the whole organic world, of the control of eternal laws, and of the causal connection, as far as yet known to us, of whole groups of phenomena, it was necessary to avoid the accumulation of isolated facts. This precaution seemed especially requisite where, in addition to the dynamic action of moving forces, the powerful influence of a specific difference of matter manifests itself in the ter- restrial portion of the universe. Tho problems presented to us in the sidereal, or uranological sphere of the Cosmos, are, considering their nature, in as far as they admit of being ob- served, of extraordinary simplicity, and capable, by means of the attractive force of matter and the quantity of its mass, of being submitted to exact calculation in accordance with the theory of motion. If, as I believe, we are justified in re- garding the revolving meteor-asteroids (aerolites) as portions of our planetary system, their fall upon the earth constitutes the sole means by which we are brought in contact with cosmical substances of a recognizable heterogeneity.* I here refer to the cause which has hitherto rendered terrestrial phenomena less amenable to th rules of mathematical de- duction than those mutually disturbing and readjusting move- ments of the cosmical bodies, in which the fundamental force of homogeneous matter is alone manifested. I have endeavored, in my delineation of the earth, to ar- range natural phenomena in such a manner as to indicate their causal connection. In describing our terrestrial sphere, I have considered its form, mean density, electro-magnetic currents, the processes of polar light, and the gradations ac- cording to which heat increases with the increase of depth. The reaction of the planet's interior on its outer crust im- plies the existence of volcanic activity ; of more or less con- tracted circles of waves of commotion (earthquake waves), and their effects, which are not always purely dynamic ; and of the eruptions of gas, of mud, and of thermal springs. The upheaval of fire-erupting mountains must be regarded as the highest demonstration of the inner terrestrial forces. We have therefore depicted volcanoes, both central and chain formations, as generative no less than as destructive agents, and as constantly forming before our eyes, for the most part, periodic rocks (rocks of eruption) ; we have likewise shown, in contrast with this formation, how sedimentary rocks are in the course of precipitation from fluids, which hold their minutest particles in solution or suspension. Such a com- parison of matter still in the act of development and solidi- fication with that already consolidated in the form of strata of the earth's crust, leads us to the distinction of geognostic epochs, and to a more certain determination of the chronolog- ical succession of those formations in which lie entombed ex- tinct genera of animals and plants the fauna and flora of a former world, whose ages are revealed by the order in which they occur. The origin, ransformation, and upheaval of ter- restrial strata, exert, at certain epochs, an alternating actior on all the special characteristics of the physical configura tion of the earth's surface ; influencing the distribution of fluids and solids, and the extension and articulation of con Cotmos, vol. i. (Harper's edit.), p 33-65, 136. INTRODUCTION. 7 tinental masses in a horizontal and vertical direction. On these relations depend the thermal conditions of oceanic cur- rents, the meteorological processes in the aerial investment of our planet, and the typical and geographical distribution of organic forms. Such a reference to the arrangement of telluric phenomena presented in the picture of nature, will, I think, suffice to show that the juxtaposition of great, and apparently complicated, results of observation, facilitates our insight into their causal connection. Our impressions of na- ture will, however, be essentially weakened, if the picture fail in warmth of color by the too great accumulation of minor details. In a carefully-sketched representation of the phenomena of the material world, completeness in the enumeration of individual features has not been deemed essential, neither does it seem desirable in the delineation of the reflex of ex- ternal nature on the inner man. Here it was necessary to observe even stricter limits. The boundless domain of the world of thought, enriched for thousands of years by the vig- orous force of intellectual activity, exhibits, among different races of men, and in different stages of civilization, sometimes a joyous, sometimes a melancholy tone of mind ;* sometimes a delicate appreciation of the beautiful, sometimes an apa- thetic insensibility. The mind of man is first led to adore the forces of nature and certain objects of the material world ; at a later period it yields to religious impulses of a higher and purely spiritual character.! The inner reflex of the outer world exerts the most varied influence on the myste- rious process of the formation of language, $ in which the original corporeal tendencies, as well as the impressions of surrounding nature, act as powerful concurring elements. Man elaborates within himself the materials presented to him by the senses, and the products of this spiritual labo- belong as essentially to the domain of the COSMOS as do the phenomena of the external world. As a reflected image of Nature, influenced by the crea- tions of excited imagination, can not retain its truthful purity, there has arisen, besides the actual and external world, an ideal and internal world, full of fantastic and partly sym- bolical myths, heightened by the introduction of fabulous ani- mal forms, whose several parts are derived from the organ- * Cosmos, vol. i., p. 23-25 ; vol. ii., p. 25 and 97. t Ibid., vol. ii., p. 38-43, and 56-60. \ Ibid, vol. i., p. 357-359; vol. ii., p. 112-117. 8 COSMOS. isms of the present world, and sometimes even from the relics of extinct species.* Marvelous flowers and trees spring from this mythic soil, as the giant ash of the Edda-Songs, the world-tree Yggdrasil, whose branches tower above the heav- ens, while one of its triple roots penetrates to the " foaming caldron springs" of the lower world. t Thus the cloud-re- gion of physical myths is filled with pleasing or with fearful forms, according to the diversity of character in nations and climates ; and these forms are preserved for centuries in the intellectual domain of successive generations. If the present work does not fully bear out its title, the adoption of which I have myself designated as bold and in- considerate, the charge of incompleteness applies especially to that portion of the COSMOS which treats of spiritual life ; that is, the image reflected by external nature on the inner world of thought and feeling. In this portion of my work I have contented myself with dwelling more especially upon those objects which lie in the direction of long-cherished studies ; on the manifestation of a more or less lively appre- ciation of nature in classical antiquity and in modern times ; on the fragments of poetical descriptions of nature, the col- oring of which has been so essentially influenced by individ- uality of national character, and the religious monotheistic view of creation ; on the fascinating charm of landscape painting ; and on the history of the contemplation of the physical universe, that is, the history of the recognition of the universe as a whole, and of the unity of phenomena a recognition gradually developed during the course of two thousand years. In a work of so comprehensive a character, the object of which is to give a scientific, and, at the same time, an ani- mated description of nature, a first imperfect attempt must rather lay claim to the merit of inciting than to that of sat- isfying inquiry. A Book of Nature, worthy of its exalted title, can nerer be accomplished until the physical sciences, notwithstanding their inherent imperfectibility, shall, by theii * M. von Olfer's Ueberreste vorweltlicher Riesenthiere in Beziehung auj Ostasiatische Sagen in the Abh. der Berl. ATead., 1832, s. 51. On the opinion advanced by Empedocles regarding the cause of the extinction of the earliest animal forms, see Hegel's Geschichte der Philosophic, bd. ii., 8. 344. t See, for the world-tree Yggdrasil, and the rushing (foaming) cal- dron-spring Hvergelmir, the Deutsche Mylhologie of Jacob Grimm, 1844, B. 530, 756; also Mallet's Northern Antiquities (Bohn's edition), 1847 p. 410, 489, aud 492, and frontispiece to ditto. INTRODUCTION. 9 gradual development and extension, have attained a higher degree of advancement, and until we shall have gained a more extended knowledge of the two grand divisions of the COSMOS the external world, as made perceptible to us by the senses ; and the inner, reflected intellectual world. I think I have here sufficiently indicated the reasons which determined me not to give greater extension to the general picture of nature. It remains for this third and fourth volume of my Cosmos to supply much that is wanting in the previ- ous portions of the work, and to present those results of ob- servation on which the present condition of scientific opinion is especially grounded. I shall here follow a similar mode of arrangement to that previously adopted, for the reasons which I have advanced, in the delineation of nature. But, before entering upon the individual facts on which special departments of science are based, I would fain offer a few more general explanatory observations. The unexpected in- dulgence with which my undertaking has been received by a large portion of the public, both at home and abroad, ren- ders it doubly imperative that I should once more define, as distinctly as possible, the fundamental ideas on which the whole work is based, and say something in regard to those demands which I have not even attempted to satisfy, be- cause, according to my view of empirical i. e., experiment- al science, they did not admit of being satisfied. These explanatory observations involuntarily associate themselves with historical recollections of the earlier attempts made to discover the one universal idea to which all phenomena, in their causal connection, might be reduced, as to a sole prin- ciple. The fundamental principle* of my work on the COSMOS, as enunciated by me more than twenty years ago, in the French and German lectures I gave at Paris and Berlin, comprehended the endeavor to combine all cosmical phenom- ena in one sole picture of nature ; to show in what manner the common conditions, that is to say, the great laws, by which individual groups of these phenomena are governed, have been recognized ; and what course has been pursued in ascending from these laws to the discovery of their causal connection. Such an attempt to comprehend the plan of the universe the order of nature must begin with a gen eralization of particular facts, and a knowledge of the con- * Cotmot. vol. i., p. 48-50, and 68-77. 10 COSMOS. ditions under which physical changes regularly and period- ically manifest themselves ; and must conduct to the thought- ful consideration of the results yielded by empirical observa- tion, but not to " a contemplation of the universe based on speculative deductions and development of thought alone, or to a theory of absolute unity independent of experience." We are, I here repeat, far distant from the period when it was thought possible to concentrate all sensuous perceptions into the unity of one sole idea of nature. The true path was indicated upward of a century before Lord Bacon's time, by Leonardo da Vinci, in these lew words : " Cominciare dall' esperienza e per mezzo di questa scoprirne la ragione."* " Commence by experience, and by means of this discover the reason." In many groups of phenomena we must still content ourselves with the recognition of empirical laws ; but the highest and more rarely attained aim of all natural in- quiry must ever be the discovery of their causal connection. t The most satisfactory and distinct evidence will always ap- pear where the laws of phenomena admit of being referred to mathematical principles of explanation. Physical cosmog- raphy constitutes merely in some of its parts a cosmology. The two expressions can not yet be regarded as identical. The great and solemn spirit that pervades the intellectual * Op. tit., vol. ii. p. 283. t In the Introductory Observations, in Cosmos, vol. i., p. 50, it should not have been generally stated that " the ultimate object of the experi- mental sciences is to discover laws, and to trace- their progressive gen- eralization." The clause " in many kinds of phenomena" should have been added. The caution with which I have expressed myself in the second volume of this work (p. 313), on the relation borne by Newton to Kepler, can not, I think, leave a doubt that I clearly distinguish be- tween the discovery and interpretation of natural laws, i.e., the explana- tion of phenomena. I there said of Kepler: " The rich abundance of accurate observations furnished by Tycho Brahe, the zealous opponent of the Copernican system, laid the foundation for the discovery of those eternal laws of the planetary movements which prepared imperishable renown for the name of Kepler, and which, interpreted by Newton, and proved to be theoretically and necessarily true, have been transferred into the bright and glorious domaiu of thought, as the intellectual rec- ognition of nature." Of Newton I said (p. 351): "We close it [the great epoch of Galileo, Kepler, Newton, and Leibnitz] with the figure of the earth as it was then recognized from theoretical conclusions. New- ton was enabled to give an explanation of the system of the universe, because he succeeded in discovering the force from whose action the laws of Kepler necessarily result." Compare on this subject (" On Laws and Causes") the admirable remarks in Sir John Herschel's address at the fifteenth meeting of the British Association at Cambridge, 1845, p. rlii. ; and Edinb. Rev., vol. 87, 1848, p. 180-183. INTRODUCTION. 11 labor, of which the limits are here defined, arises from the sublime consciousness of striving toward the infinite, and of grasping all that is revealed to us amid the boundless and inexhaustible fullness of creation, development, and being. This active striving, which has existed in all ages, must frequently, and under various forms, have deluded men into the idea that they had reached the goal, and discovered the principle which could explain all that is variable in the or- ganic world, and all the phenomena revealed to us by sen- suous perception. After men had for a long time, in accord- ance with the earliest ideas of the Hellenic people, vener- ated the agency of spirits, embodied in human forms,* in the creative, changing, and destructive processes of nature, the germ of a scientific contemplation developed itself in the physiological fancies of the Ionic school. The first principle of the origin of things, the first principle of all phenomena, was referred to two causes! either to concrete material prin- ciples, the so-called elements of Nature, or to processes of rarefaction and condensation, sometimes in accordance with mechanical, sometimes with dynamic views. The hypothe- sis of four or five materially differing elements, which was probably of Indian origin, has continued, from the era of the didactic poem of Empedocles down to the most recent times, to imbue all opinions on natural philosophy a primeval evi- dence and monument of the tendency of the human mind to seek a generalization and simplification of ideas, not only with reference to the forces, but also to the qualitative na- ture of matter. In the latter period of the development of the Ionic phys- iology, Anaxagoras of Clazomense advanced from the postu- late of simply dynamic forces of matter to the idea of a spirit independent of all matter, uniting and distributing the homo- geneous particles of which matter is composed. The world- arranging Intelligence (vovg) controls the continuously pro- gressing formation of the world, and is the primary source * In the memorable passage (Metaph., xii., 8, p. 1074, Bekker") in which Aristotle speaks of " the relics of an earlier acquired and subse- quently lost wisdom," he refers with extraordinary freedom and sig- nificance to the veneration of physical forces, and of gods in human forms : " much," says he, " has been mythically added for the persua* tion of the multitude, as also on account of the laws and for other useful ends." t The important difference in these philosophical directions rpdiroi, is clearly indicated in Arist., Phys. Auscult., 1, 4, p. 187, Bekk. (Com- pare Brandis, in the Rhein. Museum fur Philologie, Jahrg. iii., 8. 105.) 12 COSMOS. of all motion, and therefore of all physical phenomena. An- axagoras explains the apparent movement of the heavenly bodies from east to west by the assumption of a centrifugal force,* on the intermission of which, as we have already ob- served, the fall of meteoric stones ensues. This hypothesis indicates the origin of those theories of rotatory motion which more than two thousand years afterward attained considera- ble cosmical importance from the labors of Descartes, Huy- gens, and Hooke. It would be foreign to the present work to discuss whether thg world- arranging Intelligence of the philosopher of Clazomenae indicates! the Godhead itself, or the mere pantheistic notion of a spiritual principle animating all nature. In striking contrast with these two divisions of the Ionic school is the mathematical symbolism of the Pythagoreans, which in like manner embraced the whole universe. Here, in the world of physical phenomena cognizable by the senses, the attention is solely directed to that which is normal in con- figuration (the five elementary forms), to the ideas of num- bers, measure, harmony, and contrarieties. Things are re- flected in numbers "which are, as it were, an imitative repre- sentation (fj,ifj,7)aig) of them. The boundless capacity for rep- etition, and the illimitability of numbers, is typical of the character of eternity and of the infinitude of nature. The essence of things may be recognized in the form of numerical relations ; their alterations and metamorphoses as numerical combinations. Plato, in his Physics, attempted to refer the nature of all substances in the universe, and their different stages of metamorphosis, to corporeal forms, and these, again, to the simplest triangular plane figures. J But in reference * Cosmos, vol. i., p. 133-135 (note), and vol. ii., p. 309, 310 (and note). Simplicius, in a remarkable passage, p. 491, most distinctly contrasts the centripetal with the centrifugal force. He there says, " The heavenly bodies do not fall in consequence of the centrifugal force being superior to the inherent falling force of bodies and to their down- ward tendency." Hence Plutarch, in his work, De Fade in Orbe Ltmte, p. 923, compares the moon, in consequence of its not falling to the earth, to " a stone in a sling." For the actual signification of the nepiXupT)ai.s of Anaxagoras, compare Schaubach, in Anaxag. Clazom. Fragm., 1827, p. 107-109. t Schaubach, Op. cit., p. 151-156, and 185-189. Plants are likewise said to be animated by the intelligence i>ot5f ; Aristot., De Plant., i., p. 815, Bekk. t Compare, on this portion of Plato's mathematical physics, B6ckh, De Platonico Syst. Caelestium Globorum, 1810 et 1811; Martin, Eludei tur le Timie, torn, ii., p. 234-242; and Brandis, in the Geschichte der GricchiKh-Rdmuchsn Philosophic, th. ii., abth. i., 1844, $ 375. INTRODUCTION. 13 to ultimate principles (the elements, as it were, of the ele ments), Plato exclaims, with modest diffidence 1 , " God alone, and those whom he loves among men, know what they are." Such a mathematical mode of treating physical phenomena, together with the development of the atomic theory, and the philosophy of measure and harmony, have long obstructed the development of the physical sciences, and misled fanciful in- quirers into devious tracks, as is shown in the history of the physical contemplation of the universe. " There dwells a captivating charm, celebrated by all antiquity, in the simple relations of time and space, as manifested in tones, numbers, and lines."* The idea of the harmonious government of the universe re- veals itself in a distinct and exalted tone throughout the writ- ings of Aristotle. All the phenomena of nature are depicted in the Physical Lectures (Auscultationes Physicce) as mov- ing, vital agents of one general cosmical force. Heaven and nature (the telluric sphere of phenomena) depend upon the " unmoved motus of the universe."! The " ordainer" and the ultimate cause of all sensuous changes must be regarded as something non-sensuous and distinct from all matter.^ Unity in the different expressions of material force is raised to the rank of a main principle, and these expressions of force are themselves always reduced to motions. Thus we find already in " the book of the soul" the germ of the undulatory theory of light. The sensation of sight is occasioned by a vibration * Cosmos, vol. ii., p. 351, note. Compare also Gruppe, Ueber die Fragmente des Archytas, 1840, s. 33. t Aristot.,Poto., vii., 4, p. 1326, and Metapk., xii., 7, p. 1072, 10, Bekk., and xii., 10, p. 1074-5. The pseudo-Aristotelian work, De Mundo, which Osann ascribed to Chrysippus (see Cotmot, vol. ii., p. 28, 29), also contains (cap. 6, p. 397) a very eloquent passage on the world-or- derer and tcorld-sustainer. t The proofs are collected in Ritter, History of Philotophy (Bohn, 1838-46), vol. iii., p. 180, et seq. Compare Aristot., De Anima, ii., 7, p. 419. In this passage the analogy with sound is most distinctly expressed, although in other por- tions of his writings Aristotle has greatly modified his theory of vision. Thus, in De Insomniis, cap. 2, p. 459, Bekker, we find the following words : " It is evident that sight is no less an active than a passive agent, and that vision not only experiences some action from the air (the medium), but itself also acts upon the medium." He adduces in evidence of the truth of this proposition, that a new and very pure me- tallic mirror will, under certain conditions, when looked at by a woman, retain on its surface cloudy specks that can not be removed without difficulty. Compare also Martin, Etudes sur le Timfe de Platun., torn ii. p. 159-163. 14 COSMOS. a movement of the medium between the eye and the object Been and not by emissions from the object or the eye. Hear- ing is compared with sight, as sound is likewise a consequence of the vibration of the air. Aristotle, while he teaches men to investigate generalities in the particulars of perceptible unities by the force of reflect- ive reason, always includes the whole of nature, and the in- ternal connection not only of forces, but also of organic forms. In his book on the parts (organs) of animals, he clearly in- timates his belief that throughout all animate beings there is a scale of gradation, in which they ascend from lower to high- er forms. Nature advances in an uninterrupted progressive course of development, from the inanimate or " elementary" to plants and animals ; and, " lastly, to that which, though not actually an animal, is yet so nearly allied to one, that on the whole there is little difference between them."* In the transition of formations, " the gradations are almost imper- ceptible."! The unity of nature was to the Stagirite the great problem of the Cosmos. " In this unity," he observes, with singular animation of expression, " there is nothing unconnect- ed or out of place, as in a bad tragedy."}: The endeavor to reduce all the phenomena of the universe to one principle of explanation is manifest throughout the physical works of this profound philosopher and accurate ob- server of nature ; but the imperfect condition of science, and ignorance of the mode of conducting experiments, i. e., of calling forth phenomena under definite conditions, prevented the comprehension of the causal connection of even small groups of physical processes. All things were reduced to the ever-recurring contrasts of heat and cold, moisture and dry- ness, primary density and rarefaction even to an evolution of alterations in the organic world by a species of inner divis- ion (antiperistasis), which reminds us of the modern hypothesis of opposite polarities and the contrasts presented by + and . * Aristot., De partibus Amm., lib. iv., cap. 5, p. 681, lin. 12, Bekker. t Aristot., Hist. Anim., lib. ix., cap. 1, p. 588, lin. 10-24, Bekker. When any of the representatives of the four elements in the animal kingdom on oar globe fail, as, for instance, those which represent the element of the purest fire, the intermediate stages may perhaps be found to occur in the moon. (Biese, Die Phil, des Aristoteles, bd. ii., s. 186.) It is singular enough that the Stagirite should seek in another planet those intermediate links of the chain of organized beings which we find in the extinct animal and vegetable forms of an earlier world. t Aristot., Metaph., lib. xiii., cap. 3, p. 1090, lin. 20, Bekker. The uvrnrepiiraait of Aristotle plays an important part in all hit INTRODUCTION. 15 The so-calle"d solutions of the problems only reproduce the same facts in a disguised form, and the otherwise vigorous and concise style of the Stagirite degenerates in his explana- tions of meteorological or optical processes into a self-com- placent diffuseness and a somewhat Hellenic verbosity. As Aristotle's inquiries were directed almost exclusively to mo- tion, and seldom to differences in matter, we find the funda- mental idea, that all telluric natural phenomena are to be ascribed to the impulse of the movement of the heavens the rotation of the celestial sphere constantly recurring, fondly cherished and fostered,* but never declared with ab- solute distinctness and certainty. The impulse to which I refer indicates only the communi- cation of motion as the cause of all terrestrial phenomena. Pantheistic views are excluded ; the Godhead is considered as the highest "ordering unity, manifested in all parts of the universe, defining and determining the nature of all forma- tions, and holding together all things as an absolute power.f The main idea and these teleological views are not applied to the subordinate processes of inorganic or elementary nature, but refer specially to the higher organizations! of the animal and vegetable world. It is worthy of notice, that in these theories the Godhead is attended by a number of astral spirits, who (as if acquainted with perturbations and the dis- explanatious of meteorological processes ; so also in the works De Gen- eralione et Interitu, lib. ii., cap. 3, p. 330 ; in the Meteorologicis, lib. i., cap. 12, and lib. iii., cap. 3, p. 372, and in the Problems (lib. xiv., cap. 3, lib. viii., No. 9, p. 888, and lib. xiv., No. 3, p. 909), which are at all events based on Aristotelian principles. In the ancient polarity hypoth- esis, /car* avrnrepiaTaaiv, similar conditions attract each other, and dis- similar ones (-J- and ) repel each other in opposite directions. (Com pare Ideler, Meteorol. veterum Grcsc. et Rom., 1832, p. 10.) The op- posite conditions, instead of being destroyed by combining together, rather increase the tension. The ipvxpov increases the -Qeppov ; as in- versely "in the formation of hail, the surrounding heat makes the cold body still colder as the cloud sinks into warmer strata of air." Aristotle explains by his antiperistatic process and the polarity of heat, what modern physics have taught us to refer to conduction, radiation, evap- oration, and changes in the capacity of heat. See the able observations of Paul Erman in the Abhandl. tier Berliner Akademie aufdasJahr 1825, s. 128. * " By the movement of the heavenly sphere, all that is unstable in natural bodies, and all terrestrial phenomena are produced." Aristot., Mtteor., i., 2, p. 339, and De Gener. et Corrupt., ii., 10, p. 336. t Aristot., De Casio, lib. i., c. 9, p. 279 ; lib. ii., c. 3, p. 286 ; lib. ii., c 13, p. 292, Bekker. (Compare Biese, bd. i., s. 352-1, 357.) t Aristot., Phys. Auscult., lib. ii., c. 8, p. 199; De Anima, lib. iii., o 12, p. 434 ; De Animal. General., lib. v., c. 1, p. 778, Bekker. 16 COSMOS. tribution of masses) maintain the planets in their eternal oib- its.* The stars here reveal the image of the divinity in the visible world. We do not here refer, as its title might lead to suppose, to the little pseudo- Aristotelian work entitled the " Cosmos," undoubtedly a Stoic production. Although it de- scribes the heavens and the earth, and oceanic and aerial currents, with much truthfulness, and frequently with rhetor- ical animation and picturesque coloring, it shows no tenden- cy to refer cosmical phenomena to general physical princi- ples based on the properties of matter. I have purposely dwelt at length on the most brilliant pe- riod of the Cosmical views of antiquity, in order to contrast the earliest efforts made toward the generalization of ideas with the efforts of modern times. In the intellectual move- ment of centuries, whose influence on the extension of cos- mical contemplation has been defined in another portion of the present work.f the close of the thirteenth and the begin- ning of the fourteenth century were specially distinguished ; but the Opus Majus of Roger Bacon, the Mirror of Nature of Vincenzo de Beauvais, the Physical Geography (Liber Cos- mographictis) of Albertus Magnus, the Picture of the World (Imago Mundi) of Cardinal Petrus d'Alliaco (Pierre d'Ailly), are works which, however powerfully they may have influ- enced the age in which they were written, do not fulfill by their contents the promise of their titles. Among the Italian opponents of Aristotle's physics, Bernardino Telesio of Cosen- za is designated the founder of a rational science of nature. All the phenomena of inert matter are considered by him as the effects of two incorporeal principles (agencies or forces) heat and cold. All forms of organic life "animated" * See the passage in Aristot., Meteor., xii., 8, p. 1074, of which there is a remarkable elucidation in the Commentary of Alexander Aphro- iisiensis. The stars are not inanimate bodies, but must be regarded as active and living beings. (Aristot., De Casio, lib. ii., cap. 12, p. 292.) They are the most divine of created things ; TO. -Qeiorepa TUV <j>avepuv. (Aristot., De Casio, lib. i., cap. 9, p. 278, and lib. ii., cap. 1, p. 284.) ip. 6, p. 400), me nigh anner is also called divine (cap. That which the imaginative Kepler calls moving spirits (anima motruai) in his work, Mysterium Cotmographicum (cap. 20, p. 71), is the distort- ed idea of a force (virtus') whose main seat is in the sun (anima mun- di), and which is decreased by distance in accordance with the laws of light, and impels the planets in elliptic orbita. (Compare Apelt, Epoch en der Gesch. der Mcnechheit, bd. i., e. 274.) * Cotmot, vol. ii., p. 241-250. INTRODUCTION. 17 plants and animals are the effect of these two ever-divided forces, of which the one, heat, specially appertains to the ce- lestial, and the other, cold, to the terrestrial sphere. "With yet more unbridled fancy, but with a profound spirit of inquiry, Giordano Bruno of Nola attempted to comprehend the whole universe, in three works,* entitled De la causa Principio e Uno; Contcmplationi circa lo Infinite, Uni- verso e Atondi innumerabili ; and De Minima et Maximo. In the natural philosophy of Telesio, a cotemporary of Co- pernicus, we recognize at all events the tendency to reduce the changes of matter to two of its fundamental forces, which, although " supposed to act from without," yet resemble the fundamental forces of attraction and repulsion in the dy- namic theory of nature of Boscovich and Kant. The cos- mical views of the Philosopher of Nola are purely meta- physical, and do not seek the causes of sensuous phenomena in matter itself, but treat of "the infinity of space, filled with self - illumined worlds, of the animated condition of those worlds, and of the relations of the highest intelligence God to the universe." Scantily endowed with mathematical knowledge, Giorda- no Bruno continued nevertheless to the period of his fearful martyrdomf an enthusiastic admirer of Copernicus, Tycho Brahe, and Kepler. He was cotemporary with Galileo, but did not live to see the invention of the telescope by Hans Lippershey and Zacharias Jansen, and did not therefore wit- ness the discovery of the " lesser Jupiter world," the phases of Venus, and the nebulse. "With bold confidence in what he terms the lume interno, ragione naturale, altezza dell' intclletto (force of intellect), he indulged in happy conjec- tures regarding the movement of the fixed stars, the planet * Compare the acute and learned commentary on the works of the Philosopher of Nola, in the treatise Jordano Bruno par Christian Bar- tholmess, torn, ii., 1847, p. 129, 149, and 201. t He was burned at Rome on the 17th of February, 1600, pursuant to the sentence " ut qnam clementissime et citra sanguinis effusionem puniretur." Bruno was imprisoned six years in the Piombi at Venice, and two years in the Inquisition at Rome. When the sentence of death was announced to him, Bruno, calm and unmoved, gave utterance to the following noble expression: "Majori forsitan cum timore sententi- am in me fertis quam ego accipiam." When a fugitive from Italy in 1580, he taught at Geneva, Lyons, Toulouse, Paris, Oxford, Marburg, Wittenberg (which he calls the Athens of Germany), Prague, and Helm- stedt, where, in 1589, he completed the scientific instruction of Duko Henry Julius of Brunswick- Wolfenbuttel. Bartholmess, torn . i , p. 167- 178. He also taught at Padua subsequently to 1592. 18 COSMOS. ary nature of comets, and the deviation from the spherical form observed in the figure of the earth.* Greek antiquity is also replete with uranological presentiments of this na- ture, which were realized in later times. In the development of thought on cosmical relations, of which the main forms and epochs have been already enu- merated, Kepler approached the nearest to a mathematical application of the theory of gravitation, more than seventy- eight years before the appearance of Newton's immortal work, Principia Philosophies Naturalis. For while the eclectic Simplicius only expressed in general terms " that the heavenly bodies were sustained from falling in conse- quence of the centrifugal force being superior to the inher- ent falling force of bodies and to the downward traction ;" while Joannes Philoponus, a disciple of Ammonius Hermeas, ascribed the movement of the celestial bodies to " a primi- tive impulse, and the continued tendency to fall ;" and while, as we have already observed, Copernicus defined only the general idea of gravitation, as it acts in the sun, as the center of the planetary world, in the earth and in the moon, using these memorable words, " Gravitatem non aliud esse quam appetentiam quandam naturalem partibus inditam a divina providentia opificis universorum, ut in unitatem integrita- temque suam sese conferant, in formam globi coeuntes ;" Kepler, in his introduction to the book De Stella MartisJ was the first who gave numerical calculations of the forces of attraction reciprocally exercised upon each other, accord- ing to their relative masses, by the earth and moon. He * Bartholmess, torn, ii., p. 219, 232, 370. Bruno carefully collected all the separate observations made on the celestial phenomenon of the sudden appearance, in 1572, of a new star in Cassiopeia. Much dis- cussion has been directed in modern times to the relation existing be- tween Bruno, his two Calabrian fellow-countrymen, Bernardino Tele- sio and Thomas Campanella, and the platonic cardinal, Nicolaus Krebs of Cusa. See Cosmos, vol. ii., p. 310, 311, note. t " Si duo lapides in aliquo loco Mundi collocarentur propinqui in- vicem, extra orbem virtutis tertii cognati corporis ; illi lapides ad simil- itudinem duorum Magneticorum corporum coirent loco intermedio, qui- libet accedens ad alterum tanto intervallo, quanta est alterius moles in comparatione. Si luna et terra non retinerentur vi animali (!) aut alia aliqua aequipollente, quselibet in suo circuitu, Terra adscenderet ad Lu- nam quinquagesima quarta parte intervalli, Luna descenderet ad Ter- rarn quinquaginta tribus circiter partibus intervalli; ibi jungerentur, posito tamen quod substantia utriusque sit unius et ejusdem densitatis." Kepler, A&tronomia nova, seu Physica ccclestis de Motibus Stella Mar- tis, 1609. Introd., fol. v. On the older views regarding gravitation, see Cosmos, vrl. ii., p. 310. INTRODUCTION. 19 distinctly adduces the tides as evidence* that the attractiv& force of the moon (virtus tractoria) extends to the earth , and that this force, similar to that exerted by the magnet on iron, would deprive the earth of its water if the forme] should cease to attract it. Unfortunately, this great man was induced, ten years afterward, in 1619, probably from deference to Galileo, who ascribed the ebb and flow of the ocean to the rotation of the earth, to renounce his correct explanation, and depict the earth in the Harmonice Mundt as a li ving monster, whose whale-like mode of breathing oc- casioned the rise and fall of the ocean in recurring periods of sleeping and waking, dependent on solar time. When we remember the mathematical acumen that pervades one of the works of Kepler, and of which Laplace has already made honorable mention,t it is to be lamented that the discoverer of the three great laws of all planetary motion should not have advanced on the path whither he had been led by his views on the attraction of the masses of cosmical bodies. Descartes, who was endowed with greater versatility of physical knowledge than Kepler, and who laid the founda- tion of many departments of mathematical physics, under- took to comprise the whole world of phenomena, the heav- * " Si Terra cessaret attrahere ad se aquas suas, aquae marinse omnes elevarentur et in corpus Luna? iufluerent. Orbis virtutis tractoriae, qua? est in Luna, porrigitur usque ad terras, et prolectat aquas quacunque in verticem loci incidit sub Zonam torridam, quippe in occursum suum quacunque in verticem loci incidit, insensibiliter in maribus inclusis, sensibiliter ibi ubi sunt latissimi alvei Oceani propinqui, aquisque spa- ciosa reciprocationis libertas." (Kepler, 1. c.) " Undas a Luna trahi ut ferrum a Magnete." .... Kepleri Harmonice Mundi, libri quinque, 1619, lib. iv., cap. 7, p. 162. The same work which presents us with so many admirable views, among others, with the data of the establish- ment of the third law (that the squares of the periodic times of two planets are as the cubes of their mean distance), is distorted by the wildest flights of fancy on the respiration, nutrition, and heat of the earth-animal, on the soul, memory (memoria animce Terra), and crea- tive imagination (anima Tdluris imaginatio) of this monster. This great man was so wedded to these chimeras, that he warmly contested his right of priority in the views regarding the earth-animal with the mys- tic author of the Macrocosmcs, Robert Fludd, of Oxford, who is report- ed to have participated in the invention of the thermometer. (Harm. Mitndi, p. 252.) In Kepler's writings, the attraction of masses is often confounded with magnetic attraction. " Corpus solis esse magneticum. Virtutem, quae Planetas movet, residere in corpore solis." Stella Mar tit, pars iii., cap. 32, 34. To each planet was ascribed a magnetic axis, which constantly pointed to one and the same quarter of the heavens. CApelt, Joh. Kepler's Astron. Weltansicht, 1849, 8. 73. t Compare Cosmos, vol. ii., p. 327 (and iiole 20 COSMOS. enly sphere and all that he knew concerning the animate and inanimate parts of terrestrial nature, in a work entitled Traite du Monde, and also Summa Philosophies. The or- ganization of animals, and especially that of man a subject to which he devoted the anatomical studies of eleven years* was to conclude the work. In his correspondence with Father Mersenne, we frequently find him complaining of hia slow progress, and of the difficulty of arranging so large a mass of materials. The Cosmos which Descartes always called " his world" (son monde) was at length to have been sent to press at the close of the year 1633, when the report of the sentence passed by the Inquisition at Rome on Gali- leo, which was first made generally known four months aft- erward, in October, 1633, by Gassendi and Bouillaud, at once put a stop to his plans, and deprived posterity of a great work, completed with much pains and infinite care. The motives that restrained him from publishing the Cosmos were, love of peaceful retirement in his secluded abode at Deventer, and a pious desire not to treat irreverentially the decrees pronounced by the Holy Chair against the planetary movement of the earth. t In 1664, fourteen years after the death of the philosopher, some fragments were first printed under the singular title of Le Monde, ou Traite de la Lu- miere."i. The three chapters which treat of light scarcely, however, constitute a fourth part of the work ; while those sections which originally belonged to the Cosmos of Des- cartes, and treated of the movement of the planets, and their distance from the sun, of terrestrial magnetism, the ebb and flow of the ocean, earthquakes, and volcanoes, have been transposed to the third and fourth portions of the celebrated work, Principes de la Philosophic. Notwithstanding its ambitious title, the Cosmotheoros of Huygens, which did not appear till after his death, scarcely deserves to be noticed in this enumeration of cosmological efforts. It consists of the dreams and fancies of a great man on the animal and vegetable worlds, of the most remote cos- mical bodies, and especially of the modifications of form which See La Vie de M. Descartes (par Baillet), 1691, Part i., p. 197, End CEuvret de Descartes, publiees par Victor Cousin, torn, i., 1824, p. 101. t Lsttres de Descartes au P. Mersenne, du, 19 Nov., 1633, et du 5 Jan- vier, 1634. (Baillet, Part i., p. 244-247.) \ The Latin translation bears the title Mundus give Dissertalio de Lwmine itt et de aliis Sensuum Objectis primariis. See Descartes, Optu~ cula posthuma Physka et Mathematica, Amst., 1704. INTRODUCTION. 21 the human race may there present. The reader might sup- pose he were perusing Kepler's Somnium Astronomicum, or Kircher's Iter Extaticus. As Huygens, like the astronomers of our own day, denied the presence of air and water in the moon,* he is much more embarrassed regarding the exist- ence of inhabitants in the moon than of those in the remoter planets, which he assumes to be " surrounded with vapors and clouds." The immortal author of the Philosophic Naturalis Prin- cipia Mathematica (Newton) succeeded in embracing the whole uranological portion of the Cosmos in the causal con- nection of its phenomena, by the assumption of one all-con- trolling fundamental moving force. He first applied phys- ical astronomy to solve a great problem in mechanics, and elevated it to the rank of a mathematical science. The quantity of matter in every celestial body gives the amount of its attracting force ; a force which acts in an inverse ra- tio to the square of the distance, and determines the amount of the disturbances, which not only the planets, but all the bodies in celestial space, exercise on each other. But the Newtonian theory of gravitation, so worthy of our admira- tion from its simplicity and generality, is not limited in its cosmical application to the uranological sphere, but com- prises also telluric phenomena, in directions not yet fully investigated ; it affords the clew to the periodic movements in the ocean and the atmosphere,! and solves the problems of capillarity, of endosmosis, and of many chemical, elec- * " Lunam aqnis carere et afire : Marium similitudinem in Luna nul- lam reperio. Nam regiones planas quae montosis multo obscuriores eunt, quasque vulgo pro maribus haberi video et oceanorum nominibus insigniri, in his ipsis, longiore telescopic inspectis, cavitates exiguas in- esse comperio rotundas, umbris intus cadentibus; quod maris superfi- ciei convenire nequit; turn ipsi campi illi latiores non prorsus eequabi- lem superficiem praDferunt, cum diligentius eas intuemur. Quod circa maria esse non possunt, sed materia constare debent minus candicante, quam qute est partibus asperioribus in quibus rursus quanlam viridiori lumine caeteras prsecellunt." Hugenii Cosmotheorog, ed. alt. 1699, lib. xi., p. 114. Huygens conjectures, however, that Jupiter is agitated by much wind and rain, for " ventorum flatus ex ilia nubium Jovialium mutabili facie cognoscitur" (lib. i., p. 69). These dreams of Huygens regarding the inhabitants of remote planets, so unworthy of a man versed iu exact mathematics, have, unfortunately, been revived by Emauuel Kant, in his cdmirable work Allgemeine Naturgeschichte und Theorie dtt Himmelt, 1755 (s. 173-192). t See Laplace (des Oscillations de t 'Atmotphlre, du flux Solaire et Lunaire} iu the Micanique Celeste, livre iv., and in the Exposition d* Syst. du Monde, 1824, p. 291-296. 22 COSMOS. tro-magnetic, and organic processes. Newton* even distin- guished the attraction of masses , as manifested in the mo- tion of cosmical bodies and in the phenomena of the tides, from molecular attraction, which acts at infinitely small distances and in the closest contact. Thus we see that among the various attempts which have been made to refer whatever is unstable in the sensuous world to a single fundamental principle, the theory of grav- itation is the most comprehensive and the richest in cosmic- al results. It is indeed true, that notwithstanding the brill- iant progress that has been made in recent times in strechi- ometry (the art of calculating with chemical elements and in the relations of volume of mixed gases), all the physical theories of matter have not yet been referred to mathematic- ally-determinable principles of explanation. Empirical laws have been recognized, and by means of the extensively- dif- fused views of the atomic or corpuscular philosophy, many points have been rendered more accessible to mathematical investigation ; but, owing to the unbounded heterogeneous- ness of matter and the manifold conditions of aggregation of particles, the proofs of these empirical laws can not as yet by any means be developed from the theory of contact-at- traction with that certainty which characterizes the estab- lishment of Kepler's three great empirical laws derived from the theory of the attraction of masses or gravitation. At the time, however, that Newton recognized all move- ments of the cosmical bodies to be the results of one and the same force, he did not, like Kant, regard gravitation as an essential property of bodies,! but considered it either as the * Adjicere jam licet de spiritu quodam subtilissimo corpora crassa pervadente et in iisdem latente, cujus vi et actiouibus particulaj corpo- rum ad minimas distantias se mutuo attrahunt et contiguae facta cohac- rent. Newton, Principia Phil. Nat. (ed. Le Sueur et Jacquier, 1760), Schol. gen., t. iii., p. 676; compare also Newton's Optics (ed. 1718), Query 31, p. 305, 353, 367, 372. (Laplace, Syst. du Monde, p. 384, and Cosmos, vol. i., p. 63 (note).) t Hactenus phaenomena coelorum et maris nostri per vim gravitatis exposui, sed causam gravitatis nondum assignavi. Oritur utique haec vis a causa aliqua, qua: penetrat ad usque centra solis et planetarum, sine virtutis dimiuutione ; quaeque agit non pro quantitate superficierum particularum, in quas agit (ut solent causae mechanics), sed pro quanti- tate materiae solidao. Rationem harum gravitatis proprietatum ex phae- nomenis nondum potui deducere et hypotheses non fingo. Satis est quod gravitas revera existat et agat secundum leges a nobis expositas. Newton, Principia Phil. Nat., p. 676. " To tell us that every spe- cies of things is endowed witlr. an occult specific quality, by which it acts and produces manifest effects, is to tell us nothing ; but to derive INTRODUCTION. 23 result of some higher and still unknown power, or of " the centrifugal force of the aether, which fills the realms of space, and is rarer within bodies, but increases in density outward. The latter view is set forth in detail in a letter to Robert Boyle* (dated February 28, 1678), which ends with the words, " I seek the cause of gravity in the sether." Eight years afterward, as we learn from a letter he wrote to Hal ley, Newton entirely relinquished this hypothesis of the rarer and denser sether.f It is especially worthy of notice, that in 1717, nine years before his death, he should have deemed it necessary expressly to state, in the short preface to the sec- ond edition of his Optics, that he did not by any means con- sider gravity as an " essential property of bodies ;"J while two or three general principles of motion from phenomena, and after- ward to tell us how the properties and actions of all corporeal things follow from those manifest principles, would be a very great step in phi- losophy, though the causes of those principles were not yet discovered ; and therefore I scruple not to propose the principles of motion, and leave their causes to be found out." Newton's Optics, p. 377. In a previ- ous portion of the same work, at query 31, p. 351, he writes as follows : " Bodies act one upon another by the attraction of gravity, magnetism, and electricity ; and it is not improbable that there may be more at- tractive powers than these. How these attractions may be performed I do not here consider. What I call attraction may be performed by impulse, or by some other means unknown to me. I use that word lio <; to signify only in general any force by which bodies tend toward on^ another, whatsoever be the cause." * " I suppose the rarer sether within bodies, and the denser without them." Operum Newtoni, tomus iv. (ed. 1782, Sam. Horsley), p. 386. The above observation was made in reference to the explanation of the discovery made by Grimaldi of the diffraction or inflection of light. At the close of Newton's letter to Robert Boyle, February, 1678, p. 94, he says : " I shall set down one conjecture more which came into my mind: it is about the cause of gravity. . . ." His correspondence with Olden- burg (December, 1675) shows that the great philosopher was not at that time averse to the " aether hypotheses." According to these views, the impulse of material light causes the aether to vibrate ; but the vibra- tions of the sether alone, which fias some affinity to a nervous fluid, does not generate light. In reference to the contest with Hooke, consult Horsley, t. iv., p. 378-380. t See Brewster's Life of Sir Isaac Newton, p. 303-305. t Newton's words " not to take gravity for an essential property of bodies" in the " Second Advertisement" contrast with his remarks on the forces of attraction and repulsion, which he ascribes to all molecu- lar particles, in order, according to the theory of emission, to explain the phenomena of the refraction and repulsion of the rays of light from reflecting surfaces "without their actual contact." (Newton, Optict, book ii., prop. 8, p. 241, and Brewster, Op. cit., p. 301.) According to Kant (see Die Metaphysischen Anfangsgrunde der Naturwissenschaft, 1800, s. 28), we can not conceive the existence of matter without these forces of attraction and repulsion. All physical phenomena are there- 24 COSMOS. Gilbert, as early as 1600, regarded magnetism as a force in- herent in all matter. So undetermined was even Newton, the profound and experienced thinker, regarding the " ulti- mate mechanical cause" of all motion. It is indeed a brilliant effort, worthy of the human mind, to comprise, in one organic whole, the entire science of na- ture from the laws of gravity to the formative impulse (ni- sus formativus) in animated bodies ; but the present imper- fect state of many branches of physical science offers innu- merable difficulties to the solution of such a problem. The imperfectibility of all empirical science, and the boundless- ness of the sphere of observation, render the task of explain- ing the forces of matter by that which is variable in matter, an impracticable one. What has been already perceived by no means exhausts that which is perceptible. If, simply re- ferring to the progress of science in modern times, we com- pare the imperfect physical knowledge of Gilbert, Robert Boyle, and Hales, with that of the present day, and remem- ber that every few years are characterized by an increasing rapidity of advance, we shall be better able to imagine the periodical and endless changes which all physical sciences are destined to undergo. New substances and new forces will be discovered. Although many physical processes, as those of light, heat, and electro-magnetism, have been rendered accessible to a mathematical investigation by being reduced to motion or vi- brations, we are still without a solution to those often mooted and perhaps insolvable problems : the cause of chemical dif- ferences of matter ; the apparently irregular distribution of the planets in reference to their size, density, the inclination of their axes, the eccentricity of their orbits, and the num- fore reduced by him, as previously by Goodwin Knight (Pkilos. Trant- act. 1748, p. 264), to the conflict of two elementary forces. In the at- omic theories, which were diametrically opposed to Kant's dynamic views, the force of attraction was referred, in accordance with a view specially promulgated by Lavoisier, to the discrete solid elementary molecules of which all bodies are supposed to consist ; while the force of repulsion was attributed to the atmospheres of heat surrounding all elementary corpuscles. This hypothesis, which regards the so-called taloric as a constantly expanded matter, assumes the existence of two elementary substances, as in the mythical idea of two kinds of tether. (Newton, Optics, query 28, p. 339.) Here the question arises, What causes this caloric matter to expand? Considerations on the density of molecules in comparison with that of their aggregates (the entire body) lead, according to atomic hypotheses, to the result, that the dis- tance between elementary corpuscles is i'ar greater than, their diameterg. INTRODUCTION. 25 her and distance of their satellites ; the configuration of con- tinents, and the position of their highest mountain chains. Those relations in space, which we have referred to merely by way of illustration, can at present be regarded only as something existing in nature, as a fact, but which I can net designate as merely causal, because their causes and mutual connection have not yet been discovered. They are the re- sult of occurrences in the realms of space coeval with the formation of our planetary system, and of geognostic process- es in the upheaval of the outer strata of the earth into con- tinents and mountain chains. Our knowledge of the prime- val ages of the world's physical history does not extend suf- ficiently far to allow of our depicting the present condition of things as one of development.* Wherever the causal connection between phenomena has not yet been fully recognized, the doctrine of the Cosmos, or the physical description of the universe, does not constitute a distinct branch of physical science. It rather embraces the whole domain of nature, the phenomena of both the celestial and terrestrial spheres, but embraces it only under the single point of view of efforts made toward the knowledge of the universe as a whole. "f As, in the " exposition of past events in the moral and political world, the historian:): can only di- vine the plan of the government of the world, according to human views, through the signs which are presented to him, and not by direct insight," so also the inquirer into nature, in his investigation of cosmical relations, feels himself pene- trated by a profound consciousness that the fruits hitherto yielded by direct observation and by the careful analysis of phenomena are far from having exhausted the number of impelling, producing, and formative forces. * Cosmos, vol. i., p. 94-97. t Op. cit., p. 55-62. t Wilhelra von Humboldt, Gesammdte Werke, bd. i., s. 23. VOL. III. B RESULTS OF OBSERVATIONS IN THE URANOLOGICAL POR- TION OF THE PHYSICAL DESCRIPTION OF THE WORLD. WE again commence with, the depths of cosmical space, and the remote sporadic starry systems, which appear to tel- escopic vision as faintly shining nebulce. From these we gradually descend to the double stars, revolving round one common center of gravity, and which are frequently bicol- ored, to the nearer starry strata, one of which appears to in- close our own planetary system ; passing thence to the air- and-ocean-girt terrestrial spheroid which we inhabit. We have already indicated, in the introduction to the General Delineation of Nature,* that this arrangement of ideas is alone suited to the character of a work on the Cosmos, since we can not here, in accordance with the requirements of di- rect sensuous contemplation, begin with our own terrestrial abode, whose surface is animated by organic forces, and pass from the apparent to the true movements of cosmical bodies. The uranological, when opposed to the telluric domain of the Cosmos, may be conveniently separated into two di- visions, one of which comprises astro^nosy, or the region of the fixed stars, and the other our solar and planetary sys- tem. It is unnecessary here to describe the imperfect and unsatisfactory nature of such a nomenclature and such class- ifications. Names were introduced into the physical sci- ences before the differences of objects and their strict limita- tions were sufficiently known.f The most important point, however, is the connection of ideas, and the order in which the objects are to be considered. Innovations in the no- menclature of groups, and a deviation from the meanings hitherto attached to well-known names, only tend to dis- tract and confuse the mind. a. ASTROGNOSY. (THE DOMAIN OF THE FIXED STARS.) Nothing is stationary in space. Even the fixed stars move, as Halleyl: endeavored to show in reference to Sirius, * Cosmos, vol. i., p. 79-83. f Op. cit., p. 56, 57 t Halley, in the Philos. Transact, for 1717, vol. xxx., p. 736. A8TROGNOSY. 27 Arcturus, and Aldebaran, and as in modern times has been incontrovertibly proved with respect to many others. The bright star Arcturus has, during the 2100 years (since the times of Aristi.'lus and Hipparchus) that it has been ob- served, changed its position in relation to the neighboring fainter stars 2 times the moon's diameter. Encke remarks " that the star \i Cassiopeise appears to have moved 31 lunar diameters, and 61 Cygni about 6 lunar diameters, if the an- cient observations correctly indicated its position." Conclu- sions based on analogy justify us in believing that there is every where progressive, and perhaps also rotatory motion. The term " fixed stars" leads to erroneous preconceptions ; it may have referred, in its earliest meaning among the Greeks, to the idea of the stars being riveted into the crys- tal vault of heaven ; or, subsequently, in accordance with the Roman interpretation, it may indicate fixity or immo- bility. The one idea involuntarily led to the other. In Gre- cian antiquity, in an age at least as remote as that of Anax- imenes of the Ionic school, or of Alcmseon the Pythagorean, all stars were divided into wandering (darpa TrAavcjjueva or TrAavT/rd) and non-wandering fixed stars (drr^avelg aarepeg or dTrXavfj darpa).* Besides this generally adopted desig- nation of the fixed stars, which Macrobius, in his Somnium Scipionis, Latinized by Sphcera aplanesj we frequently meet in Aristotle (as if he wished to introduce a new tech- nical term) with the phrase riveted stars, Ivdedeneva darpa, instead of a^Xavr],% as a designation for fixed stars. From this form of speech arose the expressions of sidera infixa cado of Cicero, Stellas quas ^nctamus affixas of Pliny, and as- 9 Pseudo-Plut., De plac. Philos., ii., 15, 16 ; Stob., Eclog. Phys., p. 582 ; Plato, in the Timeeus, p. 40. t Macrob., Sown. Scip., i., 9-10 ; slellce inerrantes, in Cicero, De Nat. Deorum, iii., 20. t The principal passage in which we meet with the technical expres- sion hdedspeva uarpa, is in Aristot., De Caelo, ii., 8, p. 289, 1. 34, p. 290, 1. 19, Bekker. This altered nomenclature forcibly attracted my atten- tion in my investigations into the optics of Ptolemy, and his experi- ments on refraction. Professor Franz, to whose philological acquire- ments I am indebted for frequent aid, reminds me that Ptolemy (Syn- tax, vii., 1) speaks of the fixed stars as affixed or riveted; uanep Trpo- OTreQvKOTee. Ptolemy thus objects to the expression a<j>aipa air'Xavfjc (orltig inerrans) ; " in as far as the stars constantly preserve their rela live distances, they might rightly be termed airhavtiq ; but in as far an the sphere in which they complete their course, and in which they seem to have grown, as it were, has an independent motion, the designation O7r/laf!?f is inappropriate if applied to the sphere." tra fixa of Manilius, which corresponds with our term fixed stars.* This idea of fixity leads to the secondary idea of immobility, of persistence in one spot, and thus the original signification of the expressions infixum or affixum sidus was gradually lost sight of in the Latin translations of the Mid- dle Ages, and the idea of immobility alone retained. This is already apparent in a highly rhetorical passage of Seneca, regarding the possibility of discovering new planets, in which he says (Nat. Queest., vii., 24), " Credis autem in hoc max- imo et pulcherrimo corpore inter innumerabiles Stellas, quae noctem decore vario distinguunt, quse ae'ra minime vacuum et inertem esse patiuntur, quinque solas esse, quibus exer- cere se liceat ; ceteras stare fixum et immobilempopulum?" "And dost thou believe that in this so great and splendid body, among innumerable stars, which by their various beau- ty adorn the night, not suffering the air to remain void and unprofitable, that there should be only five stars to whom it is permitted to be in motion, while all the rest remain a fixed and immovable multitude ?" This fixed and immovable mul- titude is nowhere to be found. In order the better to classify the main results of actual observations, and the conclusions or conjectures to which they give rise, in the description of the universe, I will sep- arate the astrognostic sphere into the following sections : I. The considerations on the realms of space and the bodies by which they appear to be filled. II. Natural and telescopic vision, the scintillation of the stars, the velocity of light, and the photometric experiments on the intensity of stellar light. III. The number, distribution, and color of the stars ; the stellar swarms, and the Milky Way, which is interspersed with a few nebulae. IV. The newly-appeared and periodically-changing stars, and those that have disappeared. V. The proper motion of the fixed stars ; the problematical existence of dark cosmical bodies ; the parallax and meas- ured distance of some of the fixed stars. VI. The double stars, and the period of their revolution round a common center of gravity. VII. The nebulas which are interspersed in the Magellanic clouds with numerous stellar masses, the black spots (coal bags) in the vault of heaven. * Cicero, De Nat Deorutn, i., 13 ; Plin., ii., 6 and 24 ; Manillas, ii., 35 THE REALMS OF SPACE, AND CONJECTURES REGARDING THAT WHICH APPEARS TO OCCUPY THE SPACE INTERVENING BETWEEN THE HEAVENLY BODIES. THAT portion of the physical description of the universe which treats of what occupies the distant regions of the heavens, filling the space between the globular cosmic al bodies, and is imperceptible to our organs, may not unaptly be compared to the mythical commencement of ancient his- tory. In infinity of space as well as in eternity of time, all things are shrouded in an uncertain and frequently deceptive twilight. The imagination is here doubly impelled to draw from its own fullness, and to give outline and permanence to these indefinite changing forms.* This observation will, I trust, suffice to exonerate me from the reproach of confound- ing that which has been reduced to mathematical certainty by direct observation or measurement, with that which is founded on very imperfect induction. Wild reveries belong to the romance of physical astronomy ; yet the mind famil- iar with scientific labors delights in dwelling on subjects such as these, which, intimately connected with the present condition of science, and with the hopes which it inspires, have not been deemed unworthy of the earnest attention of the most distinguished astronomers of our day. By the influence of gravitation, or general gravity, as well as by light and radiating heat,t we are brought in contact, as we may with great probability assume, not only with our own Sun, but also with all the other luminous suns of the firma- ment. The important discovery of the appreciable resist- ance which a fluid filling the realms of space is capable of opposing to a comet having a period of revolution of five years, has been perfectly confirmed by the exact accordance of numerical relations. Conclusions based upon analogies may fill up a portion of the vast chasm which separates the certain results of a mathematical natural philosophy from conjectures verging on the extreme, and therefore obscure and barren confines of all scientific development of mind. From the infinity of space an infinity, however, doubted * Cosmos, vol. i., p. 87. (Compare the admirable observations of Encke, Ueber die Anordnung des Sternsystems, 1844, s. 7.) t Cotmot, vol. i., p. 154, 155. 30 COSMOS. by Aristotle* follows the idea of its immeasurability. Sep arate portions only have been rendered accessible to meas- urement, and the numerical results, which far exceed the grasp of our comprehension, become a source of mere puerile gratification to those who delight in high numbers, and im- agine that the sublimity of astronomical studies may be heightened by astounding and terrific images of physical mag- nitude. The distance of 61 Cygni from the Sun is 657,000 semi-diameters of the Earth's orbit ; a distance which light takes rather more than ten years to traverse, while it passes from the Sun to the Earth in 8' 17"-78. Sir John Herschel conjectures, from his ingenious combination of photometric calculations,! that if the stars in the great circle of the Milky Way which he saw in the field of his twenty-feet telescope were newly-arisen luminous cosmical bodies, they would have required 2000 years to transmit to us the first ray of light All attempts to present such numerical relations fail, either from the immensity of the unit by which they must be meas- ured, or from the high number yielded by the repetition of this unit. Bessel$ very truly observes that " the distance which light traverses in a year is not more appreciable to us than the distance which it traverses in ten years. There- fore every endeavor must fail to convey to the mind any idea of a magnitude exceeding those that are accessible on the earth." This overpowering force of numbers is as clear- ly manifested in the smallest organisms of animal life as in the milky way of those self-luminous suns which we call fixed stars. What masses of Polythalami are inclosed, ac- cording to Ehrenberg, in one thin stratum of chalk ! This eminent investigator of nature asserts that one cubic inch of the Bilin polishing slate, which constitutes a sort of mount- ain cap forty feet in height, contains 41,000 millions of the microscopic Galionella distans ; while the same volume con- tains more than 1 billion 750,000 millions of distinct indi- viduals of Galionella ferruginea.k Such estimates remind us of the treatise named Arenarius (tpafifiirrj^) of Archime- des of the sand-grains which might fill the universe of space ! If the starry heavens, by incalculable numbers, magnitude, space, duration, and length of periods, impress * Aristot., De Casio, 1, 7, p. 276, Bekker. t Sir John Herschel, Outlines of Astronomy, 1849, 803, p. 541. j Bessel, in Schumacher's Jahrluchfur 1839, s. 50. $ Ehrenberg, Abhandl. der Berl. Akad., 1838, s. 59 ; also in his Info rionsthiere, B. 170. THE PROPAGATION OP LIGHT. 31 man with the conviction of his own insignificance, his phys- ical weakness, and the ephemeral nature of his existence ; he is, on the other hand, cheered and invigorated by the consciousness of having been enabled, by the application and development of intellect, to investigate very many important points in reference to the laws of Nature and the sidereal arrangement of the universe. Although not only the propagation of light, but also a special form of its diminished intensity, the resisting medium acting on the periods of revolution of Encke's comet, and the evaporation of many of the large tails of comets, seem to prove that the regions of space which separate cosmical bod- ies are not void,* but filled with some kind of matter ; we must not omit to draw attention to the fact that, among the now current but indefinite expressions of " the air of Jieav- en" " cosmical (non-luminous) matter" and " ether" the latter, which has been transmitted to us from the earliest an- tiquity of Southern and Western Asia, has not always ex- pressed the same idea. Among the natural philosophers of India, ether (aka'sa) was regarded as belonging to the pant- scJiata, or five elements, and was supposed to be a fluid of infinite subtlety, pervading the whole universe, and constitu- ting the medium of exciting life as well as of propagating sound.f Etymologically considered, aka'sa signifies, accord- ing to Bopp, " luminous or shining, and bears, therefore, in its fundamental signification, the same relation to the ' ether' of the Greeks as shining does to burning." In the dogmas of the Ionic philosophy of Anaxagoras and Empedocles, this ether (alOr^p) differed wholly from the act- ual (denser) vapor-charged air (drjp) which surrounds the * Aristotle (Phys. Auseu.lt., iv., 6-10, p. 213-217, Bekker) proves, in opposition to Leucippus and Democritus, that there is no unfilled space no vacuum in the universe. t Akd'sa signifies, according to Wilson's Sanscrit Dictionary, " the subtle and ethereal fluid supposed to fill and pervade the universe, and to be the peculiar vehicle of life and sound." " The word dlcd'sa (lu- minous, shining) is derived from the root ka's (to shine), to which is added the preposition d. The quintuple of all the elements is called pantsckatd, or pantschatra, and the dead are, singularly enough, desig- nated as those who have been resolved into the five elements (prdpta pantschatra'). Such is the interpretation given in the text of Amara- koscha, Amarasinha's Dictionary." (Bopp.) Colebrooke's admirable treatise on the Sankhya Philosophy treats of these five elements ; see Transact, of the Asiat. Soc., vol. i., Loud., 1827, p. 31. Strabo refers, according to Megasthenes (xv., $ 59, p. 713, Gas.), to the all-forming fifth element of the Indians, without, however, naming it. 32 COSMOS. earth, and " probably extends as far as the moon." It was of " a fiery nature, a brightly-beaming, pure fire-air,* of great subtlety and eternal serenity." This definition perfectly co- incides with its etymological derivation from aWeiv, to burn, for which Plato and Aristotle, from a predilection for me- chanical views, singularly enough substituted another (del- 6elv), on account of the constancy of the revolving and rota- tory movement.! The idea of the subtlety and tenuity of the upper ether does not appear to have resulted from a knowledge that the air on mountains is purer and less charged with the heavy vapors of the earth, or that the dens- ity of the strata of air decreases with their increased height. In as far as the elements of the ancients refer less to mate- rial differences of bodies, or even to their simple nature (their incapacity of being decomposed), than to mere conditions of matter, the idea of the upper ether (the fiery air of heaven) has originated in the primary and normal contraries of heavy and light, lower and upper, earth vxAfire. These extremes * Empedocles, v. 216, calls the ether irapfavouv, brightly-beaming, and therefore self-luminous. t Plato, Cratyl., 410 B., where we meet with the expression aetdsrip. Aristot., De Casio, 1, 3, p. 270, Bekk., says, in opposition to Anaxagoras: aidtpa rrpoffuvofiaaav TOV UVUTUTU TOTTOV, U.TTO TOT delv act rbv aldiov Xpovov -QfUfvoi. TTJV snuwpiav avru. 'Avagayopaf t)e KaraKixpilfai ru ov6/j.aTi TovTCf) ov KO^uf bvoftd&t yap aWepa avrl irvpoc.. We find this more circumstantially referred to in Aristot., Meteor., 1, 3, p. 339, lines 21-34, Bekk. : " The so-called ether has an ancient designation, which Anaxagoras seems to identify with fire ; for, according to him, the up- per region is full of fire, and to be considered as ether ; in which, in- deed, he is correct. For the ancients appear to have regarded the body which is in a constant state of movement, as possessing a divine nature, and therefore called it ether, a substance with which we have nothing analogous. Those, however, who hold the space surrounding bodies to be fire no less than the bodies themselves, and who look upon that which lies between the earth and the stars as air, would probably re- linquish such childish fancies if they properly investigated the results of the latest researches of mathematicians." (The same etymology of this word, implying rapid revolution, is referred to by the Aristotelian, or Stoic, author of the work De Mundo, cap. 2, p. 392, Bekk.) Professor Franz has correctly remarked, "That the play of words in the designa- tion of bodies in eternal motion (aufta uei dtov') and of the divine (tfftov) alluded to in the Meteoroloeica, is strikingly characteristic of the Greek type of imagination, and affords additional evidence of the inaptitude of the ancients for etymological inquiry." Professor Buschmann calls at- tention to a Sanscrit term, dschtra, ether or the atmosphere, which looks very like the Greek aidr/p, with which it has been compared by Vans Kennedy, in his Researches into the Origin and Affinity of the principal Languages of Asia and Europe, 1828, p. 279. This word may also be ich referred to the root (as, asch), to which the Indians attach the signifi cation of shining or beaming. COSMICAL ETHER. 33 are separated by two intermediate elementary conditions, of which the one, water, approximates most nearly to the heavy earth, and the other, air, to the lighter element of fire.* Considered as a medium filling the regions of space, the ether of Empedocles presents no other analogies excepting those of subtlety and tenuity with the ether, by whose trans- verse vibrations modern physicists have succeeded so hap- pily in explaining, on purely mathematical principles, the propagation of light, with all its properties of double refrac- tion, polarization, and interference. The natural philosophy of Aristotle further teaches that the ethereal substance pen- etrates all the living organisms of the earth both plants and animals ; that it becomes in these the principle of vital heat, the very germ of a psychical principle, which, uninflu- enced by the body, stimulates men to independent activity.! These visionary opinions draw down ether from the higher regions of space to the terrestrial sphere, and represent it as a highly rarefied substance constantly penetrating through the atmosphere and through solid bodies ; precisely similar to the vibrating light-ether of Huygens, Hooke, and modern physicists. But what especially distinguishes the older Ionic from the modern hypothesis of ether is the original assump- tion of luminosity, a view, however, not entirely advocated by Aristotle. The upper fire-air of Empedocles is expressly termed brightly radiating (-rrafi^avouv), and is said to be seen by the inhabitants of the earth in certain phenomena, gleaming brightly through fissures and chasms (^da/zara) which occur in the firmament.^ The numerous investigations that have been made in re- cent times regarding the intimate relation between light, heat, electricity, and magnetism, render it far from improba- ble that, as the transverse vibrations of the ether which fills the regions of space give rise to the phenomena of light, the thermal and electro-magnetic phenomena may likewise Aristot., De Ccelo, iv., 1, and 3-4, p. 308, and 311-312, Bekk. If the Stagirite withholds from ether the character of a fifth element, which indeed is denied by Ritter (Geschichte der Philosophic, th. iii., s. 259), and by Martin (Etudes sur le Timte de Platan., t. ii., p. 150), it ia only because, according to him, ether, as a condition of matter, has no contrary. (Compare Biese, Philosophic des Aristoteles, bd. xi., s. 66.) Among the Pythagoreans, ether, as a fifth element, was represented by the fifth of the regular bodies, the dodecahedron, composed of twelve pentagons. (Martin, t. ii., p. 245-250.) t See the proofs collected by Biese, op. cit., bd. xi., s. 93. t Cosmos, vol. i., p. 153. B2 34 COSMOS. have their origin in analogous kinds of motion (currents). It is reserved for future ages to make great discoveries in ref- erence to these subjects. Light, and radiating heat, which is inseparable from it, constitute a main cause of motion and organic life, both in the non-luminous celestial bodies and on the surface of our planet.* Even far from its surface, in the interior of the earth's crust, penetrating heat calls forth electro-magnetic currents, which exert their exciting influ- ence on the combinations and decompositions of matter on all formative agencies in the mineral kingdom on the dis- turbance of the equilibrium of the atmosphere and on the functions of vegetable and animal organisms. If electricity moving in currents develops magnetic forces, and if, in ac- cordance with an early hypothesis of Sir "William Herschel,t the sun itself is in the condition of " a perpetual northern light" (I should rather say of an electro-magnetic storm), we should seem warranted in concluding that solar light, trans- mitted in the regions of space by vibrations of ether, may be accompanied by electro-magnetic currents. Direct observations on the periodic changes in the decli- nation, inclination, and intensity of terrestrial magnetism, have, it is true, not yet shown with certainty that these con- ditions are affected by the different positions of the sun or moon, notwithstanding the latter's contiguity to the earth. The magnetic polarity of the earth exhibits no variations that can be referred to the sun, or which perceptibly affect the precession of the equinoxes. t The remarkable rotatory or oscillatory motion of the radiating cone of light of Halley's comet, which Bessel observed from the 12th to the 22d of October, 1835, and endeavored to explain, led this great as- tronomer to the conviction that there existed a polar force, Compare the fine passage on me influence of the sun's rays in Sir n Herschel's Outlines of Astronomy, p. 237 : " By the vivifying ac- tion of the sun's rays, vegetables are enabled to draw support from in- organic matter, and become, in their turn, the support of animals and of man, and the sources of those great deposits of dynamical efficiency which are laid up for human use in our coo 1 , strata. By them the wa- ters of the sea are made to circulate in v*pT through the air, and irri- gate the land, producing springs and rivers. By them are produced all disturbances of the chemical equilibrium of the elements of nature, which, by a series of compositions and decompositions, give rise to new products, and originate a transfer of materials." t Philos. Transact, for 1795, vol. Ixxxv., p. 318 ; John Herschel, Out- lines of Astr., p. 238; see also Cosmos, vol. i., p. 189. t See Bessel, in Schumacher's Astr. Nachr., bd. xiii., 1836, No. 300, B. 201. RADIATING HEAT. ' 35 " whose action differed considerably from gravitation or the ordinary attracting force of the sun ; since those portions of the comet which constitute the tail are acted upon by a re- pulsive force proceeding from the body of the sun."* The splendid comet of 1744, which was described by Heinsius, led my deceased friend to similar conjectures. T/te actions of radiating heat in the regions of space are regarded as less problematical than electro-magnetic phenom- ena. According to Fourier and Poisson, the temperature of the regions of space is the result of radiation of heat from the sun and all astral bodies, minus the quantity lost by absorp- tion in traversing the regions of space filled with ether, t Frequent mention is made in antiquity by the Greek and RomanJ writers of this stellar heat ; not only because, from a universally prevalent assumption, the stars appertained to the region of the fiery ether, but because they were supposed to be themselves of a fiery nature the fixed stars and the sun being, according to the doctrine of Aristarchus of Samos, of one and the same nature. In recent times, the observa- tions of the above-mentioned eminent French mathemati- cians, Fourier and Poisson, have been the means of direct- ing attention to the average determination of the tempera- ture of the regions of space ; and the more strongly since the importance of such determinations on account of the radia tion of heat from the earth's surface toward the vault of heaven has at length been appreciated in their relation to all thermal conditions, and to the very habitability of our planet. According to Fourier's Analytic Theory of Heat, the temperature of celestial space (des espaces planetaires ou celestes) is rather below the mean temperature of the poles, or even, perhaps, below the lowest degree of cold hith- erto observed in the polar regions. Fourier estimates it at from 58 to 76 (from 40 to 48 Reaum.). The icy pole (pole glacial), or the point of the greatest cold, no more * Bessel, op. cif., 8. 186-192, 229. t Fourier, Thforie Analytique de la Chaleur, 1822, p. ix. (Annalet de Chimie et de Physique, torn, iii., 1816, p. 350; torn, iv., 1817, p. 128; torn, vi., 1817, p. 259 ; torn, xiii., 1820, p. 418.) Poisson, in his Thlorie Mathematiqve de la Chaleur ( 196, p. 436, $ 200, p. 447, and $ 228, p. 521), attempts to give the numerical estimates of the stellar heat (cha- leur stellaire) lost by absorption in the ether of the regions of space. t On the heating power of the stars, see Aristot., De Meteor., 1, 3, p. 340, lin. 28 ; and on the elevation of the atmospheric strata at which heat is at the minimum, consult Seneca, in Nat. Qu&st., ii., 10: ''So- periora enim afiris calorem vicinoruin siderum sentiurt." $ Plut., Deplac. Philos., ii., 13. corresponds with the terrestrial pole than does the thermal equator, which connects together the hottest points of al] meridians with the geographical equator. Arago concludes, from the gradual decrease of mean temperatures, that the degree of cold at the northern terrestrial pole is 13, if the maximum cold ohserved by Captain Back at Fort Reliance (62 46' lat.) in January, 1834, were actually 70 ( 56-6 Cent., or 45 0> 3 Reaum.).* The lowest temperjtlure that, as far as we know, has as yet been observed on the earth, is probably that noted by Neveroff, at Jakutsk (62 2' lat.), on the 21st of January, 1838. The instruments used in this observation were compared with his own by Middendorff, whose operations were always conducted with extreme ex- actitude. Neveroff found the temperature on the day above named to be 76 (or 48 Reaum.). Among the many grounds of uncertainty in obtaining a numerical result for the thermal condition of the regions of space, must be reckoned that of our inability at present to ascertain the mean of the temperatures of the poles of great- est cold of the two hemispheres, owing to our insufficient ac- quaintance with the meteorology of the antarctic pole, from which the mean annual temperature must be determined. I attach but little physical probability to the hypothesis of Pois- son, that the different regions of space must have a very va- rious temperature, owing to the unequal distribution of heat- radiating stars, and that the earth, during its motion with the * Arago, Sur la Temperature du P6le et des espaces Celestes, in the Annuaire du Bureau des Long, pour 1825, p. 189, et pour 1834, p. 192; also Saigey, Physique du Globe, 1832, p. 60-76. Swanberg found, from considerations on refraction, that the temperature of the regions of space was 58.5. Berzelius, Jahresbericht fur 1830, s. 54. Arago, from polar observations, fixed it at 70 ; and Pectet at 76. Saigey, by calculating the decrease of heat in the atmosphere, from 367 observa- tions made by myself in the chain of the Andes and in Mexico, found it 85 ; and from thermometrical measurements made at Mont Blanc, and during the aeronautic ascent of Gay-Lussac, 107-2. Sir John Herschel (Edinburgh Review, vol. 87, 1848, p. 223) gives it at 132. We feel considerable surprise, and have our faith in the correctness of the methods hitherto adopted somewhat shaken, when we find that Poisson, notwithstanding that the mean temperature of Melville Island (74 47' N. lat.) is 1 66', gives the mean temperature of the regions of space at only 8'6, having obtained his data from purely theoretical premises, according to which the regions of space are warmer than the outer limits of the atmosphere (see the work already referred to, $ 227, p. 520) ; while Pouillet states it, from actinometric experiments, to be as low as 223-6. See Comptet Rendus de I' Academic det Science!, torn, vii., 1838, p. 25-65. TEMPERATURE OF SPACE. 37 whole solar system, receives its internal heat from without while passing through hot and cold regions.* The question whether the thermal conditions of the celes- tial regions, and the climates of individual portions of space, have suffered important variations in the course of ages, de pends mainly on the solution of a problem warmly discussed by Sir William Herschel : whether the nebulous masses are subjected to progressive processes of formation, while the cos- mic al vapor is being condensed around one or more nuclei in accordance with the laws of attraction ? By such a con- densation of cosmical vapor, heat must be liberated, as in every transition of gases and fluids into a state of solidifica- tion. t If, in accordance with the most recent views, and the important observations of Lord Rosse and Mr. Bond, we may assume that all nebulae, including those which the high- est power of optical instruments has hitherto failed in resolv- ing, are closely crowded stellar swarms, our faith in this per- petually augmenting liberation of heat must necessarily be in some degree weakened. But even small consolidated cos- mical bodies which appear on the field of the telescope as distinguishable luminous points, may change their density by combining in larger masses ; and many phenomena pre- sented by our own planetary system lead to the conclusion that planets have been solidified from a state of vapor^ and that their internal heat owes its origin to the formative pro- cess of conglomerated matter. It may at first sight seem hazardous to term the fearfully low temperature of the regions of space (which varies be- tween the freezing point of mercury and that of spirits of wine) even indirectly beneficial to the habitable climates of the earth and to animal and vegetable life. But in proof of the accuracy of the expression, we need only refer to the ac- tion of the radiation of heat. The sun-warmed surface of our planet, as well as the atmosphere to its outermost strata, freely radiate heat into space. The loss of heat which they experience arises from the difference of temperature between the vault of heaven and the atmospheric strata, and from the feebleness of the counter-radiation. How enormous would be this loss of heat.J if the regions of space, instead of the * See Poisson, Tklorie Maih6m. de la Chaleur, p. 438. According to him, the consolidation of the earth's strata began from the center, ana advanced gradually toward the surface ; $ 193, p. 429. Compare also Cosmos, vol. i., p. 176, 177. t Cosmos, vol. i., p. 83, 84, 144. t " Were there no atmosphere, a thermometer freely exposed (at sun- 38 COSMOS. temperature they now possess, and which we designate as 76 of a mercury thermometer, had a temperature of about 1400 or even many thousand times lower ! It still remains for us to consider two hypotheses in rela- tion to the existence of a fluid filling the regions of space, of which one the less firmly-based hypothesis -refers to the limited transparency of the celestial regions ; and the other, founded on direct observation and yielding numerical results, is deduced from the regularly shortened periods of revolution of Encke's comet. Olbers in Bremen, and, as Struve has ob- served, Loys de Cheseaux at Geneva, eighty years earlier* drew attention to the dilemma, that since we could not con- ceive any point in the infinite regions of space unoccupied by a fixed star, i. e., a sun, the entire vault of heaven must ap- pear as luminous as our sun if light were transmitted to us in perfect intensity ; or, if such be not the case, we must as- sume that light experiences a diminution of intensity in its passage through space, this diminution being more excessive than in the inverse ratio of the square of the distance. As we do not observe the whole heavens to be almost uniformly illumined by such a radiance of light (a subject considered by Halleyf in an hypothesis which he subsequently rejected), the regions of space can not, according to Cheseaux, Olbers, and Struve, possess perfect and absolute transparency. The results obtained by Sir William Herschel from gauging the p set) to the heating influence of the earth's radiation, and the cooling power of its own into space, would indicate a medium temperature be- tween that of the celestial spaces (132 Fahr.) and that of the earth's surface below it, 82 Fahr., at the equator, 3* Fahr., in the Polar Sea. Under the equator, then, it would stand, on the average, at 25 Fahr., and in the Polar Sea at 68 Fahr. The presence of the atmosphere tends to prevent the thermometer so exposed from attaining these ex- treme low temperatures : first, by imparting heat by conduction ; sec- ondly, by impeding radiation outward." Sir John Herschel, in the Edinburgh Review, vol. 87, 1848, p. 222. " Si la chaleur des espaces planetaires n'existait point, notre atmosphere 6prouverait un refroidis- sement, dont on ne peut fixer la limite. Probablement la vie des plantes et des animaux serait impossible a la surface du globe, ou releguee dans une etroite zone de cette surface." (Saigey, Physique du Globe, p. 77.) * Traiti de la Comete de 1743, avec une Addition sur la force de la Lumiere et sa Propagation dans l'6ther, ct sur la distance des etoiles fixes; par Loys de Cheseaux (1744). On the transparency of the regions of space, see Olbers, in Bode's Jahrbuckfur 1826, s. 110-121 ; and Struve, Etudes d'Astr. Slellaire, 1847, p. 83-93, and note 95. Compare also Sir John Herschel, Outlines of Astronomy, $ 798, and Cosmos, vol. i., p. 151, 152. t Halley, On the Infinity of the Sphere of Fixed Stars, in the Philos. Transact., vol. xxxi., for tfie year 1720, p. 22-26. RESISTING MEDIUM. 39 stars,* and from his ingenious experiments on the space-pen- etrating power of his great telescopes, seem to show, that if the light of Sirius in its passage to us through a gaseous or ethereal fluid loses only T 7 tb of its intensity, this assump- tion, which gives the amount of the density of a fluid capa- ble of diminishing light, would suffice to explain the phe- nomena as they manifest themselves. Among the doubts advanced by the celebrated author of " The New Outlines of Astronomy" against the views of Olbers and Struve, one of the most important is that his twenty -feet telescope shows, throughout the greater portion of the Milky Way in both hem- ispheres, the smallest stars projected on a black ground. t A better proof, and one based, as we have already stated, upon direct observation of the existence of a resisting fluid,} is afforded by Encke's comet, and by the ingenious and im- portant conclusion to which my friend was led in his observ- ations on this body. This resisting medium must, however, be regarded as different from the all-penetrating light-ether, because the former is only capable of offering resistance in- asmuch as it can not penetrate through solid matter. These observations require the assumption of a tangential force to explain the diminished period of revolution (the diminished major axis of the ellipse), and this is most directly afforded by the hypothesis of a resisting fluid. The greatest action * Cosmos, vol. i., p. 86, 87. t "Throughout by far the larger portiou of the extent of the Milky Way in both hemispheres, the general blackness of the ground of the heavens, on which its stars are projected .... In those regions where the zone is clearly resolved into stars, well separated, and seen projected on a black ground, and where we look out beyond them into space. . . ." Sir John Herschel, Outlines of Astr., p. 537, 539. t Cosmos, vol. i., p. 85, 86, 107 ; compare also Laplace, Essai Philos- ophique sur les Probability's, 1825, p. 133 ; Arago, in the Annuaire du Bureau des Long, pour 1832, p. 188, pour 1836, p. 216; and Sir John Herschel, Outlines of Astr., $ 577. The oscillatory movement of the emanations from the head of some comets, as in that of 1744, and in Halley's, as observed by Bessel, be- tween the 12th and 22d of October, 1835 (Schumacher, Astron. Nachr., Nos. 300, 302, 185, 232), "may indeed, in the case of some individ- uals of this class of cosmical bodies, exert an influence on the transla- tory and rotatory motion, and lead us to infer the action of polar forces ( 201, 229), which differ from the ordinary attracting force of the sun ;" but the regular acceleration observable for sixty-three years in Encke's comet (whose period of revolution is 3 years), can not be regarded as the result of incidental emanations. Compare, on this cosmically im- portant subject, Bessel, in Schum., Astron. Nachr., No. 289, s. 6, and No. 310, s. 345-350, with Encke's Treatise on the hypothesis of the re- sisting medium, in Schum., No. 305, s. 265-274 40 COSMOS. is manifested during the twenty-five days immediately pre- ceding and succeeding the comet's perihelion passage. The value of the constant is therefore somewhat different, because in the neighborhood of the sun the highly attenuated but still gravitating strata of the resisting fluid are denser. 01- bers maintained* that this fluid could not be at rest, but must rotate directly round the sun, and therefore the resist- ance offered to retrograde comets, like Halley's, must differ wholly from that opposed to those comets having a direct course, like Encke's. The perturbations of comets having long periods of revolution, and the difference of their magni tudes and sizes, -complicate the results, and render it diffi- cult to determine what is ascribable to individual forces. The gaseous matter constituting the belt of the zodiacal light may, as Sir John Herschelf expresses it, be merely the denser portion of this comet-resisting medium. Although it may be shown that all nebulae are crowded stellar masses, indistinctly visible, it is certain that innumerable comets fill the regions of space with matter through the evaporation of their tails, some of which have a length of 56, 000, 000 of miles. Arago has ingeniously shown, on optical grounds,^ that the variable stars which always exhibit white light without any change of color in their periodical phases, might afford a means of determining the superior limit of the dens- ity to be assumed for cosmical ether, if we suppose it to be equal to gaseous terrestrial fluids in its power of refraction. The question of the existence of an ethereal fluid filling the regions of space is closely connected with one warmly agitated by WolJaston, in reference to the definite limit of the atmosphere a limit which must necessarily exist at the elevation where the specific elasticity of the air is equipoised by the force of gravity. Faraday's ingenious experiments on * Gibers, in Schum., Astr. NacJir., No. 268, a. 58. t Outlines of Astronomy, $ 556, 597. t "En assimilant la mature ires rare qui remplit les espaces celestes quant a ses proprietes refringentes aux gas terrestres, la densite de cette matiere nt saurait depasser itne certaine limite dont les observations des eloilcs chcngeantes, p. e. celles d 1 Algol oudc/3 de Persic, peuvent assigner la valeur." Arago, in the Annuaire pour 1842, p. 336-345. " On com paring the extremely rare matter occupying the regions of space with terrestrial gases, in respect to its refractive properties, we shall find that the density of this matter can not exceed a definite limit, whose value may be obtained from observations of variable stars, as, for instance, Algol or (3 Persei." See Wollaston, Philos. Transact, for 1822, p. 89,' Sir John Herschel, op. eit., $ 34, 36. FIRST TELESCOPE, 41 the limits of an atmosphere of mercury (that is, the elevation at which mercurial vapors precipitated on gold leaf cease perceptibly to rise in an air-filled space) have given consid- erable weight to the assumption of a definite surface of the atmosphere " similar to the surface of the sea." Can any gaseous particles belonging to the region of space blend with our atmosphere and produce meteorological changes ? New- ton* inclined to the idea that such might be the case. If we regard falling stars and meteoric stones as planetary as- teroids, we may be allowed to conjecture that in the streams of the so-called November phenomena,! when, as in 1799, 1833, and 1834, myriads of falling stars traversed the vault of heaven, and northern lights were simultaneously observed, our atmosphere may have received from the regions of space some elements foreign to it, which were capable of exciting electro-magnetic processes. II. NATURAL AND TELESCOPIC VISION. SCINTILLATION OF THE STARS VELOCITY OF LIGHT. RESULTS OF PHOTOMETRY. THE increased power of vision yielded nearly two hundred and fifty years ago by the invention of the telescope, has af- forded to the eye, as the organ of sensuous cosmical contem- plation, the noblest of all aids toward a knowledge of the contents of space, and the investigation of the configuration, physical character, and masses of the planets and their sat- ellites. The first telescope was constructed in 1608, seven years after the death of the great observer, Tycho Brahe. Its earliest fruits were the successive discovery of the satel- lites of Jupiter, the Sun's spots, the crescent shape of Venus, the ring of Saturn as a triple planetary formation (planeta tergeminus), telescopic stellar swarms, and the nebulae in Andromeda. J In 1634, the French astronomer Morin, emi- nent for his observations on longitude, first conceived the idea of mounting a telescope on the index bar of an instrument of measurement, and seeking to discover Arcfurus by day.$ * Newton, Princ. Mathem., t. iii. (1760), p. 671: "Vapores qui ex sole et stellis fixis et caudis cometarum oriuiitur, incidere posswit in at- mosphaeras planetarum " t Cosmos, vol. i., p. 124-135. t See Cosmos, vol. ii., p. 317-335, with notes. $ Delambre, Histoire de C Astronomic Moderne, torn, ii., p. 255, 269 42 COSMOS. The perfection in the graduation of the arc would have failed entirely, or to a considerable extent, in affording that great- er precision of observation at which it aimed, if optical and astronomical instruments had not been brought into accord, and the correctness of vision made to correspond with that of measurement. The micrometer-application of fine threads stretched in the focus of the telescope, to which that instru- ment owes its real and invaluable importance, was first de- vised, six years afterward (1640), by the young and talented Gascoigne.* While, as I have already observed, telescopic vision, ob- servation, and measurement extend only over a period of about 240 years in the history of astronomical science, we find, without including the epoch of the Chaldeans, Egyp- tians, and Chinese, that more than nineteen centuries have intervened between the age of Timochares and Aristillusf and the discoveries of Galileo, during which period the posi- tion and course of the stars were observed by the eye alone, unaided by instruments. When we consider the numerous disturbances which, during this prolonged period, checked the advance of civilization, and the extension of the sphere of ideas among the nations inhabiting the basin of the Medi- terranean, we are astonished that Hipparchus and Ptolemy should have been so well acquainted with the precession of the equinoxes, the complicated movements of the planets, the two principal inequalities of the moon, and the position of the stars ; that Copernicus should have had so great a knowledge of the true system of the universe ; and that Tycho Brahe should have been so familiar with the methods of practical astronomy before the discovery of the telescope. Long tubes, 272. Morin, in his work, Seientia Longitudinum, which appeared in 1634, writes as follows: Applicatio tubi optici ad alkidadam pro slellit fans prompte et accurate mensurandis a me excogitata est. Picard had not, up to the year 1667, employed any telescope on the mural circle; and Hevelius, when Halley visited him at Dantzic in 1679, and admired the precision of his measurement of altitudes, was observing through improved slits or openings. (Daily's Catal. of Stars, p. 38.) * The unfortunate Gascoigne, whose merits, remained so long unac- knowledged, lost his life, when scarcely twenty-three years of age, at the battle of Marston Moor, fought by Cromwell against the Royalists. See Derham, in the Philos. Transact., vol. xxx., for 1717-1719, p. 603 -610. To him belongs the merit of a discovery which was long ascribed to Picard and Auzout, and which has given an impulse previously un- known to practical astronomy, the principal object of which is to de- termine positions in the vault of heaven. t Cosmos, vol. iL, p. 177, 1F8. DIOPTRIC TUBES. 43 which were certainly employed by Arabian astronomers, and very probably also by the Greeks and Romans, may indeed, in some degree, have increased the exactness of the observa- tions by causing the object to be seen through diopters or slits. Abul-Hassan speaks very distinctly of tubes, to the extremi- ties of which ocular and object diopters were attached ; and instruments so constructed were used in the observatory founded by Hulagu at Meragha. If stars be more easily discovered during twilight by means of tubes, and if a star be sooner revealed to the naked eye through a tube than without it, the reason lies, as Arago has already observed, in the circumstance that the tube conceals a great portion of the disturbing light (rayons perturbateurs) diffused in the atmos pheric strata between the star and the eye applied to the tube. In like manner, the tube prevents the lateral impression of the faint light which the particles of air receive at night from all the other stars in the firmament. The intensity of the image and the size of the star are apparently augmented. In a fre- quently emendated and much contested passage of Strabo, in which mention is made of looking through tubes, this " en- larged form of the stars" is expressly mentioned, and is erro- neously ascribed to refraction.* * The passage in which Strabo (lib. iii., p. 138, Casaub.) attempts to refute the views of Posidonius is given as follows, according to the manuscripts : " The image of the sun is enlarged on the seas at its ris- ing as well as at its setting, because at these times a larger mass of ex- halations rises from the humid element ; and the eye, looking through these exhalations, sees images refracted into larger forms, as observed through lubes. The same thing happens when the setting sun or moon is seen through a dry and thin cloud, when those bodies likewise appear reddish." This passage has recently been pronounced corrupt (see Kramer, in Strabonis Geogr., 1844, vol. i., p. 211), and 81 vdAwv (through glass spheres) substituted for 61 avhuv (Schneider, Eclog. Phyt., vol. ii., p. 273). The magnifying power of hollow glass spheres, filled with water (Seneca, i., 6), was, indeed, as familiar to the ancients as the ac- tion of burning-glasses or crystals (Aristoph., Nub., v. 765), and that of Nero's emerald (Plin., xxxvii., 5); but these spheres most assuredly could not have been employed as astronomical measuring instruments. (Compare Cosmos, vol. ii., p. 245, and note J.) Solar altitudes, taken through thin, light clouds, or through volcanic vapors, exhibit no trace of the influence of refraction. (Humboldt, Recveil d'Obterv. Astr., vol. i., p. 123.) Colonel Baeyer observed no angular deviation in the heli- otrope light on the passage of streaks of mist, or even from artificially developed vapors, and therefore fully confirms Arago's experiments. Peters, at Pulkowa, in no case found a diiference of 0"'017 on compar- ing groups of stellar altitudes, measured in a clear sky, and through light clouds. See his Recherche* sur la Parallaxe des Etoiles, 1848, p. 80, 140-143 ; also Struve's Etudes Stellaires, p. 98. On the application of tubes for astronomical observation in Arabian instruments, see Jour- 44 COSMOS. Light, from whatever source it comes whether from th sun, as solar light, or reflected from the planets ; from the fixed stars ; from putrescent wood ; or as the product of the vital activity of glow-worms always exhibits the same con- ditions of refraction.* But the prismatic spectra yielded by different sources of light (as the sun and the fixed stars) ex- hibit a difference in the position of the dark lines (raies du spectre) which Wollaston first discovered in 1808, and the po- sition of which was twelve years afterward so accurately de- termined by Fraunhofer. While the latter observer counted 600 dark lines (breaks or interruptions in the colored spec- trum), Sir David Brewster, by his admirable experiments with nitric oxyd, succeeded, in 1833, in counting more than 2000 lines. It had been remarked that certain lines failed in the spectrum at some seasons of the year ; but Sir David Brew- ster has shown that this phenomenon is owing to different al- titudes of the sun, and to the different absorption of the rays of light in their passage through the atmosphere. In the spec- dam, Sur V Obseroatoire de Meragha, p. 27 ; and A. Sedillot, Mem. sur les Instruments Astronomiques des Arabes, 1841, p. 198. Arabian astron- omers have also the merit of having first employed large gnomons with small circular apertures. In the colossal sextant of Abu Mohammed al-Chokandi, the limb, which was divided into intervals of five minutes, received the image of the sun. " A midi les rayons du soleil passaient par une ouverture pratique dans la voflte de 1'observatoire qui couvrait {'instrument, suivaut le tuyau, et formaient sur la concavite du sextant une image circulaire, dont le centre donnait, sur 1'arc gradue, le com plement de la hauteur du soleil. Cet instrument differe de notre mural, qu'en ce qu'il etait garni d'un simple tuyau au lieu d'une lunette." " At noon, the rays of the sun passed through an opening in the dome of the observatory, above the instrument, and, following the tube, formed in the concavity of the sextant a circular image, the center of which marked the sun's altitude on the graduated limb. This instrument in no way differed from our mural circle, excepting that it was furnished with a mere tube instead of a telescope." Sedillot, p. 37, 202, 205. Dioptric rulers (pinnulce) were used by the Greeks and Arabs in determining the moon's diameter, and were constructed in such a manner that the circular aperture in the moving object diopter was larger than that of the fixed ocular diopter, and was drawn out until the lunar disk, seen through the ocular aperture, completely filled the object aperture. Delambre, Hist, de VAstron. du Moyen Age, p. 201 ; and S6dillot, p. 198. The adjustment of the dioptric rulers of Archimedes, with round aper- tures or slits, in which the direction of the shadows of two small cylin- ders attached to the same index bar was noted, seems to have been orig- inally introduced by Hipparchus. (Baily, Hist, de VAstron. Mod., 2d ed., 1785, torn, i., p. 480.) Compare also Theon Alexandria, Bas., 1538, p. 257, 262; Les Hypotyp. de Prochis Diadockus, ed. Halma, 1820, p 107, 110 ; and Ptolem. Almag., ed. Halma, torn, i., Par., 1813, p. Ivii. * According to Arago. See Moigno, Rtpert. d'Optique Moderne, 1847 p. 153. POLARIZATION OF LIGHT. 45 tra of the light reflected from the moon, from Venus, Mars, and the clouds, we recognize, as might be anticipated, all the peculiarities of the solar spectrum ; but, on the other hand, the dark lines in the spectrum of Sirius differ from those of Castor and the other fixed stars. Castor likewise exhibits dif- ferent lines from Pollux and Procyon. Amici has confirmed this difference, which was first indicated by Fraunhofer, and has ingeniously called attention to the fact that in fixed stars, which now have an equal and perfectly white light, the dark lines are not the same. A wide and important field is thus still open to future investigations,* for we have yet to distin- guish between that which has been determined with certain- ty and that which is merely accidental and depending on the absorbing action of the atmospheric strata. We must here refer to another phenomenon, which is pow- erfully influenced by the specific character of the source of light. The light of incandescent solid bodies, and the light of the electric spark, exhibit great diversity in the number and position of Wollaston's dark lines. From Wheatstone's remarkable experiments with revolving minors, it would ap- pear that the tight of frictional electricity has a greater veloc- ity than solar light in the ratio of 3 to 2 ; that is to say, a ve- locity of 95,908 miles in one second. The stimulus infused into all departments of optical science by the important discovery of polarization,! to which the in- genious Malus was led in 1808 by a casual observation of the light of the setting sun reflected from the windows of the Pa- lais du Luxembourg, has aflbrded unexpected results to sci- ence by the more thorough investigation of the phenomena of double refraction, of ordinary (Huygens's) and of chromatic po- larization, of interference, and of diffraction of light. Among these results may be reckoned the means of distinguishing between direct and reflected light, $ the power of penetrating, * On the relation of the dark lines on the solar spectrum in the Da- guerreotype, see Comptes Rendus des S6ance de I'Acadtmie des Science*, torn, xiv., 1842, p. 902-904, and torn, xvi., 1843, p. 402-407. t Cosmos, vol. ii., p. 332. t Arago's investigation of cometary light may hero be adduced as an instance of the important difference between proper and reflected light. The formation of the complementary colors, red and green, showed by the application of his discovery (in 1811) of chromatic polarization, that the light of Halley's comet (1835) contained reflected solar light. I was myself present at the earlier experiments for comparing, by means of the equal and unequal intensity of the images of the polariscope, the proper light of Capella with the splendid comet, as it suddenly emerged from the rays of the sun at the beginning of July, 1819. (See Annuaire 46 COSMOS. as it were, into the constitution of the body of the eun and of its luminous envelopes,* of measuring the pressure of at- du Bureau des Long, pour 1836, p. 232 ; Cosmos, vol. i., p. 105 ; aud Bes- eel, in Schumacher's Jahrbuchfur 1837, 1G9.) * Lettre de M. Arago a M. Alexandre de Humboldt, 1840, p. 37 : "A 1'aide d'un polariscope de mon invention, je reconnus (avant 1820) quo la lumiere de tous les corps terrestres incandescents, solides ou liquides, est de la lumiere naturelle, tant qu'elle emane du corps sous des inci- dences perpendiculaires. La lumiere, au contraire, qui sort de la surface incandescente sous un angle aigu, offre des marques manifestos de po- larisation. Je ne m'arrete pas a te rappeler ici, comment je d6duisis de ce fait la consequence curieuse que la lumiere ne s'eugendre pas seulement a la surface des corps ; qu'une portion nalt dans leur sub- ttance ineme, cette substance fut-elle du platine. J'ai seulement besoin de dire qu'en repetant la meme serie d'epreuves, et avec les mmes instruments sur la lumiere que lance une substance gazeuse enflammee, on ne lui trouve, sous quelque incllnaison que ce soit, aucun des carac- teres de la lumiere polarisee; que la lumiere des gaz, prise a la sortie de la surface enflammee, est de la lumiere naturelle, ce qui n'empeche pas qu'elle ne se polarise ensuite completement si on la soumet a des reflexions ou a des refractions conveuables. De la une methode trea simple pour decouvrir a 40 millions de lieues de distance la nature du soleil. La lumiere proveuant du hard de cet astre, la lumiere emanee de la matiere solaire sous un angle aigu, et nous arrivant sans avoir eprouve en route des reflexions ou des refractions sensibles, offre-t-elle des traces de polarisation, le soleil est un corps solide ou liquide. S'il n'y a, au contraire, aucun indice de polarisation dans la lumiere du bord, la parte incandescente du soleil est gazeuse. C'est par cet euchainement methodique d'observations qu'on peut arriver a des notions exactes sur la constitution physique -du soleil." " By the aid of my polariscope I discovered (before 1820) that the light of all terrestrial objects in a state of incandescence, whether they be solid or liquid, is natural as long as it emanates from the object in perpendicular rays. The light emanating from an incandescent surface at an acute angle presents, on the other hand, manifest proofs of polar- ization. I will not pause to remind you that this circumstance has led me to the remarkable conclusion that light is not generated on the sur- face of bodies only, but that some portion is actually engendered within the substance itself, even in the case of platinum. I need only here ob- serve, that in repeating the same series of experiments (aud with the same instruments) on the light emanating from a burning gaseous sub- stance, I could not discover any characteristics of polarized light, what- ever might be the angle at which it emanated ; and I found that the light of gaseous bodies is natural light when it issues from the burning sur- face, although this circumstance does not prevent its subsequent com- plete polarization, if subjected to suitable reflections or refractions. Hewoe we obtain a most simple method of discovering the nature of the sun at a distance of 40 millions of leagues. For if the light emanating from the margin of the sun, and radiating from the solar substance at an acute angle, reach us without having experienced any sensible reflec- tions or refractions in its passage to the earth, and if it offer traces of polarization, the sun must be a solid or a liquid body. Put if, on the contrary, the light emanating from tke sun's margin giv- no indications of polarization, the incandescent portion of the sun inuetbe %aeuu. it POLARIZATION OF LIGHT. 47 mospheric strata, and even the smallest amount of water they contain, of scrutinizing the depths of the ocean and its rocks by means of a tourmaline plate,* and, in accordance with Newton's prediction, of comparing the chemical compositionf of several substances^ with their optical effects. It will be sufficient to mention the names of Airy, Arago, Biot, Brew- ster, Cauchy, Faraday, Fresnel, John Herschel, Lloyd, Ma- lus, Neumann, Plateau, Seebeck, to remind the sci- entific reader of a succession of splendid discoveries and of their happy applications. The great and intellectual labors of Thomas Young more than prepared the way for these im- portant efforts. Arago's polariscope and the observation of the position of colored fringes of diffraction (in consequence of interference) have been extensively employed in the pros- ecution of scientific inquiry. Meteorology has made equal advances with physical astronomy in this new path. However diversified the power of vision may be in differ- ent persons, there is nevertheless a certain average of organ- is by means of such a methodical sequence of observations that we may acquire exact ideas regarding the physical constitution of the sun." (On the Envelopes of the Sun, see Arago, in the Annuaire pour 1846, p. 464.) I give all the circumstantial optical disquisitions which I have borrowed from the manuscript or printed works of my friend, in his own words, in order to avoid the misconceptions to which the variations of scientific terminology might give rise in retranslating the passages into French, or any other of the various languages in which the Cosmos has appeared. * " Sur 1'effet d'une lame de tourmaline taillee parallelement aux aretes du prisme servant, lorsqu'elle est convenablement situee, a 61i- miner en totalite les rayons reflechis par la surface de la mer et meles la lumiere provenant de 1'ecueil." " On the effect of a tourmaline plate cut parallel to the edges of the prism, in concentrating (when placed in a suitable position) all the rays of light reflected by the surface of the sea, and blended with the light emanating from the sunken rocks." See Arago, Instructions de la Bonite, in the Annuaire pour 1836, p. 339 343. t " De la possibilit6 de d6terminer les pouvoirs refringents des corps d'apres leur composition chimique." On the possibility of determining the refracting powers of bodies according to their chemical composition applied to the ratio of the oxygen to the nitrogen in atmospheric air, to the quantity of hydrogen contained in ammonia and in water, to car- bonic acid, alcohol, and the diamond). See Biot ct Arago, Mtmoire sur les Ajfinites des Corps pour la Lumiere, Mars, 1806; also Mlmoirts Mathem. et Phys. de V Inslilut, t. vii., p. 327-346 ; and my M6moire gur les Refractions Astronomiques dans la Zone Torride, in the Recueit d'Obsew. Astron., vol. i., p. 115 and 122. t Experiences de M. Arago sur la puissance Refractive des Corps D\- aphanes (de I' air sec ct de I' air humide) par le Replacement des Franges, iu Moigno, Repertoire d'Oplique Mod., 1847, p. 159-K52. 48 COSMOS. fc capacity, which was the same among former generation!, as, for instance, the Greeks and Romans, as at the present day. The Pleiades prove that several thousand years ago, even as now, stars which astronomers regard as of the sev- enth magnitude, wera invisible to the naked eye of average visual power. The group of the Pleiades consists of one star of the third magnitude, Alcyone ; of two of the fourth, Electra and Atlas ; of three of the fifth, Merope, Maia, and Taygeta ; of two hetween the sixth and the seventh magni- tudes, Pleione and Celseno ; of one between the seventh and the eighth, Asterope ; and of many very minute telescopic stars. I make use of the nomenclature and order of succes sion at present adopted, as the same names were among the ancients in part applied to other stars. The six first-named stars of the third, fourth, and fifth magnitudes were the only ones which could be readily distinguished.* Of these Ovid says (Fast., iv., 170), " Qua; septem dici, sex tamen esse solent." One of the daughters of Atlas, Merope, the only one who was wedded to a mortal, was said to have veiled herself for very shame, or even to have wholly disappeared. This is probably the star of about the seventh magnitude, which we call Celajno ; for Hipparchus, in his commentary on Aratus, observes that on clear moonless nights seven stars may ac- tually be seen. Celseno, therefore, must have been seen, for Pleione, which is of equal brightness, is too near to Atlas, a star of the fourth magnitude. The little star Alcor, which, according to Triesnecker, is situated in the tail of the Great Bear, at a distance of 11' * Hipparchus says (ad Arati Phcen., 1, p. 190, in Uranologio Pctavii), in refutation of the assertion of Aratus that there were only six stars visible in the Pleiades : " One star escaped the attention of Aratus. For when the eye is attentively fixed on this constellation on a serene and moonless night, seven stars are visible, and it therefore seems strange that Attains, in his description of the Pleiades, should have neglected to notice this oversight on the part of Aratus, as though he regarded the statement as correct." Merope is called the invisible (nava<j>avjjf') in the Catasterisms (XXIII.) ascribed to Eratosthenes. On a supposed connection between the name of the veiled (the daughter of Atlas) with the geographical myths in the Meropis of Theopompus, as well as with the great Saturnian Continent of Plutarch and the Atlantis, see my Ex amen Grit, de VHist. de la Geographic, t. i., p. 170. Compare also Ideler Untersuchungen fiber den Ursprung imd die Bedeutung der Sternnamen, 1809, p. 145; and in reference to astronomical determination of place, consult Madler, Unttrsucli. ubet die Fixstern-Systeme, th. ii., 1848, s. 38 and 166 ; also Baily in the Mem. of the Astr. Soc., vol. xiii., p. 33. VISIBILITY OF STARS. 49 48 from Mizar, is, according to Argelander, of the fifth magnitude, but overpowered by the rays of Mizar. It was called by the Arabs Saidak, " the Test," because, as the Per- sian astronomer Kazwini* remarks, " It was employed as a * See Ideler, Sternnamen, s. 19 and 25. Arago, in manuscript notices of the year 1847, writes as follows: " On observe qu'une lumiere forte fait disparaltre une lumiere faible placee dans le voisinage. Quelle peut en etre la cause ? II est possible physiologiquement que 1'ebran- lement communique k la retine par la lumiere forte s'etend au del& des points que la lumiere forte a frappes, et que cet ebranlement secon- daire absorbe et neutralise en quelque sorte 1'ebranlement provenant de la seconde et faible lumiere. Mais sans entrer dans ces causes physio- logiques, il y a une cause directe qu'on peut indiquer pour la dispari- tion de la faible lumiere : c'est que les rayons provenant de la grande n'ont pas seulement forme une image nette sur la retine, mais se sont disperses aussi sur toutes les parties de cet organe cause des imper- fections de transparence de la cornee. Les rayons du corps plus bril- lant a en traversant la cornee se comportent comme en traversant un corps legerement depoli. Une partie des ces rayons refractes reguliere- ment forme 1'image neme de a, 1'autre partie disperses eclaire la totalite de la retine. C'est done sur ce fond lumineux que se projette 1'image de 1'objet voisin b. Cette derniere image doit done ou disparaltre ou etre affaiblie. De jour deux causes contribuent 1'affaiblissement des etoiles. L'une de ces causes c'est 1'image distincte de cette portion de ratmosphere comprise dans la direction de 1'etoile (de la portion aeri- enne placee entre 1'oeil et 1'etoile) et sur laquelle 1'image de 1'etoile vient de se peindre ; 1'autre cause c'est la lumiere diffuse provenant de la dis- persion que les defauts de la cornee imprlment aux rayons emanants de tous les points de 1'atmosphere visible. De nuit les couches atmosphe- riques interposees entre 1'oeil et 1'etoile vers laquelle on vise, n'agissent pas ; chaque etoile du firmament forme une image plus nette, mais une partie de leur lumiere se trouve dispersee a, cause du manque de dia- phanite de la cornee. Le me me raisonnement s'applique 4 une deux- icme, troisieme .... millidme etoile. La retine se trouve done eclai- ree en totalite par une lumiere diffuse, proportionnelle au nombre de ces etoiles et a leur eclat. On con^oit par 1& que cette somme de lu- miere diffuse affaiblisse ou fasse entierement disparaitre 1'image do 1'etoile vers laquelle on dirige la vue." " We find that a strong light causes a fainter one placed near it to dis- appear. What can be the cause of this phenomenon ? It is physiolog- ically possible that the vibration communicated to the retina by strong light may extend beyond the points excited by it; and that this secondary vioration may in some degree absorb and neutralize that arising from the second feeble light. Without, however, entering upon these physiologic- al considerations, there is a direct cause to which we may refer the disap- pearance of the feeble light, viz., that the rays emanating from the strong light, after forming a perfect image on the retina, are dispersed over all parts of this organ in consequence of the imperfect transparency of the cornea. The rays of the more brilliant body a, in passing the cornea, are affected in th.2 same manner as if they were transmitted through a body whose surface was not perfectly smooth. Some of these regularly refracted rays form the image a, while the remainder of the dispersed rays illumine the whole jf the retina. On this luminous ground the VOL. III. C 60 COSMOS. test of the power of vision." Notwithstanding the low po- sition of the Great Bear under the tropics, I have very dis- tinctly seen Alcor, evening after evening, with the naked eye, on the rainless shores of Cumana, and on the plateaux of the Cordilleras, which are elevated nearly 13,000 feet above the level of the sea, while I have seen it less frequent- ly and less distinctly in Europe and in the dry atmosphere of the Steppes of Northern Asia. The limits within which the naked eye is unable to separate two very contiguous ob- jects in the heavens depend, as Madler has justly observed, on the relative brilliancy of the stars. The two stars of the third and fourth magnitudes, marked as a Capricorni, which are distant from each other six and a half minutes, can with ease be recognized as separate. Galle thinks that and 6 Lyrse, being both stars of the fourth magnitude, may be dis- tinguished in a very clear atmosphere by the naked eye, al- though situated at a distance of only three and a half min- utes from each other. The preponderating effect of the rays of the neighboring planet is also the principal cause of Jupiter's satellites re- maining invisible to the naked eye ; they are not all, how- ever, as has frequently been maintained, equal in brightness to stars of the fifth magnitude. My friend, Dr. Galle, has found from recent estimates, and by a comparison with neighboring stars, that the third and brightest satellite is probably of the fifth or sixth magnitude, while the others, which are of various degrees of brightness, are all of the sixth or seventh magnitude. There are only few cases on record in which persons of extraordinarily acute vision that is to say, capable of clearly distinguishing with the naked eye image of the neighboring object b is projected. This last imago must therefore either wholly disappear or be dimmed. By day two causes contribute to weaken the light of the stars ; one is the distinct image of that portion of the atmosphere included in the direction of the star (the aerial field interposed between the eye and the star), and on which the image of the star is formed, while the other is the light diffused by the dispersion which the defects of the cornea impress on the rays em- anating from all points of the visible atmosphere. At night, the strata of air interposed between the eye and the star to which we direct the instrument, exert no disturbing action ; each star in the firmament forms a more perfect image, but a portion of the light of the stars is dispered in consequence of the imperfect transparency of the cornea. The same reasoning applies to a second, a third, or a thousandth star. The retina, then, is entirely illumined by a diffused light, proportionate to the num- ber of the stars and to their brilliancy. Hence we may imagine that the aggregate of this diffused light must either weaken, or entirely ob- literate the imago of tlie star toward which the eye is directed." VISIBILITY OF STARS. 51 gtars fainter than those of the sixth magnitude have been able to distinguish the satellites of Jupiter without a tele- scope. The angular distance of the third and brightest sat- ellite from the center of the planet is 4' 42" ; that of the fourth, which is only one sixth smaller than the largest, is 8' 16" ; and all Jupiter's satellites sometimes exhibit, as Ar- ago maintains,* a more intense light for equal surfaces than * Arago, in the Annuaire pour 1842, p. 284, and in the Compte* Rendus, torn, xv., 1842, p. 750. (Schutn., Astron. Nachr., No. 702.) " I have instituted some calculations of magnitudes, in reference to your conjectures on the visibility of Jupiter's satellites," writes Dr. Galle, in letters addressed to me, " but I have found, contrary to my expecta- tions, that they are not of the fifth magnitude, but, at most, only of the sixth, or even of the seventh magnitude. The third and brightest sat- ellite alone appeared nearly equal in brightness to a neighboring star of the sixth magnitude, which I could scarcely recognize with the naked eye, even at some distance from Jupiter ; so that, considered in refer- ence to the brightness of Jupiter, this satellite would probably be of the fifth or sixth magnitude if it were isolated from the planet. The fourth satellite was at its greatest elongation, but yet I could not estimate it at more than the seventh magnitude. The rays of Jupiter would not pre- vent this satellite from being seen if it were itself brighter. From a comparison of Aldebaran \vith the neighboring star 6 Tauri, which is easily recognized as a double star (at a distance of 5J minutes), I should estimate the radiation of Jupitr at five or six minutes, at least, for or- dinary vision." These estimates correspond with those of Arago, who is even of opinion that this false radiation may amount in the case of some persons to double this quantity. The mean distances of the four satellites from the center of the main planet are undoubtedly 1' 51", 2' 57", 4' 42", and 8' 16". " Si nous supposons que 1'image de Jupiter, dans certains yeux exceptionnels, s'epanouisse seulement par des ray- ons d'une ou deux minutes d'amplitude, il ne semblera pas impossible que les satellites soient de terns en terns aper9us, sans avoir besoin de recourir a 1'artifice de 1'amplification. Pour verifier cette conjecture, j'ai fait construire une petite lunette dans laquelle 1'objectif et 1'ocu- laire ont a peu pres le menie foyer, et qni des lors lie grossit point. Cette lunette ne detruit pas eutierement les rayons divergents, mais elle en reduit considerablement la longueur. Cela a suffi your qu'un satellite convenablement 6cart6 de la plandte, soil devenu visible. Le fait a ete constate par tous les jeunes astronomes de 1'Observatoire." " If we suppose that the image of Jupiter appears to the eyes of some persons to be dilated by rays of only one or two minutes, it is nit im- possible that the satellites may from time to time be seen without the aid of magnifying glasses. In order to verify this conjecture, I caased a small instrument to be constructed in which the object-glass and the eye-piece had nearly the same focus, and which, therefore, did not mag itify. This instrument does not entirely destroy the diverging rays, al though it considerably reduces their length. This method has sufficed to render a satellite visible 'when at a sufficient distance from the planet. This observation has been confirmed by all the young astronomers at the Observatory." (Arago, in the Comptes Rendus, torn, xv., 1842, p. 52 COSMOS. Jupiter himself; occasionally, however, as shown by recent observations, they appear like gray spots on the planet. The rays or tails, which to our eyes appear to radiate from the planets and fixed stars, and which were used, since the ear- liest ages of mankind, and especially among the Egyptians, as pictorial representations to indicate the shining orbs of heaven, are at least from five to six minutes in length. (These lines are regarded by Hassenfratz as caustics on the crystalline lens : intersections des deux caustiques.} " The image of the star which we see with the naked eye is magnified by diverging rays, in consequence of which it occupies a larger space on the retina than if it were concen- As a remakable instance of acute vision, and of the great sensibility of the retina in some individuals who are able to see Jupiter's satellites with the naked eye, I may instance the case of a master tailor, named Schbn, who died at Breslau in 1837, and with reference to whom I have received some interesting communications from the learned and active director of the Breslau Observatory, Von Boguslawski. " After having (since 1820) convinced ourselves, by several rigid tests, that in serene moonless nights Schbn was able correctly to indicate the position of sev- eral of Jupiter's satellites at the same time, we spoke to him of the em- anations and tails which appeared to prevent others from seeing so clearly as he did, when he expressed his astonishment at these ob- structing radiations. From the animated discussions between himself and the by-standers regarding the difficulty of seeing the satellites with the naked eye, the conclusion was obvious, that the planet and fixed stars must always appear to Schbii like luminous points having no rays. He saw the third satellite the best, and the first very plainly when it was at the greatest digression, but he never saw the second and the fourth alone. When the air was not in a very favorable condition, the satellites appeared to him like faint streaks of light. He never mistook small fixed stars for satellites, probably on account of the scintillating and less constant light of the former. Some years before his death Schbn complained to me that his failing eye could no longer distinguish Jupiter's satellites, whose position was only indicated, even in clear weather, by light faint streaks." These circumstances entirely coin- cide with what has been long known regarding the relative luster of Jupiter's satellites, for the brightness and quality of the light probably exert a greater influence than mere distance from the main planet on persons of such great perfection and sensibility of vision. Schbii never saw the second nor the fourth satellite. The former is the smallest of all ; the latter, although the largest after the third and the most remote, is periodically obscured by a dark color, and is generally the faintest of all the satellites. Of the third and the first, which were best and most frequently seen by the naked eye, the former, which is the largest of all, is usually the brightest, and of a very decided yellow color ; the latter occasionally exceeds in the intensity of its clear yellow light the luster of the third, which is also much larger. (Madler, Astr., 1846, s. 231-234, and 439.) Sturm and Airy, in the Complex Rendut, t. xx., p. 764-6, show how, under proper conditions of refraction in the organ of vision, remote luminous poin'a may appear as light streaks. NATURAL VISION. 53 trated in a single point. The impression on the nerves is weaker. A very dense starry swarm, in which scarcely any of the separate stars belong even to the seventh magnitude, may, on the contrary, be visible to the unaided eye in con- sequence of the images of the many different stars crossing each other upon the retina, by which every sensible point of its surface is more powerfully excited, as if by one concen- trated image."* * " L'image fpanouie d'ane etoile de 7eme grandeur n'ebranle pas suffisamment la retine : elle n'y fait pas naitre une sensation apprecia- ole de lumiere. Si 1'image n'etait point epanouie (par des rayons di- vergents), la sensation aurait plus de force, et 1'etoile se verrait. La premiere classe d'etoiles invisibles a 1'ceil nu ne serait plus alors la sep- tieme: pour la trouver, il faudrait peut-etre descendre alors jusqu'a la 12etne. Considerons un groupe d'etoiles de 7eme grandeur tellement rapproch6es les unes des autres que les intervalles echappent necessaire- ment a 1'oeil. Si la vision avait de la nettete, si 1'image de chaque etoile etait tres petite et bien termin&e, 1'observateur aperceverait un champ de lumiere dont chaque point aurait Veclat concentre d'une etoile de 7eme grandeur. I? eclat concentre d'une etoile de 7eme grandeur suffit a la vision a 1'oeil nu. Le groupe serait done visible a I'ojil nu. Di- latons maintenant sur la retine 1'image de chaque etoile du groupe ; remplasons chaque point de 1'ancienne image generale par un petit cer- cle : ces cercles empieteront les uns sur les autres, et les divers points de la retine se trouveront eclaires par de la lumiere venaut simultan - ment de plusieurs etoiles. Pour peu qu'on y reflechisse, il restera evi- dent qu' excepte sur les bords de 1'image g6nerale, 1'aire lumineuse ainsi eclairee a precisement, a cause de la superposition des cercles, la mme inteusite que dans le cas ou chaque etoile n'eclaire qu'un seul point au fond de 1'ceil ; mais si chacun de ces points re<joit une lumiere 6gale en intensity a la lumiere concentree d'une etoile de 7eme gran- deur, il est clair que 1'epanouissement des images individuelles des fetoiles contigues ne doit pas empecher la visibilite de 1'ensemble. Les instruments telescopiques ont, quoiqu'a un beaucoup momdre degre, le defaut de donner aussi aux etoiles un diamitre sensible et factice. Avec ces instruments, comme & 1'ceil nu, on doit done apercevoir des groupes, composes d'etoiles inferieures en intensite a celles que les memea lu- nettes ou telescopes feraient apercevoir isolement." " The expanded image of a star of the seventh magnitude doen not cause sufficient vibration of the retina, and does not give rise to an ap- preciable sensation of light. If the image were not expanded (by di- vergent rays), the sensation would be stronger and the star discernible. The lowest magnitude at which stars are visible would not therefore be the seventh, but some magnitude as low perhaps as the twelfth de- gree. Let us consider a group of stars of the seventh magnitude so close to one another that the intervals between them necessarily escape the eye. If the sight were very clear, and the image of each star small and well defined, the observer would perceive a field of light, each point of which would be equal to the concentrated brightness of a star of the seventh magnitude. The concentrated light of a star of the sev- enth magnitude is sufficient to be seen by the naked eye. The group, therefore, would be visible to the naked eye. Let us now dilate the 54 COSMOS. Telescopes, although in a much less degree, unfortunately also give the stars an incorrect and spurious diameter ; but, according to the splendid investigations of Sir William Her- echel,* these diameters decrease with the increasing power of the instrument. This distinguished observer estimated that, at the excessive magnifying power of 6500, the appar- ent diameter of Vega Lyrae still amounted to 0"36. In ter- restrial objects, the form, no less than the mode of illumina- tion, determines the magnitude of the smallest angle of vision for the naked eye. Adams very correctly observed that a long and slender staff can be seen at a much greater distance than a square whose sides are equal to the diameter of the staff. A stripe may be distinguished at a greater distance than a spot, even when both are of the same diameter. Ara- go has made numerous calculations on the influence of form (outline of the object) by means of angular measurement of distant lightning conductors visible from the Paris Observa- tory. The minimum optical visual angle at which terres- trial objects can be recognized by the naked eye has been gradually estimated lower and lower from the time when Robert Hooke fixed it exactly at a full minute, and Tobias Mayer required 34" to perceive a black speck on white pa- per, to the period of Leeuwenhoek's experiments with spi- der's threads, which are visible to ordinary sight at an angle of 4" - 7. In the recent and most accurate experiments of Hueck, on the problem of the movement of the crystalline image of each star of the group on the retina, and substitute a small circle for each point of the former general image ; these circles will impinge upon one another, and the different points of the retina will be illumined by light emanating simultaneously from many stars. A slight consideration will show, that, excepting at the margins of the general image, the luminous air has, in consequence of the superposi- tion of the circles, the same degree of intensity as in those cases where each star illumines only one single point of the retina ; but if each of these points be illumined by a light equal in intensity to the concen- trated light of a star of the seventh magnitude, it is evident that the dilatation of the individual images of contiguous stars can not prevent the visibility of the whole. Telescopic instruments have the defect, although in a much less degree, of giving the stars a sensible and spu. rious diameter. We therefore perceive with instruments, no less than with the naked eye, groups of stars, inferior in intensity to those which the same telescopic or natural sight would recognize if they were iso- lated." Arago, in the Annuaire du Bureau des Longitudes pour Van 1842, p. 284. * Sir William Herschel, in the Philos. Transact, for 1803, vol. 93, p. "225, and for 1805, vol. 94, p. 184. Compare also Arago, in the An nuairepour 1842, p. 360-374. VISIBILITY OP OBJECTS. 55 lens, white lines on a black ground were seen at an angle of l"-2; a spider's thread at 0"-6 ; and a fine glistening wire at scarcely 0"'2. This problem does not admit gen- erally of a numerical solution, since it entirely depends on the form of the objects, their illumination, their contrast with the back-ground, and on the motion or rest, and the nature of the atmospheric strata in which the observer is placed. During my visit at a charming country-seat belonging to the Marquess de Selvalegre at Chillo, not far from Q,uito, where the long-extended crests of the volcano of Pichincha lay stretched before me at a horizontal distance, trigonomet- rically determined at more than 90,000 feet, I was much struck by the circumstance that the Indians who were stand- ing near me distinguished the figure of my traveling com- panion Bonpland (who was engaged in an expedition to the volcano) as a white point moving on the black basaltic sides of the rock, sooner than we could discover him with our tel- escopes. The white moving image was soon detected with the naked eye both by myself and by my friend the unfor- tunate son of the marquess, Carlos Montufar, who subsequent- ly perished in the civil war. Bonpland was enveloped in a white cotton mantle, the poncho of the country ; assuming the breadth across the shoulders to vary from three to five feet, according as the mantle clung to the figure or fluttered in the breeze, and judging from the known distance, we found that the angle at which the moving object could be distinctly seen varied from 7" to 12". White objects on a black ground are, according to Hueck's repeated experiments, distinguish- ed at a greater distance than black objects on a white ground. The light was transmitted in serene weather through rar- efied strata of air at an elevation 15,360 feet above the level of the sea to our station at Chillo, which was itself sit- uated at an elevation of 8575 feet. The ascending distance was 91,225 feet, or about 17 miles. The barometer and thermometer stood at very different heights at both stations, being probably at the upper one about 17*2 inches and 46'4, while at the lower station they were found, by accurate ob- servation, to be 22-2 inches and 65-7. Gauss's heliotrope light, which has become so important an element in German trigonometrical measurements, has been seen with the naked eye reflected from the Brocken on Hohenhagen, at a distance of about 227,000 feet, or more than 42 miles, being fre- quently visible at points in which the apparent breadth of a three-inch mirror was only 0"'43. 56 COSMOS. The visibility of distant objects irf modified by the absorp- tion of the rays passing from the terrestrial object to the* naked eye at unequal distances, and through strata of air more or less rarefied and more or less saturated with moist- ure ; by the degree of intensity of the light diffused by the radiation of the particles of air ; and by numerous meteoro- logical processes not yet fully explained. It appears from the old experiments of the accurate observer Bouguer that a difference of ^th in the intensity of the light is necessary to render objects visible. To use his own expression, we only negatively see mountain-tops from which but little light is radiated, and which stand out from the vault of heaven in the form of dark masses ; their visibility is solely owing to the difference in the thickness of the atmospheric strata ex- tending respectively to the object and to the horizon. Strong ly-illumined objects, such as snow-clad mountains, white chalk cliffs, and conical rocks of pumice-stone, are seen pos- itively. The distance at which high mountain summits may be recognized from the sea is not devoid of interest in relation to practical navigation, where exact astronomical determina- tions are wanting to indicate the ship's place. I have treat- ed this subject more at length in another work,* where I considered the distance at which the Peak of Teneriffe might be seen. The question whether stars can be seen by daylight with the naked eye through the shafts of mines, and on very high mountains, has been with me a subject of inquiry since my early youth. I was aware that Aristotle had maintained! * Humboldt, Relation Hist, du Voyage aux Regions Equinox., torn. i., p. 92-97; and Bouguer, Traiti d'Optique, p. 360 and 365. (Com- pare, also, Captain Beechey, in the Manual of Scientific Inquiry for the Use of the Royal Navy, 1849, p. 71.) t The passage in Aristotle referred to by Buffon occurs in a work where we should have least expected to find it De Generat. Animal., \. i., p. 780, Bekker. Literally translated, it runs as follows : " Keen- ness of sight is as much the power of seeing far as of accurately distin- guishing the differences presented by the objects viewed. These two properties are not met with in the same individuals. For he who holds his hand over his eyes, or looks through a tube, is not, on that account, more or less able to distinguish differences of color, although he will see objects at a greater distance. Hence it arises that persons in cavern* or cisterns are occasionally enabled, to see stars." The Grecian 'Ooiiy/za- ra, and more especially (ppeara, are, as an eye-witness, Professor Franz, observes, subterranean cisterns or reservoirs which communicate with the light and air by means of a vertical shaft, and widen toward the bot- tom, like the neck of a bottle. Pliny (lib. ii., cap. 14) ays, " Altituda FISIBILITT OP STARS. 57 that stars might occasionally be seen from ctverns and cis- terns, as through tubes. Pliny alludes to the same circum- stance, and mentions the stars that have been most distinctly recognized during solar eclipses. While practically engaged in mining operations, I was in the habit, during many years, of passing a great portion of the day in mines where I could see the sky through deep shafts, yet I never was able to ob- serve a star ; nor did I ever meet with any individual in the Mexican, Peruvian, or Siberian mines who had heard of stars having been seen by daylight ; although in the many latitudes, in both hemispheres, in which I have visited deep mines, a sufficiently large number of stars must have passed the zenith to have afforded a favorable opportunity for their being seen. Considering this negative evidence, I am the more struck by the highly credible testimony of a celebrated optician, who in his youth saw stars by daylight through the shaft of a chimney.* Phenomena, whose manifestation de- pends on the accidental concurrence of favoring circum- stances, ought not to be disbelieved on account of their rarity The same principle must, I think, be applied to the asser- tion of the profound investigator Saussure, that stars have been seen with the naked eye in bright daylight, on the de- clivity of Mont Blanc, and at an elevation of 12,757 feet " Ctuelques-uns des guides m'ont assure avoir vu des etoiles en plein jour ; pour moi je n'y songeais pas, en sorte que je n'ai point ete le temoin de ce phenomene ; mais I' assertion uniforme des guides ne me laisse aucun doute sur la rea- lite. II faut d'ailleurs etre entitlement a 1'ombre d'une epais- seur considerable, sans quoi 1'air trop fortement eclaire fait evanouir la faible clarte des etoiles." " Several of the guides assured me," says this distinguished Alpine inquirer, "that cogit minores videri Stellas ; affixas ccelo-solis fulgor interdiu non cerni, quum aeque ac noctu luceant ; idque manifesto m fiat defectu soils et prce- altis puteis." Cleomedes ( Cycl. Theor., p. 83, Bake) does not speak of stars seen by day, but asserts " that the sun, when observed from deep cisterns, appears larger, on account of the darkness and the damp air." * " We have ourselves heard it stated by a celebrated optician that the earliest circumstance which drew his attention to astronomy waa the regular appearance, at a certain hour, for several successive days, of a considerable star, through the shaft of a chimney." John Herschel, tlines of Astr., 61. The chimney-sweepers whom I have ques- Ontlines of Astr., $ 61. The chimney-sweepers whom I have ques- tioned agree tolerably well in the statement that " they have never seen stars by day, but that, when observed at night, through deep shafts, the sky appeared quite near, and the stars larger." I will not enter upon any discussion regarding the connection between these two illusions. C 2 58 COSMOS they had seen stars at broad daylight : not having myself been a witness of this phenomenon, I did not pay much at- tention to it, but the unanimous assertions of the guides left me no doubt of its reality.* It is essential, however, that the observer should be placed entirely in the shade, and that he should even have a thick and massive shade above his head, since the stronger light of the air would otherwise dis- perse the faint image of the stars." These conditions are therefore nearly the same as those presented by the cisterns of the ancients, and the chimneys above referred to. I do not find this remarkable statement (made on the moniing of the 2d of August, 1787) in any other description of the Swiss mountains. Two well-informed, admirable observers, the brothers Hermann and Adolph Schlagentweit, who have re- cently explored the eastern Alps as far as the summit of the Gross Glockner (13,016 feet), were never able to see stars by daylight, nor could they hear any report of such a phe- nomenon having been observed among the goatherds and chamois-hunters. Although I passed many years in the Cordilleras of Mexico, Quito, and Peru, and frequently in clear weather ascended, in company with Bonpland, to ele- vations of more than fifteen or sixteen thousand feet above the level of the sea, I never could distinguish stars by day- light, nor was my friend Boussingault more successful in his subsequent expeditions ; yet the heavens were of an azure so intensely deep, that a cyanometer (made by Paul of Geneva) which had stood at 39 when observed by Saussure on Mont Blanc, indicated 46 in the zenith under the tropics at ele- vations varying between 17,000 and 19,000 feet.f Under the serene etherially-pure sky of Cumana, in the plains near the sea-shore, I have frequently been able, after observing an eclipse of Jupiter's satellites, to find the planet again with the naked eye, and have most distinctly seen it when the gun's disk was from 18 to 20 above the horizon. The present would seem a fitting place to notice, although cursorily, another optical phenomenon, which I only observed once during my numerous mountain ascents. Before sunrise, on the 22d of June, 1799, when at Malpays, on the decliv- ity of the Peak of Teneriffe, at an elevation of about 11,400 feet above the sea's level, I observed with the naked eye * Consult Saussure, Voyage dans les Alpes (Neuchatel, 1779, 4to), torn, iv., 2007, p. 199. t Humboldt, Essai sur la G6ographie des Plantes, p. 103. Compare also my Voy. aux Regions Equinox, torn, i., p. 143, 248. UNDULATION OP THE STARS. 5 cars near the horizon flickering with a singular oscillating motion. Luminous points ascended, moved laterally, and feii back to their former position. This phenomenon lasted only from seven to eight minutes, and ceased long before the sun's disk appeared above the horizon of the sea. The same .motion was discernible through a telescope, and there was no doubt that it was the stars themselves which moved.* Did this change of position depend on the much-contested phenomenon of lateral radiation ? Does the undulation of the rising sun's disk, however inconsiderable it may appear when measured, present any analogy to this phenomenon in the lateral alteration of the sun's margin ? Independently of such a consideration, this motion seems greater near the horizon. This phenomenon of the undulation of the stars was observed almost half a century later at the same spot by a well-informed and observing traveler, Prince Adalbert of Prussia, who saw it both with the naked eye and through a telescope. I found the observation recorded in the prince's manuscript journal, where he had noted it down, before he learned, on his return from the Amazon, that I had wit- nessed a precisely similar phenomenon.! I was never able to detect any trace of lateral refraction on the declivities of the Andes, or during the frequent mirages in the torrid plains or Llanos of South America, notwithstanding the het- erogeneous mixture of unequally-heated atmospheric strata. As the Peak of Tenerifie is so near us, and is so frequently * Humboldt, in Fr. von Zach's Monatliche Correspondenz zur Erd- und Himmels-Kunde, bd. i., 1800, s. 396 ; also Voy. aux R6g. Equin., torn, i., p. 125 : " On croyait voir de petites fusses lancees dans 1'air. Des points lumineux eleves de 7 a 8 degres, paraissent d'abord se mou- voir dans le sens vertical, mais puis se convertir en une veritable oscil- lation horizontale. Ces images lumineux etaient des images de plu- sieurs etoiles agrandies (en apparence) par des vapeurs et revenant au meme point d'ou elles etaient partis." " It seemed as if a number of sinall rockets were being projected in the air ; luminous points, at an elevation of 7 or 8, appeared moving, first in a vertical, and then os- cillating iu a horizontal direction. These were the images of many stars, apparently magnified by vapors, and returning to the same point from which they had emanated." t Prince Adalbert of Prussia, Aus meinem Tagebuche, 1847, s. 213. Is the phenomenon I have described connected with the oscillations of 10"-12", observed by Carlini, in the passage of the polar star over the field of the great Milan meridian telescope ? (See Zach's Corres- endance Astronomique et Giog., vol. ii., 1819, p. 81.) Brandes (Geh- 's Umgearb. Phys. Wortersb., bd. iv., s. 549) refers the phenomenon to mirage. The star-like heliotrope light has also frequently been seen, by the admirable and skillful observer, Colonel Baeyer, to oscillate to and fro in a horizontal direction. 60 COSMOS. ascended before sunrise by scientific travelers provided with instruments, I would hope that this reiterated invitation on my part to the observation of the undulation of the stars may not be wholly disregarded. I have already called attention to the fact that the basis of a very important part of the astronomy of our planetary system was already laid before the memorable years 1608 and 1610, and therefore before the great epoch of the in- vention of telescopic vision, and its application to astronom- ical purposes. The treasure transmitted by the learning of the Greeks and Arabs was augmented by the careful and persevering labors of George Purbach, Regiomontanus (i. e. } Johann Miiller), and Bernhard Walther of Niirnberg. To their efforts succeeded a bold and glorious development of thought the Copernican system ; this, again, was followed by the rich treasures derived from the exact observations of Tycho Brahe, and the combined acumen and persevering spirit of calculation of Kepler. Two great men, Kepler and Galileo, occupy the most important turning-point in the his- tory of measuring astronomy ; both indicating the epoch that separates observation by the naked eye, though aided by greatly improved instruments of measurement, from tele- scopic vision. Galileo was at that period forty-four, and Kepler thirty-seven years of age ; Tycho Brahe, the most exact of the measuring astronomers of that great age, had been dead seven years. I have already mentioned, in a pre- ceding volume of this work (see vol. ii., p. 328), that none of Kepler's cotemporaries, Galileo not excepted, bestowed any adequate praise on the discovery of the three laws which have immortalized his name. Discovered by purely empir- ical methods, although more rich in results to the whole do- main of science than the isolated discovery of unseen cos- mical bodies, these laws belong entirely to the period of nat- ural vision, to the epoch of Tycho Brahe and his observa- tions, although the printing of the work entitled Astronomia nova seu Physica codestis de motibus Stella Martis was not completed until 1609, and the third law, that the squares of the periodic times of revolution of two planets are as the cubes of their mean distances, was first fully developed in 1619, in the Harmonice Mundi. The transition from natural to telescopic vision which characterizes the first ten years of the seventeenth century was more important to astronomy (the knowledge of the re- gions of space) than the year 1492 (that of the discoveries ASTRONOMICAL DISCOX ERIB3. t51 of Columbus) in respect to our knowledge of terrestrial space. It not only infinitely extended our insight into creation, but also, besides enriching the sphere of human ideas, raised mathematical science to a previously unattained splendor, by the exposition of new and complicated problems. Thus the increased power of the organs of perception reacts on the world of thought, to the strengthening of intellectual force, and the ennoblement of humanity. To the telescope alone we owe the discovery, in less than two and a half centuries, of thirteen new planets, of four satellite-systems (the four moons of Jupiter, eight satellites of Saturn, four, or perhaps six of Uranus, and one of Neptune), of the sun's spots and faculse, the phases of Venus, the form and height of the lunar mountains, the wintery polar zones of Mars, the belts of Jupiter and Saturn, the rings of the latter, the inte- rior planetary comets of short periods of revolution, together with many other phenomena which likewise escape the na- ked eye. While our own solar system, which so long seemed limited to six planets and one moon, has been enriched in the space of 240 years with the discoveries to which we have alluded, our knowledge regarding successive strata of the region of the fixed stars has unexpectedly been still more increased. Thousands of nebulae, stellar swarms, and double stars, have been observed. The changing position of the double stars which revolve round one common center of gravity has proved, like the proper motion of all fixed starb, that forces of gravitation are operating in those distant re- gions of space, as in our own limited mutually-disturbing planetary spheres. Since Morin and Gascoigne (not indeed till twenty-five or thirty years after the invention of the tel- escope) combined optical arrangements with measuring in- struments, we have been enabled to obtain more accurate observations of the change of position of the stars. By this means we are enabled to calculate, with the greatest pre- cision, every change in the position of the planetary bodies, the ellipses of aberration of the fixed stars and their paral- laxes, and to measure the relative distances of the double stars even when amounting to only a few tenths of a sec- onds-arc. The astronomical knowledge of the solar system has gradually extended to that of a system of the universe. We know that Galileo made his discoveries of Jupiter's satellites with an instrument that magnified only seven diam- eters, and that he never could have used one of a higher power than thirty-two. One hundred and seventy years later, 62 COSMOS. we find Sir William Herschel, in his investigations on the magnitude of the apparent diameters of Arcturus (0 //- 2 within the nebula) and of Vega Lyrte, using a power of 6500. Since the middle of the seventeenth century, constant attempts have been made to increase the focal length of the telescope. Christian Huygena, indeed, in 1655, discovered the first sat- ellite of Saturn, Titan (the sixth in distance from the center of the planet), with a twelve-feet telescope ; he subsequent- ly, however, examined the heavens with instruments of a greater focal length, even of 122 feet ; but the three object- glasses in the possession of the Royal Society of Londonj whose focal lengths are respectively 123, 170, and 210 feet, and which were constructed by Constantin Huygens, brother of the great astronomer, were only tested by the latter, as he expressly states,* upon terrestrial objects. Auzout, who in 1663 constructed colossal telescopes without tubes, and therefore without a solid connection between the object-glass and the eye-piece, completed an object-glass, which, with a focal length of 320 feet, magnified 600 times.f The most useful application of these object-glasses, mounted on poles, was that which led Dominic Cassini, between the years 1671 and 1684, to the successive discoveries of the eighth, fifth, fourth, and third satellites of Saturn. He made use of ob- ject-glasses that had been ground by Borelli, Campani, and Hartsoeker. Those of the latter had a focal length of 266 feet. During the many years I passed at the Paris Observatory, I frequently had in my hands the instruments made by Cam- pani, which were in such great repute during the reign of Louis XIV. ; and when we consider the faint light of Saturn's satellites, and the difficulty of managing instruments, worked by strings only,| we can not sufficiently admire the skill and the untiring perseverance of the observer. * The remarkable artistical skill of Constantin Huygens, who was private secretary to King William the Third, has only recently been presented in its proper light by Uytenbrock in the " Oratio de fratribiis Christiano atque Constantino Hugenio, artis dioptricae cultoribus," 1838; and by Prof. Kaiser, the learned director of the Observatory at Leyden (in Schumacher's Astron. Nachr., No. 592, s. 246). t See Arago, in the Annuaire pour 1844, p. 381. t " Nous avons place ces grands verres, tantot sur un grand m&t, tan- tot sur la tour de bois venue de Marly ; enfiu nous les avons mis dans un tuyau monte sur un support en forme d'6chelle & trois faces, ce qui a eu (dans la decouverte des satellites de Saturne) le succSs que nous en avions esp6r&." " We sometimes mounted these great instruments on a high pole," says Dominique Caasini, " and sometimes on the wood- TELESCOPES. 63 The advantages which were at that period supposed to be obtainable only by gigantic length, led great minds, as is frequently the case, to extravagant expectations. Auzout considered it necessary to refute Hooke, who is said to have proposed the use of telescopes having a length of upward of 10,000 feet (or nearly two miles),* in order to see animals in the moon. A sense of the practical inconvenience of op- tical instruments having a focal length of more than a hund- red feet, led, through the influence of Newton (in following out the earlier attempts of Mersenne and James Gregory of Aberdeen), to the adoption, especially in England, of shorter reflecting telescopes. The careful comparison made by Brad- ley and Pond, of Hadley's five-feet reflecting telescopes, with the refractor constructed by Constantin Huygens (which had, as already observed, a focal length of 123 feet), fully demonstrated the superiority of the former. Short's expens- ive reflectors were now generally employed until 1759, when. John Dollond's successful practical solution of the problem of achromatism, to which he had been incited by Leonhard Euler and Klingenstierna, again gave preponderance to re- fracting instruments. The right of priority, which appears to have incontestably belonged to the mysterious Chester More, Esq., of More Hall, in Essex (1729), was first made known to the public when John Dollond obtained a patent for his achromatic telescopes. f The triumph obtained by refracting instruments was not, however, of long duration. In eighteen or twenty years after the construction of achromatic instruments by John Dollond, by the combination of crown with flint glass, new fluctua- en tower that had been brought from Marly ; and we also placed them in a tube mounted on a three-sided ladder, a method which, in the dis- covery of the satellites of Saturn, gave us all the success we had hoped." Delambre, Hist, de VAstr. Moderne, torn, ii., p. 785. Optical instru- ments having such enormous focal lengths remind us of the Arabian in- struments olmeasurement -quadrants with a radius of about 190 feet, upon whose graduated limb the image of the sun was received as in the gnomon, through a small round aperture. Such a quadrant was erect- ed at Samarcand, probably constructed after the model of the older sex- tants of Al-Chokandi (which were about 60 feet in height). Compare Sedillot, P 'rottgomenes det Tables d'Oloug-Beg, 1847, p. Ivii. and cxxix. * See Delambre, Hist, de VAstr. Mod., t. ii., p. 594. The mystic Capuchin monk, Schyrle von Rheita, who, however, was well versed in optics, had already spoken in his work, Oculus Enoch et Elite (Autv., 1645), of the speedy practicability of constructing telescopes that should magnify 4000 times, by means of which the lunar mountains might b accurately laid down. Compare also Cosmos, vol. ii., p. 323 (note). t Edinb. Encyclopedia, vol. xx., p. 479. tions of opinion were excited by the just admiration award- ed, both at home and abroad, to the immortal labors of a German, William Herschel. The construction of numerous seven-feet and twenty-feet telescopes, to which powers of from 2200 to 6000 could be applied, was followed by that of his forty-feet reflector. By this instrument he discovered, in August and September, 1789, the two innermost satellites of Saturn Enceladus, the second in order, and^soon after- ward, Mimas, the first, or the one nearest to the ring. The discovery of the planet Uranus in 1781 was made with Herschel's seven-feet telescope, while the faint satellites of this planet were first observed by him in 1787, with a twen- ty-feet "front view" reflector.* The perfection, unattained till then, which this great man gave to his reflecting tele- scopes, in which light was only once reflected, led, by the uninterrupted labor of more than forty years, to the most important extension of all departments of physical astron- omy in the planetary spheres, no less than in the world of nebulae and double stars. The long predominance of reflectors wus followed, in the earlier part of the nineteenth century, by a successful emu- lation in the construction of achromatic refractors, and heli- ometers, paralactically moved by clock-work. A homoge- neous, perfectly smooth flint glass, for the construction of object-glasses of extraordinary magnitude, was manufactured in the institutions of Utzschneider and Fraunhofer at Mu- nich, and subsequently in those of Merz and Mahler ; and in the establishments of Guinand and Bontems (conducted for MM. Lerebours and Cauchoix) in Switzerland and France. It will be sufficient in this historical sketch to mention, by way of example, the large refractors made under Fraunho- fer's directions for the Observatories of Dorpat and Berlin, in which the clear aperture was 9' 6 inches in diameter, with a focal length of 14 '2 feet, and those executed by Merz and Mahler for the Observatories of Pulkowa and Cambridge, in the United States of America ;t they are both adjusted with * Consult htruve, Etudes d'Astr. Stellaire, 1847, note 59, p. 24. I have retained the designations of forty, twenty, and seven-feet Herschel reflecting telescopes, although in other parts of the work (the original German) I have used French measurements. I have adopted these designations not merely on account of their greater convenience, but also because they have acquired historical celebrity from the important labors both of the elder and younger Herschel in England, and of the latter at Feldhausen, at the Cape of Good Hope. t See Schumacher's Astr. Nachr., No. 371 and 611. Cauchoix and TELESCOPES. 65 object-glasses of 15 inches in diameter, and a focal length of 22-5 feet. The heliometer at the Konigsberg Observa- tory, which continued for a long time to be the largest in existence, has an aperture of 6'4 inches in diameter. This instrument has been rendered celebrated by the memorable labors of Bessel. The well-illuminated and short dyalitic refractors, which were first executed by Plosl in Vienna : and the advantages of which were almost simultaneously recognized by Rogers in England, are of sufficient merit to warrant their construction on a large scale. During this period, to the efforts of which I have refer- red, because they exercised so essential an influence on the extension of cosmical views, the improvements made in in- struments of measurement (zenith sectors, meridian circles, and micrometers) were as marked in respect to mechanics as they were to optics and to the measurement of time. Among the many names distinguished in modern times in relation to instruments of measurement, we will here only mention those of Ramsden, Troughton, Fortin, Reichenbach, Gam- bey, Ertel, Steinheil, Repsold, Pistor, and Oertling ; in rela- tion to clironometers and astronomical pendulum clocks, we may instance Mudge, Arnold, Emery, Earnshaw, Breguet, .Rirgens^n, Kessels, "Winnerl, and Tiede ; while the noble la- Ws of \Yilliam and John Herschel, South, Struve, Bessel, and Dawes, in relation to the distances and periodic motions of the double stars, specially manifest the simultaneous per- fection acquired in exact vision and measurement. Struve's classification of the double stars gives about 100 for the num- ber whose distance from one another is below 1", and 336 for those between 1" and 2" ; the measurement in every case being several times repeated.* During the last few years, two men, unconnected with any industrial profession the Earl of Rosse, at Parson's Town (about fifty miles west of Dublin), and Mr. Lassell, at Starfield, near Liverpool, have, with the most unbounded liberality, inspired with a noble enthusiasm for the cause of science, constructed under their own immediate superintend- ence two reflectors, which have raised the hopes of astron- omers to the highest degree. t Lassell's telescope, which has Lerebours have also constructed object-glasses of more than 13 - 3 inches in diameter, and nearly 25 feet focal length. * Struve, Stellarum duplicium el multiplicium Mensurce Micrometricee, p. 2, 41. t Mr. Airy has recently given a comparative description of the meth- ods of constructing these two telescopes, including an account of the 66 COSMOS. an aperture only two feet in diameter, with a focal length of twenty feet, has already been the means of discovering one satellite of Neptune, and an eighth of Saturn, besides which two satellites of Uranus have been again distinguish- ed. The new colossal telescope of Lord Rosse has an aper- ture of six feet, and is fifty-three feet in length. It is mount- ed in the meridian between two walls, distant twelve feet 011 either side from the tube, and from forty-eight to fifty-six feet in height. Many nebulae, which had been irresolvable by any previous instruments, have been resolved into stellar swarms by this noble telescope ; while the forms of other nebulae have now, for the first time, been recognized in their true outlines. A marvelous effulgence is poured forth from the speculum. The idea of observing the stars by daylight with a tele- scope first occurred to Morin, who, with Gascoigne (about 1638, before Picard and Auzout), combined instruments of measurement with the telescope. Morin himself says,* "It was not Tycho's great observations in reference to the posi- tion of the fixed stars, when, in 1582, twenty-eight years before the invention of the telescope, he was led to compare Venus by day with the sun, and by night with the stars," but " the simple idea that Arcturus and other fixed stars might, like Venus, when once they had been fixed in the field of the telescope before sunrise, be followed through the heavens after the sun had risen, that led him to a discovery which might prove of importance for the determination of longitude at sea." No one was able before him to distin- guish the fixed stars in the presence of the sun. Since the mixing of the metal, the contrivances adopted for casting and polishing the specula and mounting the instruments. Abatr. of the Astr. Soc., vol. ix., No. 5, March, 1849. The effect of Lord Rosse's six feet metal- lic reflector is thus referred to (p. 120): "The astronomer royal, Mr. Airy, alluded to the impression made by the enormous light of the tel- escope ; partly by the modifications produced in the appearances of nebulas already figured, partly by the great number of stars seen even at a distance from the Milky Way, and partly from the prodigious brill- iancy of Saturn. The account given by another astronomer of the ap- pearance of Jupiter was, that it resembled a coach-lamp in the tele- scope; and this well expresses the blaze of light which is seen in the instrument." Compare also Sir John Herschel, Oull. of Astr., 870. " The sublimity of the spectacle afforded by the magnificent reflecting telescope constructed by Lord Rosse of some of the larger globular clus- ters of nebulae, is declared by all who have witnessed it to be such as no words can express. This telescope has resolved or rendered resolv- able multitudes of nebula? which had resisted all inferior powers." * Delambre, Hist, de V Astr on. Moderne, t. ii., p. 255. TELESCOPES. 67 employment, by Homer, of great meridian telescopes in 1691, observations of the stars by day have been frequent and fruit- ful in results, having been, in some cases, advantageously applied to the measurement of the double star's. Struve states* that he has determined the smallest distances of ex- tremely faint stars in the Dorpat refractor, with a power of only 320, in so bright a crepuscular light that he could read with ease at midnight. The polar star has a companion of the ninth magnitude, which is situated at only 18" distance : it was seen by day in the Dorpat refracting telescope by Struve and Wrangel,f and was in like manner observed on one occasion by Encke and Argelander. Many conjectures have been hazarded regarding the cause of the great power of the telescope at a time when the dif- fused light of the atmosphere, by multiplied reflection, ex- erts an obstructing action.^ This question, considered as an * Strove, Metis. Microm., p. xliv. t Schumacher's Jahrbuchfur 1839, s. 100. j La lumiere atmospherique diffuse ne peut s'expliquer par le reflet des rayons solaires sur la surface de separation des couches de difleren- tes densites dont on suppose 1'atmosphere composee. En eflet, suppo- sons le soleil place a 1'horizon, les surfaces de separation dans la direc- tion du zenith seraient horizontales, par consequent la reflexion serait horizontale aussi, et nous ne verrions aucune lumiere au zenith. Dana la supposition des couches, aucun rayon ne nous arriverait par voie d'une premiere reflexion. Ce ne seraient que les reflexions multiples qui pourraient agir. Done pour expliquer la lumiere diffuse, il faut se figurer 1'atmosphere composee de molecules (spheriques, par exemple) dont chacune uonne une image du soleil a peu pres comme les boulea de verres que nous plasons dans nos jardius. L'air pur est bleu, par- ceque d'apres Newton, les molecules de 1'air ont Vepaisseur qui convi- ent a la reflexion des rayons bleus. II est done naturel que les petites images du soleil que de tous cotes reflechissent les molecules sphe- riques de 1'air et qui sont la lumiere diffuse aient une teinte bleue : mais ce bleu n'est pas du bleu pur, c'est uu blanc dans lequel le bleu predomine. Lorsque le ciel n'est pas dans toute sa purete et que 1'air est me!6 de vapeurs visibles, la lumiere diffuse resoit beaucoup de blanc. Comme la lune est jaune, le bleu de 1'air pendant la nuit est un peu verdatre, c'est-a-dire, melange de bleu et de jaune." " We can not explain the diffusion of atmospheric light by the reflec- tion of solar rays on the surface of separation of the strata of different density, of which we suppose the atmosphere to be composed. In fact, if we suppose the sun to be situated on the horizon, the surfaces of sep- aration in the direction of the zenith will be horizontal, and consequent- ly the reflection would likewise be horizontal, and we should not be able to see any light at the zenith. On the supposition that such strata exist, no ray would reach us by means of direct reflection. Repeated reflections would be necessary to produce any effect. In order, there- fore, to explain the phenomenon of diffused light, we must suppose the atmosphere to be composed of molecules (of a spherical form, for in 68 COSMOS. optical problem, excited the strongest interest in the mind of Bessel, whose too early death was so unfortunate for the cause of science. In his long correspondence with myself, he frequently reverted to this subject, admitting that he could not arrive at any satisfactory solution. 1 feel confident it will not be unwelcome to my readers if I subjoin, in the form of a note, some of the opinions of Arago,* as expressed stance), each of which presents an image of the sun somewhat in the game manner as an ordinary glass ball. Pure air is blue, because, ac- cording to Newton, the molecules of the air have the thickness neces sary to reflect blue rays. It is therefore natural that the small images of the sun, reflected by the spherical molecules of the atmosphere, should present a bluish tinge ; this color is not, however, pure blue, but white, in which the blue predominates When the sky is not perfectly pure and the atmosphere is blended with perceptible vapors, the diffused light is mixed with a large proportion of white. As the moon is yellow, the blue of the air assumes somewhat of a greenish tinge by night, or, in other words, becomes blended with yellow." MSS. of 1847. * D'vn des Effcts dcs Lunettes sur la Visibility des etoiles. (Lctlre de M. Arago a M. de Humboldt en Die., 1847.) " L'ceil n'est done que d'une sensibilite circonscrite, bornee. Quand la lumiere qui frappe la retine, n'a pas assez d'intensite, 1'oeil ne sent rien. C'est par un manque d'intensite que beaucoup d.' etoiles, mdme dans les nuits les plus profondes echappent a nos observations. Les lu- nettes ont pour effet, quant aux etoiles, d'augmenter 1'intensite de 1'image. Le faisceau cylindrique de rayons paralleles venant d'une etoile, qui s'appuie sur la surface de la lentille objective, et qui a cette surface cir- culaire pour base, se trouve considerablement resserre a la sortie de la lentille oculaire. Le diametre du premier cylindre est au diametre du second, comme la distance focale de 1'objectif est a la distance fo- cale de Poculaire, ou bieu comme le diametre de 1'objectif est au dia- metre de la portion d'oculaire qu'occupe le faisceau emergent. Les iii- tensites de lumiere dans les deux cylindres en question (dans les deux cylindres, incident et emergent) doiveut etre entr'elles comme les eten- dues superficielles des bases. Ainsi la lumiere emergente sera plus con- densee, plug intense que la lumiere naturelle tombant sur 1'objectif, dans le rapport de la surface de cet objectif a la surface circulaire de la base du faisceau emergent. Le faisceau Emergent, quand la lunette grosrit, etant plus etroit que le faisceau cylindrique qui tombe sur 1'objectif, il est evident que la pupille, quelle que soit son cMverture, recueillera plus de rayons par 1'intermediaire de la lunette que sans elle. La lunette augmentera done toujours 1'intensite de la lumiere des ttoilei. " Le cas le plus favorable, quant a 1'effet des lunettes, est evidemment celui ou 1'ceil re9oit la totalite du faisceau emergent, le cas ou ce fais- ceau a moins de diametre que la pupille. Alors toute la lumiire que 1'objectif embrasse, concourt, par 1'entremise du telescope, a la forma- tion de 1'image. A 1'ceil nu, au contraire, une portion seule de cetto meme lumiere est mise a profit ; c'est la petite portion que la surface de la pupille decoupe dans le faisceau incident naturel. L'intensit^ do 1'image telescopique d'une etoile est done & 1'intensite de 1'image & 1'ceil nu, comme la surface de 1'objectif est a celle de la pupille. " Ce qui precede est relatif a la visibilite d'uu seul point, d'uno seulo TELESCOPES. 69 in one of the numerous manuscripts to which I was permit- ted free access during my frequent sojourn in Paris. Ac- etoile. Venons a 1'observation d'un objet ayant des dimensions an gulairw seusibles, a 1'observation d'une planete. Dans les cas les plus lavorables, c'est-a-dire lorsque la pupille re^oit la totalite du pinceaa emergeut, 1'intensite de 1'image de ckaque point de la planete se calcu- lera par la proportion que nous venons de donner. La quantite totalt de lumiere concourant a former V ensemble de 1'image a Tceil nu, sera done aussi a la quantite totale de lumiere qui forme 1'image de la pla- nete A 1'aide d'uue lunette, comme la surface de la pupille est 4 la sur- face de 1'obiectif. Les intensites comparatives, non plus de pointe isoles, mais des deux images d'une planete, qui se forment sur la retina a 1'oeil nu, et par I'iutermediaire d'une lunette, doivent evidemment diminuer proportionuellement aux etendues superficielles de ces deux im- ages. Les dimensions lineaires des deux images sont entr'elles comme le diametre de 1'objectif est au diametre du faisceau emergent. Le nouibre de fois que la surface de 1'image amplifiee surpasse la surface de 1'image a 1'oeil nu, s'obtiendra done en divisant le carre du diametre de 1'objeclifpa.r le carre du diametre du faisceau Emergent, ou bien la sur- face de I'objeclif par la surface de la base circulaire du faisceau emergent. " Nous avons deja obtenu le rapport des quantites totales de lumiere qui engendrent les deux images d'une planete, en divisant la surface de 1'objectif par la surface de la pupille. Ce nombre est plus petit que le quotient auquel on arrive en divisant la surface de 1'objectif par la sur- face du faisceau Emergent. II en resulte, quant aux planetes, qu'une lunette lait moins gagtier en intensite de lumiere, qu'elle ne fait perdre en agrandissaut la surface des images sur la retine; 1'intensite de ces images doit done aller continuellement en s'afiaiblissant a mesure que le pouvoir amplificatif de la lunette ou du telescope s'accroit. " L'atmosphere peut etre consideree comme une planete a dimen- sions indefiuies. La portion qu'on en verra dans une lunette, subira done aussi la loi d'affaiblissement que nous venons d'indiquer. Le rap* port entre 1'intensite de la lumiere d'une planete et le champ de lumiere atinospherique a travers lequel on la verra, sera le memo a 1'ceil nu et dans les lunettes de tous les grossissements, de toutes les dimensions. Les lunettes, sous le rapport de Vintensite, ne favorisent done pas la visi- bilite des planetes. " II n'eu est point ainsi des etoiles. L'intensite de 1'image d'une etoile est plus forte avec une lunette qu'a I'o3il nu ; au contraire, le champ de la vision, uuiformement eclaire dans les deux cas par la lumiere atmos- pherique, est plus clair a I'osil nu que dan la lunette. II y a done deux raisons, sans sortir des considerations d'intensite, pour que dans une lu- ette de 1'image de 1'etoile predomine sur celle de 1'atmosphere, nota- lement plus qu'a I'oail nu. " Cette predominance doit aller graduellement en augmentant avec le grossissement. En efiet, abstraction faite de certaine augmentation du diametre de 1'etoile, consequence de divers effets de diffraction ou d 1 'interferences, abstraction faite aussi d'une plus forte reflexion que la lumiere subit sur les surfaces plus obliques des oculaires de tres courts foyers, V intensite dc la lumiere de Vetoile est constante tant que 1'ouver- ture de 1'objectif ne varie pas. Comme ou 1'a vu, la clarte du champ de la lunette, au contraire, diminue sans cesse a mesure que le pouvoir amplificatif s'accroit. Done toutes autres circoustances restant egales, uue etoile sera d'autant p. 1 '!* visible, sa predouiiuence sur la lumiere tin 70 COSMOS. cording to the ingenious explanation of my friend, high rnag nify ing powers facilitate the discovery and recognition of the champ du telescope sera d'autaut plus tranchee qu'on lera usage d'un grossissemont plus fort." " The eye is endowed with only a limited sensibility ; for when the light which strikes the retina is not sufficiently strong, the eye is not sensible of any impression. In consequence of deficient intensity, many stars escape our observation, even in the darkest nights. Telescopic glasses have the effect of augmenting the intensity of the images of the stars. The cylindrical pencil of parallel rays emanating from a star, and striking the surface of the object-glass, on whose circular surface it rests as on abase, is considerably contracted on emerging from the eye- piece. The diameter of the first cylinder is to that of the second as the focal distance of the object-glass is to the focal distance of the eye- piece, or as the diameter of the object-glass is to the diameter of tho part of the eye-piece covered by the emerging rays. The intensities of the light in these two cylinders (the incident and emerging cylin- ders) must be to one another as the superficies of their bases. Thus, the emerging light will be more condensed, more intense, than the nat- ural light falling on the object-glass, in the ratio of the surface of this object-glass to the circular surface of the base of this emerging pencil. As the emerging pencil is narrower in a magnifying instrument than the cylindrical pencil falling on the object-glass, it is evident that the pupil, whatever may be its aperture, will receive more rays, by the interven- tion of the telescope, than it could without. The intensity of the light of the stars will, therefore, always be augmented when seen through a telescope. " The most favorable condition for the use of a telescope is undoubt edly that in which the eye receives the whole of the emerging rays, and, consequently, when the diameter of the pencil is less than that of the pupil. The whole of the light received by the object-glass then co- operates, through the agency of the telescope, in the' formation of the image. In natural vision, on the contrary, a portion only of this light is rendered available, namely, the small portion which enters the pupil naturally from the incident pencil. The intensity of the telescopic im age of a star is, therefore, to the intensity of the image seen with the naked eye, as the surface of the object-glass is to that, of the pupil. " The preceding observations relate to the visibility of one point or one star. We will now pass on to the consideration of an o*bject having sensible angular dimensions, as, for instance, a planet. Under the most favorable conditions of vision, that is to say, when the pupil receives the whole of the emerging pencil, the intensity of each point of the plan- et's image may be calculated by the proportions we have already given. The total quantity of light contributing to form the whole of the image, as seen by the naked eye, will, therefore, be to the total quantity of the light forming the image of the planet by the aid of a telescope, as the surface of the pupil is to the surface of the object-glass. The compar- ative intensities, not of mere isolated points, but of the images of a plan- et formed respectively on the retina of the naked eye, and by the in- tervention of a telescope, must evidently diminish proportionally to the superficial extent of these two images. The linear dimensions of the two images are to one another as the diameter of the object-glass is to that of the emerging pencil. We therefore obtain the number of times that the surface of the magnified image exceeds the surface of the iui- TELESCOPES. 71 fixed stars, since they convey a greater quantity of intense light to the eye without perceptibly enlarging the image ; age when seen by the naked eye by dividing the square of the diameter of the object-glass by the square of the diameter of the emerging pencil, or rather the surface of the abject-glass by the surface of the circular bate of the emerging pencil. " By dividing the surface of the object-glass by the surface of the pu pil, \ve have already obtained the ratio of the total quantities of light produced by the two images of a planet. This number is lower than the quotient which we obtain by dividing the surface of the object- glass by the surface of the emerging pencil. It follows, therefore, with respect to planets, that a telescope causes us to gain less in intensity of light than is lost by magnifying the surface of the images on the retina; the intensity of these images must therefore become continually fainter, in proportion as the magnifying power of the telescope increases. " The atmosphere may be considered as a planet of indefinite dimen- sions. The portion of it that we see in a telescope will therefore also be subject to the same law of diminution that we have indicated. The relation between the intensity of the light of a planet and the field of at- mospheric light through which it is seen, will be the same to the naked eye and in telescopes, whatever may be their dimensions and magnify- ing powers. Telescopes, therefore, do not favor the visibility of planets in respect to the intensity of their light. " The same is* not the case with respect to the stars. The intensity of the image of a star is greater when seen with the telescope than with the naked eye ; the field of vision, on the contrary, uniformly illumined in both cases by the atmospheric light, is clearer in natural than in tel- escopic vision. There are two reasons, then, which, in connection with the consideration of the intensity of light, explain why the image of a star preponderates in a telescope rather than in the naked eye over that of the atmosphere. " This predominance must gradually increase with the increased magnifying power. In fact, deducting the constant augmentation of the star's diameter, consequent upon the different effects of diffraction or interference, and deducting also the stronger reflection experienced by the light on the more oblique surfaces of ocular glasses of short focal lengths, the intensity of the light of the star is constant as long as the aperture of the object-glass does not vary. As we have already seen, the brightness of the field of view, on the contrary, diminishes inces- santly in the same ratio in which the magnifying power increases. All other circumstances, therefore, being equal, a star will be more or less visible, and its prominence on the field of the telescope will be more or less marked, in proportion to the magnifying powers we employ." Arago, Manuscript 0/1847. I will further add the following passage from the Annuaire du Bu- reau des Long, pour 1846 (Notices Scient. par M. Arago), p. 381 : " L'experience a montre que pour le commun des hommes, deux espaces eclaires et contigus ne se distingueut pas 1'un de Pautre, a inoius que leurs intensites comparatives ne presentent, au minimum, une dif ference de $$. Quand une lunette est tournee vers le firmament, son champ semble nniformement eclaire : c'est qu' alors il existe, dans un plan passant par le foyer et perpendiculaire a 1'axe de 1'objectiC un image indefinie de la region atmospherique vers laquelle la lunette est dirigee. Supposous qu'un astre. c'est-a-dire uu objet situe bieii au- 72 COSMOS. while, in accordance with another law, they influence the aerial space on which the fixed star is projected. The tele- scope, by separating, as it were, the illuminated particles of air surrounding the object-glass, darkens the field of view, and diminishes the intensity of its illumination. We are en- abled to see, however, only by means of the difference be- tween the light of the fixed star and of the aerial field or the mass of air which surrounds the star in the telescope. Plan- etary disks present very different relations from the simple ray of the image of a fixed star ; since, like the aerial field (fair aerienne), they lose in intensity of light by dilatation in the magnifying telescope. It must be further observed, that the apparent motion of the fixed star, as well as of the planetary disk, is increased by high magnifying powers. This circumstance may facilitate the recognition of objects by day, in instruments whose movements are not regulated paralactically by clock-work, so as to follow the diurnal mo- tion of the heavens. Different points of the retina are suc- cessively excited. " Very faint shadows are not observed," Arago elsewhere remarks, " until we can give them motion." In the cloudless sky of the tropics, during the driest sea- son of the year, I have frequently been able to find the pale disk of Jupiter with one of Dollond's telescopes, of a magni- fying power of only 95, when the sun was already from 15 to 18 above the horizon. The diminished intensity of the light of Jupiter and Saturn, when seen by day in the great Berlin refractor, especially when contrasted with the equally reflected light of the inferior planets, Venus and Mercury, frequently excited the astonishment of Dr. Galle. Jupiter's delA de 1'atmosphere, se trouve dans la direction de la lunette : son image ne sera visible qu'autant qu'elle augmentera de ^Vi au moins, 1'intensite de la portion de 1'image focale indifinie de 1'atmosphere, sur laquelle sa propre image limitfe ira se placer. Sans cela le champ visuel continuera a paraitre partout de la meme intensity." " Experience has shown that, in ordinary vision, two illuminated and contiguous spaces can not be distinguished from each other unless their comparative intensities present a minimum difference of inj-th. When a telescope is directed toward the heavens, its field of view appears uniformly illumined: there then exists in a plane passing through the focus, and perpendicular to the axis of the object-glass, an indefinite im- age of the atmospheric region toward which the instrument is pointed. If we suppose a star, that is to say, an object very far beyond the atmos- phere, situated in the direction of the telescope, its image will not be visible except it exceed, by at least g^-th, the intensity of that portion of the indefinite focal image of the atmosphere on which its limited proper image is thrown. Otherwise the visual field will continue to appear esery where of the same intensity. ' SCINTILLATION OF THE STARS. 73 occultations have occasionally been observed by daylight, with the aid of powerful telescopes, as in 1792, t by Flau- gergues, and in 1820, by Struve. Argelander (on the 7th of December, 1849, at Bonn) distinctly saw three of the sat ellites of Jupiter, a quarter of an hour after sunrise, with one of Fraunhofer's five-feet telescopes. He was unable to distinguish the fourth ; but, subsequently, this and the other satellites were observed emerging from the dark margin of the moon, by the assistant astronomer Schmidt, with the eight-feet heliometer. The determination of the limits of the telescopic visibility of small stars by daylight, in differ- ent climates, and at different elevations above the sea's level, is alike interesting in an optical and a meteorological point of view. Among the remarkable phenomena whose causes have been much contested, in natural as well as in telescopic vision, we must reckon the nocturnal scintillation of the stars. Accord- ing to Arago's investigations, two points must be specially dis- tinguished in reference to this phenomenon* firstly, change * The earliest explanations given by Arago of scintillation occur in the appendix to the 4th book of my Voyage avx Regions Equinoxialet, torn, i., p. 623. I rejoice that I am able to enrich this section on nat- ural and telescopic vision with the following explanations, which, for the reasons already assigned, I subjoin in the original text. Des causes de la scintillation des ttoiles. lation, c'est le changement de couleur. Ce changement est beaucoup plus frequent que 1'observation ordinaire 1'indique. En effet, en agi- tant la lunette, on transforme 1'image dans uue ligne ou un cercle, et tous les points de cette ligne ou de ce cercle paraissent de couleure dif- ferentes. C'est la resultante de la superposition de toutes ces images que 1'on voit, lorsqu'on laisse la lunette immobile. Les rayons qui so reunissent au foyer d'une lentille, vibrent d'accord ou en disaccord, s'ajoutent ou se detruisent, suivant que les couches qu'ils ont traver- sees, ont telle ou telle refringence. L'ensemble des rayons rouges petit se detruire seul, si ceux de droite et de gauche, et ceux de haut et de bas, ont traverse des milieux inegalement refringents. Nous avons dit seul, parceque la difference de refringence qui correspond a la destruc tion du rayon rouge, n'est pas la meme que cella qui amene la destruc- tion du rayon vert, et reciproquement. Main tenant, si des rayons rou ges sont detruits, ce qui reste sera le blanc moins le rouge, c'est-a-dire du vert. Si le vert au contraire est detruit par interference, 1'image sera du blanc moins le vert, c'est-a-dire du rouge. Pour expliquer pourquoi les planetes a grand diametre ne scintillent pas ou tres peu, il faut se rappeler que le disque peut tre consider^ comme une aggregation d'^toiles ou de petits points qui scintillent isol^ment; mais les images de differentes couleurs que chacun de ces points pris isol^ment don- nerait, empietant les unes sur les autres, formeraient du blanc. Lors- qu'on place un diaphragme ju un bouchou perce d'uii trou sur 1'objec- VOL III. D 74 COSMOS. in the intensity of the light, from a sudden decrease to perfect extinction and rekindling ; secondly, change of color. Both tif d'une lunette, les etoiles acquiere'nt un disque entoure d'une serie d'anneaux lumineux. Si 1'on enfonce 1'oculaire, le disque de 1'eioilo augmente de diamctre, et il se produit dans son centre un trou obscur ; si on 1'enfonce davantage, un point lumineux se substitue au point noir. Un nouvel enfoncemeut donne naissance a un centre noir, etc. Pro nons la lunette lorsque le centre de 1'image est noir, et visons a uno toile qui ne sciatillo pas : le centre restera noir, comme il l'6tait au- paravant. Si au contraire on dirige la lunette & une 4toile qui scintille, on verra le centre de 1'image lumineux et obscur par intermittence. Dans la position ou le centre de 1'image est occup6 par un point lumi- neux, on verra ce point disparaltre et renaitre successivement. Cette disparition ou reapparition du point central est la preuve directe de I' interference variable des rayons. Pour bien concevoir 1'absence de lumiere au centre de ces images dilatees, il faut se rappeler quo les rayons regulierement refractes par 1'objectif ne se reunisseut et ne peu- vent par consequent interferer qu'au foyer : par consequent les images dilatees que ces rayons peuvent produire, resteraient toujours pleines (sans trou). Si dans une certaine position de 1'oculaire un trou se pre- sente au centre de 1'image, c'est que les rayons r6gulierement refrac- ted inierferent avec des rayons diffractes sur les bords du diaphragme circulaire. Le phenomene n'est pas constant, parceque les rayons qui interferent dans un certain moment, n'interferent pas un instant apres, lorsqu'ils ont traverse des couches atmospheriques dont le pouvoir r6- fringent a varie. On trouve dans cette experience la preuve manifesto du role que joue dans le ph^nomene de la scintillation 1'inegale refran- gibilit^ des couches atmospheriques traversees par les rayons dont le faisceau est tres etroit. II r^sulte de ces considerations que 1'explica- tion des scintillations ne pent etre rattachee qu'aux phenomenes des interference* lumineuses. Les rayons des etoiles, apres avoir traverse nne atmosphere ou il existe des couches in^galement chaudes, inegale- ment denses, inegalement humides, vont se reiinir au foyer d'une len- tille, pour y former des images d'intensite et de couleurs perp6tuelle- ment changeantes, c'est--dire des images telles que la scintillation les presente. II y a aussi scintillation hors du foyer des lunettes. Les ex- plications proposers par Galileo, Scaliger, Kepler, Descartes, Hooke, Huygens, Newton et John Michell, que j'ai examin6 dans un tnemoife presente a 1'Institut en 1840 (Comptes Rcndus, t. x., p. 83), sont inad- missibles. Thomas Young, auquel nous devons les premieres lois des interferences, a cru inexplicable le phenomene de la scintillation. La faussete de 1'ancienne explication" par des vapeurs qui voltigent et d6- placent, est deja prouvee par la circonstance que nous voyons la scin- tillation des yeux, ce qui supposerait un deplacement d'une minute. Les ondulations du bord du soleil sont de 4" a 5", et peut-etre des pie- ces qui manquent, done encore effet de 1'interference des rayons." On the causes of the scintillation of the stars. " The most remarkable feature in the phenomenon of the stars' scin- tillation is their change of color. This change is of much more frequent occurrence than would appear from ordinary observation. Indeed, on shaking the telescope, the image is transformed into a line or circle, and nil the points of this line or circle appear of different colors. We havo here the results of the superposition of all the images seen when the toleacope is at rest. The rays united in the focus of a leas vibrate in SCINTILLATION OF THE STARS. 75 these alterations are more intense in reality than they appear to the naked eye ; for when the several points of the retina harmony or at variance with one another, and increase or destroy one another according to the various degrees of refraction of the strata through which they have passed. The whole of the red rays alone can destroy ono another, if the rays to the right and left, above and below them, have passed through unequally refracting media. We have used the term alone, because the difference of refraction necessary to destroy the red ray is not the same as that which is able to destroy the green ray, and vice versa. Now, if the red rays be destroyed, that which re- mains will be white minus red, that is to say, green. If the green, on the other hand, be destroyed by interference, the image will be white minus green, that is to say, red. To understand why planets having large diameters should be subject to little or no scintillation, it must be remem- bered that the disk may be regarded as an aggregation of stars or of small points, scintillating independently of each other, while the images of different colors presented by each of these points taken alone would impinge upon one another and form white. If we place a diaphragm or a cork pierced with a hole on the object-glass of a telescope, the stars present a disk surrounded by a series of luminous rings. On push- ing in the eye-piece, the disk of the star increases in diameter, and a dark point appears in its center ; when the eye-piece is made to recede still further into the instrument, a luminous point will take the place of the dark point. On causing the eye-piece to recede still further, a black center will be observed. If, while the center of the image is black, we point the instrument to a star which does not scintillate, it will remain bkck as before. If, on the other hand, we point it to a scin- tillating star, we shall see the center of the image alternately luminous and dark. In the position in which the center of the image is occu- pied by a luminous point, we shall see this point alternately vanish and reappear. This disappearance and reappearance of the central point is a direct proof of the variable interference of the rays. In order to comprehend the absence of light from the center of these dilated im- ages, we must remember that rays regularly refracted by the object- glass do not reunite, and can not, consequently, interfere except in the focus ; thus the images produced by these rays will always be uniform and without a central point. If, in a certain position of the eye-piece, a point is observed in the center of the image, it is owing to the inter- ference of the regularly refracted rays with the rays diffracted on the margins of the circular diaphragm. The phenomenon is not constant, for the rays which interfere at one moment no longer do so in the next, after they have passed through atmospheric strata possessing a varying power of refraction. We here meet with a manifest proof of the im- portant part played in the phenomenon of scintillation by the unequal refrangibility of the atmospheric strata traversed by rays united in a very narrow pencil." " It follows from these considerations that scintillation mast necessa- rily be referred to the phenomena of luminous interferences alone The rays emanating from the stars, after traversing an atmosphere composed of strata having different degrees of heat, density, and humidity, com- bine in the focus of a lens, where they form images perpetually chang- ing in intensity and color, that is to say, the images presented by scin- tillation. There is another form of scintillation, independent of the fo cus of the telescope. The explanations of this phenomenon advanced 76 COSMOS. are once excited, they retain the impression of light which they have received, so that the disappearance, obscuration and change of color in a star are not perceived by us to their full extent. The phenomenon of scintillation is more striking- ly manifested in the telescope when the instrument is shaken, for then different points of the retina are successively excited, and colored and frequently interrupted rings are seen. The principle of interference explains how the momentary colored effulgence of a star may be followed by its equally instanta- neous disappearance or sudden obscuration, in an atmosphere composed of ever-changing strata of different temperatures, moisture, and density. The undulatory theory teaches us generally that two rays of light (two systems of waves) em- anating from one source (one center of commotion), destroy each other by inequality of path ; that the light of one ray added to the light of the other produces darkness. When the retardation of one system of waves in reference to the other amounts to an odd number of semi-undulations, both systems endeavor to impart simultaneously to the same molecule of ether equal but opposite velocities, so that the effect of their combination is to produce rest in the molecule, and therefore darkness. In some cases, the refrangibility of the different strata of air intersecting the rays of light exerts a greater in- fluence on the phenomenon than the difference in length of their path.* The intensity of scintillations varies considerably in the dif- ferent fixed stars, and does not seem to depend solely on their altitude and apparent magnitude, but also on the nature of their own light. Some, as for instancfe Vega, flicker less than Arcturus and Procyon. The absence of scintillation in plan- ets with larger disks is to be ascribed to compensation and to the naturalizing mixture of colors proceeding from different points of the disk. The disk is to be regarded as an aggregate oy Galileo, Scaliger, Kepler, Descartes, Hooke, Huygens, Newton, and John Michell, which I examined in a memoir presented to the Institute in 1840 (Comptes Rendus, t. x., p. 83), are inadmissible. Thomas Young, to whom we owe the discovery of the first laws of interference regarded scintillation as an inexplicable phenomenon. The erroneous- ness of the ancient explanation, which supposes that vapors ascend and displace one another, is sufficiently proved by the circumstance that we see scintillations with the naked eye, which presupposes a displace ment of a minute. The undulations of the margin of the sun are from 4" to 5", and are perhaps owing to chasms or interruptions, and there- fore also to the effect of interference of the rays of light." (Extrac/i from Arago's MSS. of 1847.) * See Arago, in the Annuaire pour 1831 p. 1C8. SCINTILLATION CP THE STARS. 77 of stars which naturally compensate for the light destroyed by interference, and again combine the colored rays into white light. For this reason, we most rarely meet with traces of scintillation in Jupiter and Saturn, but more frequently in Mercury and Venus, for the apparent diameters of the disks of these last-named planets diminish to 4"*4 and 9"'5. The diameter of Mars may also decrease to 3"-3 at its conjunc- tion. In the serene cold winter nights of the temperate zone, the scintillation increases the magnificent impression produced by the starry heavens, and the more so from the circumstance that, seeing stars of the sixth and seventh magnitude flicker- ing in various directions, we are led to imagine that we per- ceive more luminous points than the unaided eye is actually capable of distinguishing. Hence the popular surprise at the few thousand stars which accurate catalogues indicate as vis- ible to the naked eye ! It was known in ancient times by the Greek astronomers that the flickering of their light dis- tinguished the fixed stars from the planets ; but Aristotle, in accordance with the emanation and tangential theory of vi- sion, to which he adhered, singularly enough ascribes the scin- tillation of the fixed stars merely to a straining of the eye. " The riveted stars (the fixed stars)," says he,* " sparkle, but not the planets ; for the latter are so near that the eye is able to reach them ; but in looking at the fixed stars (rrpdf 6e rouf fj,VovTa$), the eye acquires a tremulous motion, owing to the distance and the effort." In the time of Galileo, between 1572 and 1604 an epoch remarkable for great celestial events, when three starsf of greater brightness than stars of the first magnitude suddenly appeared, one of which, in Cygnus, remained luminous for twenty-one years Kepler's attention was specially directed to scintillation as the probable criterion of the non-planetary nature of a celestial body. Although well versed in the sci- ence of optics, in its then imperfect state, he was unable to rise above the received notion of moving vapors.J In the Chinese Records of the newly appeared stars, according to the great collection of Ma-tuan-lin, their strong scintillation is occasionally mentioned. The more equal mixture of the atmospheric strata, in and near the tropics, and the faintness or total absence of scintil- * Aristot., De Ccelo, ii., 8, p. 290, Bekker. t Cosmos, vol. ii., p. 326. t Causa scintillationis, in Kepler, De Stella nova in pede Serpcntara, 1606, cap. xviii., p. 92-97. lation of the fixed stars when they have risen 12 or 15 above the horizon, give the vault of heaven a peculiar char- acter of mild effulgence and repose. I have already referred in many of iny delineations of tropical scenery to this charac- teristic, which was also noticed by the accurate observers La Condamine and Bouguer, in the Peruvian plains, and by Garcin,* in Arabia, India, and on the shores of the Persian Gulf (near Bender Abassi). As the aspect of the starry heavens, in the season of the serene and cloudless nights of the tropics, specially excited my admiration, I have been careful to note in my journals the height above the horizon at which the scintillation of the stars ceased in different hygrometric conditions. Cumana and the rainless portion of the Peruvian coast of the Pacific, before the season of the garua (mist) had set in, were pecul- iarly suited to such observations. On an average, the fixed stars appear only to scintillate when less than 10 or 12 above the horizon. At greater elevations, they shed a mild, planetary light; but this difference is most strikingly per- ceived when the same fixed stars are watched in their grad- ual rising or setting, and the angles of their altitudes meas- ured or calculated by the known time and latitude of the place. In some serene and calm nights, the region of scin- tillation extended to an elevation of 20 or even 25 ; but a connection could scarcely ever be traced between the differ- ences of altitude or intensity of the scintillation and the hy- grometric and thermometric conditions, observable in the low- er and only accessible region of the atmosphere. I have ob- served, during successive nights, after considerable scintilla- tion of stars, having an altitude of 60 or 70, when Saus- sure's hair-hygrometer stood at 85, that the scintillation en- tirely ceased when the stars were 15 above the horizon, al- though the moisture of the atmosphere was so considerably increased that the hygrometer had risen to 93. The intri- cate compensatory phenomena of interference of the rays of light are modified, not by the quantity of aqueous vapor con- tained in solution in the atmosphere, but by the unequal dis- tribution of vapors in the superimposed strata, and by the upper currents of cold and warm air, which are not percept- ible in the lower regions of the atmosphere. The scintilla- tion of stars at a great altitude was also strikingly increased during the thin yellowish red mist which tinges the heavens * Lettre de M. Garcin, Dr. en Med. a M. de RSavmur, in Hist, de TAcadtmie Royale des Sciences,' Annie 1743, p. 28-32. SCINTILLATION OF THE STARS. 79 Bhort\y before an earthquake. These observations only refer to the serenely bright and rainless seasons of the year with- in the tropics, from 10 to 12 north and south of the equa- tor. The phenomena of light exhibited at the commence- ment of the rainy season, during the sun's zenith-passage, depend on very general, yet powerful, and almost tempestu- ous causes. The sudden decrease of the northeast trade- wind, and the interruption of the passage of regular upper currents from the equator to the poles, and of lower currents from the poles to the equator, generate clouds, and thus daily give rise, at definite recurring periods, to storms of wind and torrents of rain. I have observed during several successive years that in regions where the scintillation of the fixed stars is of rare occurrence, the approach of the rainy season is an- nounced many days beforehand by a flickering light of the stars at great altitudes above the horizon. This phenome- non is accompanied by sheet lightning, and single flashes on the distant horizon, sometimes without any visible cloud, and at others darting through narrow, vertically ascending col- umns of clouds. In several of my writings I have endeav- ored to delineate these precursory characteristics and physi- ognomical changes in the atmosphere.* The second book of Lord Bacon's Novum Organum gives us the earliest views on the velocity of light and the prob- ability of its requiring a certain time for its transmission. He speaks of the time required by a ray of light to traverse the enormous distances of the universe, and proposes the * See Voyage aux Regions Equin., t. i., p. 511 and 512, and t. ii., p. 202-208; also my Views of Nature, p. 16, 138. En Arabie, de meme qu'a Bender-Abassi, port fameux du Golfe Persique, 1'air est parfaitement serein presque toute 1'annee. Le prin- temps, l'et, et 1'automne se passent, sans qu'on y voie la moindre rosee. Dans ces raemes temps tout le monde couche dehors sur le haut dea maisons. Quand on est ainsi couche, il n'est pas possible d'exprimer le plaisir qu'on prend 4 contempler la beaute du ciel, 1'eclat des etoiles. C'est une lumiere pure, ferme et eclatante, sans 4tincellement. Ce n'est qu'au milieu de 1'hiver que la scintillation, quoique tres foible, s'y fait apercevoir." " In Arabia," says Garciu, "as also at Bender-Abassi, a celebrated port on the Persian Gulf, the air is perfectly serene throughout nearly the whole of the year. Spring, summer, and autumn pass without ex- hibiting a trace of dew. During these seasons all the inhabitants sleep on the roofs of their houses. It is impossible to describe the pleasure experienced in contemplating the beauty of the sky, and the brightness of the stars, while thus lying in the open air. The light of the stars is pure, steady, and brilliant ; and it is only in the middle of the winter that a slight degree of scintillation is observed." Garcin, in Hist, dt PAcad. de* Sc., 1743, p. 30. 80 COSMOS. question whether those stars yet exist which we now see shining.* We are astonished to meet with this happy con- jecture in a work whose intellectual author was far behind his cotemporaries in mathematical, astronomical, and phys- ical knowledge. The velocity of reflected solar light was first measured by Homer (November, 1675) by comparing the periods of occultation of Jupiter's satellites ; while the velocity of the direct light of the fixed stars was ascertained (in the autumn of 1727) by means of Bradley's great discov- ery of aberration, which afforded objective evidence of the translatory movement of the earth, and of the truth of the Copernican system. In recent times, a third method of measurement has been suggested by Arago, which is based on the phenomena of light observed in a variable star, as, for instance, Algol in Perseus. f To these astronomical meth- ods may be added one of terrestrial measurement, lately con- ducted with much ingenuity and success by M. Fizeau in the neighborhood of Paris. It reminds us of Galileo's early * In speaking of the deceptions occasioned by the velocity of sound and light, Bacon says : " This last instance, and others of a like nature, have sometimes excited in us a most marvelous doubt, no less than whether the image of the sky and stars is percei ved as at the actual moment of its existence, or rather a little after, and whether there is not (with regard to the visible appearance of the heavenly bodies) a true and apparent place which is observed by astronomers in parallaxes. It appeared so incredible to us that the images or radiations of heavenly bodies could suddenly be conveyed through such immense spaces to the eight, and it seemed that they ought rather to be transmitted in a def- inite time. That doubt, however, as far as regards any great difference between the true and apparent time, was subsequently completely set at rest when we considered . . . ." The works of Francis Bacon, vol. xiv., Lond., 1831 (Novum Organutn), p. 177. He then recalls the cor- rect view he had previously announced precisely in the manner of the ancients. Compare Mrs. Somerville's Connection of the Physical Sci- ences, p. 36, and Cosmos, vol. i., p. 154, 155. t See Arago's explanation of his method in the Annuaire du Bureau des Longitudes pour 1842, p. 337-343. " L'observation attentive des phases d'Algol i six mois d'intervalle servira a determiner directement la vitesse de la lumiere de cette etoile. Pres du maximum et du mini- mum le changement d'intensite s'opere lentement ; il est au contraire rapide a certames epoques interme'diares entre celles qui correspondent aux deux etats extremes, quand Algol, soil en diminuant, soit en aug- mentant d'dclat, passe pour la troisieme grandeur." " The attentive observation of the phases of Algol at a six-months in- terval will serve to determine directly the velocity of that star's light Near the maximum and the minimum the change of intensity is very slow ; it is, on the contrary, rapid at certain intermediate epochs be- tween those corresponding to the two extremes, when Algol, either di minishing or increasing in Brightness, appears of the third magnitude. SCINTILLATION OF THE STARS. 81 and fruitless experiments with two alternately obscured lan- terns. Horrebow and Du Hamel estimated the time occupied in the passage of light from the sun to the earth at its mean dis- tance, according to Romer's first observations of Jupiter's satel- lites, at 14' 7", then 11' ; Cassini at 14' 10" ; while Newton* * Newton, Optics, 2d ed. (London, 1718), p. 325. " Light moves from the sun to us in seven or eight minutes of time." Newton com- pares the velocity of sound (1140 feet in 1") with that of light. As, from observations on the occultations of Jupiter's satellites (Newton's death occurred about half a year before Bradley's discovery of aberra- tion), he calculates that light passes from the sun to the earth, a distance, as he assumed, of 70 millions of miles, in 7' 30" ; this result yields a ve- locity of light equal to 155,555| miles in a second. The reduction of these [ordinary] to geographical miles (60 to 1) is subject to variations according as we assume the figure of the earth. According to Encke's accurate calculations in the Jahrbuch fur 1852, an equatorial degree is equal to 69-1637 English miles. According to Newton's data, we should therefore have a velocity of 134,944 geographical miles. Newton, how- ever, assumed the sun's parallax to be 12". If this, according to Encke's calculation of the transit of Venus, be 8"-57116, the distance is greater, and we obtain for the velocity of light (at seven and a half minutes) 188,928 geographical, or 217,783 ordinary miles, in a second of time ; therefore too much, as before we had too h'ttle. It is certainly very re- markable, although the circumstance has been overlooked by Delambre (Hist, de V Astronomic Moderne, torn, ii., p. 653), that Newton (proba- bly basing his calculations upon more recent English observations of the first satellite) should have approximated within 47" to the true re- sult (namely, that of Struve, which is now generally adopted), while the time assigned for the passage of light over the semi-diameter of the earth's orbit continued to vacillate between the very high amounts of 11' and 14' 10", from the period of Earner's discovery in 1675 to the be- ginning of the eighteenth century. The first treatise in which RSmer, the pupil of Picard, communicated his discovery to the Academy, bears the date of November 22, 1675. He found, from observations of forty emersions and immersions of Jupiter's satellites, "a retardation of light amounting to 22 minutes for an interval of space double that of the sun's distance from the earth." (Memoirs de VAcad. de 1666-1699, torn, x., 1730, p. 400.) Cassini does not deny the retardation, but he does not concur in the amount of time given, because, as he erroneously argues, different satellites presented different results. Du Hamel, secretary to the Paris Academy (Regies Scientiarum Academics Historia, 1698, p. 143), gave from 10 to 11 minutes, seventeen years after RSmer had left Pans, although he refers to him ; yet we know, through Peter Horre- bow (Basis Astronomic sive Trvluum Roemerianum, 1735, p. 122-129), that Romer adhered to the result of 11', when in 1704, six years before his death, he purposed bringing out a work on the velocity of light; the same was the case with Huygens (Tract, de Lumine, cap. i., p. 7) Cassini's method was very different ; he found 7' 5" for the first satel- lite, and 14' 12" for the second, having taken 14' 10" for the basis of his tables for Jupiter pro pcragrando diametri semissi. The error waa therefore on the increase. (Compare Horrebow, Triduum, p. 129 ; Gas- sini, Hypotheses et Satellites de Jupiter iu the M6m de VAcad., 166G- D 2 82 COSMOS. approximated very remarkably to the truth when he gave it at 7' 30". Delambre,* who did not take into account any of the observations made in his own time, with the excep- tion of those of the first satellite, found 8' 13"-2. Encke has very justly noticed the great importance of undertaking a special course of observations on the occultations of Jupi- ter's satellites, in order to arrive at a correct idea regarding the velocity of light, now that the perfection attained in the construction of telescopes warrants us in hoping that we may obtain trustworthy results. Dr. Busch,t of Konigsberg, who based his calculations on Bradley's observations of aberration, as rediscovered by Bi- gaud of Oxford, estimated the passage of light from the sun to the earth at 8' 12"- 14, the velocity of stellar light at 167,976 miles in a second, and the constant of aberration at 20"-2116 ; but it would appear, from the more recent ob- servations on aberration carried on during eighteen months by Struve with the great transit instrument at Pulkowa,t that the former of these numbers should be considerably in- 1699, torn, viii., p. 435, 475; Delambre, Hist, de VAstr. Mod., toin. ii., p. 751, 782 ; Du Hamel, Physica, p. 435.) * Delambre, Hist, de VAstr. Mod., torn, ii., p. 653. t Reduction of Bradley's Observations at Kew and Wangled, 1836, p. 22; Schumacher's Astr. Nachr., bd. xiii., 1836, No. 309 (compare Mis- cellaneous Works and Correspondence of the Rev. James Bradley, by Prof. Rigaud, Oxford, 1832). On the mode adopted for explaining ab- erration in accordance with the theory of undulatory light, see Doppler, in iheAbhl. derKon. bohmischen Gesellschaft der Wiss.,5te Folge., bd. iii., s. 754-765. It is a point of extreme importance in the history of great astronomical discoveries, that Picard, more than half a century before the actual discovery and explanation by Bradley of the cause of aberration, probably from 1667, had observed a periodical movement of the polar star to the extent of about 20", which could " neither be the effect of parallax or of refraction, and was very regular at opposite seasons of the year." (Delambre, Hist, de I' Astr. Moderns, torn, ii., p. 616.) Picard had nearly ascertained the velocity of direct light before his pupil, R6mer, made known that of reflected light. \ Schum., Astr. Nachr , bd. xxi., 1844, No. 484 ; Struve, Eludes d'Astr. Stellaire, p. 103, 107 (compare Cosmos, vol. i., p. 153, 154). The re- suit given in the Annuaire pour 1842, p 87, for the velocity of light in a second, is 308,000 kilomenes, or 77,000 leagues (each of 4000 metres), which corresponds to 215,834 miles, and approximates most nearly to Struve's recent result, while that obtained at the Pulkowa Observatory is 189,746 miles. On the difference in the aberration of the light 01 the polar star and that of its companion, and on the doubts recently expressed by Struve, see Madler, Astronomic, 1849, s. 393. William Richardson gives as the result of the passage of light from the Bun to the earth 8' 19"-28, from which we obtain a velocity of 215,392 milei in a second. (Mem. of the Astren. Soc., vol. iv., Part i.. p. 68.) SCINTILLATION OF THE STARS. 83 creased. The result of these important observations gave 8' 17"'78 ; from which, with a constant of aberration of 20"-4451, and Encke's correction of the sun's parallax in the year 1835, together with his determination of the earth's radius, as given in his Astronomisches Jahrbuch fur 1852, we obtain 166,196 geographical miles for the velocity of light in a second. The probable error in the velocity seems scarcely to amount to eight geographical miles. Struve's result for the time which light requires to pass from the sun to the earth differs about 7TTr th from Delambre's (8' 13"'2), which has been adopted by Bessel in the Tab. Regiom., and has hitherto been followed in the Berlin Astronomical Al- manac. The discussion on this subject can not, however, be regarded as wholly at rest. Great doubts still exist as to the earlier adopted conjecture that the velocity of the light of the polar star was smaller than that of its compan- ion in the ratio of 133 to 134. M. Fizeau, a physicist, distinguished alike for his great acquirements and for the delicacy of his experiments, has submitted the velocity of light to a terrestrial measurement, by means of an ingeniously constructed apparatus, in which artificial light (resembling stellar light) generated from oxy- gen and hydrogen is made to pass back, by means of a mir- ror between Suresne and La Butte Montmartre, over a dis- tance of 28,321 feet, to the same point from which it ema- nated. A disk having 720 teeth, which made 12-6 rotations in a second, alternately obscured the ray of light and allowed it to be seen between the teeth on the margin. It was sup- posed from the marking of a counter (compteur) that the artificial light traversed 56,642 feet, or the distance to and from the stations in T ^ 7 ?th part of a second, whence we ob- tain a velocity of 191,460 miles in a second.* This result, therefore, approximates most closely to Delambre's (which was 189,173 miles), as obtained from Jupiter's satellites. Direct observations and ingenious reflections on the ab- sence of all coloration during the alternation of light in the variable stars a subject to which I shall revert in the se- * Fizean gives his result in leagues, reckoning 25 (and consequently 4452 metres) to the equatorial degree. He estimates the velocity of light at 70,000 such leagues, or about 210,000 miles in the second. On the earlier experiments of Fizeau, see Comptes Rendvt, torn, xxix., p. 92. In Moigno, Rupert. tfOptique Moderne, Part iii., p. 1162, we find this velocity given at 70,843 leagues (of 25=1), or about 212,529 miles, which approximates most nearly to the result of Bradley, as given by Busch. 84 COSMOS. quel led Arago to the result that, according to the undu- latory theory, rays of light of different color, which conse quently have transverse vibrations of very different length and velocity, move through space with the same rapidity. The velocity of transmission and refraction differ, therefore, in the interior of the different bodies through which the col- ored rays pass ;* for Arago's observations have shown that * " D'apres la theorie mathematique dans le systeme des ondes, les rayons de differentes couleurs, les rayons dont les ondulations sont ine- gales, doivent neanmoins se propager dans I'ether avec la meme vi- tesse. H n'y a pas de difference a cet egard entre la propagation des ondes sonores, lesquelles se propagent dans 1'air avec la memo rapidite. Cette 6galit6 de propagation des ondes sonores est bien etablio experi- mentalement par la similitude d'effet que produit une musique donnee & toutes distances du lieu ou 1'on 1'execute. La principale difficulte, je dirai 1'unique difficulte, qu'on cut elev6e contre le systeme des ondes, consistait done a expliquer, comment la vitesse de propagation des ray- ons de differentes couleurs dans les corps differents pouvait etre dissem- blable et servir a rendre compte de 1'inegalite de refraction de ces ray- ons ou de la dispersion. On a montre r6cemment que cette difBculte n'est pas insurmontable ; qu'on peut constituer I'ether dans les corps inegalement denses de maniere que des rayons a ondulations dissem- blables s'y propagent avec des vitesses inegales : reste a determiner, si les conceptions des geometres a cet egard sont conformes a la nature des choses. Voici les amplitudes des ondulations deduites experimen- talement d'une serie de fails relatif aux interferences : mm. Violet 0-000423 Jaune 0-000551 Rouge 0-000620 La vitesse de transmission des rayons de diffe>entes couleurs dans le espaces celestes est la meme dans le systeme des ondes et tout-a-fait Ind^pendante de 1'etendue ou de la vitesse des ondulations." " According to the mathematical theory of a system of waves, rayi of different colors, having unequal undulations, must nevertheless be transmitted through ether with the same velocity. There is no differ- ence hi this respect from the mode of propagation of waves of sound which are transmitted through the atmosphere with equal velocity. This equality of transmission in waves of sound may be well demon strated experimentally by the uniformity of effect produced by music at all distances from the source whence it emanates. The principal, I may say the only objection, advanced against the undulatory theory, consisted in the difficulty of explaining how the velocity of the propa- gation of rays of different colors through different bodies could be dis similar, while it accounted for the inequality of thd "^fraction of the rays or of their dispersion. It has been recently shown* that this diffi culty is not insurmountable, and that the ether may be supposed to bo transmitted through bodies of unequal density in such a manner that rays of dissimilar systems of waves may be propagated through it with unequal velocities ; but it remains to be determined whether the views advanced by geometricians on this question are in unison with the act- ual nature of things. The following are the lengths of the undulations VELOCITY OF LIGHT. 85 refraction in the prism is not altered by the relation of the velocity of light to that of the earth's motion. All the meas- urements coincide in the result, that the light of those stars toward which the earth is moving presents the same index of refraction as the light of those from which it is receding. Using the language of the emission hypothesis, this celebra- ted observer remarks, that bodies send forth rays of all ve- locities, but that among these different velocities one only is capable of exciting the sensation of light.* as experimentally deduced from a series of facts in relation to inter- ference : mm. Violet 0-000423 Yellow 0-000551 Red 0-000620 The velocity of the transmission of rajs of different colors through ce- lestial space is equal in the system of waves, and is quite independent of the length or the velocity of the undulations." Arago, MS. of 1849. Compare also the Annuaire pour 1842, p. 333-336. The length of the luminous wave of the ether, and the velocity of the vibrations, determ- ine the character of the colored rays. To the violet, which is the most refrangible ray, belong 662, while to the red (or least refrangible ray with the greatest length of wave) there belong 451 billions of vibra- tions in the second. * " J'ai prouve, il y a bien des annees, par des observations directes que les rayons des Stoiles vers lesquelles la Terre marche, et les ray- ons des etoiles dont la terre s'eloigne, se refractent exactement de la meme quantite. Un tel resultat ne pent se concilier avec la tkforie de remission qu'a 1'aide d'une addition importante a faire a cette theorie : il faut admettre que les corps lumineux emettent des rayons de toutes les vitesses, et que les seuls rayons d'une vitesse determined sont visi- bles, qu'eux seuls produisent dans Tcei! la sensation de lumiere. Dans la theorie de 1'emission, le rouge, le jaune, le vert, le bleu, le violet so- laires sont respectivement accompagnes de rayons pareils, mais obscurs par defaut ou par exces de vitesse. A plus de vitesse correspond une moindre refraction, comtne moins de vitesse entraine une refraction plus grande. Ainsi chaque rayon rouge visible est accompagne de rayons obscurs de la meme nature, qui se refractent les uns plus, les autres moins que lui : ainsi il existe des rayons dans les stries noiret de la por- tion rouge du spectre ; la meme chose doit etre admise des stries situ ees dans les portions jaunes, vertes, bleues et violettes." " I showed many years ago, by direct observations, that the rays of those stars toward which the earth moves, and the rays of those stars from which it recedes, are repeated in exactly the same degree. Such a result can not be reconciled with the theory of emistion, unless we make the important admission that luminous bodies emit rays of all ve- locities, and that only rays of a determined velocity are visible, these alone being capable of impressing the eye with the sensation of light. In the theory of emission, the red, yellow, green, blue, and violet so- lar rays are respectively accompanied by like rays, which are, how- ever, dark from deficiency or excess of velocity. Excessive velocity is 86 COSMOS. On comparing the velocities of solar, stellar, and terres- trial light, which are all equally refracted in the prism, with the velocity of th& light of frictional electricity, we are disposed, in accordance with Wheatstone's ingeniously con- ducted experiments, to regard the lowest ratio in which the latter exceeds the former as 3 : 2. According to the lowest results of Wheatstone's optical rotatory apparatus, electric light traverses 288,000 miles in a second.* If we reckon 189,938 miles for stellar light, according to Struve's observ- ations on aberration, we obtain the difference of 95,776 miles as the greater velocity of electricity in one second. These results are apparently opposed to the views ad- vanced by Sir William Herschel, according to which solar and stellar light are regarded as the effects of an electro- magnetic process a perpetual northern light. I say ap- parently, for no one will contest the possibility that there may be several very different magneto-electrical processes in the luminous cosmical bodies, in which light the product of the process may possess a different velocity of propaga tion. To this conjecture may be added the uncertainty of the numerical result yielded by the experiments of Wheat- stone, who has himself admitted that they are not sufficient- ly established, but need further confirmation before they can associated with a slight degree of refraction, while a smaller amount of velocity involves a slighter degree of refraction. Thus every visible red ray is accompanied by dark rays of the same nature, of which some are more, and others less, refracted than the former ; there are conse- quently rays in the black lines of the red portion of the spectrum ; and the same must be admitted in reference to the lines situated in the yel low, green, blue, and violet portions." Arago, in the Comptes Rendus de VAcad. des Sciences, t. xvi., 1843, p. 404. Compare also t. viii., 1839, p. 326, and Poisson, Traite de Mccanique, ed. ii., 1833, t. i., $ 168. Ac- cording to the undulatory theory, the stars emit waves of extremely various transverse velocities of oscillations. * Wheatstone, in the Philos. Transact, of the Royal Soc.for 1834, p. 589, 591. From the experiments described in this paper, it would ap pear that the human eve is capable of perceiving phenomena of light, whose duration is limited to the millionth part of a second (p. 591). On the hypothesis referred to in the text, of the supposed analogy be- tween the light of the sun and polar light, see Sir John Herschel's Re- mit* of Aitron. Observ. at the Cape of Good Hope, 1847, p. 351. Arago, in the Comptes Rendus pour 1838, t. vii., p. 956, has referred to the in- genious application of Breguet's improved Wheatstone's rotatory ap- paratus for determining between the theories of emission and undula- tion, since, according to the former, light moves more rapidly through water than through air, while, according to the latter, it moves more rapidly through air than through water. (Compare also Comptes Ren- dus pour 1850, t. xxx., p. 489-495, 556.) VELOCITY OF ELECTRICITY. 87 be satisfactorily compared with the results deduced from ob- servations on aberration and on the satellites. The attention of physicists has been powerfully attracted to the experiments on the velocity of the transmission of electricity, recently conducted in the United States by Walk- er during the course of his electro-telegraphic determina- tions of the terrestrial longitudes of Washington, Philadel- phia, New York, and Cambridge. According to Steinheil's description of these experiments, the astronomical clock of the Observatory at Philadelphia was brought to correspond so perfectly with Morse's writing apparatus on the tele- graphic line, that this clock marked its own course by points on the endless paper fillets of the apparatus. The electric telegraph instantaneously conveys each of these clock times to the other stations, indicating to these the Philadelphia time by a succession of similar points on the advancing pa- per fillets. In this manner, arbitrary signs, or the instant of a star's transit, may be similarly noted down at the sta- tion by a mere movement of the observer's finger on the stop. "The special advantage of the American method consists," as Steinheil observes, " in its rendering the determination of time independent of the combination of the two senses, sight and hearing, as the clock notes its own course, and indicates the instant of a star's transit (with a mean error, according to Walker's assertion, of only the 70th part of a second). A constant difference between the compared clock times at Philadelphia and at Cambridge is dependent upon the time occupied by the electric current in twice traversing the closed circle between the two stations." Eighteen equations of condition, from measurements made on conducting wires of 1050 miles, gave for the velocity of transmission of the hydro-galvanic current 18,700 miles,* which is fifteen times less than that of the electric current in Wheatstone's rotatory disks. As in Walker's remarkable experiments two zvires were not used, but half of the con- * Steinheil, in Schumacher's Astr. Naekr., No. 679 (1849), s. 97-100; Walker, in the Proceedings of the American Philosophical Society, vol. v., p. 128. (Compare earlier propositions of Pouillet in the Comptes Rendus, t. xix., p. 1386.) The more recent ingenious experiments of Mitchel, Director of the Observatory at Cincinnati (Gould's Astron. Journal, Dec., 1849, p. 3, On the Velocity of the Electric Wave'), and the investigations of Fizeau and Gounelle at Paris, in April, 1850, differ both from Wheatstone's and Walker's results. The experiments re- corded in the Comptes Rendut, t. xxx., p. 439, exhibit striking differ ences between iron and copper as conducting media. 88 COSMOS. duction, to use a conventional mode of expression, passed through the moist earth, we should seem to be justified in concluding that the velocity of the transmission of electricity depends upon the nature as well as the dimensions* of the medium. Bad conductors in the voltaic circuit become more powerfully heated than good conductors ; and the experi- ments lately made by Eiessf show that electric discharges are phenomena of a very various and complicated nature. The views prevailing at the present day regarding what is usually termed " connection through the earth" are opposed to the hypothesis of linear, molecular conduction between the extremities of the wires, and to the conjectures of the impediments to conduction, of accumulation, and disruption in a current, since what was formerly regarded as interme- diate conduction in the earth is now conjectured to belong exclusively to an equalization or restoration of the electric tension. Although it appears probable, from the extent of accura- cy at present attainable in this kind of observation, that the constant of aberration, and, consequently, the velocity of light, is the same for all fixed stars, the question has fre- quently been mooted whether it be not possible that there are luminous cosmical bodies whose light does not reach us, in consequence of the particles of air being turned back by the force of gravitation exercised by the enormous masses of these bodies. The theory of emission gives a scientific form to these imaginative speculations.^ I here only refer * See PoggendorflPs Annalen, bd. Ixxiii., 1848, s. 337, and Pouillet, Comptes Rendus, t. xxx., p. 501. t Riess, in PoggendorfTs Ann., bd. 78, s. 433. On the non-conduc tion of the intermediate earth, see the important experiments of Guille- miu, Sur le courant dans une pile isolte ct sans communication entre let pdles in the Comptes Rendus, t. xxix., p. 521. " Quand on remplace un fil par la terre, dans les telegraphes electriques, la terre sort plut6t de reservoir commun, quo de moyen d'union entre les deux extremi- tes du fil." " When the earth is substituted for half the circuit in the electric telegraph, it serves rather as a common reservoir than as a means of connection between the two extremities of the wire." t Madler, Astr., a. 380; also Laplace, according to Moigno, Repertoire d'Optique Moderne, 1847, t. i., p. 72 : " Selon la theorie de l'6mission on croit pouvoir demontrer que si le diametre d'une 6toile fixe serait 250 fois plus grand que celui du soleil, sa densite restant la meme, 1'attrac- tion exercee a sa surface detruirait la quantite de mouvement, de la moldcule lumineuse Praise, de sorte qu'elle serait invisible a de gramles distances." " It seems demonstrable by the theory of emission that if the diameter of a fixed star be 250 times greater than that of the sun its density remaining the same the attraction exercised on the surface STELLAR LIGHT. 89 to such views because it will be necessary in the sequel that we should consider certain peculiarities ef motion ascribed to Procyon, which appeared to indicate a disturbance from dark cosmical bodies. It is the object of the present portion of this work to notice the different directions to which scien- tific inquiry had inclined at the period of its composition and publication, and thus to indicate the individual character of an epoch in the sidereal as well as the telluric sphere. The photometric relations (relations of brightness) of the self-luminous bodies with which the regions of space are filled, have for more than two thousand years been an ob- ject of scientific observation and inquiry. The description of the starry firmament did not only embrace determinations of places, the relative distances of luminous cosmical bodies from one another and from the circles depending on the ap- parent course of the sun and on the diurnal movement of the vault of heaven, but it also considered the relative in- tensity of the light of the stars. The earliest attention of mankind was undoubtedly directed to this latter point, in- dividual stars having received names before they were ar- ranged Avith others into groups and constellations. Among the wild tribes inhabiting the densely- wooded regions of the Upper Orinoco and the Atabapo, where, from the impene- trable nature of the vegetation, I could only observe high culminating stars for determinations of latitude, I frequently found that certain individuals, more especially old men, had designations for Canopus, Achernar, the feet of the Centaur, and a in the Southern Cross. If the catalogue of the con- stellations known as the Catasterisjns of Eratosthenes can lay claim to the great antiquity so long ascribed to it (between Autolycus of Pitane and Timocharis, and therefore nearly a would destroy the amount of motion emitted from the luminous mole- cule, so that it would be invisible at great distances." If, with Sir William Herschel, we ascribe to Arcturus an apparent diameter of 0"-1, it follows that the true diameter of this star is only eleven times greater than that of our sun. (Cosmos, vol. i., p. 148.) From the above con- siderations on one of the causes of non-luminosity, the velocity of light must be very different in cosmical bodies of different dimensions. This has, however, by no means been confirmed by the observations hitherto made. Arago says in the Comptes Rendus, t. viii., p. 326, " Les expe- riences sur 1'egale deviation prismatique des etoiles, vers lesquelles la terre marche ou dont elle s'eloigne, rend compte de I'6galit6 de vitesse apparente de toutes les etoiles." "Experiments made on the equal prismatic deviation of the stars toward which the earth is moving, and from which it is receding, explain the apparent equality of velocity in the ray of all the stars." 90 COSMOS. century and a half before the time of Hipparchus), we pos- sess in the astronomy of the Greeks a limit for the period when the fixed stars had not yet been arranged according to their relative magnitudes. In the enumeration of the stars belonging to each constellation, as given in the Catas- terisms, frequent reference is made to the number of the largest and most luminous, or of the dark and less easily rec- ognized stars ;* but we find no relative comparison of the stars contained in the different constellations. The Catas- terisms are, according to Bernhardy, Baehr, and Letronne, more than two hundred years less ancient than the catalogue of Hipparchus, and are, besides, a careless compilation and a mere extract from the Poeticum Astronomicum (ascribed to Julius Hyginus), if not from the poem 'Epju^f of the older Eratosthenes. The catalogue of Hipparchus, which we pos- sess in the form given to it in the Almagest, contains the ear- liest and most important determination of classes of magni- tude (gradations of brightness) of 1022 stars, and therefore of about one fifth of all the stars in the firmament visible to the naked eye, and ranging from the first to the sixth mag- nitude inclusive. It remains undetermined whether these estimates are all due to Hipparchus, or whether they do not rather appertain in part to the observations of Timocharis or Aristyllus, which Hipparchus frequently used. This work constituted the important basis on which was established the science of the Arabs and of the astronomers of the Middle Ages : the practice, transmitted to the nine- teenth century, of limiting the number of stars of the first magnitude to 15 (although Madler counts 18, and Riimker, after a more careful observation of the southern celestial hem- isphere, upward of 20), takes its origin from the classifica- tion of the Almagest, as given at the close of the table of stars in the eighth book. Ptolemy, referring to natural vi- sion, called all stars dark which were fainter than those of his sixth class ; and of this class he singularly enough only instances 49 stars distributed almost equally over both hem- ispheres. Considering that the catalogue enumerates about one fifth of all the fixed stars visible to the naked eye, it should, according to Argelander's investigations, have given * Eratosthenes, Catasterismi, ed. Schaubach, 1795, and Eratotthenica, ed. G. Bernhardy, 1822, p. 110-116. A distinction is made between stars hafinpovc (jieyuTiOvf) and afjtavpovf (cap. 2, 11, 41). Ptolemy also limits ol {ipopfyuToi to those stars which do not regularly belong to a con- stellation. MAGNITUDES OF STARS. 91 640 stars of .the sixth magnitude. The nebulous stars (ve- deXoeLdds') of Ptolemy and of the Pseudo-Eratosthenian Ca- iasterisms are mostly small stellar swarms,* appearing like nebulae in the clearer atmosphere of the southern hemisphere. I more particularly base this conjecture on the mention of a nebula in the right hand of Perseus. Galileo, who, like the Greek and Arabian astronomers, was unacquainted with the nebula in Andromeda which is visible to the naked eye, says in his Nuntius sidereus that stellce nebulosts are nothing more than stellar masses scattered in shining groups through the ether (areolce, sparsim per cethera fulgent).^ The ex- pression (r&v fj,eydA.d)v raft^), the order of magnitudes, al- though referring only to luster, led, as early as the ninth cen- tury, to hypotheses on the diameters of stars of different bright- ness ;J as if the intensity of light did not depend on the dis- tance, volume, and mass, as also on the peculiar character of the surface of a cosmical body in more or less favoring the process of light. At the period of the Mongolian supremacy, when, in the fifteenth century, astronomy nourished at Samarcand, under Timur Ulugh Beg, photometric determinations were facili- tated by the subdivision of each of the six classes of Hippar- chus and Ptolemy into three subordinate groups ; distinctions, for example, being drawn between the small, intermediate, and large stars of the second magnitude an attempt which reminds us of the decimal gradations of Struve and Argelan- der. This advance in photometry, by a more exact determ- ination of degrees of intensity, is ascribed in Ulugh Beg's tables to Abdurrahman Sufi, who wrote a work " on the knowledge of the fixed stars," and was the first who men- tions one of the Magellanic clouds under the name of the White Ox. Since the discovery and gradual improvement of telescopic vision, these estimates of the gradations of light have been extended far below the sixth class. The desire of comparing the increase and decrease of light in the newly- * Plot. Almas., ed Halma, torn, ii., p. 40, and in Eratosth. Catast., cap. 22, p. 18: rj de KtQa/.Tj Kai ij apnr) uvaifTOf dparai, 6ia de ve0e?.u<5otf avarpo<j>jjc do/ccl TLGIV opuadai. Thus, too, Geminus, Pheen. (ed. Hilder, 1590), p. 46. t Cosmos, vol. ii., p. 330, 331. t Muhamedis Alfragani Chronologica et Ast. Elementa, 1590, cap. xxiv., p. 118. $ Some MSS. of the Almagest refer to such subdivisions or interme- diate classes, as they add the words pei&v or ehdaauv to the determ- ination of magnitudes. (Cod. Paris, No. 2389.) Tycho expressed thia increase or diminution by points. 92 COSMOS. appeared stars in Cygnus and Ophiuchus (tie former of which continued luminous for twenty-one years), with the bright- ness of other stars, called attention to photometric determina- tions. The so-called dark stars of Ptolemy, which were he- low the sixth magnitude, received numerical designations according to the relative intensity of their light. " Magni- tudes, from the eighth down to the sixteenth," says Sir John Herschel, " are familiar to those who are in the practice of using powerful instruments.* But at this faint degree of brightness, the denominations for the different gradations in the scale of magnitudes are very undetermined, for Struve occasionally classes among the twelfth or thirteenth stars which Sir John Herschel designates as belonging to the eighteenth or twentieth magnitudes. The present is not a fitting place to discuss the merits of the very different methods which have been adopted for the measurement of light within the last hundred and fifty years, from Auzout and Huygens to Bouguer and Lambert ; and from Sir William Herschel, K-umford, and Wollaston, to Stein- heil and Sir John Herschel. It will be sufficient for the ob- ject of this work briefly to indicate the different methods. These were a comparison of the shadows of artificial lights, differing in numbers and distance ; diaphragms ; plane-glass- es of different thickness and color ; artificial stars formed by reflection on glass spheres ; the juxtaposition of two seven- feet telescopes, separated by a distance which the observer could pass in about a second ; reflecting instruments in which two stars can be simultaneously seen and compared, when the telescope has been so adjusted that the star directly ob- served gives two images of like intensity ;t an apparatus hav * Sir John Herschel, Outlines of Astr., p. 520-27. t This is the application of reflecting sextants to the determination of the intensity of stellar light ; of this instrument I made greater use when in the tropics than of the diaphragms recommended to me by Borda. I began my investigation under the clear skies of Cumana, and continued them subsequently till 1803, but under less favorable condi- tions, on the elevated plateaux of the Andes, and on the coasts of the Pacific, near Guayaquil. I had formed an arbitrary scale, in which I marked Sirius, as the brightest of all the fixed stars, equal to 100; the stars of the first magnitude between 100 and 80, those of the second magnitude between 80 and 60, of the third between 60 and 45, of the fourth between 45 and 30, and those of the fifth between 30 and 20. I especially measured the constellations of Argo and Grus, in which I thought I had observed alterations since the time of Lacaille. It seemed to me, after a careful combination of magnitudes, using other stars as intermediate gradations, that Sirius was as much brighter than Canopus, as a Centauri than Achernar. My numbers can not, on accoupt of tho PHOTOMETRIC METHODS. 93 ing (iii front of the object-glass) a mirror and diaphragms, whose rotation is measured on a ring ; telescopes with di- vided object-glasses, on either half of which the stellar light is received through a prism ; astrometers* in which a prism reflects the image of the moon or of Jupiter, and concentrates it through a lens at different distances into a star more or less bright. Sir John Herschel, who has been more zealous- ly engaged than any other astronomer of modern times in making numerical determinations in both hemispheres of the intensity of light, confesses that the practical application of exact photometric methods must still be regarded as a " de- above-mentioned mode of classification, be compared directly with those which Sir John Herschel made public as early as 1838. (See my Recneil d'Observ. Astr., vol. i., p. Ixxi., and Rclat. Hist, du Voyage aux Regions Equin., t. i., p. 518 and 624; also Lettre de M. de Humboldt a M. Schumacher en Fevr., 1839, in the Astr. Nachr., No. 374.) In this letter I wrote as follows : " M. Arago, qui possede des moyens photo- metriques entierement difierents de ceux qui ont ete publics jusqu'ici, m'avait rassure sur la partie des erreurs qui pouvaient provenir du change- ment d'inclinaison d'un miroir entame sur la face interieure. H blame d'ailleurs le principe de ma methode et le regarde comme peu suscep- tible de perfectionnement, non seulement a cause de la difference des angles entre 1'etoile vue directement et celle qui est amenee par reflex- ion, mais surtout parceque le resultat de la mesure d'intensite dfepend de la partie de 1'ceil qui se trouve en face de 1'oculaire. II y a erreur lorsque la pupille n'est pas tres exactement a la hauteur de la limite in- ferieure de la portion non eutamee du petit miroir." " M. Arago, who possesses photometric data differing entirely from those hitherto pub- lished, had instructed me in reference to those errors which might arise from a change of inclination of a mirror silvered on its inner surface. He moreover blames the principle of my method, and regards it as lit- tle susceptible of correctness, not only on account of the difference of angles between the star seen directly and by reflection, but especially because the result of the amount of intensity depends on the part of the eye opposite to the ocular glass. There will be an error in the observ- ations when the pupil is not exactly adjusted to the elevation of the lower limit of the unplated part of the small mirror." * Compare Steinheil, Elemente der Helligkeils-Messungen am Sternen- himmel Munchen, 1836 (Schum., Astr. Nachr., No. 609), and John Her- schel, Results of Astronomical Observations made during the Years 1834 -1838 at the Cape of Good Hope (Lond., 1847), p. 353-357. Seidel at- tempted in 1846 to determine by means of Steinheil's photometer the quantities of light of several stars of the first magnitude, which attain the requisite degree of latitude in our northern latitudes. Assuming Vega to be =1, he finds for Sirius 5-13 ; for Rigel, whose luster appears to be on the increase, 1-30; for Arcturus, 0-84; for Capella, 0-83; for Procyon, 071; for Spica, 0-49; for Atair, 0-40; for Aldebaran, 0-36; for Deneb, 0-35; for Regulns, 0-34; for Pollux, 0-30; he does not give the intensity of the light of Betelgeux, on account of its being a varia- ble star, as was particularly manifested between 1836 and 1839. (Ont tiite*, p. 523 ) 94 COSMOS. eideratum in astronomy," and that " photometry is yec /*. * infancy." The increasing interest taken in variable swrs, and the recent celestial phenomenon of the extraordinary in- crease of light exhibited in the year 1837 in a star of the con- stellation Argo, has made astronomers more sensible of the importance of obtaining certain determinations of light. It is essential to distinguish between the mere arrangement of stars according to their luster, without numerical estimates of the intensity of light (an arrangement adopted by Sir John Herschel in his Manual of Scientific Inquiry prepared for the Use of the Navy), and classifications in which intensity of light is expressed by numbers, under the form of so-called relations of magnitude, or by more hazardous estimates of the quantities of radiated light.* The first numerical scale, based on estimates calculated with the naked eye, but improved by an ingenious elaboration of the materials! probably deserves the preference over any other approximative method practi- cable in the present imperfect condition of photometrical in- struments, however much the exactness of the estimates must be endangered by the varying powers of individual observers the serenity of the atmosphere the different altitudes of widely-distant stars, which can only be compared by means of numerous intermediate stellar bodies and above all by the unequal color of the light. Very brilliant stars of the first magnitude, such as Sirius and Canopus, a Centauri and Acher- nar, Deneb and Vega, on account of their white light, admit far less readily of comparison by the naked eye than fainter stars below the sixth and seventh magnitudes. Such a com- parison is even more difficult when we attempt to contrast yellow stars of intense light, like Procyon, Capella, or Atair, with red ones, like Aldebaran, Arcturus, and Betelgeux.J * Compare, for the numerical data of the photometric results, four tables of Sir John Herschel's Astr. Obs. at the Cape, a), p. 341 ; b), p. 367-371 ; c), p. 440 ; and d), in his Outlines of Astr., p. 522-525, 645- 646. For a mere arrangement without numbers, see the Manual of Scientific Inquiry prepared for the Use of the Navy, 1819, p. 12. In order to improve the old conventional mode of classing the stars accord- ing to magnitudes, a scale of photometric magnitudes, consisting in the addition of 0-41, as explained more in detail in Astr. Obs. at the Cape, p. 370. has been added to the vulgar scale of magnitudes in the Outlines of Astronomy, p. 645, and these scales are subjoined to this portion of the present work, together with a list of northern and southern stars. t Argelander, Durchmusterung des nordl. Himmels zwischen 45 und 80 Decl. 1846, s. xxiv.-xxvi. ; Sir John Herschel, Astr. Obscrv. al the Cape of Good Hope, p. 327, 340, 365. t Op. cit., p. 304, aud Outl., p. 522. PHOTOMETRY. 95 Sir John Herschcl has endeavored to determine the rela- tion between the intensity of solar light and that of a star of the first magnitude by a photometric comparison of the moor, with the double star a Centauri of the southern hemisphere, which is the third in brightness of all the stars. He thus fulfilled (as had been already done by "Wollaston) a wish ex- pressed by John Michell* as early as 1767. Sir John Her- schel found from the mean of eleven measurements conduct- ed with a prismatic apparatus, that the full moon was 27,408 times brighter than a Centauri. According to Wollaston, the light of the sun is 801,072 times brighter than the full moon ;f whence it follows that the light transmitted to us from the sun is to the light which we receive from a Centauri as 22,000 millions to 1. It seems, therefore, very probable, when, in accordance with its parallax, we take into account the distance of the star, that its (absolute) proper luminosity exceeds that of our sun by 2f- times. Wollaston found the brightness of Sirius 20,000 million times fainter than that of the sun. From what we at present believe to be the paral- lax of Sirius (0-"230), its actual (absolute) intensity of light exceeds that of the sun 63 times4 Our sun therefore be- longs, in reference to the intensity of its process of light, to the fainter fixed stars. Sir John Herschel estimates the in- tensity of the light of Sirius to be equal to the light of nearly * Philos. Transact., vol. Ivii., for the year 1767, p. 234. t Wollaston, in the Philos. Transact, for 1829, p. 27. Herschel'a Outlines, p. 553. Wollaston's comparison of the light of the sun with that of the moon was made in 1799, and was based on observations of the shadows thrown by lighted wax tapers, while in the experiments made on Sirius in 1826 and 1827, images reflected from thermometer bulbs were employed. The earlier data of the intensity of the sun'a light, compared with that of the moon, differ widely from the results here given. They were deduced by Michelo and Euler, from theoret- ical grounds, at 450,000 and 374,000, and by Bouguer, from measure- ments of the shadows of the light of wax tapers, at only 300,000. Lam- bert assumes Venus, in her greatest intensity of light, to be 3000 times fainter than the full moon. According to Steinheil, the sun must be 3,286,500 times further removed from the earth than it is, in order to appear like Arcturus to the inhabitants of our planet (Struve, Stellarum Compositarnm Mcnsuree Micrometricee, p. clxiii.); and, according to Sir John Herschel, the light of Arcturus exhibits only half the intensity of 3anopus. Herschel, Observ. at the Cape, p. 34. All these conditions of intensity, more especially the important comparison of tho bright ness of the sun, the full moon, and of the ash-colored light of our satel- lite, which varies so greatly according to the different positions of the earth considered as a reflecting body, deserve further and serious in- vestigation. \ Quit. <>f Astr., p. 553 ; Aslr. Observ. at the Cape, p. 363. 96 COSMOS. two hundred stars of the sixth magnitude. Since it is very probable, from analogy with the experiments already made, that all cosmical bodies are subject to variations both in their movements through space and in the intensity of their light, although such variations may occur at very long and unde- termined periods, it is obvious, considering the dependence of all organic life on the sun's temperature and on the intens- ity of its light, that the perfection of photometry constitutes a great and important subject for scientific inquiry. Such an improved condition of our knowledge can render it alone possible to transmit to future generations numerical determ- inations of the photometric condition of the firmament. By these means we shall be enabled to explain numerous geog- nostic phenomena relating to the thermal history of our at- mosphere, and to the earlier distribution of plants and ani- mals. Such considerations did not escape the inquiring mind of William Herschel, who, more than half a century ago, be- fore the close connection between electricity and magnetism had been discovered, compared the ever-luminous cloud-en- velopes of the sun's body with the polar light of our own ter- restrial planet.* Arago has ascertained that the most certain method for the direct measurement of the intensity of light consists in observing the complementary condition of the colored rings seen by transmission and reflection. I subjoin in a note,t in * William Herschel, On the Nature of the Sun and Fixed Stars, in the Philos. Transact, for 1795, p. 62 ; and On the Changes that happen to the Fixed Stars, in the Philos. Transact, for 1796, p. 186. Gomparo also Sir John Herschel, Obsero. at the Cape, p. 350-352. t Extract of a Letter from M. Arago to M. de Humboldt, May, 1850. (a.) Mesurcs Photom6triqucs. " II n'existe pas de photometre proprement dit, c'est-a-dire d'instru- ment donuant 1'intensite d'une lumiere isol^e ; le photometre de Les- lie, a 1'aide duquel il avail eu 1'audace de vouloir comparer la lumiere de la lune a la lumiere du soleil, par des actions calonfiques, est com- pletement defectueux. .T'ai prouve, en effet, que ce preteudu photo- metre monte quand on 1'expose a la lumiere du soleil, qu'il descend sous 1'action de la lumiere du feu ordinaire, et qu'il reste complete- ment stationnaire lorsqu'il re9oit la lumiere d'une lampe d'Argand'. Tout ce qu'on a pu faire jusqu'ici, c'est de comparer entr'elles deux lu- mieres en presence, et cette comparaison n'est meme a 1'abri de toute objection que lorsqu'on ramene ces deux lumieres a I'egalit^ par un amublissement graduel de la lumiere la plus forte. C'est comme crite- rium de cette egalit6 que j'ai employ^ les anneaux colores. Si on place I'une sur 1'autre deux lentilles d'un long foyer, il se forme autour de leur point de contact des anneaux colored tant par voie de reflexion que par voi 5 de transmission. Les anueaux reflechia sout conipleineutaires PHOTOMETRY. 97 his own words, the results of my friend's photometric method, to which he has added an account of the optical principle oa which his cyanometer is based. en couleur des anneaux transmis; ces deux series d'anneaux se neu- tralisent mutuellement qoand les deux lumieres qui les Ibrment et qui arrivent simultanement sur les deux lentilles, sont egales entr'elles. " Dans le cas contraire on voit des traces ou d'anneaux reflechis ou d'anneaux transrais, suivant quo la lumiere qui forme les premiers, est plus forte ou plus foible que la lumiere a laquelle on doit les seconds. C'est dans ce sens settlement que les anneaux colores jouent un role dans les mesures de la lumiere auxquelles je me suis livre." (6.) Cyanometre. " Mon cyanometre est une extension de mon polariscope. Ce der- nier instrument, comma tu sais, se compose d'un tube ferme 1'une de ses extremites par une plaque de cristal de roche perpendiculaire a I'axe, de 5 millimetres d'epaisseur ; et d'un prisme doue de la double refraction, place du cote de 1'oeil. Parmi les couleurs variees que donne cet appareil, lorsque de la lumiere polarisee le traverse, et qu'on fait tourner le prisme sur lui-meme, se trouve par un heureux basard la nuance du bleu de ciel. Cette couleur bleue fort afiaiblie, c'est-4-dire tres melangee de blanc lorsque la lumiere est presque neutre, aug- mente d'intensite progressivement, a mesure que les rayons qui pene- trent dans 1'instrumeut, renferment une plus grande proportion de ray- ons polarises. " Supposons done que le polariscope soit dirige sur une feuille de pa- pier blanc ; qu'entre cette feuille et la lame de cristal de roche il ex- iste une pile de plaques de verre susceptible de changer d'inclinaison, co qui rendra la lumiere eclairante du papier plus ou nioins polarisee ; la couleur bleue fournie par 1'instrument va en augmentant avec 1'in- clinaison de la pile, et 1'on s'arrete lorsque cette couleur parait la meme que celle de la region de 1'atmosphere dont on veut determiner la teinte cyanometrique, et qu'ou regarde & 1'ceil nu immecliatement a cote de 1'instrumeut. La mesure de cette teiute est don nee par 1'inclinaison de la pile. Si cette derniere partie de 1'instrument se compose du meme nombre de plaques et d'une meme espece de verre, lea observations faites dans divers lieux seront parfaitement comparables entr'elles." (a.) Photometric Measurements. " There does not exist a photometer properly so called, that is to say, no instrument giving the intensity of an isolated light ; for Leslie's photometer, by means of which he boldly supposed that he could com pare the light of the moon with that of the sun, by their caloric actions, is utterly defective. I found, in fact, that this pretended photometer rose on being exposed to the light of the sun, that it fell when exposed to a moderate fire, and that it remained altogether stationary when brought near the light of an Argand lamp. All that has hitherto been done has been to compare two lights when contiguous to one another ; but even this comparison can not be relied on unless the two lights be equalized, the stronger being gradually reduced to the intensity of the feebler. For the purpose of judging of this inequality I employed col- ored rings. On placing on one another two lenses of a great focal length, colored rings wdl be formed round their point of contact as much by means of reflection as of transmission. The colors of the r& VOL, III E 98 COSMOS. The so-called relations of the magnitude cf the fixed star* as given in our catalogues and maps of the stars, sometimes indicate as of simultaneous occurrence that which belongs to very different periods of cosmical alterations of light. The order of the letters which, since the beginning of the seven- teenth century, have been added to the stars in the general- ly consulted Uranometria Bayeri, are not, as was long sup- posed, certain indications of these alterations of light. Arge- lander has ably shown that the relative brightness of the stars can not be inferred from the alphabetical order of the letters, and that Bayer was influenced in his choice of these letters by the form and direction of the constellations.* fleeted rings are complementary to those of the transmitted rings ; these two series of rings neutralize one another when the two lights by which they aro formed, and which fall simultaneously on the two lenses, are equal. " In the contrary case, we meet with traces of reflected or transmit- ted rings, according as the light by which the former are produced is stronger or fainter than that from which the latter are formed. It is only in this manner that colored rings can ba said to come into play in those photometric measurements to which I bavi diracted my atten- tion." (b.) Cyanometer. " My cyanometer is an extension of my polariscope. This latter in- strument, as you know, consists of a tube closed at ono end by a plate of rock crystal, cut perpendicular to its axis, and 5 millimetres in thick- ness ; and of a double refracting prism placed near the part to which the eye is applied. Among the varied colors yielded by this apoaratus, when it is traversed by polarized light and the prism turns on itself, wo fortunately find a shade of azure. This blue, which is very faint, that is to say, mixed with a large quantity of white when the light is almost neutral, gradually increases in intensity in proportion to the quantity of polarized rays which enter the instrument. " Let us suppose the polariscope directed toward a sheet of white paper, and that between this paper and the plate of rock crystal there is a pile of glass plates capable of being variously inclined, by which means the illuminating light of the paper would be more or less polar- ized ; the blue color yielded by the instrument will go on increasing with the inclination of the pile ; and the process must be continued un- til the color appears of the same intensity with the region of the atmos- phere whose cyanometrical tinge is to be determined, and which is seen by the naked eye in the immediate vicinity of the instrument. The amount of this color is given by the inclination of the pile ; and if this portion of the apparatus consist of the same number of plates formed of the same kind of glass, observations made at different places may readily be compared together." * Argelander, Defde Uranomelria: Bayeri, 1842, p. 14-23. "In ea- dem classe littera prior majorem splendorem nullo modo indicat" ( 9). Bayer did not, therefore, show that the light of Castor was more intense in 1603 than that of Pollux. PHOTOMETRIC SCALE. 99 PHOTOMETRIC ARRANGEMENT OF THE FIXED STARS. I close this section with a table taken from Sir John Herschel's Out ines of Astronomy, p. 645 and 64G. I am indebted for the mode of its arrangement, and for the following lucid exposition, to my learned friend Dr. Galle, from whose communication, addressed to me in March, 1850, I extract the subjoined observations : " The numbers of the photometric scale in the Outlines of Astron- omy have been obtained by adding throughout 0-41 to the results calcu- lated from the vulgar scale. Sir John Herschel arrived at these more exact determinations by observing their " sequences" of brightness, and by combining these observations with the average ordinary data of mag- nitudes, especially on those given in the catalogue of the Astronomical Society for the year 1827. See Observ. at the Cape, p. 304-352. The actual photometric measurements of several stars as obtained by the Astrometer (op. cit., p. 353), have not been directly employed in this catalogue, but have only served generally to show the relation existing between the ordinary scale (of 1st, 2d, 3d, &c., magnitudes) to the act- ual photometric quantities of individual stars. This comparison has given the singular result that our ordinary stellar magnitudes ( 1, 2, 3 . . .) decrease in about the same ratio as a star of the first magnitude when removed to the distances of 1, 2, 3 ... by which its brightness, accord- ing to photometric law, would attain the values 1, Jth, ^th, -pg-th . . . (Observ. at the Cape, p. 371, 372 ; Outlines, p. 521, 522) ; in order, how- ever, to make this accordance still greater, it is only necessary to raise our previously adopted stellar magnitudes about half a magnitude (or, more accurately considered, 0-41), so that a star of the 2-00 magnitude would in future be called 2-41, and star of 2-50 would become 2-91, and so forth. Sir John Herschel therefore proposes that this " photo- metric" (raised) scale shall in future be adopted (Observ. at the Cape, p. 372, and Outlines, p. 522) a proposition in which we can not fail to concur ; for while, on the one hand, the difference from the vulgar scale would hardly be felt (Observ. at the Cape, p. 372), the table in the Out- lines (p. (>45) may, on the other hand, serve as a basis for stars down to the fourth magnitude. The determinations of the magnitudes of the stars according to the rule, that the brightness of the stars of the first, second, third, fourth magnitude is exactly as 1, jth, ith, -p^th ... as is now shown approximatively, is therefore already practicable. Sir John Herschel employs a Centauri as the standard star of the first magnitude for his photometric scale, and as the unit for the quantity of light (Out- lines, p. 523; Observ. at the Cape, p. 372). If, therefore, we take the square of a star's photometric magnitude, we obtain the inverse ratio of the quantity of its light to that of a Centauri. Thus, for instance, if K Orionis have a photometric magnitude of 3, it consequently has ^th of the light of a Centauri. The number 3 would at the same time in- dicate that K Orionis is 3 times more distant from us than a Centauri, provided both stars be bodies of equal magnitude and brightness. If another star, as, for instance, Sirius, which is four times as bright, were chosen as the unit of the photometric magnitudes indicating distances, the above conformity to law would not be so simple and easy of recog- nition. It is also worthy of notice, that the distance of a Centauri has been ascertained with some probability, and that this distance is the smallest of any yet determined. Sir John Henchel demonstrates ( Out- lines, p. 521) the inferiority of other scales to the photometric, which 100 COSMOS. progresses in order of the squares, 1, th, th, J ff th ... He likewise treats of geometric progressions, as, for instance, 1, 1, Jth, |th, ... or 1, ^d, th, ^th. .... The gradations employed by yourself in your ob- servations under the equator, during your travels in America, are ar- ranged in a kind of arithmetical progression (Recueil d'Observ. Atlron., vol. i., p. Ixxi., and Schumacher's Astron. Nachr., No. 374). These scales, however, correspond less closely than the photometric scale of progression (by squares) -with the vulgar scale. In the following table the 190 stars have been given from the Outlines, without reference to their declination, whether southern or northern, being arranged solely in accordance with their magnitudes." List of 190 stars from the first to the third magnitude, arranged accord- ing to the determinations of Sir John Herschel, giving the ordinary magnitudes with greater accuracy, and likewise the magnitudes in ac- cordance with his proposed photometric classification : STARS OF THE FIRST MAGNITUDE. Star. Magnitude. Star. Magnitude. Sirius Vulg. 008 0-29 059 077 082 1-0 10 1-0 Phot. 0-49 0-70 1-00 1-18 1-23 1-4 1-4 1-4 a Orionis Vulg. 1-0 1-09 1-1 1-17 1-2 1-2 1-28 1-38 Phot. 1-43 150 15 1-58 1-6 1-6 1 69 1-79 tl Argus (Var.) a Eridani Canopus Aldebaran a Centauri.... . . 8 Centauri Arcturus a Crucis Rigel An tares Capella a Aquila? ......... a. Lyra? Spica Procyon STARS OF THE SECOND MAGNITUDE. Star. Magnitude. Star. Magnitude. Fomalhaut Vulg.l Phot 1-541-95 1-57 1-98 1-6 2-0 1-6 20 1-66207 1-73214 l-84 ! 225 1-862-27 1-872-28 1-90 ! 231 1-94|235 1-95236 1-96:2-37 2-01242 2-03 2 44 2-072-48 2-082-49 2182-59 2-18259 2-18259 a Triang austr. 2^ 2-26 2-28 228 229 230 232 233 234 2-36 2-40 241 2-42 243 2-45 2-46 2-46 2-48 M Phot. 264 2-67 269 2-69 270 271 273 2-74 275 2-77 281 2-82 2-83 284 2-86 2-87 28"" 289 291 ft Crucis e Sagittarii Pollux 3 Tauri Regulus Polaris a Gruis . .. ....... 6 Scorpii a Hydra e Orionis 6 Canis e Can is a Pavonis y Leonis a Cygni 8 Gruis Castor a Arietis ........ E Ursse (Var ) a Sagittarii a Ursae (Var ) 6 Argus f Orionis Ursae 8 Argus . ... . . 8 Andromeda? /? Ceti A Argus . ........ e Argus i? Ursae (Var ) y Andromedse y Orionis PHOTOMETRIC SCALE. STARS OF THE THIRD MAGNITUDE. 101 Masmtmle. Magnitude. y Cassiopeiae Vulg. J 52 254 2-54 2-57 2-58 259 259 2-61 2-62 2-62 2 62 263 263 263 263 265 265 2-68 269 2-71 2-71 2-72 2-77 2-78 2-80 280 2-82 2-82 2-85 2-85 286 2-88 Phot. 2-93 2-95 2-95 2-98 2-99 300 300 3-02 3-03 3-03 3-03 3-04 3-04 3-04 3-04 306 3-06 3-09 3-10 3-12 3-12 3-13 3-18 3-19 3-21 3-21 3-23 323 326 326 3-27 'VQ f Sagittarii Vulg. 3-01 3-01 302 305 3-06 307 3-08 3-08 3-09 311 3-11 312 313 3-14 3-14 3-15 3-17 3-18 320 320 322 322 3-23 324 3-26 3-26 326 326 327 327 3-28 3 28 3-29 330 331 331 3-^2 332 332 332 333 334 335 335 3-35 335 33G 336 336 337 Phot. 342 3-42 343 346 347 348 349 349 350 352 352 353 354 355 355 356 358 359 361 3-61 3 63 3-63 3-64 365 3-67 367 3-67 367 368 368 369 369 370 3-71 372 372 373 373 373 373 374 375 3-76 3 76 376 376 377 377 377 a-78 a Andromedae ..... 77 Bootis 6 Centauri 77 Draconis a Cassiopeiae ........ TT Ophiuchi /3 Canis i3 Draconis * Orionis 3 Librae y Geminorum y Virginis i Orionis . . u Argns . . Algol (Var ) 3 Arietis y Pegasi 6 Sa^ittarii (3 Leonis a Libras a Ophiuchi ... ... * Sagittarii 3 Cassiopeiae 3 Lupi y Cygni . ... e Virginis 1 . a Pegasi a Columbae 3 Pegasi i9- Aurigae . y Centauri 3 Herculis a Coronas i Centauri y Ursae 6 Capricorni e Scorpii 6 Corvi f Argus a Can. ven. B Ursa 3 Ophiuchi Phcenicis 6 Cygni Argus e Persei Bootis TJ Tauri Lupi 3 Eridani Centauri & Argus Canis 3 Hydri Aquarii f Persei Scorpii . . f Herculis Cygni E Corvi TI Ophiuchi 2893-30 290331 290.331 2913-32 2 92 3 33 294335 2943-35 2 95 3 36 296337 2-96337 2-973-38 *-973-38 298339 298,339 2 99 3 40 2-99340 3-003-41 3003-41 i Aurigae y Corvi y Urs. Min a Cephei T) Pegasi 6 Centauri 3 Arae a Serpentis a Toucani 6 Leonis 3 Caprieorni K Argus p Argus 3 Corvi Aquilae B Scorpii 3 Cygni f Centauri y Persei.. C Ophiuchi .. u Ursae.. . a Aquani Tf Scorpii 3 Leporis 6 Cassiopeias y Lupi d Centauri a Leporis ............. I Persei ^ Ursse 6 Ophiuchi .. e Aurigae (Var.) .. 102 Star. M^utudp. Star. MHgmtii.l..-. v Scorpii Vulg. 3-37 337 339 3-40 340 340 3-41 3-41 342 3-42 3-42 343 343 343 344 344 344 Phot 3-78 3-78 3-80 3-81 381 3-81 3-82 3-82 383 383 383 3-84 384 3-84 385 385 385 <5 Geminorum Z& 3-45 3-45 345 3-45 3-46 346 3-46 3-46 3-47 348 348 349 349 350 350 3-50 Phot. 3-85 3-86 386 3-86 3-86 3-87 3-87 387 3-87 3-88 389 389 3-90 3-90 391 391 391 i Orionis o Orionis ... y Lyncis (3 Cephei tfUrsae f Hydra rr Sagittarii ...... y Hydrae TT Herculis . f3 Triang. austr /3 Can. min. 1 t Ursae f Tauri tj Aurigae ... (5 Draconis y Lyrae ......... p Geminorum T) Geminorum y Bootis y Cephei e Geminorum K Ursas a Muscae e Cassiopeiae .... a Hydri? 1? Aquilae T Scorpii a Scorpii 6 Herculis r Argus " The following short table of the photometric quantities of seventeen stars of the first magnitude (as obtained from the photometric scale of magnitudes) may not be devoid of interest :" Sirius 4-165 7i Argus Canopus 2-041 aCentauri 1-000 Arcturus 0-718 Rigel 0-661 Capella 0-510 aLyrae 0-510 Procyon 0-510 " The following is the photometric quantity of stars strict- ly belonging to the 1st, 2d 6th magnitudes, in which the quantity of the light of a Centauri is regarded as the unit :" a Orionis .... a Eridani .... Aldebaran. . j3 Centauri . . . a Crucis Antares .... a Aquilae .... Spica . . 0-489 .0-444 0-444 0-401 .0-391 .0-391 .0-350 0-312 Magnitude on the vulgar scale. 1-00 2-00 3-00 Quantity of light 0-500 0-172 0-086 Magnitude on the vulgar scale. 4-00 5-00 6-00 Quantity of light. 0-051 0-034 0-024 III. NUMBER, DISTRIBUTION, AND COLOR OF THE FIXED STARS. STEL- LAR MASSES (STELLAR SWARMS). THE MILKY WAY INTERSPERSED WITH A FEW NEBULOUS SPOTS. We have already, in the first section of this fragmentary Astrognosy, drawn attention to a question first mooted by Olbers.* If the entire vault of heaven were covered with innumerable strata of stars, one behind the other, as with a wide-spread starry canopy, and light were undiminished in its passage through space, the sun would be distinguishable only by its spots, the moon would appear as a dark disk, and amid the general blaze not a single constellation would be visible. During my sojourn in the Peruvian plains, be- tween the shores of the Pacific and the chain of the Andes, I was vividly reminded of a state of the heavens which, though diametrically opposite in its cause to the one above referred to, constitutes an equally formidable obstacle to hu- man knowledge. A thick mist obscures the firmament in this region for a period of many months, during the season called el tiempo de la garua. Not a planet, not the most brilliant stars of the southern hemisphere, neither Canopus, the Southern Cross, nor the feet of the Centaur, are visible. It is frequently almost impossible to distinguish the position of the moon. If by chance the outline of the sun's disk be visible during the day, it appears devoid of rays, as if seen through colored glasses, being generally of a yellowish red, sometimes of a white, and occasionally even of a bluish green color. The mariner, driven onward by the cold south cur- rents of the sea, is unable to recognize the shores, and in the absence of all observations of latitude, sails past the harbors which he desired to enter. A dipping needle alone could, as I have elsewhere shown, save him from this error, by the local direction of the magnetic curves. f Bouguer and his coadjutor, Don Jorge Juan, complained, long before me, of the " unastronomical sky of Peru." A graver consideration associates itself with this stratum of vapors, in which there is neither thunder nor lightning, in consequence of its incapacity for the transmission of light or electric charges, and above which the Cordilleras, free and cloudless, raise their elevated plateaux and snow-covered * Vide supra, p. 38, and note. t Cotmos, vol. i., p. 178, and note. 104 COSMOS. summits. According to what modern geology has taught us to conjecture regarding the ancient history of our atmosphere, its primitive condition, in respect to its mixture and density, must have been unfavorable to the transmission of light. When we consider the numerous processes which, in the pri- mary world, may have led to the separation of the solids, fluids, and gases around the earth's surface, the thought in- voluntarily arises how narrowly the human race escaped be- ing surrounded with an untransparent atmosphere, which, though perhaps not greatly prejudicial to some classes of vegetation, would yet have completely veiled the whole of the starry canopy. All knowledge of the structure of the universe would thus have been withheld from the inquiring spirit of man. Excepting our own globe, and perhaps the sun and the moon, nothing would have appeared to us to have been created. An isolated triad of stars the sun, the moon, and the earth would have appeared the sole occu- pants of space. Deprived of a great, and, indeed, of the sub- limest portion of his ideas of the Cosmos, man would have been left without all those incitements which, for thousands of years, have incessantly impelled him to the solution of important problems, and have exercised so beneficial an in- fluence on the most brilliant progress made in the higher spheres of mathematical development of thought. Before we enter upon an enumeration of what has already been achieved, let us dwell for a moment on the danger from which the spiritual development of our race has escaped, and the physical impediments which would have formed an im- passable barrier to our progress. In considering the number of cosmical bodies which fill the celestial regions, three questions present themselves to our notice. How many fixed stars are visible to the naked eye ? How many of these have been gradually catalogued, and their places determined according to longitude and lat- itude, or according to their right ascension and declination ? "What is the number of stars from the first to the ninth and tenth magnitudes which have been seen in the heavens by means of the telescope ? These three questions may, from the materials of observation at present in our possession, be determined at least approximatively. Mere conjectures based on the gauging of the stars in certain portions of the Milky Way, differ from the preceding questions, and refer to the theoretical solution of the question : How many stars might be distinguished throughout the whole heavens with NUMBER OF THE FIXED STARS. 105 Herschel's twenty-feet telescope, including the stellar light, " which is supposed to require 2000 years to reach oui earth ?"* The numerical data which I here publish in reference to this subject are chiefly obtained from the final results of my esteemed friend Argelander, director of the Observatory at Bonn. I have requested the author of the Durchmusterung des nordlichen Himmeh (Suney of the Northern Heav- ens) to submit the previous results of star catalogues to a new and careful examination. In the lowest class of stars visible to the naked eye, much uncertainty arises from or- ganic difference in individual observations ; stars between the sixth and seventh magnitude being frequently confound- ed with those strictly belonging to the former class. We obtain, by numerous combinations, from 5000 to 5800 as the mean number of the stars throughout the whole heavens vis- ible to the unaided eye. Argelanderf determines the distri- * On the space-penetrating power of telescopes, see Sir John Her- schel, Outlines of Astr., 803. t I can not attempt to include in a note all the grounds on which Argelander's views are based. It will suffice if I extract the following remarks from his own letters to me: "Some years since (1843) you recommended Captain Schwink to estimate from his Mappa Coelestis the total number of stars from the first to the seventh magnitude in- clusive, which the heavens appeared to contain ; his calculations give 12,1 48 stars for the space between 30 south and 90 north declination ; and consequently, if we conjecture that the proportion of stars is the same from 30 S. D. to the South Pole, we should have 16,200 stars of the above-named magnitudes throughout the whole firmament. This estimate seems to me to approximate very nearly to the truth. It is well known that, on considering the whole mass, we find each class contains about three times as many stars as the one preceding. (Struve, Catalogvs Sldlarum duplicium, p. xxxiv. ; Argelander, Banner Zonen, s. xxvi.) I have given in my Uranometria 1441 stars of the sixth mag- nitude north of the equator, whence we should obtain about 3000 for the whole heavens; this estimate does not, however, include the stars of the 6-7 mag., which would be reckoned among those of the sixth, if only entire classes were admitted into the calculation. I think the number of the last-named stars might be assumed at 1000, according to the above rule, which would give 4000 stars for the sixth, and 12 000 for the seventh, or 18,000 for the first to the seventh inclusive. From other considerations on the number of the stars of the seventh magni- tude, as given in my zones namely, 2257 (p. xxvi.), and allowing for those which have been twice or oftener observed, and for those which have probably been overlooked, I approximated somewhat more nearly to the truth. By this method I found 2340 stars of the seventh magni- tude between 45 and 80 N. D., and, therefore, nearly 17,000 for the whole heavens. Struve, in his Description de V Observatoire de Paul- kova, p. 268, gives 13,400 for the number of stars down to the seventh magnitude in the region of the heavens explored by him (from 15 E 2 106 COSMOS. bution of the fixed stars according to difference of magnitude, down to the ninth, in about the following proportion . to +90), whence we should obtain 21,300 for the whole firmament. According to the introduction to Weisse's Catal. e Zonis Regiomonta- nit, dcd., p. xxxii., Struve found in the zone extending from 15 to -{-15 by the calculus of probabilities, 3903 stars from the first to the seventh, and therefore 15,050 for the entire heavens. This number is lower than mine, because Bessel estimated the brighter stars nearly half a magnitude lower than I did. We can here only arrive at a mean result, which would be about 18,000 from the first to the seventh mag- nitudes inclusive. Sir John Herschel, in the passage of the Outlines of Astronomy, p. 521, to which you allude, speaks only of ' the whole num- ber of stars already registered, down to the seventh magnitude inclu- sive, amounting to from 12,000 to 15,000.' As regards the fainter stars, Struve finds within the above-named zone (from 15 to +15), for the faint stars of the eighth magnitude, ] 0,557 ; for those of the ninth, 37,739 ; and, consequently, 40,800 stars of the eighth, and 145,800 of the ninth magnitude for the whole heavens. Hence, according to Struve, we have, from the first to the ninth magnitude inclusive, 15,100+ 40,800+145,800=201,700 stars. He obtained these numbers by a careful comparison of those zones or parts of zones which comprise the same regions of the heavens, deducing by the calculus of probabilities the number of stars actually present from the numbers of those com- mon to, or different in, each zone. As the calculation was made from a very large number of stars, it is deserving of great confidence. Bes- sel has enumerated about 61,000 different stars from the first to the ninth inclusive, in his collective zones between 15 and +45, after deducting such stars as have been repeatedly observed, together with those of the 9 - 10 magnitude; whence we may conclude, after taking into account such as have probably been overlooked, that this portion of the heavens contains about 101,500 stars of the above-named magni- tudes. My zones between -j-45 and -|-80 contain about 22,000 stars (Durchmu sterung des ndrdl. Himmels, s. xxv.), which would leave about 19,000 after deducting 3000 for those belonging to the 9-10 magnitude. My zones are somewhat richer than Bessel's, and I do not think we can fairly assume a larger number than 2850 for the stars actually existing between their limits (+45 and +80), whence we should obtain 130,000 stars to the ninth magnitude inclusive, between 15 and -J-80 . This space is, however, only 0-62181 of the whole heavens, and we therefore obtain 209,000 stars for the entire number, supposing an equal distribution to obtain throughout the whole firmament ; these numbers, again, closely approximate to Struve's estimate, and, indeed, not improbably exceed it to a considerable ttent, since Struve reck- oned stars of the 9-10 magnitude among thosrof the ninth. The num- bers which, according to my view, may be ashamed for the whole firm- ament, are therefore as follows : first mag., 20 ; second, 65 ; third, 190 ; fourth, 425; fifth, 1100; sixth, 3200; seventh, 13,000; eighth, 40,000; ninth, 142,000 ; and 200,000 for the entire number of stars from the first to the ninth magnitude inclusive. If you would contend that Lalande {Hist. Celeste, p. iv.) has given the number of stars observed by himself with the naked eye at 6000, I would simply remark that this estimate contains very many that have been repeatedly observed, and that after deducting these, we obtain only about 3800 stars for the portion of the heavens between 26 30* NUMBER OF THH FIXED STARS. 107 1st Mag. SdMag. 3d Mag. 4th Mag. 5th Mag. 20 65 190 425 1100 6th Mag. 7th Mag. 8th Mag. 9th Mag. 3200 13,000 40,000 142,000 The number of stars distinctly visible to the naked eye (amounting in the horizon of Berlin to 4022, and in that of Alexandria to 4638) appears at first sight strikingly small.* If we assume the moon's mean semi-diameter at 15' 33"' 5, it would require 195,291 surfaces of the full moon to cover the whole heavens. If we further assume that the stars are uniformly distributed, and reckon in round numbers 200,000 stars from the first to the ninth magnitude, we shall have nearly a single star for each full-moon surface. This result explains why, also, at any given latitude, the moon does not more frequently conceal stars visible to the naked eye. If the calculation of occultations of the stars were extended to those of the ninth magnitude, a stellar eclipse would, according to Galle, occur on an average every 44' 30", for in this period the moon traverses a portion of the heavens equal in extent to its own surface. It is singular that Pliny, who was un- doubtedly acquainted with Hipparchus's catalogue of stars, and +90 observed by Lalande. As this space is 723 1 of the whole heavens, we should again have for this zone 5255 stars visible to the naked eye. An examination of Bode's Uranography (containing 17,240 stars), which is composed of the most heterogeneous elements, does not give more than 5600 stars from the first to the sixth magnitude inclusive, after deducting the nebulous spots and smaller stars, as well as those of the G-7th magnitude, which have been raised to the sixth. A simi- lar estimate of the stars registered by La Caille between the south pole and the tropic of Capricorn, and varying from the first to the sixth mag- nitude, presents for the whole heavens two limits of 3960 and 5900, and thus confirms the mean result already given by yourself. You will perceive that I have endeavored to fulfill your wish for a more thor- ough investigation of these numbers, and I may further observe that M. Heis, of Aix-la-Chapelle, has for many years been engaged in a very careful revision of my Uranometrie. From the portions of this work already complete, and from the great additions made to it by an observ er gifted with keener sight than myself, I find 2836 stars from the first to the sixth magnitude inclusive for the northern hemisphere, and there- fore, on the presupposition of equal distribution, 5672 as the number of stars visible throughout the whole firmament to the keenest unaided vision." {From the Manuscripts of Professor Argelander, March, 1850.) * Schubert reckons the number of stare, from the first to the sixth magnitude, at 7000 for the whole heavens (which closely approximates to the calculation made by myself in Cosmos, vol. i., p. 150), and up- ward of 5000 for the horizon of Paris. He gives 70,000 for the whole sphere, including stars of the ninth magnitude. (Astronomic, th. in., s. 54.) These numbers are all much too high. Argelander finds only 58,000 from the first to the eighth magnitude. 108 COSMOS. and who comments on his boldness in attempting, as it were, " to leave heaven as a heritage to posterity," should have enumerated only 1600 stars visible in the fine sky of Italy !* In this enumeration he had, however, descended to stars of the fifth, while half a century later Ptolemy indicated only 1025 stars down to the sixth magnitude. Since it has ceased to be the custom to class the fixed stars merely according to the constellations to which they belong, and they have been catalogued according to determinations of place, that is, in their relations to the great circles of the equator or the ecliptic, the extension as well as the accuracy of star catalogues has advanced with the progress of science and the improved construction of instruments. No catalogues of the stars compiled by Timocharis and Aristyllus (283 B.C.) have reached us ; but although, as Hipparchus remarks in the fragment " on the length of the year," cited in the sev- enth book of the Almagest (cap. 3, p. xv., Halma), their ob- servations were conducted in a very rough manner (navv oAoo^epoif), there can be no doubt that they both determ- ined the declination of many stars, and that these determin- ations preceded by nearly a century and a half the table of fixed stars compiled by Hipparchus. This astronomer is said to have been incited by the phenomenon of a new star to attempt a survey of the whole firmament, and endeavor to determine the position of the stars ; but the truth of this statement rests solely on Pliny's testimony, and has often been regarded as the mere echo of a subsequently invented tradition.! It does indeed seem remarkable that Ptolemy should not refer to the circumstance, but yet it must be ad- mitted that the sudden appearance of a brightly luminous * " Patrocinatur vastitas cceli, immensa discreta altitudine, in duo at- que septuaginta signa. Haec sunt rerum et animantium effigies, in quas digessere ccelum periti. In his quidem mille sexcentas adnotavere stel- las, iusignes videlicet effectu visuve" .... Plin., ii., 41. "Hipparchus nunquam satis laudatus, ut quo nemo magis approbaverit cognationera cum homine siderum animasque nostras partem esse coeli, novam stel lam et aliam in aevo suo genitam deprehendit, ejusque motu, qua die fulsit, ad dubitationem est adductus, anne hoc saepius fieret moveren- turque et ese quas putamus affixas ; itemque ausus rem etiam Deo im- probam, adnumerare posteris Stellas ac sidera ad nomen expungere, or- ganis excogitatis, per quse singularum loca atque magnitudines signaret, ut facile discerni posset ex eo, non modo an obirent nascerenturve, sed an omnino aliqua transirent moverenturve, item an crescerent minue- renturque, coelo in hereditate cunctis relicto, si quisquam qvi cretioneru earn caperet inventus esset." Plin., ii., 26. t Delambre, Hist, de VAstr. Anc., torn, i., p. 290, and Hist, de VAstr. Mod., torn, ii., p. 186. NUMBER OF THE FIXED STARS. 109 star in Cassiopeia (November, 1572) led Tycho Brahe to compose his catalogue of the stars. According to an ingen- ious conjecture of Sir John Herschel,* the star referred to by Pliny may have been the new star which appeared in Scorpio in the month of July of the year 134 before our era (as we learn from the Chinese Annals of the reign of "Wou-ti, of the Han dynasty). Its appearance occurred exactly six years before the epoch at which, according to Ideler's investiga- tions, Hipparchus compiled his catalogue of the stars. Ed- ward Biot, whose early death proved so great a loss to science, found a record of this celestial phenomenon in the celebra- ted collection of Ma-tuan-lin, which contains an account of all the comets and remarkable stars observed between the years B.C. 613 and A.D. 1222. The tripartite didactic poem of Aratus,t to whom we are indebted for the only remnant of the works of Hipparchus that has come down to us, was composed about the period of Eratosthenes, Timocharis, and Aristyllus. The astronomical non-meteorological portion of the poem is based on the ura- nography of Eudoxus of Cnidos. The catalogue compiled by Hipparchus is unfortunately not extant ; but, according to Ideler,f it probably constituted the principal part of his work, cited by Suidas, " On the arrangement of the region of the fixed stars and the celestial bodies," and contained 1080 de- terminations of position for the year B.C. 128. In Hippar- chus' s other Commentary on Aratus, the positions of the stars, which are determined more by equatorial armillse than by the astrolabe, are referred to the equator by right ascension and declination ; while in Ptolemy's catalogue of stars, which is supposed to have been entirely copied from that of Hip- parchus, and which gives 1025 stars, together with five so- called nebulae, they are referred by longitudes and latitudes * Outlines, $ 831 ; Edward Biot, Sur les Etoilet Extraordinaire* ob- servles en Chine, in the Connaissance des temps pour 1846. t It is worthy of remark that Aratus was mentioned with approba- tion almost simultaneously by Ovid (Amor., i., 15) and by the Apostle Paul at Athens, in an earnest discourse directed against the Epicureans and Stoics. Paul (Acts, ch. xvii., v. 28), although he does not mention Aratus by name, undoubtedly refers to a verse composed by him (Phccn., v. 5) on the close communion of mortals with the Deity. \ Ideler, Untersuchungen fiber den Unsprung der Stemnamen, s. xxx. xxxv. Daily, in the Mem. of the Astron. Soc., vol. xiii., 1843, p. 12 and 15, also treats of the years according to our era, to which we must refer the observations of Aristyllus, as well as the catalogues of the stars com- piled by Hipparchus (128. and not 140, B.C.) and by Ptolemy (138 AD.). 110 COSMOS. to the ecliptic.* On comparing the number of fixed stars m the Hipparcho-Ptolemaic Catalogue, Almagest, ed. Halma, t. ii., p. 83 (namely, for the first mag., 15 stars ; second, 45 ; third, 208 ; fourth, 474 ; fifth, 217 ; sixth, 49), with the numbers of Argelander as already given, we find, as might be expected, a great paucity of stars of the fifth and sixth magnitudes, and also an extraordinarily large number of those belonging to the third and fourth. The vagueness in the determinations of the intensity of light in ancient and modern times renders direct comparisons of magnitude ex- tremely uncertain. Although the so-called Ptolemaic catalogue of the fixed stars enumerated only one fourth of those visible to the naked eye at Rhodes and Alexandria, and, owing to erroneous re- ductions of the precession of the equinoxes, determined their positions as if they had been observed in the year 63 of our era, yet, throughout the sixteen hundred years immediately following this period, we have only three original catalogues of stars, perfect for their time ; that of Ulugh Beg (1437), * Compare Delambre, Hist, de I'Astr. Anc., torn, i., p. 184; torn, ii., p. 260. The assertion that Hipparchus, in addition to the right ascen- sion and declination of the stars, also indicated their positions in his catalogue, according to longitude and latitude, as was done by Ptolemy, is wholly devoid of probability and in direct variance with the Alma- gest, book vii., cap. 4, where this reference to the ecliptic is noticed as something new, by which the knowledge of the motions of the fixed stars round the pole of the ecliptic may be facilitated. The table of stars with the longitudes attached, which Petrus Victorius found in a Medicean Codex, and published with the life of Aratus at Florence in 1567, is indeed ascribed by him to Hipparchus, but without any proof. It appears to be a mere rescript of Ptolemy's catalogue from an old manuscript of the Almagest, and does not give the latitudes. As Ptole- my was imperfectly acquainted with the amount of the retrogression of the equinoctial and solstitial points (Almag., vii., c. 2, p. 13, Halma), and assumed it about T ^ too slow, the catalogue which he determined for the beginning of tne reign of Antoninus (Ideler, op. cit., B. xxxiv.) indicates the positions of the stars at a much earlier epoch (for the year 63 A.D.). (Regarding the improvements for reducing stars to the time of Hipparchus, see the observations and tables as given by Encke in Schumacher's Astron. Nachr., No. 608, s. 113-126.) The earlier epoch to which Ptolemy unconsciously reduced the stars in his catalogue cor- responds tolerably well with the period to which we may refer the Pseudo-Eratosthenian Catasterisms, which, as I have already elsewhere observed, are more recent than the time of Hyginus, who lived in the Augustine age, but appear to be taken from him, and have no connec- tion with the poem of Hermes by the true Eratosthenes. (Eratostheni- ea, ed. Bernhardy, 1822, p. 114, 116, 129.) These Pseudo-Eratosthe- nian Catasterisms contain, moreover, scarcely 700 individual stars dis- tributed among the mythical constellations. EARLY CATALOGUES. -Ill that of Tycho Brahe (1600), and that of Hevelius (1660). During the short intervals of repose which, amid tumultuous revolutions and devastations of war, occurred between the ninth and fifteenth centuries, practical astronomy, under Arabs, Persians, and Moguls (from Al-Mamun, the son of the great Haroun Al-Raschid, to the Timurite, Mohammed Tar- aghi Ulugh Beg, the son of Shah Rokh), attained an emi- nence till then unknown. The astronomical tables of Ebn- Junis (1007), called the Hakemitic tables, in honor of the Fatimite calif, Aziz Ben-Hakem Biamrilla, afford evidence, as do also the Ilkhanic tables* of Nassir-Eddin Tusi (who founded the great observatory at Meragha, near Tauris, 1259), of the advanced knowledge of the planetary motions the improved condition of measuring instruments, and the mul- tiplication of more accurate methods differing from those em- ployed by Ptolemy. In addition to clepsydras,f pendulum- oscillationsj were already at this period employed in the measurement of time. The Arabs had the great merit of showing how tables might be gradually amended by a comparison with observa- tions. Ulugh Beg's catalogue of the stars, originally written in Persian, was entirely completed from original observations made in the Gymnasium at Samarcand, with the exception of a portion of the southern stars enumerated by Ptolemy, 4 * Cosmos, vol. ii., p. 222, 223. The Paris Library contains a manu- script of the Ilkhanic Tables by the hand of the son of Nassir-Eddin. They derive their name from the title " Ilkhan," assumed by the Tar- tar princes who held rule in Persia. Reinaud, Introd. de la Gfogr. d'Aboulfeda, 1848, p. cxxxix. t [For an account of clepsydras, see Beckmann's Inventions, voL i., 341, et seq. (Bohn's edition).] Ed. t Sedillot fils, Prolegomenes des Tables Astr. d' Oloug-Beg, 1847, p. cxxxiv., note 2. Delambre, Hist, de I' Astr. du May en Age, p. 8. $ In my investigations on the relative value of astronomical determ- inations of position in Central Asia (Asie Centrale, t. iii., p. 581-596), I have given the latitudes of Samarcand and Bokhara according to the different Arabic and Persian MSS. contained in the Paris Library. I have shown that the former is probably more than 39 52', while most of the best manuscripts of Ulugh Beg give 39 37', and the Kitab aL- athual of Alfares, and the Kanum of Albyruni, give 40. I would again draw attention to the importance, in a geographical no less than an as- tronomical point of view, of determining the longitude and latitude of Samarcand by new and trustworthy observations. Burnes's Travels have made us acquainted with the latitude of Bokhara, as obtained from observations of culmination of stars, which gave 39 43' 41". There is, therefore, only an error of from 7 to 8 minutes in the two fine Persian and Arabic MSS. (Nos. 164 and 2460) of the Paris Library. Major Ren- nell, whoso combinations are generally so successful, made an error of 112 COSMOS. and not visible in 39 52' lat. (?) It contains only 1019 positions of stars, which are reduced to the year 1437. A subsequent commentary gives 300 other stars, observed by Abu-Bekri Altizini in 1533. Thus we pass from Arabs, Per- sians, and Moguls, to the great epoch of Copernicus, and nearly to that of Tycho Brahe. The extension of navigation in the tropical seas, and in high southern latitudes, has, since the beginning of the six- teenth century, exerted a powerful influence on the gradual extension of our knowledge of the firmament, though in a less degree than that effected a century later by the appli- cation of the telescope. Both were the means of revealing new and unknown regions of space. I have already, in other works, considered* the reports circulated first by Americus Vespucius, then by Magellan, and Pigafetta (the companion of Magellan and Elcano), concerning the splendor of the southern sky, and the descriptions given by Vicente Yanez Pinzon and Acosta of the black patches (coal-sacks), and by Anghiera and Andrea Corsali of the Magellanic clouds. A merely sensuous contemplation of the aspect of the heavens here also preceded measuring astronomy. The richness of the firmament near the southern pole, which, as is well known, is, on the contrary, peculiarly deficient in stars, was so much exaggerated that the intelligent Polyhistor Cardanus indicated in this region 10,000 bright stars which were said to have been seen by Vespucius with the naked eye.f Friedrich Houtman and Petrus Theodori of Embden (who, according to Olbers, is the same person as Dircksz Keyser) now first appeared as zealous observers. They measured distances of stars at Java and Sumatra ; and at this period the most southern stars were first marked upon the celestial maps of Bartsch, Hondius, and Bayer, and by Kepler's in- dustry were inserted in Tycho Brahe' s Rudolphine tables. Scarcely half a century had elapsed from the time of Ma- gellan's circumnavigation of the globe before Tycho com- menced his admirable observations on the positions of the fixed stars, which far exceeded in exactness all that had hitherto been done in practical astronomy, not excepting even about 19' in determining the latitude of Bokhara. (Humboldt, A fie Centrale, t. iii., p. 592, and Sedillot, in the Proligorllenes d' Olov.g-Beg, p. cxxiii.-cxxv.) * Cosmos, vol. ii., p. 285-29C ; Humboldt, Examen Crit. de VHisloirt de la Gtogr., t. iv., p. 321-336 : t. v., p. 226-238. t Cardani Paralipomenon, lib. viii., cap. 10. (Opp., t. ix., ed. Lugd . 1663, p. 508.) PROGRESS OP ASTRONOMY. 113 the laborious observations of the Landgrave "William IV. at Cassel. Tycho Brahe's catalogue, as revised and published by Kepler, contains no more than 1000 stars, of which one fourth at most belong to the sixth magnitude. This cata- logue, and that of Hevelius, which was less frequently em- ployed, and contained 1564 determinations of position for the year 1660, were the last which were made by the unaided eye, owing their compilation in this manner to the capricious disinclination of the Dantzig astronomer to apply the telescope to purposes of measurement. This combination of the telescope with measuring instru- ments the union of telescopic vision and measurements at length enabled astronomers to determine the position of stars below the sixth magnitude, and more especially between the seventh and the twelfth. The region of the fixed stars might now, for the first time, be said to be brought within the reach of observers. Enumerations of the fainter tele- scopic stars, and determinations of their position, have not only yielded the advantage of making a larger portion of the regions of space known to us by the extension of the sphere of i nervation, but they have also (what is still more import- ant) indirectly exercised an essential influence on our knowl- edge of the structure and configuration of the universe, on tiiv.- discovery of new planets, and on the more rapid determ- ination of their orbits. When William Herschel conceived the happy idea of, as it were, casting a sounding line in the depths of space, and of counting during his gaugings the stars which passed through the field of his great telescope,* at different distances from the Milky Way, the law was discov- ered that the number of stars increased in proportion to their vicinity to the Milky Way a law which gave rise to the idea of the existence of large concentric rings filled with millions of stars which constitute the many-cleft Galaxy. The knowledge of the number and the relative position of the faintest stars facilitates (as was proved by Galle's rapid and felicitous discovery of Neptune, and by that of several of the smaller planets) the recognition of planetary cosmic al bodies which change their positions, moving, as it were, be- tween fixed boundaries. Another circumstance proves even more distinctly the importance of very complete catalogues of the stars. If a new planet be once discovered in the vault of heaven, its notification in an older catalogue of po- Cosmos, vol. i., p. 87-89. 114 COSMOS. eitions will materially facilitate the difficult calculation of its orbit. The indication of a new star which has subse- quently been lost sight of, frequently affords us more assist- ance than, considering the slowness of its motion, we can hope to gain by the most careful measurements of its course through many successive years. Thus the star numbered 964 in the catalogue of Tobias Mayer has proved of great im- portance for the determination of Uranus, and the star num- bered 26,266 in Lalande's catalogue* for that of Neptune Uranus, before it was recognized as a planet, had, as is now well known, been observed twenty-one times ; once, as al- ready stated, by Tobias Mayer, seven times by Flamstead, once by Bradley, and twelve times by Le Monnier. It may be said that our increasing hope of future discoveries of plan- etary bodies rests partly on the perfection of our telescopes (Hebe, at the time of its discovery in July, 1847, was a star of the 8-9 magnitude, while in May, 1849, it was only of the eleventh magnitude), and partly, and perhaps more, on the completeness of our star catalogues, and on the exactness of our observers. The first catalogue of the stars which appeared after the epoch when Morin and Gascoigne taught us to combine tele- scopes with measuring instruments, was that of the southern stars compiled by Halley. It was the result of a short resi- dence at St. Helena in the years 1677 and 1678, but, singu- larly enough, does not contain any determinations below the sixth magnitude.! Flamstead had, indeed, begun his great Star Atlas at an earlier period ; but the work of this cele- brated observer did not appear till 1712. It was succeeded by Bradley's observations (from 1750 to 1762), which led to the discovery of aberration and nutation, and have been ren- dered celebrated by the Fundamenta Astronomice of our countryman Bessel (1818),^ and by the stellar catalogues of * Bally, Cat. of those stars in the "Histoire Celeste" of Jerome de Lalande,for which tables of reduction to the epoch 1800 have been pub- lished by Prof. Schumacher, 1847, p. 1195. On what we owe to the perfection of star catalogues, see the remarks of Sir John Herschel in Cat. of the British Assoc., 1845, p. 4, 10. Compare also on stars that have disappeared, Schumacher, Astr. Nachr., No. 624, and Bode, Jahrb. fur 1817, s 249. t Memoirs of the Royal Astron. Soc., vol. xiii., 1843, p. 33 and 168. t Bessel, Fundamenta Astronomies pro anno 1755, deducta ex observa- tionibus viri incomparabilis James Bradley in Specula astronomica Ore- novicensi, 1818. Compare also Bessel, Tabula Regiomontancn reduclio- num observationum astronomicarum ab anno 1750 usque ad annum 1850 computatoE (1830). STAR CATALOGUES. 115 La Caille, Tobias Mayer, Cagnoli, Piazzi, Zach, Pond, Taylor, Groombridge, Argelander, Airy, Brisbane, and Riimker. We here only allude to those works which enumerate a great and important part* of the stars of the seventh to the tenth magnitude which occupy the realms of space. The catalogue known under the name of Jerome de Lalandt's, but which is, however, solely based on observations made by his nephew, Francois de Lalande, and by Burckhardt between the years 1789 and 1800, has only recently been duly appre- ciated. After having been carefully revised by Francis Baily, under the direction of the " British Association for the Ad- vancement of Science" (in 1847), it now contains 47,390 stars, many of which are of the ninth, and some even below that magnitude. Harding, the discoverer of Juno, catalogued above 50,000 stars in twenty-seven maps. Bessel's great work on the exploration of the celestial zones, which comprises 75,000 observations (made in the years 1825-1833 between 15 and +45 declination), has been continued from 1841 to 1844 with the most praiseworthy care, as far as +80 decl., by Argelander at Bonn. Weisse of Cracow, under the auspices of the Academy of St. Petersburgh, has reduced 31,895 stars for the year 1825 (of which 19,738 belonged to the ninth magnitude) from Bessol's zones, between 15 and + 15 decl. ;t and Argelander' s exploration of the northern heavens from +45 to +80 decl. contains about 22,000 well-determined positions of stars. * I here compress into a note the numerical data taken from star cat- alogues, containing lesser masses and a smaller number of positions, with the names of the observers, and the number of positions attached : La Caille, in scarcely ten months, during the years 1751 and 1752, with instruments magnifying only eight times, observed 9766 southern stars, to the seventh magnitude inclusive, which were reduced to the year 1750 by Henderson ; Tobias Mayer, 998 stars to 1756 ; Flamstead, orig- inally only 2866, to which 564 were added by Daily's care (Mem. of the Astr. Soc., vol. iv., p. 1291-64); Bradley, 3222, reduced by Bessel to the year 1755; Pond, 1112; Piazzi,,7646 to 1800; Groombridge, 4243, mostly circumpolar stars, to 1810 ; Sir Thomas Brisbane, and Rflmker, 7385 stars, observed in New Holland in the years 1822-1828 ; Airy, 2156 stars, reduced to the year 1845 ; Riimker, 12,000 on the Hamburg hori- zon; Argelander (Cat. of Abo), 560; Taylor (Madras), 11,015. The British Association Catalogue of Stars (1845), drawn up under Baily's superintendence, contains 8377 stars from the first to 7i magnitudes. For the southern stars we have the rich catalogues of Henderson, Fal- lows, Maclear, and Johnson at St. Helena. t Weisse, Positiones media stellarum fixarum in Zonis Regiomontanit a Bessclio inter 15 ct -J-15 decl. observatanim ad annum 1825 re dvcla (1846); with an important Preface by Struve. 116 COSMOS. I can not, I think, make mere honorable mention of the great work of the star maps of the Berlin Academy than by quoting the words used by Encke in reference to this un- dertaking, in his oration to the memory of Bessel : '' With the completeness of catalogues is connected the hope that, by a careful comparison of the different aspects of the heav- ens with those stars which have been noted as fixed points, we may be enabled to discover all moving celestial bodies, whose change of position can scarcely, owing to the faint- ness of their light, be noted by the unaided eye, and that we may in this manner complete our knowledge of the so- lar system. While Harding's admirable atlas gives a per- fect representation of the starry heavens as far as Lalande's Histoire Celeste, on which it is founded, was capable of af- fording such a picture Bessel, in 1824, after the comple- tion of the first main section of his zones, sketched a plan for grounding on this basis a more special representation of the starry firmament, his object being not simply to exhibit what had been already observed, but likewise to enable as- tronomers, by the completeness of his tables, at once to rec- ognize every new celestial phenomenon. Although the star maps of the Berlin Academy of Sciences, sketched in ac- cordance with Bessel's plan, may not have wholly completed the first proposed cycle, they have nevertheless contributed in a remarkable degree to the discovery of new planets, since they have been the principal, if not the sole means, to which, at the present time (1850), we owe the recognition of seven new planetary bodies."* Of the twenty-four maps designed to represent that portion of the heavens which extends 15 on either side of the equator, our Academy has already con- tributed sixteen. These contain, as far as possible, all stars down to the ninth magnitude, and many of the tenth. The present would seem a fitting place to refer to the average estimates which have been hazarded on the num- ber of stars throughout the whole heavens, visible to us by the aid of our colossal space-penetrating telescopes. Struve assumes for Herschel's twenty-feet reflector, which was em- ployed in making the celebrated star-gauges or sweeps, that a magnifying power of 180 would give 5.800,000 for the number of stars lying within the zones extending 30 on ei- ther side of the equator, and 20,374,000 for the whole heav- ens. Sir Wilh'am Herschel conjectured that eighteen mill- * Encke, Geddchtnissrede auf Bessel, s. 13. DISTRIBUTION OF THE FIXED STARS. 117 ions of stars in the Milky Way might be seen by his still more powerful forty-feet reflecting telescope.* After a careful consideration of all the fixed stars, wheth- er visible to the naked eye or merely telescopic, whose po- sitions are determined, and which are recorded in catalogues, we turn to their distribution and grouping in the vault of neaven. As we have already observed, these stellar bodies, from the inconsiderable and exceedingly slow (real and apparent) change of position exhibited by some of them partly owing to precession and to the different influences of the progression of our solar system, and partly to their own proper motion may be regarded as landmarks in the boundless regions of space, enabling the attentive observer to distinguish all bod- ies that move among them with a greater velocity or in an opposite direction consequently, all which are allied to tel- escopic comets and planets. The first and predominating interest excited by the contemplation of the heavens is di- rected to the fixed stars, owing to the multiplicity and over- whelming mass of these cosmical bodies ; and it is by them that our highest feelings of admiration are called forth. The orbits of the planetary bodies appeal rather to inquiring reason, and, by presenting to it complicated problems, tend to promote the development of thought in relation to astron- omy. Amid the innumerable multitude of great and small stare, which seem scattered, as it were by chance, throughout the vault of heaven, even the rudest nations separate single (and almost invariably the same) groups, among which cer- tain bright stars catch the observer's eye, either by their proximity to each other, their juxtaposition, or, in some cases, by a kind of isolation. This fact has been confirmed by re- cent and careful examinations of several of the languages of so-called savage tribes. Such groups excite a vague sense of the mutual relation of parts, and have thus led to their receiving names, which, although varying among different races, were generally derived from organic terrestrial ob- jects. Amid the forms with which fancy animated the waste and silent vault of heaven, the earliest groups thus distinguished were the seven-starred Pleiades, the seven stars of the Great Bear, subsequently (on account of the repetition of the same form) the constellation of the Lesser Bear, the * Compare Struve, Etudes d'Astr. Sttllaire, 1847, p. 66 and 72 ; Cot- mo$, vol. i., p. 100; ami Madlec Astr., 4te Aufl., $ 417. 118 COSMOS belt of Orion (Jacob's stafT), Cassiopeia, the Swan, the Soor pion, the Southern Cross (owing to the striking difference in its direction before and after its culmination), the South- ern Crown, the Feet of the Centaur (the Twins, as it were, of the Southern hemisphere), &c. "Wherever steppes, grassy plains, or sandy wastes present a far-extended horizon, those constellations whose rising or setting corresponds with the busy seasons and requirements of pastoral and agricultural life have become the subject of attentive consideration, and have gradually led to a symbol- izing connection of ideas. Men thus became familiarized with the aspect of the heavens before the development of measuring astronomy. They soon perceived that besides the daily movement from east to west, which is common to all celestial bodies, the sun has a far slower proper motion in an opposite direction. The stars which shine in the even- ing sky sink lower every day, until at length they are wholly lost amid the rays of the setting sun ; while, on the other hand, those stars which were shining in the morning sky before the rising of the sun, recede further and further from it. In the ever-changing aspect of the starry heavens, suc- cessive constellations are always coming to view. A slight degree of attention suffices to show that these are the same which had before vanished in the west, and that the stars which are opposite to the sun, setting at its rise, and rising at its setting, had about half a year earlier been seen in its vicinity. From the time of Hesiod to Eudoxus, and from the latter to Aratus and Hipparchus, Hellenic literature abounds in metaphoric allusions to the disappearance of the stars amid the sun's rays, and their appearance in the morn- ing twilight their heliacal setting and rising. An atten- tive observation of these phenomena yielded the earliest ele- ments of chronology, which were simply expressed in num- bers, while mythology, in accordance with the more cheerful or gloomy tone of national character, continued simultane- ously to rule the heavens with arbitrary despotism. The primitive Greek sphere (I here again, as in the his- tory of the physical contemplation of the universe,* follow the investigations of my intellectual friend Letronne) had be- come gradually filled with constellations, without being in any degree considered with relation to the ecliptic. Thus Homer and Hesiod designate by name individual stars and * Cosmos, vol. ii., j>. 167 ZODIACAL SIGNS. 119 groups ; the former mentions the constellation of the Bear (" otherwise known, as the Celestial Wain, and which alone never sinks into the bath of Oceanos"), Bootes, and the Dog of Orion ; the latter speaks of Sirius and Arcturus, and both refer to the Pleiades, the Hyades, and Orion.* Homer's twice repeated assertion that the constellation of the Bear alone never sinks into the ocean, merely allows us to infer that in his age the Greek sphere did not yet comprise the constella- tions of Draco, Cepheus, and Ursa Minor, which likewise do not set. The statement does not prove a want of acquaint- ance with the existence of the separate stars forming these three catasterisms, but simply an ignorance of their arrange ment into constellations. A long and frequently misunder- stood passage of Strabo (lib. i., p. 3, Casaub.) on Homer, II., xviii., 485-489, specially proves a fact important to the question that in the Greek sphere the stars were only grad- ually arranged in constellations. Homer has been unjustly accused of ignorance, says Strabo, as if he had known of only one instead of two Bears. It is probable that the lesser one had not yet been arranged in a separate group, and that the name did not reach the Hellenes until after the Phoanicians had specially designated this constellation, and made use of it for the purposes of navigation. All the scholia on Homer, Hyginus, and Diogenes Laertius ascribe its introduction to Thales. In the Pseudo-Eratosthenian work to which we have already referred, the lesser Bear is called <&otviKTj (or, as it were, the Phoenician guiding star). A century later (01. 71), Cleostratus of Tenedos enriched the sphere with the constellations of Sagittarius, TO^OTT/C, and Aries, K.pi6$. The introduction of the Zodiac into the ancient Greek sphere coincides, according to Letronne, with this period of the domination of the Pisistratidae. Eudemus of Rhodes, one of the most distinguished pupils of Aristotle, and author of a "History of Astronomy," ascribes the introduction of this zo- diacal belt (TI TOV fadiaicov dia^dxrig, also wt Jio^ /cvwAof) to (Enopides of Chios, a cotemporary of Anaxagoras.f The * Ideler, Unters. uber die Sternnamen, s. xi., 47, 139, 144, 2 13 Le- tronne, Sur V Origins du Zodiaque Grec, 1340, p. 25. t Letronne, op. cit., p. 25 ; and Carteron, Analyse de Rechercn.es de tl. Letronne sur les Representations Zodiacales, 1843, p. 119. "It ia Yery doubtful whether Eudoxus (Ol. 103) ever made use of the word Cudianof. We first meet with it in Euclid, and in the Commentary of Hipparchus on Aratus (Ol. 160). The name ecliptic, eKfautTiicdf, is also very recent." Compare Martin in the Commentary to Thconi* Smyrn<ri Plalonici Liber de Aslronotnia, 1849, p. 50, GO. 120 COSMOS. idea of the relation of the planets and fixed stars to the sun'* course, the division of the ecliptic into twelve equal parts (Dodecatomeria), originated with the ancient Chaldeans, and very probably came to the Greeks, at the beginning of the fifth, or even in the sixth century before our era, direct from Chaldea, and not from the Valley of the Nile.* The Greeks merely separated from the constellations named in their prim- itive sphere those which were nearest to the ecliptic, and could be used as signs of the zodiac. If the Greeks had bor- rowed from another nation any thing more than the idea and number of the divisions (Dodecatomeria) of a zodiac if they had borrowed the zodiac itself, with its signs they would not at first have contented themselves with only eleven con- stellations. The Scorpion would not have been divided into two groups ; nor would zodiacal constellations have been in- troduced (some of which, like Taurus, Leo, Pisces, and Virgo, extend over a space of 35 to 48, while others, as Cancer, Aries, and Capricornus, occupy only from 19 to 23), which are inconveniently grouped to the north and south of the ecliptic, either at great distances from each other, or, like Tau- rus and Aries, Aquarius and Capricornus, so closely crowded together as almost to encroach on each other. These cir- cumstances prove that catasterisms previously formed were converted into signs of the zodiac. The sign of Libra, according to Letronne's conjecture, was introduced at the time of, and perhaps by, Hipparchus. It is never mentioned by Eudoxus, Archimedes, Autolycus, or even by Hipparchus in the few fragments of his writings which have been transmitted to us (excepting indeed in one * Letronne, Orig. du Zod., p. 25 ; and Analyse Crit. des Reprtt. Zod., 1846, p. 15. Ideler and Lepsius also consider it probable " that the knowledge of the Chaldean zodiac, as well in reference to its divi- sions as to the names of the latter, had reached the Greeks in the sev- enth century before our era, although the adoption of the separate signs of the zodiac in Greek astronomical literature was gradual and of a sub- sequent d*te." (Lepsius, Chronologic der ^Egypter, 1849, s. 65 and 124.) Ideler is inclined to believe that the Orientals had names, but not constellations for the Dodecatomeria, and Lepsius regards it as a natural assumption " that the Greeks, at the period when their sphere was for the most part unfilled, should have added to their own the Chaldean constellations, from which the twelve divisions were named." But are we not led on this supposition to inquire why the Greeks had at first only eleven signs instead of introducing all the twelve belong- ing to the Chaldean Dodecatomeria ? If they introduced the twelve signs, they are hardly likely to hava removed one in order to replace it at a subseq :en* period. ZODIACAL SIGNS. 121 passage, probably falsified by a copyist).* The earliest no- tice of this new constellation occurs in Geminus and Varro scarcely half a century before our era ; and as the Romans, from the time of Augustus to Antoninus, became more strong- ly imbued with a predilection for astrological inquiry, those constellations which " lay in the celestial path of the sun" acquired an exaggerated and fanciful importance The Egyp- tian zodiacal constellations found at Dendera, Esneh, the Propylon of Panopolis, and on some mummy-cases, belong to the first half of this period of the Roman dominion, as was maintained by Visconti and Testa, at a time when the nec- essary materials for the decision of the question had not been collected, and the wildest hypothesis still prevailed regard- ing the signification of these symbolical zodiacal signs, and their dependence on the precession of the equinoxes. The great antiquity which, from passages in Manu's Book of Laws, Valmiki's Ramayana and Amarasinha's Dictionary, Augustus "William von Schlegel attributed to the zodiacal circles found in India, has been rendered very doubtful by Adolph Holtzmann's ingenious investigations.! * On the passage referred to in the text, and interpolated by a copy ist of Hipparchus, see Letronne, Orie. du Zod., 1840, p. 20. As early as 1812, when I was much disposed to believe that the Greeks had been long acquainted with the sign of Libra, I directed attention in an elaborate memoir (on all the passages in Greek and Roman writers of antiquity, in which the Balance occurs as a sign of the zodiac) to that passage in Hipparchus (Comment, in Aratum, lib. iii., cap. 2) which re- fers to the tiripiov held by the Centaur (in his fore-foot), as well as to the remarkable passage of Ptolemy, lib. ix., cap. 7 (Halma, t. ii., p. 170). In the latter the Southern Balance is named with the affix KOTO f , and is opposed to the pincers of the Scorpion in an observ- ation, which was undoubtedly not made at Babylon, but by some of the astrological Chaldeans, dispersed throughout Syria and Alexandria. ( Vues des Cordilleret et Monument des Peuples Indigenes de I'Amerique, t. ii., p. 380.) Buttman maintained, what is very improbable, that the 7/A<u originally signi6ed the two scales of the Balance, and were sub- sequently by some misconception converted into the pincers of a scor- pion. (Compare Ideler, Untersuchungen uber die astronomischen Beo- bachtungen der Alien., 8. 374, and Ueber die Sternnamen, s. 174-177, with Carteron, Recherches de M. Letronne, p. 113.) It is a remarkable circumstance connected with the analogy between many of the names of the twenty-seven " houses of the moon," and the Dodecatomeria of the zodiac, that we also meet with the sign of the Balance among the Indian Nakschatras (Moon-houses), which are undoubtedly of very great antiquity. ( Vites des Cordilleres, t. ii., p. 6-12.) t Compare A. W. von Schlegel, Ueber Sternbilder des Thierkreises im alien Indien, in the Zeitschrift fur die Kunde det Morgenlandes, bd. i., Heft 3, 1837, and his Commentatio de Zodiari Antiquitate et Origins, 1839, with Adolph Holtzmaun, Ueber den Griechtichen Ursprung det In Vo*. III. F 122 COSMOS. The artifical grouping of the stars into constellations, which arose incidentally during the lapse of ages the fre- quently inconvenient extent and indefinite outline the com- plicated designations of individual stars in the different con- stellations the various alphabets which have been required to distinguish them, as in Argo together with the tasteless blending of mythical personages with the sober prose of philo- sophical instruments, chemical furnaces, and pendulum clocks, in the southern hemisphere, have led to many propositions for mapping the heavens in new divisions, without the aid of imaginary figures. This undertaking appears least haz- ardous in respect to the southern hemisphere, where Scorpio, Sagittarius, Centaurus, Argo, and Eridanus alone possess any poetic interest.* The heavens of the fixed stars (orbis inerrans of Apule- ius), and the inappropriate expression of fixed stars (astro, fixa of Manilius), reminds us, as we have already observed in the introduction to the Astrognosy,f of the connection, or, rather, confusion of the ideas of insertion, and of absolute im- mobility or fixity. When Aristotle calls the non-wandering celestial bodies (dnXavrj darpa) riveted (ivdede^iva), when Ptolemy designates them as ingrafted (TrpoanefivKorec;), these terms refer specially to the idea entertained by Anaximenes of the crystalline sphere of heaven. The apparent motion of all the fixed stars from east to west, while their relative distances remained unchanged, had given rise to this hypoth- esis. " The fixed stars (drrAavr/ aarpa) belong to the higher and more distant regions, in which they are riveted, like nails, dischcn Thierkreises, 1841, s. 9, 16, 23. " The passages quoted from Amorakoscha and Ramayana," says the latter writer, "admit of un- doubted interpretation, and speak of the zodiac in the clearest terms ; but if these works were composed before the knowledge of the Greek signs of the zodiac could have reached India, these passages ought to be carefully examined for the purpose of ascertaining whether they may not be comparatively modern interpolations." * Compare Buttman, in Berlin Astron. Jahrbuchfur 1822, s. 93, Ol- bers on the more recent constellations in Schumacher's Jahrbuch fur 1840, s. 283-251, and Sir John Herschel, Revision and Rearrangement of the Constellations, with special reference to those^ of the Southern Hem- uphere, in the Memoirs of the Astr. Soc., vol. xii., p. 201-224 (with a very exact distribution of the southern stars from the first to the fourth magnitude). On the occasion of Lalande's formal discussion with Bode on the introduction of his domestic cat and of a reaper (Messier!), Ol- bers complains that in order " to find space in the firmament for Kirg Frederic's glory, Andromeda must lay her right arm in a different place from that which it had occupied for 3000 years !" t Vide supra, p. 26-28, and note. THE FIXED STARS. 123 to the crystalline heavens ; the planets (aarpa or TrAavT/rd), which move in an opposite direction, belong to a lower and nearer region."* As we find in Manilius, in the earliest ages of the Caesars, that the term Stella fixa was substituted for infixa or affiza, it may be assumed that the schools of Rome attached thereto at first only the original signification of riveted ; but as the word Jixus also embraced the idea of immobility, and might even be regarded as sy- nonymous with immotus and immobilis, we may readily con- ceive that the national opinion, or, rather, usage of speech, should gradually have associated with Stella fixa the idea of immobility, without reference to the fixed sphere to which it was attached. In. this sense Seneca might term the world of the fixed stars fixum et immobilem populum. Although, according to Stobseus, and the collector of the " Views of the Philosophers," the designation " crystal vault of heaven" dates as far back as the early period of Anax- imenes, the first clearly-defined signification of the idea on which the term is based occurs in Empedocles. This phi- losopher regarded the heaven of the fixed stars as a solid mass, formed from the ether which had been rendered crys- talline and rigid by the action of fire.f According to his * According to Democritus and his disciple Metrodorus, Stob., Eclog. Phys., p. 582. t Plut., De plac. Phil., ii., 11; Diog. Laert., viii., 77; Achilles Tat., ad. Arat., cap. 5, EMTT, Kpvara^un TOVTOV (TOV ovpavbv) elvai (fujaiv, ix TOV KayeTudovc. ffv/.AeyevTO ; in like manner, we only meet with the expression crystal-likti in Diog. Laert., viii., 77, and Galenus, Hist. Phil., I'-i (Sturz, Empedocles Agrigent., t. i., p. 321). Lactautius, De Opificio Dei, c. 17 : " An, si milii quispiam dixerit tcncum esse ccelum, aut vi- treum, aut, ut Empedocles ait, aCrem glacialum, statimne assentiat quia cajliim ex qua materia sit, ignorem." " If any one were to tell me that the heavens are made of brass, or of glass, or, as Empedocles asserts, of frozen air, I should incontinently assent thereto, for I am ignorant of what substance the heavens are composed." We have no early Hel- lenic testimony of the use of this expression of a glass-like or vitreous heaven (cesium vitreum'), for only one celestial body, the sun, is called by PhilolaUs a glass-like body, which throws upon us the rays it has received from the central fire. (The view of Empedocles, referred to in the text, of the reflection of the sun's light from the body of the moon (supposed to be consolidated in the same manner as hailstones), is frequently noticed by Plutarch, apud Euseb. Prtep. Evangel., 1, p. 24, D, and De Facie in Orbe Lunte, cap. 5.) Where Uranos is described as xaZiceof and otdrjpeof by Homer and Pindar, the expression refers only to the idea of steadfast, permanent, and imperishable, as in speak- ing of brazen hearts and brazen voices. V61cker uber Homerische Geo- graphie, 1830. s. 5. The earliest mention, before Pliny, of the word Kov<Tra/lAof when applied to ice-like, transparent rock-crystal, occurs in Uionysius Periegetes, 781, Lilian, xv., 8, and Strabo, xv., p. 717 Ca- 124 COSMOS. theory, the moon is a body conglomerated (like hail) by the action of fire, and receives its light from the sun. The original saub. The opinion that the idea of the crystalline heavens being a gla- cial vault (atr glacintvs of Lactantius) arose among the ancients, from their knowledge of the decrease of temperature, with the increase of height in the strata of the atmosphere, as ascertained from ascending great heights and from the aspect of snow-covered mountains, is refuted by the circumstance that they regarded the fiery ether as lying beyond the confines of the actual atmosphere, and the stars as warm bodies. (Aristot., Meteor., 1,3; De Casio, 11, 7, p. 289.) In speaking of the music of the spheres (Aristot., De Casio, 11, p. 290), which, according to the views of the Pythagoreans, is not perceived by men, because it is continuous, whereas tones can only be heard when they are inter- rupted by silence, Aristotle singularly enough maintains that the move- ment of the spheres generates heat in the air below them, while they are themselves not heated. Their vibrations produce heat, but no sound. " The motion of the sphere of the fixed stars is the most rapid (Aristot., De Caelo, ii., 10, p. 291) ; as ths sphere and the bodies attached to it are impelled in a circle, the subjacent space is heated by this movement, aud hence heat is diffused to the surface of the earth." (Meteorol., 1, 3, p. 340.) It has always struck me as a circumstance worthy of remark, that the Stagirite should constantly avoid the word crystal heaven; for the expression, " riveted stars" (kv6ede[ieva aarpa), which he uses, in- dicates a general idea of solid spheres, without, however, specifying the nature of the substance. We do riot meet with any allusion to the sub- ject in Cicero, but we find in his commentator, Macrobius (Cic. Som- nium Scipionis, 1, c. 20, p. 99, ed. Bip.), traces of freer ideas on the dim- inution of temperature with the increase of height. According to him, eternal cold prevails in the outermost zones of heaven. " Ita enim not solum ten-am sed ipsum quoque coelum, quod vere mundus vocatur, temperari a sole certissimum est, ut extremitates ejus, quae via solis longissime recesserunt, omni careant beneficio caloris, et una frigoris perpetuitate torpescant." " For as it is most certain that not only the earth, but the heavens themselves, which are truly called the universe, are rendered more temperate by the sun, so also their confines, which are most distant from the sun, are deprived of the benefits of heat, and languish in a state of perjpetual cold." These confines of heaven (ex- tremitates cceli), in which the Bishop of Hippo (Augustinus, ed. Antv., 1700, i., p. 102, and iii., p. 99) placed a region of icy-cold water near Saturn the highest, and therefore the coldest, of all the planets, are within the actual atmosphere, for beyond the outer limits of this space lies, according to a somewhat earlier expression of Macrobius (1, c. 19, p. 93), the fiery ether which enigmatically enough does not prevent this eternal cold: " Stellte supra coalum locate, in ipso purissimo asthere suut, in quo omne quidquid est, lux naturalis et sua est, quae tota cum igne suo its sphasrae solis incumbit, ut cceli zonas, quas procul a sole suut, perpetno frigore oppressae sint." " The stars above the heavens are situated in the pure ether, in which all things, whatever they may be, have a natural and proper light of their own" (the region of self-lumin- ous stars), " which so impends over the sphere of the sun with all its fire, that those zones of heaven which are far from the sun are oppress- ed by perpetual cold." My reason for entering so circumstantially into the physical and meteorological ideas of the Greeks and Romans is sim- ply because these subjects, except in the works of Ukert, Henri Martin, THB FIXED STARS. 125 idea of transparency, congelation, and solidity would not, ac- cording to the physics of the ancients,* and their ideas of the solidification of fluids, have referred directly to cold and ice : but the affinity between jcpvaroAAof, fpuoc, and *pwrr<uv&>. as well as this comparison with the most transparent of all bodies, gave rise to the more definite assertion that the vault of heaven consisted of ice or of glass. Thus we read in Lac- tantius : " Ccelum aerem glaciatum esse" and " vitreum coe- lum." Empedocles undoubtedly did not refer to the glass of the Phoenicians, but to air, which was supposed to be con- densed into a transparent solid body by the action of the fiery ether. In this comparison with ice (cpvoTaAAof), the idea of transparency predominated ; no reference being here made to the origin of ice through cold, but simply to its conditions of transparent condensation. "While poets used the term crystal, prose writers (as found in the note on the passage cited from Achilles Tatius, the commentator of Aratus) lim- ited themselves to the expression crystalline or crystal-like, Kpvo-a/J.oi6fi$. In like manner, Trayoc (from tr^ywaBfu, to become solid) signifies a piece of ice its condensation be ing the sole point referred to. The idea of a crystalline vault of heaven was handed down to the Middle Ages by the fathers of the Church, who* believed the firmament to consist of from seven to ten glassy strata, incasing one another like the different coatings of an onion. This supposition still keeps its ground in some of the monasteries of Southern Europe, where I was greatly sur- prised to hear a venerable prelate express an opinion in ref- erence to the fall of aerolites at Aigle, which at that time formed a subject of considerable interest, that the bodies we called meteoric stones with vitrified crusts were not portions of the fallen stone itself, but simply fragments of the crys- and the admirable fragment of the Meteorologia Vetenm of Julias Ide- ler. have hitherto been very imperfectly, and, for the most part, super ficially considered. * The ideas that fire has the power of making rigid (Aristot., ProbL, xiv.. 11). and that the formation of ice itself may be promoted by beat, are deeply rooted in the physics of the ancients, and based on a fanci- ful theory of contraries (AnJiperittatit) on obscure conceptions of po- larity (of exciting opposite qualities or conditions). ( Vide mpra, p. 14, and note.) The quantity of hail produced was considered to be proportional to the degree of heat of the atmospheric strata. (Aristot., Meteor., i.. 12.) In the winter fishery on the shores of the Euxin-s warm water was used to increase the ice formed in the neighborhood of an upright tube. (Alex. Aphrodu., foL 86, and Plat, De\ do, c. 12.) 126 COSMOS. tal vault shattered by it in its fall. Kepler, from his con- siderations of comets which intersect the orbits of all the planets,* boasted, nearly two hundred and fifty years ago, that he had destroyed the seventy-seven concentric spheres of the celebrated Girolamo Fracastoro, as well as all tki? more ancient retrograde epicycles. The ideas entertained by such great thinkers as Eudoxus, Mensechmus, Aristotle, and Apollonius Pergaeus, respecting the possible mechanism and motion of these solid, mutually intersecting spheres by which the planets were moved, and the question whether they regarded these systems of rings as mere ideal modes of representation, or intellectual fancies, by means of which diffi- cult problems of the planetary orbits might be solved or de- termined approximately, are subjects of which I have already treated in another place,t and which are not devoid of interest in our endeavors to distinguish the different periods of devel- opment which have characterized the history of astronomy. Before we pass from the very ancient, but artificial zodi- acal grouping of the fixed stars, as regards their supposed insertion into solid spheres, to their natural and actual ar- rangement, and to the known laws of their relative distri- bution, it will be necessary more fully to consider some of the sensuous phenomena of the individual cosmical bodies their extending rays, their apparent, spurious disk, and their differences of color. In the note referring to the invisibility of Jupiter's satellites,^: I have already spoken of the influ- ence of the so-called tails of the stars, which vary in num- ber, position, and length in different individuals. Indistinct- ness of vision (la vue indistincte) arises from numerous or- ganic causes, depending on aberration of the sphericity of * Kepler expressly says, in his Stella Mortis, fol. 9 : " Solidos orbes rejeci." "I have rejected the idea of solid orbs;" and in the Stella Nova, 1606, cap. 2, p. 8: " Planetse in puro aethere, perinde atque aves in aftre cursus suos conficiunt." " The planets perform their course in the pure ether as birds pass through the air." Compare also p. 122. He inclined, however, at an earlier period, to the idea of a solid icy vault of heaven congealed from the absence of solar heat : " Orbis ex. aqua factus gelu concreta propter solis absentiam." (Kepler, Epit. Astr. Copern., i., 2, p. 51.) " Two thousand years before Kepler, Empedocles maintained that the fixed stars were riveted to the crystal heavens, but that the planets were free and unrestrained" (roif Se Trhav- firae avtiadai). (Plut., plac. Phil., ii., 13; Eraped., 1, p. 335, Sturz; Euseb., Preep. Evang., xv., 30, col. 1688, p. 839.) It is difficult to con- ceive how, according to Plato in the Titnueus ( Tim., p. 40, B ; see Bohn's edition of Plato, vol. ii., p. 344; but not according to Aristotle), the fixed stars, riveted as they are to solid spheres, could rotate independently. t Cosmos, vol. ii., p 315, 316. t Vide supra, p. 51, and note. VELOCITY OF LIGHT. tne eye, diffraction at the margins of the pupil, or at the eyelashes, and on the more or less widely-diffused irritabili- ty of the retina from the excited point.* I see very regu- * "Le3 principales causes de la vue iudistincte sont: aberration de sphericite de 1'oeil, diffraction sur les bords de la pupille, communica- tion d'irritabilite 4 des points voisius sur la retine. La vue confuse est celle ou le foyer ne tombe pas exactement sur la retine, mais tombe au-clevant ou derriere la retine. Les queues des etoiles sont 1'effet de la vision iudistincte, autant qu'elle depend de la constitution du cristal- lin. D'apres un tres ancien m^moire de Hassenfratz (1809) ' les queues au nombre de 4 ou 8 qu'offrent les etoiles ou une bougie vue 4 25 me- tres de distance, sont les caustiques du cristallin formees par 1'interseo tiou des rayons refractes.' Ces caustiques se meuvent a mesure que nous iuclinons la tete. La propriete de la lunette de terminer 1'image lait qu'elle concentre dans un petit espace la lumiere qui sans cela en aurait occupe uu plus grand. Cela est vrai pour les etoiles fixes el pour les disques des planetes. La lumiere des etoiles qui n'ont pas de disque reels, conserve la me me intensite, quel que soil le grossissement. Le foud de 1'air duquel se detache 1'etoile dans la lunette, devient plus noir par le grossisseraent qui dilate les molecules de 1'air qu'embrasse le champ de la lunette. Les planetes a vrais disques deviennent elles- m^mes plus pales par cet effet de dilatation. Quand la peinture focale est uette, quand les rayons partis <fun point de 1'objet se sont concen- tr6s en un seul point dans 1'image, 1'oculaire donne des resultats satis- faisants. Si au contraire les rayons emanes d'un point ne se reiinissent pas au foyer en un seul point, s'ils y forment un petit cercle, les images de deux points contigus de 1'objet empietent necessairement 1'une sur 1'autre; leurs rayons se confondeut. Cette confusion la lentille ocu- laire ue saurait la faire disparaitre. L'office qu'elle remplit exclusive- ment, c'est de grossir ; elle grossit tout ce qui est dans 1'image, les de- fauts comme le reste. Les etoiles n'ayant pas de diametres angulaires sensibles, ceux qu'elles conservent toujours, tiennent pour la plus grande partie au manque de perfection des instrumens (. la courbure moins reguliere donnee aux deux faces de la lentille objective) et & quelques defauts et aberrations de notre ceil. Plus une 6toile semble petite, tout etant egal quant au diametre de 1'objectif, au grossiasement em- ploy6 et A 1'eclat de Petoile observee, et plus la lunette a de perfection. Or le meilleur moyen de juger si les etoiles sont tres petites, si des points sont representes au foyer par des simples points, c'est evidem- ment de viser ^. des etoiles excessivement rapproch6es entr'elles et de voir si dans les etoiles doubles connues les images se confondent, si elles empietent 1'une sur 1'autre, ou bien si on les apercoit bien nette- ment separees." " The pi-incipal causes of indistinct vision are, aberration of the sphe- ricity of the eye, diffraction at the margins of the pupil, and irritation transmitted to contiguous points of the retina. Indistinct vision exists where the focus does not fall exactly ou the retina, but either somewhat before or behind it. The tails of the stars are the result of indistinct- ness of vision, as far as it depends on the constitution of the crystalline lens. According to a very old paper of Hassenfratz (1809), ' the four or eight tails which surround the stars or a candle seen at a distance of 25 metres [82 feet], are the caustics formed on the crystalline lens by the intersection ofrefracted rays.' These caustics follow the move* 128 COSMOS. laxly eight rays at angles of 45 in stars from the first to the third magnitude. As, according to Hassenfratz, these radi- ations are caustics intersecting one another on the crystal- line lens, they necessarily move according to the direction in which the head is inclined.* Some of my astronomical friends see three, or, at most, four rays ahove, and none be- IOAV the star. It has always appeared extraordinary to me that the ancient Egyptians should invariably have given only five rays to the stars (at distances, therefore, of 72) ; so that a star in hieroglyphics signifies, according to Hora- pollo, the number five.f The rays of the stars disappear when the image of the radiating star is seen through a very small aperture made ments of the head. The property of the telescope, in giving a definite outline to images, causes it to concentrate in a small space the light which would otherwise be more widely diffused. This obtains for the fixed stars and for the disks of planets. The light of stars having no actual disks, maintains the same intensity, whatever may be the mag- nifying power of the instrument. The aerial field from which the star is projected in the telescope is rendered more black by the magnifying property of the instrument, by which the molecules of air included in the field of view are expanded. Planets having actual disks become fainter from this effect of expansion. When the focal image is clearly defined, and when the rays emanating from one point of the object are concentrated into one point in the image, the ocular focus affords satis- factory results. But if, on the contrary, the rays emanating from one point do not reunite in the focus into one point, but form a small circle, the images of two contiguous points of the object will necessarily im- pinge upon each other, and their rays will be confused. This confusion can not be removed by the ocular, since the only part it performs is that of magnifying. It magnifies every thing comprised in the image, including its defects. As the stars have no sensible angular diameters, those which they present are principally owing to the imperfect con- struction of the instrument (to the different curvatures of the two sides of the object-glass), and to certain defects and aberrations pertaining to the eye itself. The smaller the star appears, the more perfect is the instrument, providing all relations are equal as to the diameter of the object-glass, the magnifying power employed, and the brightness of the star. Now the best means of judging whether the stars are very small, and whether the points are represented in the focus by simple points, is undoubtedly that of directing the instrument to stars situated very near each other, and of observing whether the images of known double stars are confused, and impinging on each other, or whether they can be seen separate and distinct." (Arago, MS. of 1834 and 1847.) * Hassenfratz, Sur les rayons divergens des Etoiles in Delam6therie, Journal de Physique, torn. Ixix., 1809, p. 324. t Horapollinis Niloi Hieroglyphica, ed. Con. Leemans, 1835, cap. 13, p. 20. The learned editor notices, however, in refutation of Jomard's assertion (Descr. de VEgypte, torn, vii., p. 423), that a star, as the nu- merical hieroglyphic for 5, has not yet been discovered on any monu- ment or papyrus-roll. (Horap., p. 194.) RAYS OF THE STARS. 129 with a needle in a card, and I have myself frequently ob- served both Canopus and Sirius in this manner. The same thing occurs in telescopic vision through powerful instru- ments, when the stars appear either as intensely luminous points, or as exceedingly small disks. Although the fainter scintillation of the fixed stars in the tropics conveys a cer- tain impression of repose, a total absence of stellar radiation would, in my opinion, impart a desolate aspect to the firma- ment, as seen by the naked eye. Illusion of the senses, op- tical illusion, and indistinct vision, probably tend to augment the splendor of the luminous canopy of heaven. Arago long since proposed the question why fixed stars of the first mag- nitude, notwithstanding their great intensity of light, can not be seen when rising above the- horizon in the same man- ner as under similar circumstances we see the outer margin of the moon's disk.* Even the most perfect optical instruments, and those hav- ing the highest magnifying powers, give to the fixed stars spurious disks (diametres factices) ; " the greater aperture," according to Sir John Herschel, " even with the same mag- nifying power, giving the smaller disk."t Occultations of the stars by the moon's disk show that the period occupied in the immersion and emersion is so transient that it can not be estimated at a fraction of a second of time. The frequent occurrence of the so-called adhesion of the immersed star to the moon's disk is a phenomenon depending on inflection of light in no way connected with the question of the spurious diameter of the star. We have already seen that Sir Will- iam Herschel, with a magnifying power of 6500, found the diameter of Vega 0"'36. The image of Arcturus was so di- minished in a dense mist that the disk was below 0"'2. It is worthy of notice that, in consequence of the illusion occa- sioned by stellar radiation, Kepler and Tycho, before the in- vention of the telescope, respectively ascribed to Sirius| a diameter of 4' and of 2' 20". of the Pacific, that the age of the moonTmight be determinecl before first quarter by looking at it through a piece of silk and counting the multiplied images. Here we have a phenomenon of diffraction ob- served through fiue slits. t Outlines, $ 816. Arago has caused the spurious diameter of Alde- baran to increase from 4" to 15" in the instrument by diminishing the object-glass. t Delambre, Hist, de I'Astr. Moderne, torn, i., p. 193 ; Arago, mre, 1842, p. 366. F2 130 COSMOS. The alternating light and dark rings which surround the email spurious disks of the stars when magnified two or three hundred times, and which appear iridescent when seen through diaphragms of different form, are likewise the result of interference and diffraction, as we learn from the observ- ations of Arago and Airy. The smallest objects which can be distinctly seen in the telescope as luminous points, may be employed as a test of the perfection in construction and illuminating power of optical instruments, whether refractors or reflectors. Among these we may reckon multiple stars, such as e Lyrse, and the fifth and sixth star discovered by Struve in 1826, and by Sir John Herschel in 1832, in the trapezium of the great nebula of Orion,* forming the quad- ruple star 6 of that constellation. A difference of color in the proper light of the fixed stars, as well as in the reflected light of the planets, was recog- nized at a very early period ; but our knowledge of this re- markable phenomenon has been greatly extended by the aid of telescopic vision, more especially since attention has been so especially directed to the double stars. We do not here allude to the change of color which, as already observed, ac- companies scintillation even in the whitest stars, and still less to the transient and generally red color exhibited by stellar light near the horizon (a phenomenon owing to the character of the atmospheric medium through which we see it), but to the white or colored stellar light radiated from each cosmic al body, in consequence of its peculiar luminous process, and the different constitution of its surface. The Greek astronomers were acquainted with red stars only, while modern science has discovered, by the aid of the tele- * " Two excessively minute and very close companions, to perceive loth of which is one of the severest tests which can be applied to a tel- escope." (Outlines, 837. Compare also Sir John Herschel, Observ- ations at the Cape, p. 29 ; and Arago, in the Annuaire pour 1834, p. 302-305.) Among the different planetary cosmical bodies by which the illuminating power of a strongly magnifying optical instrument may be tested, we may mention the first and fourth satellites of Uranus, re- discovered by Lassell and Otto Struve in 1847, the two innermost and the seventh satellite of Saturn (Mimas, Enceladus, and Bond's Hyperi- on), and Neptune's satellite discovered by Lassell. The power of pen- etrating into celestial space occasioned Bacon, in an eloquent passage in praise of Galileo, to whom he erroneously ascribes the invention of telescopes, to compare these instruments to ships which cany men upon an unknown ocean : " Ut propriora exercere possiut cum ccelestibus commercia." ( Works of Francis Bacon, 1740, vol. i., Novum Orga- num, p. 361.) COLOR OF THE STABS. 131 Biope, in the radiant fields of the starry heaven, as in the blossoms of the phanerogamia, and in the metallic oxyds, almost all the gradations of the prismatic spectrum between the extremes of refrangibility of the red and the violet ray. Ptolemy enumerates in his catalogue of the fixed stars six (vnoictppoi) fiery red stars, viz. :* Arcturus, Aldebaran, Pol- lux, Antares, a Orionis (in the right shoulder), and Sirius. Cleomedes even compares Antares in Scorpio with the fiery red Mars,f which is called both Trvppdf and nvpoeidfj^. Of the six above-named stars, five still retain a red or red- dish light. Pollux is still indicated as a reddish, but Castor as a greenish star.J Sirius therefore affords the only ex- ample of an historically proved change of color, for it has at present a perfectly white light. A great physical revolu- tion must therefore have occurred at the surface or in the photosphere of this fixed star (or remote sun, as Aristarchus * The expression vnonififtof, which Ptolemy employs indiscriminate- ly to designate the six stars named in his catalogue, implies a slightly- marked transition from fiery yellow to fiery red; it therefore refers, strictly speaking, to a. fiery reddish color. He seems to attach the gen- eral predicate av66f, fiery yellow, to all the other fixed stars. (Almag., viii., 3d ed., Halma, torn, ii., p. 94.) ~K.if>f>6( is, according to Galen (Meth. Med., 12), a pale fiery red inclining to yellow. Gellius com- pares the word with melinus, which, according to Servius, has the same meaning as " gilvus" and " fulvus." As Sirius is said by Seneca (Nat. Quasi., i., 1) to be redder than Mars, and belongs to the stars called in the Almagest vnoKifipoi, there can be no doubt that the word implies the predominance, or, at all events, a certain proportion of red rays. The assertion that the affix 7roi*c/^of , which Aratus, v. 327, attaches to Sirius, has been translated by Cicero as " rutilus," is erroneous. Cicero says, indeed, v. 348: " Namque pedes subter rutilo cum lumine claret, Fervidus ille Canis stellarum luce refulgena ;" but " rutilo cum lumine" is not a translation of iroiK&of, but the mere addition of a free translation. (From letters addressed to me by Pro- fessor Franz.) " If," as Arago observes (Annuaire, 1842, p. 351), " the Roman orator, in using the term rutilus, purposely departs from the strict rendering of the Greek of Aratus, we must suppose that he rec- ognized the reddish character of the light of Sirius." t Cleom., Cycl. Theor., i., ii., p. 59. t Madler, Aslr., 1849, s. 391. $ Sir John Herschel, in the Edinb. Review, vol. 87, 1848, p. 189, and in Schum., A$tr. Nachr., 1839, No. 372: " It seems much more likely that in Sirius a red color should be the effect of a medium interfered, than that in the short space of 2000 years so vast a body should have actually undergone such a material change in its physical constitution. It may be supposed owing to the existence of some sort of cosmical cloudiness, subject to internal movements, depending on causes of which we are ignorant." (Compare Arago, in the Annuaire pour 1842. p. 350- 353.) 132 COSMOS. of Samos called the fixed ttars) before the process could have been disturbed by means of which the less refrangible red rays had obtained the preponderance, through the abstraction or absorption of other complementary rays, either in the pho- tosphere of the star itself, or in the moving cosmical clouds by which it is surrounded. It is to be wished that the epoch of the disappearance of the red color of Sirius had been re- corded by a definite reference to the time, as this subject has excited a vivid interest in the minds of astronomers since the great advance made in modern optics. At the time of Tycho Brahe the light of Sirius was undoubtedly already white, for when the new star which appeared in Cassiopeia in 1572, was observed in the month of March, 1573, to change from its previous dazzling white color to a reddish hue, and again became white in January, 1574, the red ap- pearance of the star was compared to the color of Mars and Aldebaran, but not to that of Sirius. M. Sedillot, or other philologists conversant with Arabic and Persian astronomy, may perhaps some day succeed in discovering evidence of the earlier color of Sirius, in the periods intervening from El-Batani (Albategnius) and El-Fergani (Alfraganus) to Ab- durrahman Sufi and Ebn-Junis (that is, from 880 to 1007), and from Ebn-Junis to Nassir-Eddin and Ulugh Beg (from 1007 to 1437). El-Fergani (properly Mohammed Ebn-Kethir El-Fergani), who conducted astronomical observations in the middle of the tenth century at Rakka (Aracte) on the Euphrates, in- dicates as red stars (stellcn ruffce, of the old Latin translation of 1590) Aldebaran, and, singularly enough,* Capella, which is now yellow, and has scarcely a tinge of red, but he does not mention Sirius. If at this period Sirius had been no longer red, it would certainly be a striking fact that El-Fer * In Muhamedis Alfragani Chronologica et Astronomica Elementa, ed. Jacobus Christmannus, 1590, cap. 22, p. 97, we read, " Stella ruffa in Tauro Aldebaran ; Stella ruffa in Geminit quse appellatur Hajok, hoc est Capra." Alhajoc, Aijuk are, however, the ordinary names for Ca- pella Aurigae, in the Arabic and Latin Almagest. Argelander justly ob- serves, in reference to this subject, that Ptolemy, in the astrological work (Tcrpu&CAof cvvTagif), the genuine character of which is testi- fied by the style as well as by ancient evidence, has associated planets with stars according to similarity of color, and has thus connected Mar tis stella, Quce urit slcut congruit igneo ipsius colori, with Auriga? stella or Capella. (Compare Ptol., Quadripart. Construct., libri iv., Basil, 1551, p. 383.) Riccioli (Almageslum Novum, ed. 1650, torn, i., pars i. lib. 6, cap. 2, p. 394) also reckons Capella, together with Antares, Aide baran, and Arcturus, among red stars. SIRIUS. 133 gani, who invariably follows Ptolemy, should not here indi- cate the change of color in so celebrated a star. Negative proofs are, however, not often conclusive, and, indeed, El- Fergani makes no reference in the same passage to the color of Betelgeux (a Orionis), which is now red, as it was in the age of Ptolemy. It has long been acknowledged that, of all the brightest luminous fixed stars of heaven, Sirius takes the first and most important place, no less in a chronological point of view than through its historical association with the earliest development of human civilization in the valley of the Nile. The era of Sothis the heliacal rising of Sothis (Sirius) on which Biot has written an admirable treatise, indicates, according to the most recent investigations of Lepsius,* the complete arrange- ments of the Egyptian calendar into those ancient epochs, in- cluding nearly 3300 years before our era, " when not only the summer solstice, and, consequently, the beginning of the rise of the Nile, but also the heliacal rising of Sothis, fell on the day of the first water-month (or the first Pachon)." I will collect in a note the most recent, and hitherto unpublished, etymological researches on Sothis or Sirius from the Coptic, Zend, Sanscrit, and Greek, which may, perhaps, be accept- able to those who, from love for the history of astronomy, seek in languages and their affinities monuments of the earlier conditions of knowledge. t 10-195, 3. e compete arrangement o te gyptan caen referred to the earlier part of the year 3285 before our era, i. e., a century and a half after the building of the great pyramid of Ch Chufu, and 940 years before the period generally assigned to the D See Chronologie der^Egypter, by Richard Lepsius, bd. i., 1849, s. 190-195, 213. The complete arrangement of the Egyptian calendar is i. e., about f Cheops- the Deluge. (Compare Cosmos, vol. ii., p. 114, 115, note.) In the calculations based on the circumstance of Colonel Vyse having found that the inclination of the narrow subterranean passage leading into the interior of the pyr- amid very nearly corresponded to the angle 26 15', which in the time of Cheops (Chufu) was attained by the star a Draconis, which indicated the pole, at its inferior culmination at Gizeh, the date of the building of the pyramid is not assumed at 3430 B.C., as given in Cosmos according to Letronne, but at 3970 B.C. (Outlines of Astr., 319.) This diflerence of 540 years tends to strengthen the assumption that a Drac. was re- garded as the pole star, as in 3970 it was still at a distance of 3 44' from the pole. t I have extracted the following observations from letters addressed to me by Professor Lepsius (February, 1850). "The Egyptian name of Sirius is Sothis, designated as a female star ; hence q Hudif is identi- fied in Greek with the goddess Sole (more frequently Sit in hieroglyph- ics), and in the temple of the great Ramses at Thebes with Isis-Sothis (Lepsius, Ckron. der ^gypter,bd. i., s. 119, 136). The signification of the root is found in Coptic, and is allied with a numerous family of words, 134 . COSMOS. Besides Sirius, Vega, Deneb, Regulus, and Spica are at the present time decidedly white ; and among the small double the members of which, although they apparently differ very widely from each other, admit of being arranged somewhat in the following order. By the three-fold transference of the verbal signification, we obtain from the original meaning, to throw out -projicere (sagittam, telurn) first, seminare, to sow; next, eztendere, to extend or spread (as spun threads); and, lastly, what is hero most important, to radiate light and to shine (as stars and fire). From this series of ideas we may deduce the names of the divinities, Satis (the female archer); Sothis, the radiating, and Seth, the fiery. We may also hieroglyphically explain sit or seti, the arrows as well as the ray ; seta, to spin ; setu, scattered seeds. Sothit is especially the brightly radiating, the star regulating the seasons of the year and periods of time. The small triangle, always represented yellow, which is a symbolical sign for Sothis, is used to designate the radiating sun when arranged in numerous triple rows issuing in a down- ward direction from the sun's disk. Seth is the fiery scorching god, ia contradistinction to the warming, fructifying water of the Nile, the god- dess Satis who inundates the soil. She is also the goddess of the cat- aracts, because the overflowing of the Nile began with the appearance of Sothis in the heavens at the summer solstice. In Vettius Valens the star itself is called 2i?0 instead of Sothis ; but neither the name nor the subject admits of our identifying Thoth with Seth or Sothis, as Ideler has done. (Handbuch der Chronologic, bd. i., 8. 126.)" (Lepsius, bd. i., s. 136.) I will close these observations taken from the early Egyptian periods with some Hellenic, Zend, and Sanscrit etymologies : " Se/p, the sun," says Professor Franz, " is an old root, differing only in pronunciation from tfep, &pog, heat, summer, in which we meet with the same change in the vowel sound as in relpof and repof or repaf. The correctness of these assigned relations of the radicals aelp and &ep, depof, is proved not only by the employment of tfepetTarof in Aratus, v. 149 (Ideler, Sternnamen, s. 241), but also by the later use of the forms aeipof, aei- piof, and asipivof, hot, burning, derived from oeip. It is worthy of no- tice that oeipd or deipiva Ifiana is used the same as tiepiva iftdria, light summer clothing. The form oelpiof seems*, however, to have had a wider application, for it constitutes the ordinary term appended to all stars in- fluencing the summer heat: hence, according to the version of the poet Archilochus, the sun was aeipioc aarrip, while Ibycus calls the stars gen- erally adpia, luminous. It can not be doubted that it is the sun to which Archilochus refers in the words iroJUoiif /*v avrov aeipioe Karavavel 6i>f kXXufiTruv. According to Hesychius and Suidas, Set'ptof does indeed signify both the sun and the Dog-star; but I fully coincide with M. Mar- tin, the new editor of Theon of Smyrna, in believing that the passage of Hesiod (Opera et Dies, v. 417) refers to the sun, as maintained by Tzetzes and Proclus, and not to the Dog-star. From the adjective oei- piof, which has established itself as the ' epilheton perpetuum' of the Dog-star, we derive the verb aeipidv, which may be translated ' to sparkle.' Aratus, v. 331, says of Sirius, bt-a aeipiuei, ' it sparkles strong- ly.' When standing alone, the word Setpjyv, the Siren, has a totally dif- ferent etymology ; and your conjecture, that it has merely an accidental similarity of sound with the brightly shining star Sirius, is perfectly well founded. The opinion of those who, according to Theon Smyrna^us (Liber de Astronomia, 1850, p. 202), derive 'Zeipfjv from oeipid&iv (a THE COLOR OF THE STARS 135 stars, Struve enumerates about 300 in which both stars are white.* Procyon, Atair, the Pole Star, and more especially ft Ursae Min. have a more or less decided yellow light. "We have already enumerated among the larger red or reddish stars Betelgeux, Arcturus, Aldebaran, Antares, and Pollux. Rum ker finds y Crucis of a fine red color, and my old friend, Cap tain Berard, who is an admirable observer, wrote from Mada gascar in 1847 that he had for some years seen a Crucis grow ing red. The star 77 Argus, which has been rendered cele- brated by Sir John Herschel's observations, and to which 1 shall soon refer more circumstantially, is undergoing a change in color as well as in intensity of light. In the year 1843, Mr. Mackay noticed at Calcutta that this star was similar in color to Arcturus, and was therefore reddish yellow ;t but in letters from Santiago de Chili, in Feb., 1850, Lieutenant Gil- liss speaks of it as being of a darker color than Mars. Sir John Herschel, at the conclusion of his Observations at the Cape, gives a list of seventy-six ruby-colored small stars, of the seventh to the ninth magnitude, some of which appear in the telescope like drops of blood. The majority of the vari- able stars are also described as red and reddish, f the excep- moreover unaccredited form of acipiav'), is likewise entirely erroneous. While the motion of heat and light is implied by the expression aeipiof, the radical of the word Zeipijv represents the flowing tones of this phe nomenon of nature. It appears to me probable that "Zeipriv is connect- ed with elpeiv (Plato, Cratyl., 398, D, TO yap slpetv hiyeiv kar'C), in which the original sharp aspiration passed into a hissing sound." (From let ters of Prof. Franz to me, January, 1850.) The Greek 2e<p, the sun, easily admits, according to Bopp, " of be- ing associated with the Sanscrit word star, which does not indeed sig- nify the sun itself, but the heavens (as something shining'). The ordi- nary Sanscrit denomination for the sun is surya, a contraction of svdrya, which is not used. The root svar signifies in general to shine. The Zend designation for the sun is hvare, with the h instead of the s. The Greek $fp, $^pof, and i?ep//6f comes from the Sanscrit word gharma (N 7 om. gharmas'), warmth, heat." The acute editor of the Rigveda, Max MQller, observes, that " the special Indian astronomical name of the Dog-star, Lubdkaka, which sig- nifies a hunter, when considered in reference to the neighboring con- stellation Orion, seems to indicate an ancient Arian community of ideas regarding these groups of stars." He is, moreover, principally inclined " to derive Zetptof from the Veda word rira (whence the adjective sair- ya) and the root tri, to go, to wander ; so that the sun and the bright- est of the stars, Sirius, were originally called wandering stars." (Com. pare also Pott, Etymologische Forschungen, 1833, s. 130.) * Stvuve, Stellarum compositarum Mensuree Micrometricts, 1837, ]X Ixxiv. et Ixxxiii. t Sir John Herschel, Observation! at (he Cape, p. 34. t Madler's Attronomie, s. 436. 136 SOSMOS. tions being Algol in Caput Medusae, ft Lyraj and e Auriga, which have a pure white light. Mira Ceti, in which a pe- riodical change of light was first recognized, has a strong red- dish light ;* but the variability observed in Algol and ft Lyrse proves that this red color is not a necessary condition of a change of light, since many red stars are not variable. The faintest stars in which colors can be distinguished belong, ac- cording to Struve, to the ninth and tenth magnitudes. Blue stars were first mentioned by Mariotte,t 1686, in his Traite des Couleurs. The light of a Lyrse is bluish ; and a smaller stellar mass of 3^- minutes in diameter in the southern hem- isphere consists, according to Dunlop, of blue stars alone. Among the double stars there are many in which the princi- pal star is white, and the companion blue ; and some in which both stars have a blue lightj (as 6 Serp. and 59 Androm.). Occasionally, as in the stellar swarm near of the Southern Cross, which was mistaken by Lacaille for a nebulous spot, more than a hundred variously-colored red, green, blue, and bluish-green stars are so closely thronged together that they appear in a powerful telescope " like a superb piece of fancy jewelry. " The ancients believed they could recognize a remarkable symmetry in the arrangement of certain stars of the first magnitude. Thus their attention was especially directed to the four so-called regal stars, which are situated at oppo- site points of the sphere, Aldebaran and Antares, Regulus and Fomalhaut. We find this regular arrangement, of which I have already elsewhere treated, II specially referred to in a late Roman writer, Julius Firmicus Maternus,1T who belonged to the age of Constantine. The differences of right ascension in these regal stars, stellce regales, are llh. 57m. and 12h. 49m. The importance formerly attached to this subject is probably owing to opinions transmitted from the East, which gained a footing in the Roman empire un- der the Caesars, together with a itrong national predilection for astrology. The leg, or north star of the Great Bear (the celebrated star of the Bull's leg in the astronomical repre- * Cosmos, vol. ii., p. 330. t Arago, Annuaire pour 1842, p. 348. t Struve, Stella comp., p. Lxxxii. $ Sir John Herschel, Observations at the Cape, p. 17, 102. (" Nebula and Clusters, No. 3435.") II Humboldt, Vues des CordUleres et Monument des Peuples Indigenes de VAmerique, torn, ii., p. 55. IT Julii Firmici Maierni Astron., libri viii., Basil, 1551, lib. vi., cap i., p. 150. SOUTHERN STARS. 137 sentations of Dendera, and in the Egyptian Book of the Dead), is perhaps the star indicated in an obscure passage of Job (ch. ix., ver. 9), in which Arcturus, Orion, and the Plei- ades are contrasted with " the chambers of the south," and in which the four quarters of the heavens in like manner are indicated by these four groups.* "While a large and splendid portion of the southern heav- ens beyond stars having 53 S. Decl. were unknown in an- cient times, and even in the earlier part of the Middle Ages, the knowledge of the southern hemisphere was gradually completed about a century before the invention and appli- cation of the telescope. At the time of Ptolemy there were visible on the horizon of Alexandria, the Altar, the feet of the Centaur, the Southern Cross, then included in the Cen- taur, and, according to Pliny, also called Ccesaris Thronus, in honor of Augustus,! and Canopus (Canobus) in Argo, which is called Ptolemceon by the scholiast to Germanicus.J * Lepsius, Chronol. der ^Egypter, bd. i., s. 143. In the Hebrew text mention is made of Asch, the giant (Orion?), the many stars (the Pleiades, Gemut?), and "the Chambers of the South." The Septua- gint ^ives : 6 iroiuv 'EAetd<5a /cat 'Ecmepov Kal 'ApKrovpov /cat ra^eta vorov. The early English translators, like the Germans and Dutch, under- go >d the first group referred to in the verse to signify the stars in the Great Bear. Thus we find in Coverdale's version, " He maketh the waynes of heaven, the Orions, the vii. stars, and the secret places of the south." Adam Clarke's Commentary on the Old Testament. (TR.) t Ideler, Sternnamen, s. 295. t Martianus Capella changes Ptolemceon into Ptolemceus; both names were devised by the flatterers at the court of the Egyptian sovereigns. Amerigo Vespucci thought he had seen three Canopi, one of which was quite dark (fosco), Canopus ingens et niger of the Latin translation ; most probably one of the black coal-sacks. (Humboldt, Examen Crit. de la Geogr., torn, v., p. 227, 229.) In the above-named Elem. Chronol. et Astron. by El-Fergani (p. 100), it is stated that the Christian pilgrims used to call the Sohel of the Arabs (Canopus) the star of St. Catharine, because they had the gratification of observing it, and admiring it as a guiding star when they journeyed from Gaza to Mount Sinai. In a fine episode to the Ramayana, the oldest heroic poem of Indian antiquity, the stars in the vicinity of the South Pole are declared for a singular reason to have been more recently created than the northern. When Brahminical Indians were emigrating from the northwest to the coun tries around the Ganges, from the 30th degree of north latitude to the lands of the tropics, where they subjected the original inhabitants to their dominion, they saw unknown stars rising above the horizon as they advanced toward Ceylon. In accordance with ancient practice, they combined these stars into new constellations. A bold fiction rep- resented the later-seen stars as having been subsequently created by the miraculous power of Visvamitra, who threatened " the ancient gods that he would overcome the northern hemisphere with his more richly- 138 COSMOS. In the catalogue of the Almagest, Achernar, a star of the first magnitude, the last in Eridanus (Achir el-nahr, in Arabic), is also given, although it was 9 below the hori- zon. A report of the existence of this star must therefore have reached Ptolemy through the medium of those who had made voyages to the southern parts of the Red Sea, or be- tween Ocelis and the Malabar emporium, Muziris.* Though improvements in the art of navigation led Diego Cam, to- gether with Martin Behaim, along the western coasts of Af- rica, as early as 1484, and carried Bartholomew Diaz in 1487, and Gama in 1497 (on his way to the East Indies), far beyond the equator, into the Antarctic Seas, as far as 35 south lat., the first special notice of the large stars and nebulous spots, the first description of the " Magellanic clouds" and the " coal-sacks," and even the fame of " the wonders of the heavens not seen in the Mediterranean," be- long to the epoch of Vicente Yanez Pinzon, Amerigo Ves- pucci, and Andrea Corsali, between 1500 and 1515. The distances of the stars of the southern hemisphere were meas- ured at the close of the sixteenth and the beginning of the seventeenth century. t Laws of relative density in the distribution of the fixed stars in the vault of heaven first began to be recognized when Sir William Herschel, in the year 1785, conceived the happy idea of counting the number of stars which passed starred southern hemisphere." (A. W. von Schlegel, in the Zeilschrift fur die Kunde des Morgcnlandes, bd. i., B. 240.) While this Indian myth figuratively depicts the astonishment excited in wandering na- ' , by the aspect of i tions by the aspect of a new heaven (as the celebrated Spanish poet, Garcilaso de la Vega, says of travelers, " they change at once their coun- try and stars," mttdan de pays y de estrellas), we are powerfully re- minded of the impression that must have been excited, even in the rudest nations, when, at a certain part of the earth's surface, they ob- served large, hitherto unseen stars appear in the horizon, as those in the feet of the Centaur, in the Southern Cross, in Eridauus or in Argo, while those with which they had been long familiar at home wholly disappeared. The fixed stars advance toward us, and again recede, owing to the precession of the equinoxes. We have already mentioned that the Southern Cross was 7 above the horizon, in the countries around the Baltic, 2900 years before our era; at a time, therefore, when the great pyramids had already existed five hundred years. (Compare Cosmog, vol. i., p. 149, and vol. ii., p. 282.) " Cauopus, on the other hand, can never have been visible at Berlin, as its distance from the south pole of the ecliptic amounts to only 14. It would have required a distance of 1 more to bring it within the limits of visibility for our horizon." * Cosmos, vol. ii., p. 571, 572. t Olbers, in Schumacher's Jahrb.f&r 1840, s.249, and Cosmos, vol. i., p. 51. DISTRIBUTION OF STARS. 139 at different heights and in various directions over the field of view, of 15' in diameter, of his twenty-feet reflecting tel- escope. Frequent reference has already been made in the present work to his laborious process of " gauging the heav- ens." The field of view each time embraced only ^^ T V^? tn of the whole heavens ; and it would therefore require, ac- cording to Struve, eighty-three years to gauge the whole sphere by a similar process.* In investigations of the par- tial distribution of stars, we must specially consider the class of magnitude to which they photometrically belong. If we limit our attention to the bright stars of the first three or four classes of magnitudes, we shall find them distributed on the whole with tolerable uniformity, t although in the south- ern hemisphere, from Orionis to a Crucis, they are locally crowded together in a splendid zone in the direction of a great circle. The various opinions expressed by different travelers on the relative beauty of the northern and south- ern hemispheres, frequently, I believe, depends wholly on the circumstance that some of these observers have visited the southern regions at a period of the year when the finest por- tion of the constellations culminate in the daytime. It fol- lows, from the gaugings of the two Herschels in the north- ern and southern hemispheres, that the fixed stars from the fifth and sixth to the tenth and fifteenth magnitudes (par- ticularly, therefore, telescopic stars) increase regularly in density as we approach the galactic circle (6 yaAafmf KV- /cAof) ; and that there are therefore poles rich in stars, and others poor in stars, the latter being at right angles to the principal axis of the Milky Way. The density of the stellar light is at its minimum at the poles of the galactic circle ; and it increases in all directions, at first slowly, and then rap- idly, in proportion to the increased galactic polar distance. By an ingenious and careful consideration of the results of the gauges already made, Struve found that on the average there are 29-4 tunes (nearly 30 times) as many stars in the center of the Milky Way as in regions surrounding the ga- lactic poles. In northern galactic polar distances of 0, 30, 60, 75, and 90, the relative numbers of the stars in a tel- escopic field of vision of 15' diameter are 4-15, 6-52, 17'68, 30-30, and 122-00. Notwithstanding the great similarity in the law of increase in the abundance of the stars, we again find in the comparison of these zones an absolute pre- * Etudes tfAstr. Stellaire, note 74, p. 31. t Outlines of Astr., 785 140 COSMOS. ponderance* on the side of the more beautiful southern heavens. When in 1843 I requested Captain Schwinck (of the En- gineers) to communicate to me the distribution according to right ascension of the 12,148 stars (from the first to the sev- enth inclusive), which, at Bessel's suggestion, he had noted in his Mappa Caslestis, he found in four groups Right Ascension, 50 to 140 3147 stars. 140 230 2627 " 230 320 3523 " 320 50 2851 " These groups correspond with the more exact results of the Etudes Stellaires, according to which the maxima of stars of the first to the ninth magnitude occur in the right ascen- sion 6h. 40m. and. I8h. 40m., and the minima in the right ascension of Ih. 30m. and 13h. 30m. t It is essential that, in reference to the conjectural struc- ture of the universe and to the position or depth of these strata of conglomerate matter, we should distinguish among the countless number of stars with which the heavens are studded, those which are scattered sporadically, and those which occur in separate, independent, and crowded groups. The latter are the so-called stellar dusters or swarms, which frequently contain thousands of telescopic stars in recogniza- ble relations to each other, and which appear to the unaided eye as round nebulae, shining like comets. These are, the nebulous stars of EratosthenesJ and Ptolemy, the nebulosce of the Alphonsine Tables in 1483, and the same of which Galileo said in the Nuncius Sidereus, " Sicut areolee spar- sim per sethera subfulgent." These clusters of stars are either scattered separately throughout the heavens, or closely and irregularly crowded together, in strata, as it were, in the Milky Way, and in the Magellanic clouds. The greatest accumulation of globular clusters, and the most important in reference to the config- uration of the galactic circle, occurs in a region of the south- ern heavens^ between Corona Australis, Sagittarius, the * Op. cit., 795, 796 ; Struve, Eludes cTAstr. Stell., p. 66, 73 (and note 75). t Struve, p. 59. Schwinck finds in his maps, R. A. 0-90, 2858 stars; R. A. 9QO-180 , 3011 stars; R. A. 180-270, 2688 stars; R. A 270-360, 3591 stars ; sura total, 12,148 stars to the seventh magnitude t On the nebula in the right hand of Perseus (near the hilt of his sword), see Eratosth., Catant., c. 22, p. 51, Schaubach. $ John Herschel's Observations at the Cape, $ 105, p. 136. CLUSTERS OF STARS. 141 tail of Scorpio, and the Altar (R. A. 16h. 45m.-l9h.). All clusters in and near the Milky Way are not, however, round and globular ; there are many of irregular outline, with but few stars and not a very dense center. In many globular clusters the stars are uniform in magnitude, in others they vary. In some few cases they exhibit a fine reddish cen- tral star* (R. A. 2h. 10m. ; N Decl. 56 21'). It is a dif- ficult problem in dynamics to understand how such island- worlds, with their multitude of suns, can rotate free and un disturbed. Nebulous spots and clusters of stars appear sub- ject to different laws in their local distribution, although the former are now very generally assumed to consist of very small and still more remote stars. The recognition of these laws must specially modify the conjectures entertained of what has been boldly termed the " structure of the heav- ens." It is, moreover, worthy of notice, that, with an in- strument of equal aperture and magnifying power, round nebulous spots are more easily resolved into clusters of stars than oval ones.f I will content myself with naming the following among the isolated systems of clusters and swarms of stars. The Pleiades : doubtless known to the rudest nations from the earliest times ; the mariner's stars Pleias, and rov rrAetv (from TrAetv, to sail), according to the etymology of the old scholiast of Aratus, who is probably more correct than those modern writers who would derive the name from rrAeof, plenty. The navigation of the Mediterranean lasted from May to the beginning of November, from the early rising to the early setting of the Pbiades. Prsesepe in Cancer : according to Pliny, nubecula quam Prczsepia vacant inter Asellos, a ve</>eA*ov of the Pseudo^ Eratosthenes. The cluster of stars on the sword-hilt of Perseus, frequent- ly mentioned by Greek astronomers. Coma Berenices, like the three former, visible to the naked eye. A cluster of stars near Arcturus (No. 1663), telescopic : R. A. I3h. 34m. 12s., N. Decl. 29 14' ; more than a thousand stars from the tenth to the twelfth magnitude. Cluster of stars between 77 and Herculis, visible to the naked eye in clear nights. A magnificent object in the tel- escope (No. 1968), with a singular radiating margin ; R. A. Outlines, $ 864-869, p. 591-596; Madler's Astr., s. 764. t Observation at the Cape, $ 29, p. 19. 142 COSMOS. 16h. 35m. 37s., N. Dccl. 36 47' ; first described by Halley in 1714. A cluster of stars near w Centauri ; described by Halley as early as 1677 ; appearing to the naked eye as a round cometic object, almost as bright as a star of the fourth or fifth magni- tude ; in powerful instruments it appears composed of count- less stars of the thirteenth to the fifteenth magnitude, crowd- ed together and most dense toward the center; R. A. 13h. 16m. 38s., S. Decl. 46 35' ; No. 3504 in Sir John Herschel's catalogue of the clusters of the southern hemisphere, 15' in diameter. (Observations at the Cape, p. 21, 105 ; Outlines ofAstr., p. 595.) Cluster of stars near K of the Southern Cross (No. 3435), composed of many-colored small stars from the twelfth to the sixteenth magnitude, distributed over an area of Jj-th of a square degree ; a nebulous star, according to Lacaille, but so completely resolved by Sir John Herschel that no nebulous mass remained ; the central star deep red. (Observations at the Cape, p. 17, 102, pi. i., fig. 2.) Cluster of stars, 47 Toucani, Bode ; No. 2322 of Sir John Herschel's catalogue, one of the most remarkable objects in the southern heavens. I was myself deceived by it for sev- eral evenings, imagining it to be a comet, when, on my ar- rival at Peru, I saw it in 12 south lat. rise high above the horizon. The visibility of this cluster to the naked eye is in- creased by the circumstance that, although in the vicinity of the lesser Magellanic cloud, it is situated in a part of the heavens containing no stars, and is from 15' to 20' in diam- eter. It is of a pale rose color in the interior, concentrically inclosed by a white margin composed of small stars (four- teenth to sixteenth magnitude) of about the same magnitude, and presenting all the characteristics of the globular form.* A cluster of stars in Andromeda's girdle, near v of this con- stellation. The resolution of this celebrated nebula into small stars, upward of 1500 of which have been vecognized, apper- tains to the most remarkable discoveries in the observing as- tronomy of the present day. The merit of this discovery is due to Mr. George Bond, assistant astronomerf at the Observatory * " A stupendous object a most magnificent glolular cluster," says Sir John Herschel, " completely insulated, upon a ground of the sky per- fectly black throughout the whole breadth of the sweep." Observations at the Cape, p. 18 and 51, PI. iii., fig. 1 ; Outlines, $ 895, p. 615. t Bond, in the Memoirs of the American Academy of Arts and Sciences, iiew series, vol. iii., p. 75. CLUSTERS 9F STARS. 143 of Cambridge, United States (March, 1848), and testifies to the admirable illuminating power of the refractor of that Ob- servatory, which has an object-glass fifteen inches in diam- eter ; since even a reflector with a speculum of eighteen inch es in diameter did not reveal " a trace of the presence of a star."* Although it is probable that the cluster in Adrom- eda was, at the close of the tenth century, already recorded as a nebula of oval form, it is more certain that Simon Ma- rius (Mayer of Guntzenhausen), the same who first observed the change of color in scintillation,! perceived it on the 1 5th of December, 1612 ; and that he was the first who described it circumstantially as a new starless and wonderful cosmical body unknown to Tycho Brahe. Half a century later, Bouil- laud, the author of Astronomia Philolaica, occupied himself with the same subject. This cluster of stars, which is 2^- in length and more than 1 in breath, is specially distinguish- ed by two remarkable very narrow black streaks, parallel to each other, and to the longer axis of the cluster, which, ac- cording to Bond's investigations, traverse the whole length like fissures. This configuration vividly reminds us of the singular longitudinal fissure in an unresolved nebula of the southern hemisphere, No. 3501, which has been described and figured by Sir John Herschel. (Observations at the Cape, p. 20, 105, pi. iv., fig. 2.) Notwithstanding the important discoveries for which we are indebted to Lord Rosse and his colossal telescope, I have not included the great nebula in Orion's belt in this selection of remarkable clusters of stars, as it appeared to me more ap- propriate to consider those portions of it which have been re- solved in the section on Nebulae. The greatest accumulation of clusters of stars, although by no means of nebulas, occurs in the Milky WayJ (Galaxias, * Outlines, $ 874, p. 601. t Delambre, Hist, de VAstr. Moderne, t. i., p. 697. t We are indebted for the first and only complete description of the Milky Way, in both hemispheres, to Sir John Herschel, in his Results of Astronomical Observations, made during the Years 1834-1838, at the Cape of Good Hope, 316-335, and still more recently in the Outlines of Astronomy, 787-799. Throughout the whole of that section of the Cosmos which treats of the directions, ramifications, and various con- tents of the Milky Way, I have exclusively followed the above-named astronomer and physicist. (Compare also Struve, Etudes d'Astr. Stel- laire, p. 35-79 ; Madler, Ast., 1849, 213 ; Cosmos, vol. i., p. 103, 150.) I need scarcely here remark that in my description of the Milky Way, in order not to confuse certainties with uncertainties, I have not refer- red to what I had myself observed with instruments of a very inferior 144 COSMOS. the celestial river of the Arabs*), which forms almost a great circle of the sphere, and is inclined to the equator at an an- gle of 63. The poles of the Milky Way are situated in Right Ascension 12h. 47m., N. Decl. 27 ; and R. A. Oh. 47m., S. Decl. 27 ; the south galactic pole therefore lies near Coma Berenices, and the northern between Phoenix and Cetus. While all planetary local relations are referred to the eclip- tic the great circle in which the plane of the sun's path in- tersects the sphere we may as conveniently refer many of the local relations of the fixed stars, as, for instance, that of their accumulation or grouping, to the nearly complete circle of the Milky Way. Considered in this light, the latter is to the sidereal world what the ecliptic is to the planetary world of our solar system. The Milky Way cuts the equator in Monoceros, between Procyon and Sinus, R. A. Gh. 54m. (for 1800), and in the left hand of Antinous, R. A. 19h. 15m. The Milky Way, therefore, divides the celestial sphere into two somewhat unequal halves, whose areas are nearly as 8 to 9. In the smaller portion lies the vernal solstice. The Milky Way varies considerably in breadth in different parts of its course.f At its narrowest, and, at the same time, most brilliant portion, between the prow of Argo and the Cross, and nearest to the Antarctic pole, its width is scarcely 3 or 4 ; at other parts it is 16, and in its divided portion, be- tween Ophiuchus and Antinous, as much as 22.$ William Herschel has observed that, judging from his star-gaugings, the Milky Way would appear in many regions to have 6 or 7 greater width than we should be disposed to ascribe to it from the extent of stellar brightness visible to the naked eye." Huygens, who examined the Milky Way with his twenty- three feet refractor, declared, as early as the year 1656, that the milky whiteness of the whole Galactic zone was not to illuminating power, in reference to the very great inequality of the light of the whole zone, during my long residence iu the southern hem- isphere, and which I have recorded in my journals. * The comparison of the ramified Milky Way with a celestial river led the Arabs to designate parts of the constellation of Sagittarius, whose bow falls in a region rich in stars, as the cattle going to drink, and to associate with them the ostrich, which has so little need of water. (Ide- ler, Untersuchungen uber den Ursprung und die Dedeutung der Sternna* men, 78, 183, and 187 ; Niebuhr, Beschreibung von Arabien, e. 112.) t Outlines, p. 529; Schubert, Ast., th. iii., s. 71. t Struve, Etudes d'Astr. Stellaire, p. 41. $ Cosmos, vol. L, p. 150. MILKY WAY. 145 be ascribed to irresolvable nebulosity. A more careful ap- plication of reflecting telescopes of great dimensions and pow- er of light has since proved, with more certainty, the cor rectness of the conjectures advanced by Democritus and Ma- nilius, in reference to the ancient path of Phaeton, that this milky glimmering light was solely owing to the accumu lated strata of small stars, and not to the scantily inter spersed nebulae. This effusion of light is the same at points where the whole can be perfectly resolved into stars, and even in stars which are projected on a black ground, wholly free from nebulous vapor.* It is a remarkable feature of the Milky "Way that it should so rarely exhibit any globular clusters and nebulous spots of a regular or oval form ;t while both are met with in great numbers at a remote distance from it ; as, for instance, in the Magellanic clouds, where isolated stars, globular clusters in all conditions of condensa- tion, and nebulous spots of a definite oval or a wholly irreg- ular form, are intermingled. A remarkable exception to the rarity of globular clusters in the Milky Way occurs in a region between R. A. 16h. 45m. and 18h. 44m., between the Altar, the Southern Crown, the head and body of Sagitta- rius, and the tail of the Scorpion.^ We even find between and 6 of the latter one of those annular nebulae, which are of such extremely rare occurrence in the southern hemi- sphere. In the field of view of powerful telescopes (and we must remember that, according to the calculations of Sir William * "Stars standing on a clear black ground." (Observations at the Cape, p. 391.) " This remarkable belt (the Milky Way, when exam- ined through powerful telescopes) is found (wonderful to relate !) to consist entirely of stars scattered by millions, like glittering dust on the )lack ground of the general heavens." Outlines, p. 182, 537, and 539. t " Globular clusters, excepting in one region of small extent (be- tween 16h. 45m. and 19h. in R. A.), and pebulce of regular elliptic forms, are comparatively rare in the Milky Way, and are found con- gregated in the greatest abundance in a part of the heavens the most remote possible from that circle." (Outlines, p. 614.) Huygens him- self, as early as 1656, had remarked the absence of nebulosity and of all nebulous spots in the Milky Way. In the same place where he mentions th 3 first discovery and delineation of the great nebulous spots in the belt of Orion, by a twenty-eight feet refractor (1656), he says (as I have already remarked in vol. h., p. 330, and note), viam lacteam perspicillis inspectam nullas habere nebulas, and that the Milky Way, like all that has been regarded as nebulous stars, is a great cluster of stars The passage is to be found in Hugenii Opera varia, 1724, p. 540. t Observations at the Cape, $ 105, 107, and 328. On the annular nel> ulsB, No. 3686, see p. 114. VOL. Ill ? 146 . COSMOS. Herschel, a twenty-feet instrument penetrates 900, and a forty-feet one 2800 distances of Sinus), the Milky Way ap- pears as diversified in its sidereal contents as it is irregular and indefinite in its outlines and limits when seen by the unaided eye. While in some parts the Milky Way exhibits, throughout a large space, the greatest uniformity in the light and apparent magnitudes of the stars, in others the most brilliant patches of closely-crowded luminous points are in- terrupted by granular or reticular darker* intervals contain- ing but few stars ; and in some of these intervals in the in- terior of the Galaxy not the smallest star (of the 18m. or 20m.) is to be discovered. It almost seems as though, in these regions, we actually saw through the whole starry stratum of the Milky Way. In gauging with a field of view of 15' diameter, fields presenting on an average forty or fifty stars are almost immediately succeeded by others exhibiting from 400 to 500. Stars of the higher magnitudes often oc- cur in the midst of the most minute telescopic stars, while all the intermediate classes are absent. Perhaps those stars which we regard as belonging to the lowest order of mag- nitudes do not always appear as such, solely on account of their enormous distance, but also because they actually have a smaller volume and less considerable development of light. In order rightly to comprehend the contrast presented by the greater brilliancy, abundance, or paucity of stars, it will be necessary to compare regions most widely separated from each other. The maximum of the accumulation and the greatest luster of stars are to be found between the prow of Argo and Sagittarius, or, to speak more exactly, between the Altar, the tail of the Scorpion, the hand and bow of Sagit- tarius, and the right foot of Ophiuchus. " No region of the heavens is fuller of objects, beautiful and remarkable in themselves, and rendered still more so by their mode of as- sociation" and grouping. t Next in brightness to this por- * " Intervals absolutely dark and completely void of any ttar of the smallest telescopic magnitude." Outlines, p. 536. t " No region of the heavens is fuller of objects, beautiful and re- markable in themselves, and rendered still more so by their mode of association, and by the peculiar features assumed by the Milky Way, which are without a parallel in any other part of its course." 'Observ- ations at the Cape, p. 386. This vivid description of Sir John Hersche] entirely coincides with the impressions I have myself experienced. Capt. Jacob, of the Bombay Engineers, in speaking of the intensity of light in the Milky Way, in the vicinity of the Southern Cross, remarks with striking truth, " Such is the general blaze of starlight near the Cross from that part of the sky, that a person is immediately made MILKY WAY. 147 tion of the southern heavens is the pleasing and richly-star- red region of our northern hemisphere in Aquila and Cyg- nus, where the Milky Way branches off in different direc- tions. While the Milky Way is the narrowest under the foot of the Cross, the region of minimum brightness (where there is the greatest paucity of stars in the Galactic zone) is in the naighborhood of Monoceros and Perseus. The magnificent effulgence of the Milky Way in the south- ern hemisphere is still further increased by the circumstance that between the star r\ Argus, which has become so cele- brated in consequence of its variability, and a Crucis, undei the parallels of 59 and 60 south lat, it is intersected at an angle of 20 by the remarkable zone of very large and probably very proximate stars, to which belong the constella- tions Orion, Canis Major, Scorpio, Centaurus, and the South- ern Cross. The direction of this remarkable zone is indi- cated by a great circle passing through e Orionis and the foot of the Cross. The picturesque effect of the Milky Way, if I may use the expression, is increased in both hemispheres by its various ramifications. It remains undivided for about two fifths of its length. According to Sir John Herschel's observations, the branches separate in the great bifurcation at a Centauri,* and not at )3 Cent., as given in our maps of the stars, or, as was asserted by Ptolemy,t in the constella- tion of the Altar ; they reunite again in Cygnus. In order to obtain a general insight into the whole course and direction of the Milky Way with its subdivisions, we will briefly consider its parts, following the order of their Right Ascension. Passing through y and e Cassiopeiae, the Milky Way sends forth toward e Persei a southern branch, which loses itself in the direction of the Pleiades and Hyades. The main stream, which is here very faint, passes on through Auriga, over the three remarkable stars e, , rj, the Hsedi of that constellation, preceding Capell a, between the feet of Gem- ini and the horns of the Bull (where it intersects the eclip- aware of its having risen above the horizon, though he should not be at the time looking at the heavens, by the increase of general illumination of the atmosphere, resembling the effect of the young moon." (See Piazzi Smyth, On the Orbit of a Centauri, in the Transact, of the Royal Soc. of Edinburgh, vol. xvi., p. 445.) * Outlines, $ 789. 791 ; Observations at the Cape, $ 325. t Almagest, lib. viii., cap. 2 (t. ii., p. 84, 90, Halma). Ptolemy's de- scription is admirable in some parts, especially when compared with Aristotle's treatment of the subject of the Milky Way, in Meteor (lib i., p. 29, 34, according to Ideler's edition). 148 COSMOS. tic nearly in the solstitial colure), and thence over Orion's club to the neck of Monoceros, intersecting the equinoctial (in 1800) at R. A. 6h. 54m. From this point the brightness considerably increases. At the stern of Argo one branch runs southward to y Argus, where it terminates abruptly. The main stream is continued to 33 S. Decl., where, after separating in a fan-like shape (20 in breadth), it again breaks off, so that there is a wide gap in the Milky "Way in the line from y to A Argus. It begins again in a similar fan-like expansion, but contracts at the hind feet of the Cen- taur and before its entrance into the Southern Cross, where it is at its narrowest part, and is only 3 or 4 in width. Soon after this the Milky Way again expands into a bright and broad mass, which incloses /3 Centauri as well as o and ft Crucis, and in the midst of which lies the black pear- shaped coal-sack, to which I shall more specially refer in the seventh section. In this remarkable region, somewhat below the coal-sack, the Milky Way approaches nearest to the South Pole. The above-mentioned bifurcation, which begins at a Cen- tauri, extended, according to older views, to the constellation Cygnus. Passing from a Centauri, a narrow branch runs northward in the direction of the constellation Lupus, where it seems gradually lost ; a division next shows itself at y Normse. The northern branch forms irregular outlines till it reaches the region of the foot of Ophiuchus, where it wholly disappears ; the most southern branch then becomes the main stream, and passes through the Altar and the tail of the Scorpion, in the direction of the bow of Sagittarius, where it intersects the ecliptic in 276 long. It next runs in an irregular patchy and winding stream through Aquila, Sagitta, and Vulpecula up to Cygnus ; between e, a, and y, of which constellation a broad dark vacuity appears, which, as Sir John Herschel says, is not unlike the southern coal- sack, and serves as a kind of center for the divergence of three great streams.* One of these, which is very vivid and conspicuous, may be traced running backward, as it were, through j3 Cygni and f Aquilae, without, however, blending with the stream already noticed, which extends to the foot of Ophiuchus. A considerable offset or protuberant append- age is also thrown off by the northern stream from the head * Outlines, p. 531. The strikingly dark spot between a and y Cas- siopeia; is also ascribed to the contrast with the brightness by which it is surrounded. See Struve, Eludes Stell., note 58. MILKY WAY. 149 of Cepheus, and therefore near Cassiopeia (from which con- stellation we began our description of the Milky Way), to- ward Ursa Minor and the pole. From the extraordinary advancement which the applica- tion of large telescopes has gradually effected in our knowl edge of the sidereal contents and of the differences in the concentration of light observable in individual portions of the Milky Way, views of merely optical projection have been re- placed by others referring rather to physical conformation. Thomas Wright, of Durham,* Kant, Lambert, and at first also Sir William Herschel, were disposed to consider the form of the Wilky Way, and the apparent accumulation of the stars within this zone, as a consequence of the flattened form and unequal dimensions of the world-island (starry stratum) in which our solar system is included. The hy- pothesis of the uniform magnitude and distribution of the fixed stars has recently been attacked on many sides. The bold and gifted investigator of the heavens, Wm. Herschel, in his last works,! expressed himself strongly in favor of the assumption of an annulus of stars ; a view which he had contested in the talented treatise he composed in 1784. The most recent observations have favored the hypothesis of a system of separate concentric rings. The thickness of these rings seems very unequal ; and the different strata, whose combined stronger or fainter light we receive, are undoubt- edly situated at very differentjiltitudes, i. e., at very unequal distances from us ; but the relative brightness of the sep- arate stars which we estimate as of the tenth to the six- teenth magnitude, can not be regarded as affording sufficient data to enable us in a satisfactory manner to deduce numer- ically from them the radius of their spheres of distances.^ In many parts of the Milky Way, the space-penetrating power of instruments is sufficient to resolve whole star- clouds, and to show the separate luminous points projected on the dark, starless ground of the heavens. We here act- * De Morgan has given an extract of the extremely rare work of Thomas Wright of Durham ( Theory of the Universe, London, 1750), p 241 in the Philot. Magazine, ser. iii., No. 32. Thomas Wright, to whose researches the attention of astronomers has been so permanently di reeled since the beginning of the present century, through the ingen ions speculations of Kant and William Herschel, observed only with a reflector of one foot focal length. t Pfaff, in Will. HertchePi sdmmtl. Schriften, bd. i. (1826), a. 78-8l ; Struve, Etvdet Stell., p. 35-44. $ Encke, in Schumacher's Attr. Nochr., No. 622, 1847 341-34C 150 COSMOS. ually look through as into free space. " It leads us," says Sir John Herschel, " irresistibly to the conclusion that in these regions we see fairly through the starry stratum."* In other regions we see, as it were, through openings and fissures, remote world-islands, or outhranching portions of the annular system ; in other parts, again, the Milky Way has hitherto been, fathomless, even with the forty-feet telescope. f Investigations on the different intensity of light in the Milky Way, as well as on the magnitudes of the stars, which regu- larly increase in number from the galactic poles to the circle itself (an increase especially observable for 30 on either side of the Milky Way in stars below the eleventh magnitude, J and therefore in |^ths of all the stars), have led the most recent investigator of the southern hemisphere to remarkable views and probable results in reference to the form of the galactic annular system, and what has been boldly called the sun's place in the world-island to which this annular system belongs. The place assigned to the sun is eccentric, and probably near a point where the stratum bifurcates or spreads itself out into two sheets, in one of those desert re- gions lying nearer to the Southern Cross than to the oppo- site node of the Milky Way.ll "The depth at which our system is plunged in the sidereal stratum constituting the galaxy, reckoning from the southern surface or limit of that * Outlines, p. 536, 537, where we find the following words on the same subject : " In such cases it is equally impossible not to perceive that we are looking through a sheet of stars nearly of a size, and of no great thickness compared with the distance which separates them from us." t Struve, Etudes Stell., p. 63. Sometimes the largest instruments reach a portion of the heavens, in which the existence of a starry stra- tum, shining at a remote distance, is only announced by " a uniform dotting or stippling of the field of view." See, in Observations at the Cape, p. 390, the section " On some indications of very remote tele- scopic branches of the Milky Way, or of an independent sidereal sys- tem or systems bearing a resemblance to such branches." t Observations at the Cape, 314. $ Sir William Herschel, in the Philos. Transact, for 1785, p. 21 ; Sir John Herschel, Observations at the Cape, 293. Compare also Struve, Descr. de I' Observatoire de Poulkova, 1845, p. 267-271. || " I think," says Sir John Herschel, " it is impossible to view this splendid zone from a Centauri to the Cross without an impression amounting almost to conviction that the Milky Way is not a mere stra- tum, but annular ; or, at least, that our system is placed within one of the poorer or almost vacant parts of its general mass, and that eccen- trically, so as to be much nearer to the region about the Cross than to that diametrically opposite to it." (Mary Somerville, On the Connec- *ion of the Physical Sciences, 1846, p. 419.) NEW STARS. 151 stratum, is about equal to that distance which, on a general average, corresponds to the light of a star of the ninth or tenth magnitude, and certainly does not exceed that corre spending to the eleventh."* Where, from the peculiar nature of individual problems, measurements and the direct evi- dence of the senses fail, we see but dimly those results which intellectual contemplation, urged forward by an intuitive im- pulse, is ever striving to attain. IV. NEW STARS AND STARS THAT HAVE VANISHED. VARIABLE STARS, WHOSE RECURRING PERIODS HAVE BEEN DETERMINED. VARIA- TIONS IN THE INTENSITY OF THE LIGHT OF STARS WHOSE PERI- ODICITY IS AS YET UNINVESTIGATED. NEW STARS. The appearance of hitherto unseen stars in the vault of heaven, especially the sudden appearance of strongly-scintillating stars of the first magnitude, is an oc- currence in the realms of space which has ever excited as- tonishment. This astonishment is the greater, in proportion as such an event as the sudden manifestation of what was before invisible, but which nevertheless is supposed to have previously existed, is one of the very rarest phenomena in nature. While, in the three centuries from 1500 to 1800, as many as forty-two comets, visible to the naked eye, have appeared to the inhabitants of the northern hemisphere, on an average, fourteen in every hundred years only eight new stars have been observed throughout the same period. The rarity of the latter becomes still more striking when, we extend our consideration to yet longer periods. From the completion of the Alphonsine Tables, an important epoch in the history of astronomy, down to the time of William Herschel that is, from 1252 to 1800 the number of visi- ble comets is estimated at about sixty-three, while that of new stars does not amount to more than nine. Consequent- ly, for the period during which, in the civilized countries of Europe, we may depend on possessing a tolerably correct enumeration of both, the proportion of new stars to comets visible to the naked eye is as one to seven. We shall pres- ently show that if from the tailless comets we separate the new stars which, according to the records of Ma-tuan-lin, * Observations at the Cape, $ 315. 152 COSMOS. have been observed in China, and go back to the middle o" the second century before the Christian era, that for about 2000 years scarcely more than twenty or twenty-two of such phenomena can be adduced with certainty. Before I proceed to general considerations, it seems not inappropriate to quote the narrative of an eye-witness, and, by dwelling on a particular instance, to depict the vividness of the impression produced by the sight of a new star. " On my return to the Danish islands from my travels in Germa- ny," says Tycho Brahe, " I resided for some time with my uncle, Steno Bille (ut aulicse vitae fastidium lenirem), in the old and pleasantly situated monastery of Herritzwadt ; and here I made it a practice not to leave my chemical labora- tory until the evening. Raising my eyes, as usual, during one of my walks, to the well-known vault of heaven, I ob- served, with indescribable astonishment, near the zenith, in Cassiopeia, a radiant fixed star, of a magnitude never be- fore seen. In my amazement, I doubted the evidence of my senses. However, to convince myself that it was no illusion, and to have the testimony of others, I summoned my assist- ants from the laboratory, and inquired of them, and of all the country people that passed by, if they also observed the star that had thus suddenly burst forth. I subsequently heard that, in Germany, wagoners and other common peo- ple first called the attention of astronomers to this great phe- nomenon in the heavens a circumstance which, as in the case of non-predicted comets, furnished fresh occasion for the usual raillery at the expense of the learned. " This new star," Tycho Brahe continues, " I found to be without a tail, not surrounded by any nebula, and perfectly like all other fixed stars, with the exception that it scintil- lated more strongly than stars of the first magnitude. Its brightness was greater than that of Sirius, a Lyrse, or Jupi- ter. For splendor, it was only comparable to Venus when nearest to the earth (that is, when only a quarter of her disk is illuminated). Those gifted with keen sight could, when the air was clear, discern the new star in the daytime, and even at noon. At night, when the sky was overcast, so that all other stars were hidden, it was often visible through the clouds, if they were not very dense (nubes non admo- dum densas). Its distances from the nearest stars of Cassi- opeia, which, throughout the whole of the following year, I measured with great care, convinced me of its perfect immo- bility. Already, in December, 1572, its brilliancy began to NiJW STARS. 153 diminish, and the star gradually resettled Jupiter ; but by January, 1573, it had become less bright than that planet. Successive photometric estimates gave the following results : for February and March, equality with stars of the first mag- nitude (stellarum affixarum primi honoris for Tycho Brahe seems to have disliked using Manilius's expression of stellse fixae) ; for April and May, with stars of the second magni- tude ; for July and August, with those of the third ; for Oc- tober and November, those of the fourth magnitude. To- ward the month of November, the new star was not bright- er than the eleventh in the lower part of Cassiopeia's chair. The transition to the fifth and sixth magnitude took place between December, 1573, and February, 1574. In the fol- lowing month the new star disappeared, and, after having shone seventeen months, was no longer discernible to the naked eye." (The telescope was not invented until thirty seven years afterward.) The gradual diminution of the star's luminosity was, more- over, invariably regular ; it was not (as is the case in the present day with 77 Argus, though indeed that is not to be called a new star) interrupted by several periods of rekind- ling or by increased intensity of light. Its color also changed with its brightness (a fact which subsequently gave rise to many erroneous conclusions as to the velocity of colored rays in their passage through space). At its first appearance, as long as it had the brilliancy of Venus and Jupiter, it was for two months white, and then it passed through yellow into red. In the spring of 1573, Tycho Brahe compared it to Mars ; afterward he thought that it nearly resembled Be- telgeux, the star in the right shoulder of Orion. Its color, for the most part, was like the red tint of Aldebaran. In the spring of 1573, and especially in May, its white color re- turned (albedinem quandam sublividam induebat, qualis Sa- turni stellse subesse videtur). So it remained in January, 1574 ; being, up to the time of its entire disappearance in the month of March, 1574, of the fifth magnitude, and white, but of a duller whiteness, and exhibiting a remarkably strong scintillation in proportion to its faintness. The circumstantial minuteness of these statements* is of * De admiranda Nova Stella, anno 1572, exorta in, Tychonis Brahe AttronomicE instauratce Progymnatmata, 1603, p. 298-304, and 578. In the text I have closely followed the account which Tycho Brahe him- self gives. The very doubtful statement (which is, however, repeated in several astronomical treatises) that his attention was first called to 154 COSMOS. itself a proof of the interest which this natural phenomenon, could not fail to awaken, by calling forth many important questions, in an epoch so brilliant in the history of astronomy. For (notwithstanding the general rarity of the appearance of new stars) similar phenomena, accidentally crowded togeth- er within the short space of thirty-two years, were thrice re- peated within the observation of European astronomers, and consequently served to heighten the excitement. The im- portance of star catalogues, for ascertaining the date of the sudden appearance of any star, was more and more recog- nized ; the periodicity* (their reappearance after many cen- turies) was discussed ; and Tycho Brahe himself boldly ad- vanced a theory of the process by which stars might be formed and molded out of cosmical vapor, which presents many points of resemblance to that of the great William Herschel. He was of opinion that the vapory celestial mat- ter, which becomes luminous as it condenses, conglomerates into fixed stars : " Coeli materiam tenuissimam, ubique nostro visui et planetarum circuitibus perviam, in unum globum con- densatam, stellam effingere." This celestial matter, which is universally dispersed through space, has already attained to a certain degree of condensation in the Milky Way, which glimmers with a soft silvery brightness. Accordingly, the place of the new star, as well as of those which became sud- denly visible in 945 and 1264, was on the very edge of the Milky Way (quo factum est quod nova stella in ipso galaxise margine constiterit). Indeed, some went so far as to believe that they could discern the very spot (the opening or hiatus) whence the nebulous celestial matter had been drawn from the Milky Way.f All this reminds one of the theories of the phenomenon of the new star by a concourse of country people, need not, therefore, be here noticed. * Cardanus, in his controversy with Tycho Brahe, went back to the star of the Magi, which, as he pretended, was identical with the star of 1572. Ideler, arguing from his own calculations of the conjunctions of Saturn with Jupiter, and from similar conjectures advanced by Kep- ler on the appearance of the new star in Ophiucus in 1604, supposes that the star of the Magi, through a confusion of atnrjp with uarpnv, which is so frequent, was not a single great star, but a remarkable con- junction of stars the close approximation of two brightly-shining plan- ets at a distance of less than a diameter of the moon. Tychonis Pro- gymnasmata, p. 324-330; contrast with Ideler, Handbuch der Malhe- matischen nnd Technischen Chronologic, bd. ii., s. 399-407. t Progymn., p. 324-330. Tycho Brahe, in his theory of the forma- tion of new stars from the Cosmical vapor of the Milky Way, builds much on the remarkable passages of Aristotle on the connection of the TEMPORARY STARS. 155 transition of the cosmical vapor into clusters of stars, of an agglomerative force, of a concentration to a central nucleus, and of hypotheses of a gradual formation of solid bodies out of a vaporous fluid views which were generally received in the beginning of the nineteenth century, but which at pres- ent, owing to the ever-changing fluctuations in the world of thought, are in many respects exposed to new doubts. Among newly-appeared temporary stars, the following (though with variable degrees of certainty) may be reckoned. I have arranged them according to the order in which they respectively appeared. (a) 134 B.C. in Scorpio. (b) 123 A.D. .... in Ophiuchus. (c) 173 " .... in Centaurus (d) 369 " 7 (e) 386 " .... in Sagittarius. [f) 389 " . in Aquila. (s\ 393 " in Scorpio. \O / ft) 827 " in Scorpio. (i) 945 .... between Cepheus and Cassiopeia. A) 1012 " in Aries. Z) 1203 ' in Scorpio. m) 1230 in Ophiuchus. n) 1264 between Cepheus and Cassiopeia. (o) 1572 1 in Cassiopeia. (p) 1578 (o) 1584 ' in Scorpio. r) 1600 ' in Cygnus. s) 1604 t) 1609 ' in Ophiuchus. u) 1670 in Vulpes. v) 1848 ' in Ophiuchus. EXPLANATORY REMARKS. (<z) This star first appeared in July, 134 years before our era. We have taken it from the Chinese Records of Ma-tuan-lin, for the transla- tion of which we are indebted to the learned linguist Edward Biot ( Connaistance des Temps pour Van 1846, p. 6 1). Its place was between P and p of Scorpio. Among the extraordinary foreign-looking stars of these records, called also guest-stars (etoiles hdtes, " Ke-sing," strangers of a singular aspect), which are distinguished by the observers from comets with tails, fixed new stars and advancing tailless comets are cer- tainly sometimes mixed up. But in the record of their motion (Ke-sing tails of comets (the vapory radiation from their nuclei) with the galaxy to which I have already alluded. (Cotmos, vol. i., p. 103.) 156 COSMOS. of 1092, 1181, and 1458), and in the absence of any si ch record, as also in the occasional addition, " the Ke-sing dissolved" (disappeared), there is contained, if not an infallible, yet a very important criterion. Besides, we must bear in mind that the light of the nucleus of all comets, wheth- er with or without tails, is dull, never scintillates, and exhibits only a mild radiance, while the luminous intensity of what the Chinese call extraordinary (stranger) stars has been compared to that of Venus a circumstance totally at variance with the nature of .comets in general, and especially of those without tails. The star which appeared in 134 B.C., under the old Han dynasty, may, as Sir John Herschel remarks, have been the new star of Hipparchus, which, according to the state- ment of Pliny, induced him to commence his catalogue of the stars. Delambre twice calls this statement a fiction, " une historiette." (Hist, de VAstr. Anc., torn, i., p. 290; and Hist, de VAstr. Mod., torn, i., p. 186.) Since, according to the express statement of Ptolemy (Almag., vii., p. 2, 13, Halmd), the catalogue of Hipparchus belongs to the year 128 B.C., and Hipparchus (as I have already remarked elsewhere) carried on his observations in Rhodes (and perhaps also in Alexandria) from 162 to 127 B.C., there is nothing irreconcilable with this conjecture. It is very probable that the great Nicean astronomer had pursued his observations For a considerable period before he conceived the idea of forming a reg- ular catalogue. The words of Pliny, " suo sevo genita," apply to the whole term of his life. After the appearance of Tycho Brahe's star in 1572, it was much disputed whether the star of Hipparchus ought to be classed among new stars, or comets without tails. Tycho Brahe himself was of the former opinion (Progymn., p. 319-325). The words " ejus- que motn addubitationem adcluctus" may undoubtedly lead to the sup- position of a faint, or altogether tailless comet; but Pliny's rhetorical style admitted of such vagueness of expression. (b) A Chinese observation. It appeared in December, A.D. 123, between a Herculis and a Ophiuchi. Ed. Biot, from Ma-tuan-lin. (It is also asserted that a new star appeared in the reign of Hadrian, about A.D. 130.) (r) A singular and very large star. This also is taken from Ma-tuan- lin, as well as the three following ones. Jt appeared on the 10th of December, 173, between a and /? Centauri and at the end of eight months disappeared, after exhibiting the five colors one after another. " Successivement" is the term employed by Ed. Biot in his translation. Such an expression would almost tend to suggest a series of colors similar to those in the above-described star of Tycho Brahe ; but Sir John Herschel more correctly takes it to mean a colored scintillation (Ozttlines, p. 563), and Arago interprets in the same way a nearly similar expression employed by Kepler when speaking of the new star (1604) in Ophiuchus. (Annuaire pour 1842, p. 347.) (d) This star was seen from March to August, 369. (e) Between A and <j> Sagittarii. In the Chinese Record it is expressly observed, " where the star remained (i. e., without movement) from April to July, 386. (/) A new star, close to a Aquilso. In the year 389, in the reign of the Emperor Honorius, it shone forth with the brilliancy of Venus, ac- cording to the statement of Cuspinianus, who had himself seen it. It totally disappeared in about three weeks.* * Other accounts place the appearance in the year 388 or 398 Jacques Cassini, Element d'Astronomie, 1740 (Etottes Nouvelles), p. 59. TEMPORARY STARS. 157 (g) March, 393. This star was also in Scorpio, in the tail of that coustellation. From the Records of Ma-tuan-lin. (h) The precise year (827) is doubtful. It may with more certainty be assigned to the first half of the ninth century, when, in the reign of Calif Al-Mamun, the two famous Arabian astronomers, Haly and Gia- far Ben Mohammed Albumazar, observed at Babylon a new star, whose light, according to their report, "equaled that of the moon in her quar- ters." This natural phenomenon likewise occurred in Scorpio. The atar disappeared after a period of four months. (t) The appearance of this star (which is said to have shone forth in the year 945, under Otho the Great), like that of 1264, is vouched for solely by the testimony of the Bohemian astronomer Cyprianus Leovi- tius, who asserts that he derived his statements concerning it from a manuscript chronicle. He also calls attention to the fact that these two phenomena (that in 945 and that in 1264) took place between the con- stellations of Cepheus and Cassiopeia, close to the Milky Way, and near the spot where Tycho Brahe's star appeared in 1572. Tycho Brahe (Progym., p. 331 and 709) defends the credibility of Cyprianus Leovi- tius against the attacks of Pontanus and Camerarius, who conjectured that the statements arose from a confusion of new stars with long-tailed comets. (&) According to the statement of Hepidannus, the monk of St. Gall (who died A.D. 1088, whose annals extend from the year A.D. 709 to 1044), a new star of unusual magnitude, and of a brilliancy that dazzled the eye (oculos verberans), was, for three months, from the end of May in the year 1012, to be seen in the south, in the constellation of Aries. In a most singular manner it appeared to vary in size, and occasionally it could not be seen at all. " Nova stella apparuit insolitae magnitudinis, aspectu fulgurans et oculos verberans non sine terrore. Qua? mirum in modum aliquando contractior, aliquando diffusior, etiam extinguebatur interdum. Visa est autem per tres menses in intimis finibus Austri, ul- tra omnia signa qute videntur in ccelo." (See Hepidanni, Annales bre- ves, in Duchesne, Histories Francorum Scriptores, t. iii., 1641, p. 477. Compare also Schnurrer, Chronik der Seuchen, th. i., s. 201.) To the manuscript made use of by Duchesne and Goldast, which assigns the phenomenon to the year 1012, modern historical criticism has, howev- er, preferred another manuscript, which, as compared with the former, exhibits many deviations in the dates, throwing them six years back. Thus it places the appearance of this star in 1006. (See Annales San- gallenses majores, in Pertz, Afonumenta Germanise historica Scriptorum, t. i., 1826, p. 81.) Even the authenticity of the writings of Hepidannus has been called into question by modern critics. The singular phenom- enon of variability has been termed by Chladni the conflagration and extinction of a fixed star. Hind (Notices of the Asfron. Soc., vol. viii., 1848, p. 156) conjectures that this star of Hepidannus is identical with a new star, which is recorded in Ma-tuan-lin, as having been seen in China, in February, 1011, between a and <p of Sagittarius. But in that case there must be an error in Ma-tuan-lin, not only in the statement of the year, but also of the constellation in which the star appeared. (A Toward the end of July, 1203, in the tail of Scorpio. According to the Chinese Record, this new star was "of a bluish-white color, without luminous vapor, and resembled Saturn." (Edouard Biot, in the Connaissance des Temps pour 1846, p. 68.) (TO) Another Chinese observation, from Ma-tuan-lin, whose astronom- ical records, containing an accurate account of the positions f comet* 158 COSMOS. and fixed stars, go back to the year 613 B.C., to the times of Thalea and the expedition of Golaeus of Samoa. This new star appeared in the middle of December, 1230, between Ophiuchus and the Serpent. It dissolved toward the end of March, 1231. () This is the star mentioned by the Bohemian astronomer, Gypri- anus Leovitius (and referred to under the ninth star, in the year 945). About the same time (July, 1264), a great comet appeared, whose tail swept over one half of the heavens, and which, therefore, could not be mistaken for a new star suddenly appearing between Cepheus and Gas- siopeia. (o) This is Tycho Brahe's star of the llth of November, 1572, in the Chair of Cassiopeia, R. A. 3 26' ; Decl. 63 3' (for 1800). (p) February, 1578. Taken from Ma-tuan-lin. The constellation ia not given, but the intensity and radiation of the light must have been extraordinary, since the Chinese Record appends the remark, " a star as large as the sun !" (?) On the 1st of July, 1584, not far from TT of Scorpio ; also a Chinese observation. (r) According to Bayer, the star 34 of Cygnus. Wilhelm Jansen, the celebrated geographer, who for a time had been the associate of Tycho Brahe in his observations, was the first, as an inscription on his celes- tial globe testifies, to draw attention to the new star in the breast of the Swan, near the beginning of the neck. Kepler, who, after the death of Tycho Brahe, was for some time prevented from carrying on any observations, both by his travels and want of instruments, did not ob- serve it till two years later, and, indeed (what is the more surprising, since the star was of the third magnitude), then first heard of its exist- ence. He thus writes : " Cum mense Maio, anni 1602, primum litteris monerer de novo Cygni phffinomeno." (Kepler, De Stella Nova tertii honoris in Cygno, 1606, which is appended to the work De Stella Nova in Serpent., p. 152, 154, 164, and 167.) In Kepler's treatise it is no- where said (as we often find asserted in modern works) that this star of Cygnus upon its first appearance was of the first magnitude. Kep- ler even calls it " parva Cygni stella," and speaks of it throughout as one of the third magnitude. He determines its position in R. A. 300 46' ; Decl. 36 52' (therefore for 1800 : R. A. 302 36' ; Decl. -f 37 27'). The star decreased in brilliancy, especially after the year 1619, and van- ished in 1621. Dominique Cassini (see Jacques Cassini, Ellmens d'Astr., p. 69) saw it, in 1655, again attain to the third magnitude, and then dis- appear. Hevelius observed it again in November, 1665, at first ex- tremely small, then larger, but never attaining to the third magnitude. Between 1677 and 1682 it decreased to the sixth magnitude, and as such it has remained in the heavens. Sir John Herschel classes it among the variable stars, in which he differs from Argelander. () After the star of 1572 in Cassiopeia, the most famous of the new stars is that of 1604 in Ophiuchus (R. A. 259 42' ; and S. Decl. 21 15', for 1800). With each of these stars a great name is associated. The star in the right foot of Ophiuchus was originally discovered, on the 1 Oth of October, 1604, not by Kepler himself, but by his pupil, the Bohemian astronomer, John Bronowski. It was larger than all stars of the first order, greater than Jupiter and Saturn, but smaller than Venus. Her- licias asserts that he had previously seen it on the 27th of September. Its brilliancy was less than that of the new star discovered by Tycho Brahe in 1572. Moreover, unlike the latter, it was not discernible in the daytime. But its scintillation was considerably greater, and espe- TEMPORARY STARS. 159 cially excited the astonishment of all who saw it. As scintillation is always accompanied with dispersion of color, much has been said of its colored and continually-changing light. Arago (Annuaire pour 1834, p. 209-301, and Ann. pour 1842, p. 345-347) has already called atten- tion to the fact that the star of Kepler did not by any means, like that of Tycho Brahe, assume, at certain long intervals, different colors, such as yellow, red, and then again white. Kepler says expressly that his star, as soon as it rose above the exhalations of the earth, was white. When he speaks of the colors of the rainbow, it is to convey a clear idea of its colored scintillation. His words are: " Exemplo adamantis multanguli, qui solis radios inter convertendum ad spectantium oculos variabili fulgore revibraret, colores Iridis (stella nova in Ophiucho) sue- sessive vibratu continue reciprocabat." (De Nova Stella Serpent., p. 5 and 125.) In the beginning of January, 1605, this star was even brighter than Antares, but less luminous than Arcturus. By the end of March in the same year it was described as being of the third magnitude. Its proximity to the sun prevented all observation for four months. Be- tween February and March, 1606, it totally disappeared. The inaccu- rate statements as to the great variations in the position of the new star, advanced by Scipio Claramontius and the geographer Blaew, are scarcely (as Jacques Cassini, Elemens d'Astr., p. 65, long since observed) deserv- ing of notice, since they have been refuted by Kepler's more trustworthy treatise. The Chinese Record of Ma-tuan-lin mentions a phenomenon which exhibits some points of resemblance, as to time and position, with this sudden appearance of a new star in Ophiuchus. On the 30th of September, 1604, there was seen in China a reddish-yellow (" ball- like?") star, not far from TT of Scorpio. It shone in the southwest till November of the same year, when it became invisible. It reappeared on the 14th of January, 16t)5, in the southeast; but its light became slightly duller by March, 1606. (Connaissance des Temps pour 1846, p. 59.) The locality, TT of the Scorpion, might easily be confounded with the foot of Ophiuchus ; but the expressions southwest and south- east, its reappearance, and the circumstance that its ultimate total dis appearance is not mentioned, leave some doubts as to its identity. () This also is a new star of considerable magnitude, and seen in the southwest. It is mentioned in Ma-tuau-lin. No further particulars are recorded. (M) This is the new star discovered by the Carthusian monk Anthel- mus on the 20th of June, 1670, in the head of Vulpes (R. A. 294 27'; Decl. 26 47'), and not far from /? Cygni. At its first appearance it was not of the first, but merely of the third magnitude, and on the 10th of August it diminished to the fifth. It disappeared after three months, but showed itself again on the 17th of March, 1671, when it was of the fourth magnitude. Dominique Cassini observed it very closely in April, 1671, and found its brightness very variable. The new star is reported to have regained its original splendor after ten months, but in Februa- ry, 1672, it was looked for in vain. It did not reappear until the 29th of March in the same year, and then only as a star of the sixth magni- tude ; since that time it has never been observed. (Jacques Cassini, Element d'Astr., p. 6971.) These phenomena induced Dominique Cassini to search for stars never before seen (by him !). He main tained that he had discovered fourteen such stars of the fourth, fifth, and sixth magnitudes (eight in Cassiopeia, two ir Eridanus, and four near the North Pole). From the absence of any precise data as to their respective positions, and especially since, like those said to have been 160 COSMOS. discovered by Maraldi between 1694 and 1709, their existence is more than questionable, they can not be introduced in our present list. (Jacques Cassini, Elimens tfAstron., p. 73-77 ; Delambre, Hist, de VAstr. Mod., t. ii., p. 780.) (t?) One hundred and seventy-eight years elapsed after the appear- ance of the new star in Vulpes without a similar phenomenon having occurred, although in this long interval the heavens were most care- fully explored, and its stars counted, by the aid of a more diligent use of telescopes and by comparison with more correct catalogues of the stars. On the 28. h of April, 1848, at Mr. Bishop's private observatory (South Villa, Regent's Park), Hind made the important discovery of a new reddish-yellow star of the fifth magnitude in Ophiuchus (R. A. 16 50' 59" ; S. Decl. 12 39' 16", for 1848). In the case of no other new star have the novelty of the phenomenon and the invariability of its po- sition been demonstrated with greater precision. At the present time (1850) it is scarcely of the eleventh magnitude, and, according to Lich tenberger's accurate observations, it will most likely soon disappear. (Notices of the Astr.Soc.,vo\. viii., p. 146 and 155-158.) The above list of new stars, which, within the last two thousand years, have suddenly appeared and again disap- peared, is probably more complete than any before given, and may justify a few general remarks. We may distinguish three classes : new stars which suddenly shine forth, and then, after a longer or shorter time, disappear ; stars whose brightness is subject to a periodical variability, which has been already determined ; and stars, like 77 Argus* which suddenly exhib- it an unusual increase of brilliancy, the variations of which are still undetermined. All these phenomena are, most prob- ably, intrinsically related to each other. The new star in Cygnus (1600), which, after its total disappearance (at least to the naked eye), again appeared and continued as a star of the sixth magnitude, leads us to infer the affinity of the two first kinds of celestial phenomena. The celebrated star dis- covered by Tycho Brahe in Cassiopeia in 1572 was consid- ered, even while it was still shining, to be identical with the new star of 945 and 1264. The period of 300 years which Goodricke conjectured, has been reduced by Keill and Pigott to 150 years. The partial intervals of the actual phenom- ena, which perhaps are not very numerically accurate, amount to 319 and 308 years. Arago* has pointed out the great improbability that Tycho Brahe's star of 1572 belongs to those which are periodically variable. Nothing, as yet, seems to justify us in regarding all new stars as variable in long periods, which from their very length have remained unknown to us. If, for instance, the self-luminosity of all the suns of the firmament is the result of an electro-mag- * Arago, Annuaire pour 1842, p. 332. NEW STARS 161 netic process in their photospheres, we may consider this process of light as variable in many ways, without assuming any local or temporary condensations of the celestial ether, or any intervention of the so-called cosmical clouds. It may either occur only once or recur periodically, and either regu- larly or irregularly. The electrical processes of light on our earth, which manifest themselves either as thunder-storms in the regions of the air, or as polar effluxes, together with much apparently irregular variation, exhibit nevertheless a certain periodicity dependent both on the seasons of the year and the hours of the day ; and this fact is, indeed, frequent- ly observed in the formation for several consecutive days, during perfectly clear weather, of a small mass of clouds in particular regions of the sky, as is proved by the frequent failures in attempts to observe the culmination of stars. The circumstance that almost all these new stars burst forth at once with extreme brilliancy as stars of the first mag- nitude, and even with still stronger scintillation, and that they do not appear, at least to the naked eye, to increase gradually in brightness, is, in my opinion, a singular pecul- iarity, and one well deserving of consideration. Kepler* at- tached such weight to this criterion, that he refuted the idle pretension of Antonius Laurentinus Politianus to having seen tiio star in Ophiuchus (1604) before Bronowski simply by the circumstance that Laurentinus had said, " Apparuit nova Stella parva et postea de die in diem crescendo apparuit lu- mine non multo inferior Venere, superior Jove." There are only three stars, which may be looked upon in the light of exceptions, that did not shine forth at once as of the first magnitude ; viz., the star which appeared in Cygnus in 1600, and that in Vulpes in 1670, which were both of the third, and Hind's ne tar in Ophiuchus in 1848, which is of the fifth magnitude. It is much to be regretted, as we have already observed, that after the invention of the telescope in the long period of 178 years, only two new stars have been seen, whereas these phenomena have sometimes occurred in such rapid suc- cession, that at the end of the fourth century four were ob- served in twenty-four years ; in the thirteenth century, three in sixty-one years ; and during the era of Tycho Brahe and Kepler, at the end of the sixteenth and beginning of the sev- enteenth centuries, no less than six were observed within a * Kepler, De Stella Nova in pede Serp., p. 3. 162 COSMOS. period of thirty-seven years. Throughout this examination I have kept in view the Chinese observations of extraordinary stars, most of which, according to the opinion of the most eminent astronomers, are deserving of our confidence. Why it is that of the new stars seen in Europe, that of Kepler in Ophiuchus (1604) is in all probability recorded in the rec- ords of Ma-tuan-lin, while that of Tycho in Cassiopeia (1572) is not noticed, I, for my part, am as little able to explain as I am to account for the fact that no mention was made in the sixteenth century, among European astronomers, of the great luminous phenomenon which was observed in China in February, 1578. The difference of longitude (1 14) could only, HI a few instances, account for their not being visible. Whoever has been engaged in such investigations, must be well aware that the want of record either of political events or natural phenomena, either upon the earth or in the heav- ens, is not invariably a proof of their never having taken place ; and on comparing together the three different cata- logues which are given in Ma-tuan-lin, we actually find com- ets (those, for instance, of 1385 and 1495) mentioned in one but omitted in the others. Even the earlier astronomers (Tycho Brahe and Kepler), as well as the more modern (Sir John Herschel and Hind), have called attention to the fact that the great majority (four fifths, I make it) of all the new stars described both in Eu- rope and China have appeared in the neighborhood of or within the Milky Way. If that which gives so mild and nebulous a light to the annular starry strata of the Milky Way is, as is more than probable, a mere aggregation of small telescopic stars, Tycho Brahe's hypothesis, which wo have already mentioned, of the formation of new, suddenly- shining fixed stars, by the globular condensation of celestial vapor, falls at once to the ground. What the influence of gravitation may be among the crowded strata and clusters of stars, supposing them to revolve round certain central nu- clei, is a question not to be here determined, and belongs to the mythical part of Astrognosy. Of the twenty-one new stars enumerated in the above list, five (those of 134, 393, 827, 1203, and 158 1) appeared in Scorpio, three in Cassi- opeia and Cepheus (945, 1264, 1572), and four in Ophiu- chus (123, 1230, 1604, 1848). Once, however (1012), one was seen in Aries at a great distance from the Milky Way (ths star seen by the monk of St. Gall). Kepler himself who, however, considers as a new star that described by Fa VANISHED STARS. 163 bricius as suddenly shining in the neck of Cetus in the year 1596, and as disappearing in October of the same year, like- wise advances this position as a proof to the contrary. (Kep- ler, De Stella Nova Serp., p. 112.) Is it allowable to in- fer, from the frequent lighting up of such stars in the same constellations, that in certain regions of space those, name- ly, where Cassiopeia and Scorpio are to be seen the condi- tions of their illuminations are favored by certain local re- lations ? Do such stars as are peculiarly fitted for the ex- plosive temporary processes of light especially lie in those directions ? The stars whose luminosity was of the shortest duration were those of 389, 827, and 1012. In the first of the above- named years, the luminosity continued only for three weeks ; in the second, four months ; in the third, three. On the other hand, Tycho Brahe's star in Cassiopeia continued to shine for seventeen months ; while Kepler's star in Cygnus (1600) was visible fully twenty-one years before it totally disappeared. It was again seen in 1655, and still of the third magnitude, as at its first appearance, and afterward dwindled down to the sixth magnitude, without, however (according to Argelander's observations), being entitled to rank among periodically variable stars. STARS THAT HAVE DISAPPEARED. The observation and enumeration of stars that have disappeared is of importance for discovering the great number of small planets which prob- ably belong to our solar system. Notwithstanding, however, the great accuracy of the catalogued positions of telescopic fixed stars and of modern star-maps, the certainty of convic- tion that a star in the heavens has actually disappeared since a certain epoch can only be arrived at with great caution. Errors of actual observation, of reduction, and of the press,* * On instances of stars which have not disappeared, see Argelander, in Schumacher's Astronom. Nachr., No. 624, e. 371. To adduce an ex- ample from antiquity, I may point to the fact that the carelessness with which Aratus compiled his poetical catalogue of the stars has led to the often-renewed question whether Vega Lyrze is a new star, or one which varies in long periods. For instance, Aratus asserts that the constella- tion of Lyra consists wholly of small stars. It is singular that Hippar- chus, in his Commentary, does not notice this mistake, especially as he censures Aratus for his statements as to the relative intensity of light in the stars of Cassiopeia and Ophiuchus. All this, however, is only ac- cidental and not demonstrative ; for when Aratus also ascribes to Cyg- nus none but stars " of moderate brilliancy," Hipparchus expressly re- futes this error, and adds the remark that the bright star in the Swan 164 COSMOS. often disfigure the very best catalogues. The disappearance of a heavenly body from the place in which it had before been distinctly seen, may be the result of its own nution as much as of any such diminution of its photometric process (whether on its surface or in its photosphere), as would ren- der the waves of light too weak to excite our organs of sight. What we no longer see is not necessarily annihilated. The idea of destruction or combustion, as applied to disappearing stars, belongs to the age of Tycho Brahe. Even Pliny, in the fine passage where he is speaking of Hipparchus, makes i a question : Stellae an obirent nascerenturve ? The ap- parent eternal cosmical alternation of existence and destruc- tion is not annihilation ; it is merely the transition of matter into new forms, into combinations which are subject to new processes. Dark cosmical bodies may by a renewed process of light again become luminous. PERIODICALLY VARIABLE STARS. Since all is in motion in the vault of heaven, and every thing is variable both in space and time, we are led by analogy to infer that as the fixed stars universally have not merely an apparent, but also a proper motion of their own, so their surfaces or luminous at- mospheres are generally subject to those changes which re- cur, in the great majority, in extremely long, and, therefore, unmeasured and probably undeterminable periods, or which, in a few, occur without being periodical, as it were, by a sudden revolution, either for a shorter or for a longer time. The latter class of phenomena (of which a remarkable in- stance is furnished in our own days by a large star in Argo) will not be here discussed, as our proper subject is those fixed stars whose periods have already been investigated and as- certained. It is of importance here to make a distinction between three great sidereal phenomena, whose connection has not as yet been demonstrated ; namely, variable stars of known periodicity ; the instantaneous lighting up in the heav- ens of so-called new stars ; and sudden changes in the lu- minosity of long-known fixed stars, which previously shone (Deneb) is little inferior in brilliancy to Lyra (Vega Lyra;). Ptolemy classes Vega among stars of the first magnitude, and in the Cataster isms of Eratosthenes (cap. 25), Vega is caEed Tievnov KOI TiOfnrpnv. Con sidering the many inaccuracies of a poet, who never himself observed the stars, one is not much disposed to give credit to the assertion that it was only between the years 272 and 127 B.C., i. e., between the times of Aratus and Hipparchus, that the star Vega Lyrae (Fidicula of Pliny, xviii., 25) became a star of the first magnitude. PERIODICAL STARS. 165 with uniform intensity. We shall first of all dwell exclu- sively on the first kind of variability ; of this, the earliest in- stance accurately observed is furnished (1638) by Mira, a star in the neck of Cetus. The East-Friesland pastor, David Fabricius (the father of the discoverer of the spots on the sun), had certainly already observed this star on the 13th of August, 1596, as of the third magnitude, and in October of the same year he saw it disappear. But it was not until for- ty-two years afterward that the alternating, recurring vari- ability of its light, and its periodic changes, were discovered by the Professor Johann Phocylides Holwarda, Professor of Franeker. This discovery was further followed in the same century by that of two other variable stars, ft Persei (1669), described by Montanari, and % Cygni (1687), by Kirch. The irregularities which have been noticed in the periods, together with the additional number of stars of this class which have been discovered, have, since the beginning of the nineteenth century, awakened the most lively interest in this complicated group of phenomena. From the difficulty of the subject, and from my own wish to be able to set down in the present work the numerical elements of this variability (as being the most important result of all observations), so far as in the present state of the science they have been ascertain- ed, I have availed myself of the friendly aid of that astrono- mer who of all our cotemporaries has devoted himself with the greatest diligence, and with the most brilliant success, to the study of the periodically varying stars. The doubts and questions called forth by my own labors I confidently laid before my worthy friend Argelander, the director of the Observatory at Bonn, and it is to his manuscript communi cations that I am solely indebted for all that follows, which for the most part has never before been published. The greater number of the variable stars, although not all, are of a red or reddish color. Thus, for instance, besides /3 Persei (Algol in the head of Medusa), /3 Lyrae and e Aurigse have also a white light. The star 77 Aquilae is rather yellow- ish ; so also, in a still less degree, is Geminorum. The old assertion that some variable stars (and especially Mira Ceti) are redder when their brilliancy is on the wane than on the increase, seems to be groundless. Whether, in the double star a Herculis (in which, according to Sir John Herschel, the greater star is red, but according to Struve yellow, while its companion is said to be dark blue), the small companion, estimated at between the fifth to the seventh magnitude, ia 166 COSMOS. itself also variable, appears very problematical. Struve* himself merely says, Suspicor minorem esse variabilem, Variability is by no means a necessary concomitant of red- ness. There are many red stars : some of them very red as Arcturus and Aldebaran in which, however, no variabil- ity has as yet been discovered. And it is also more than doubtful in the case of a star of Cepheus (No. 7582 of the catalogue of the British Association), which, on account of its extreme redness, has been called by William Herschel the Garnet Star (1782). It would be difficult to indicate the number of periodically variable stars for the reason that the periods already determ- ined are all irregular and uncertain, even if there were no other reasons. The two variable stars of Pegasus, as well as a Hydra, s Aurigse, and a Cassiopeise, have not the cer- tainty that belongs to Mira Ceti, Algol, and 6 Cephei. In inserting them, therefore, in a table, much will depend on the degree of certainty we are disposed to be content with. Argelander, as will be seen from the table at the close of this investigation, reckons the number of satisfactorily de- termined periods at only twenty-four.t The phenomenon of variability is found not only both in red and in some white stars, but also in stars of the most di- versified magnitude ; as, for example, in a star of the first magnitude, a Orionis ; by Mira Ceti, a Hydra, a Cassiopeiae, and (3 Pegasi, of the second magnitude ; ft Persei, of the 2'3d magnitude ; and in 77 Aquilse, and (3 Lyrse, of the 3'4th mag- nitude. There are also variable stars, and, indeed, in far greater numbers, of the sixth to the ninth magnitude, such as the variabiles Coronae, Virginis, Cancri, et Aquarii. The star % Cygni likewise presents very great fluctuations at its maximum. * Compare Madler, Astr., s. 438, note 12, with Struve, Stellarum, compos. Mensurce Microm., p. 97 and 98, star 2140. "I believe," says Argelander, "it is extremely difficult with a telescope having a great power of illumination to estimate rightly the brightness of two such different stars as the two components of a Herculis. My experience is strongly against the variability of the companion; or, during my many observations in the daytime with the telescopes of the meridian circles of Abo, Helsingfors, and Bonn, I have never seen a Herculis single, which would assuredly have been the case if the companion at its minimum were of the seventh magnitude. I believe the latter to be constant, and of the fifth or 5-6th magnitude." t Madler's Table (Astron., s. 435) contains eighteen stars, with widely differing numerical elements. Sir John Herschel enumerates more than forty-five, including those mentioned in the notes. Outlines, 819-826. VARIABLE STARS. 167 That the periods of the variable stars are very irregular has. been, long known ; but that this variability, with all its apparent irregularity, is subject to certain definite laws, was first established by Argelander. This he hopes to be able to demonstrate in a longer and independent treatise of his own. In the case of % Cygni, he considers that two perturb- ations in the period the one of 100, the other of 8^ are more probable than a single period of 108. Whether such disturbances arise from changes in the process of light which is going on in the atmosphere of the star itself, or from the periodic times of some planet which revolves round the fixed star or sun % Cygni, and by attraction influences the form of its photosphere, is still a doubtful question. The greatest irregularity in change of intensity has unquestionably been exhibited by the variabilis Scuti (Sobieski's shield) ; for this star diminishes from the 5 - 4th down to the ninth magnitude ; and, moreover, according to Pigott, it once totally disappeared at the end of the last century. At other times the fluctua- tions in its brightness have been only from the 6 - 5th to the sixth magnitude. The maximum of the variations of% Cygni have been between the 6'7th and fourth magnitude ; of Mira, from the fourth to the 2- 1st magnitude. On the other hand, in the duration of its periods 6 Cephei shows an extraordi- nary, and, indeed, of all variable stars, the greatest regularity, as is proved by the 87 minima observed between the 10th of October, 1840, and 8th of January, 1848, and even later. In the case of e Aurigse, the variation of its brilliancy, dis- covered by that indefatigable observer, Heis, of Aix-la-Cha pelle,* extends only from the 3'4th to the 4'5th magnitude. A great difference in the maximum of brightness is exhib- ited by Mira Ceti. In the year 1779, for instance (on the 6th of November), Mira was only a little dimmer than Alde- baran, and, indeed, not unfrequently brighter than stars of the second magnitude ; whereas at other times this variable star scarcely attained to the intensity of the light of 6 Ceti, which is of the fourth magnitude. Its mean brightness is equal to that of y Ceti (third magnitude). If we designate by the brightness of the faintest star visible to the naked eye, and that of Aldebaran by 50, then Mira has varied in its maximum from 20 to 47. Its probable brightness may be expressed by 30 : it is oftener below than above this limit. The measure of its excess, however, when it does occur, ia * Argelander, in Schumacher's Aslron. Nachr., bd. xxvi. (1848), No, 624, s. 369. 168 COSMOS. in proportion more considerable. No certain period of these oscillations has as yet been discovered. There are, however, indications of a period of 40 years, and another of 160. The periods of va nation in different stars vary as 1:250. The shortest period is unquestionably that exhibited by (3 Persei, being 68 hours and 49 minutes ; so long, at least, as that of the polar star is not established at less than two days. Next to ft Persei come 6 Cephei (5d. 8h. 49m.), 77 Aquilaa (7d. 4h. 14m.), and $ Geminorum (lOd. 3h. 35m.). The longest periods are those of 30 Hydrae Hevelii, 495 days ; X Cygni, 406 days ; Variabilis Aquarii, 388 days ; Serpentis S., 367 days ; and Mira Ceti, 332 days. In several of the variable stars it is well established that they increase in brill- iancy more rapidly than they diminish. This phenomenon is the most remarkable in 6 Cephei. Others, as, for instance, ft Lyrse, have an equal period of augmentation and diminu- tion of light. Occasionally, indeed, a difference is observed in this respect in the same stars, though at different epochs in their process of light. Generally Mira Ceti (as also 6 Ce- phei) is more rapid in its augmentation than in its diminu tion ; but in the former the contrary has also been observed Periods within periods have been distinctly observed in the case of Algol, of Mira Ceti, of ft Lyrao, and with great probability also in % Cygni. The decrease of the period of Algol is now unquestioned. Goodricke was unable to per- ceive it, but Argelander has since done so ; in the year 1842 he was enabled to compare more than 100 trustworthy ob- servations (comprising 7600 periods), of which the extremes differed from each other more than 58 years. (Schumacher's Astron. Nachr., Nos. 472 and 624.) The decrease in the period is becoming more and more observable.* For the * " If," says Argelander, " I take for the epoch the minimum bright- ness of Algol, in 1800, on the 1st of January, at 18h. 1m. mean Paris time, I obtain the duration of the periods for 1987, 2d. 20h. 48m., or 59s.-416iOs.-316 1406, " 58s.-737iOs.-094 58s.-393JtOs.-175 58s.-154-tOs.-039 58s.-193-tOs.-096 57s.-971iOs.-045 558.-182-j-Os.-348 " In this table the numbers have the following signification : if we designate the minimum epoch of the 1st of Jan., 1800, by 0, that im- mediately preceding by 1, and that immediately following by -^-1, and BO on, then the duration between 1987 and 1986 would be exactly 2d. 20h. 48m. 59s -416. but *Jie duration between -f-5441 and +5442 VARIABLE STARS. 169 periods of the maximum of Mira (including the maximum of brightness observed by Fabricius in 1596), a formula* has been established by Argelander, from which all the maxima can be so deduced that the probable error in a long period of variability, extending to 33 Id. 8h., does not in the mean ex- ceed 7 days, while, on the hypothesis of a uniform period, it would be 15 days. The double maximum and minimum of (3 Lyrae, in each of its periods of nearly 13 days, was from the first correctly ascertained by its discoverer, Goodricke (1784) ; but it has been placed still more beyond doubtf by very recent observ- ations. It is remarkable that this star attains to the same brightness in both its maxima, but in its principal minimum it is about half a magnitude fainter than in the other. Since the discovery of the variability of (3 Lyrae, the period in a period has probably been on the increase. At first the vari- ability was more rapid, then it became gradually slower ; and this decrease in the length of time reached its limit between the years 1840 and 1844. During that time its period was nearly invariable ; at present it is again decidedly on the de- crease. Something similar to the double maximum of (3 Lyrse occurs in 6 Cephei. There is a tendency to a second maxi- would be 2d. 20h. 48m. 55s. -182 ; the former applies to the year 1784, the latter to the year 1842. "The numbers which follow the signs ^ are the probable errors. That the diminution becomes more and more rapid is shown as well by the last number as by all my observations since 1847." * Argelander's formula for representing all observations of the maxima of Mira Ceti is, as communicated by himself, as follows : 1751, Sep., 9-76 -f331d.-3363 E. +10d.-5, sin. ( 3 T 6 T E. +86 23') +18d.-2, sin. (ff E. +231 42') +33d.-9, sin. (ff E. +170 19') -f 65d.-3, sin. (fp E. -j-6 37') where E. represents the number of maxima which have occurred since Sept. 9, 1751, and the co-efficients are given in days. Therefore, for the current year (E. being =109), the following is the maximum: 1751, Sep., 9-76+36115d.-G5-|-8d.-44 12d.-24. 4-18d.-59+27d.-34=1850, Sep., 8d.-54. " The strongest evidence in favor of this formula is, that it represents even the maximum of 1596 {Cosmos, vol. ii., p. 330), which, on the supposition of a uniform period, would deviate more than 100 days. However, the laws of the variation of the light of this star appear so complicated, that in particular cases e. g., for the accurately observed maximum of 1840 the formula was wrong by many days (nearly twen- ty-five)." t Compare Argelander's essay, written on the occasion of the cen- tenary jubilee of the KOnigsberg University, and entitled De Stella 8 Lyra; Variabili, 1844. VOL. III. H 170 COSMOS. uaum, in so far as its diminution of light does not \ &/ uniformly ; but, after having been for some time tolerably rapid, it comes to a stand, or at least exhibits a very Incon- siderable diminution, which suddenly becomes rapicj again. In some stars it would almost appear as though th,j light were prevented from fully attaining a second maximum. In % Cygni it is very probable that two periods of variability prevail a longer one of 100 years, and a shorter on; of 8. The question whether, on the whole, there is greater reg- ularity in variable stars of very short than in those of very long periods, is difficult to answer. The variations from a uniform period can only be taken relatively ; i. e., in parts of the period itself. To commence with long periods, ^ Cygni, Mira Ceti, and 30 Hydrse must first of all be consideied. In X Cygni, on the supposition of a uniform variability, the devi- ations from a period of 406-0634 days (which is the most probable period) amount to 39-4 days. Even though a por- tion of these deviations may be owing to errors of observa- tion, still at least 29 or 30 days remain beyond doubt ; i. e., one fourteenth of the whole period. In the case of Mira Ceti,* in a period of 331 '340 days, the deviations amount to 55'5 days, even if we do not reckon the observations of David Fabricius. If, allowing for errors of observation, we limit the estimate to 40 days, we still obtain one eighth ; conse- quently, as compared with % Cygni, nearly twice as great a deviation. In the case of 30 Hydrse, which has a period of 495 days, it is still greater, probably one fifth. It is only during the last few years (since 1840, and still later) that the variable stars with very short periods have been observed steadily and with sufficient accuracy, so that the problem in question, when applied to them, is still more difficult of solu- tion. From the observations, however, which have as yet been taken, less considerable deviations seem to occur. In the case of 77 Aquilse (with a period of 7d. 4h.) they only amount to one sixteenth or one seventeenth of the whole pe- riod ; in that of ft Lyrse (period 12d. 21h.) to one twenty- seventh or one thirtieth ; but the inquiry is still exposed to much uncertainty as regards the comparison of long and short periods. Of j3 Lyra between 1700 and 1800 periods have been observed ; of Mira Ceti, 279 ; of % Cygni, only 145. The question that has been mooted, whether stars which * The work of Jacques Cassini (Eltmens Astronomic, 1740, p. 66- 69) belongs to the earliest systematic attempts to investigate tb? mean duration of the period of the variation of Mira Ceti. VARIABLE STARS. 171 have long appeared to be variable in regular periods ever cease to be so, must apparently be answered in the nega- tive. As among the constantly variable stars there are some which at one time exhibit a very great, and at anoth- er a very small degree of variability (as, for instance, vari- abilis Scuti), so, it seems, there are also others whose vari- ability is at certain times so very slight, that, with our lim- ited means, we are unable to detect it. To such belongs variabilis Coronae bor. (No. 5236 in the Catalogue of the British Association), recognized as variable by Pigott, who observed it for a considerable time. In the winter of 1795-6 this star became totally invisible ; subsequently it again appeared, and the variations of its light were observed by Koch. In 1 8 1 7 , Harding and Westphal found that its bright- ness was nearly constant, while in 1824 Olbers was again enabled to perceive a variation in its luminosity. Its con- stancy now again returned, and from August, 1843, to Sep- tember, 1845, was established by Argelander. At the end of September, a fresh diminution of its light commenced. By October, the star was no longer visible in the comet-seek- er ; but it appeared again in February, 1846, and by the be- ginning of June had reached its usual magnitude (the sixth). Since then it has maintained this magnitude, if we overlook some small fluctuations whose very existence has not been established with certainty. To this enigmatical class of stars belong also variabilis Aquarii, and probably Janson and Kep- ler's star in Cygnus of 1600, which we have already men- tioned among the new stars. 172 TABLE OP THE VARIABLE STARS, BY F. ARGELANDEE. No.! Name of tli* Star. Length of Period. Brigbtnes s in the Minimum. NameofniscoTeraraod Date of DiKjorery. 1 o Ceti J). H. M. 331 20 Magnit. 4to2-l 23 6-7 to 4 5 to 4 5 34 3-4 43 3 6 6 5 to 5-4 7to6'7 9 to 6 7 67 8 to 7-8 7 2 1 2 34 4-3 2 8 7-8 Magnit. 4 5-4 45 5-4 34 9 to 6 3-2 1-2 23 4-5 5-4 2*3 Holwarda, 1639. Montanari, 1669. Gottfr. Kirch, 1687. Maraldi, 1704. Koch, 1782. E. Pigott, 1784. Goodricke, 1784. Ditto, 1784. Wm. Herschel 1795. E. Pigott, 1795. Ditto, 1795. Harding, 1809. Ditto, 1810. Ditto, 1826. Ditto, 1828. Schwerd, 1829. Birt, 1831. John Herschel, 1836. Ditto, 1837. Heis, 1846. Schmidt, 1847. Ditto, 1848. Hind, 1848. Ditto, 1848. 210 Perse i 22049 406 1 30 495 31218 7-414 122145 5 849 66 8 323 71 17 145 21 388 13 359 367 5 380 79 3 196 55 i 10 335 4023 350 i 3 \ x Cygni 430 Hydra He v. . 5LeonisR.,420M. 6 17 A(| u il;i- 7/3 Lyra 8 6 Cephei . 9 a Herculis 10 Coronje R. 11 Scuti R. 12 Virginia R 13Aquarii R 14Serpentis R ISSerpentis S l6Cancri R 17 a Cassiopeiae ... 18 a Orionis 19 o Hydra 20eAurigae 21 f Geminorum ... 22 {3 Pegasi 23PegasiR 24CancriS EXPLANATORY REMARKS. The in the column of the minima indicates that the star is then fainter than the tenth magnitude. For the purpose of clearly and con- veniently designating the smaller variable stars, which for the most part have neither names nor other designations, I have allowed myself to ap- C,d to them capitals, since the letters of the Greek and the smaller in alphabet have, for the most part, been already employed by Bayer. Besides the stars adduced in the preceding table, there are almost as many more which are supposed to be variable, since their magnitudes are set down differently by different observers. But as these estimates were merely occasional, and have not been conducted with much pre- cision, and as different astronomers have different principles in estima- ting magnitudes, it seems the safer course not to notice any euch cases until the same observer shall have found a decided variation in them at different times. With all those adduced in the table, this is the case ; and the fact of their periodical change of light is quite established, even where the period itself has not been ascertained. The periods given in the table are founded, for the most part, on my own examination of all the earlier observations that have been published, and on my own ob- servations within the last ten years, which have not as yet been pub- lished. Exceptions will be mentioned in the following notices of the several stars. In these notices the positions are those for 1850, and are expressed in VARIABLE STARS. 173 right ascension and declination. The frequently-repeated term grada- tion indicates a difference of brightness, which may be distinctly recog- nized even by the naked eye, or, in the case of those stars which are invisible to the unaided sight, by a Frauenhofer's comet-seeker of twen- ty-five and a half inches focal length. For the brighter stars above the sixth magnitude, a gradation indicates about the tenth part of the dif- ference by which the successive orders of magnitude differ from one an- other ; for the smaller stars the usual classifications of magnitude are considerably closer. (l)o Ceti, R. A. 32 57', Decl. 3 40' ; also called Mira, on account of the wonderful change of light which was first observed in this star. As early as the latter half of the seventeenth century, the periodicity of this star was recognized, and Bouillaud fixed the duration of its period at 333 days ; it was found, however, at the same time, that this dura- tion was sometimes longer and sometimes shorter, and that the star, at its greatest brilliancy, appeared sometimes brighter and sometimes faint- er. This has been subsequently fully confirmed. Whether the star ever becomes perfectly invisible is as yet undecided; at one time, at the epoch of its minimum, it has been observed of the eleventh or twelfth magnitude ; at another, it could not be seen even with the aid of a three or a four-feet telescope. This much is certain, that for a long period it is fainter than stars of the tenth magnitude. But few observations of the star at this stage have as yet been taken, most having commenced when it had begun to be visible to the naked eye as a star of the sixth magnitude. From this period the star increases in brightness at first with great rapidity, afterward more slowly, and at last with a scarcely perceptible augmentation ; then, again, it diminishes at first slowly, aft- erward rapidly. On a mean, the period of augmentation of light from the sixth magnitude extends to fifty days ; that of its decrease down to the same degree of brightness takes sixty-nine days ; so that the star is visible to the naked eye for about four months. However, this is only the mean duration of its visibility ; occasionally it has lasted as long as five months, whereas at other times it has not been visible for more than three. In the same way, also, the duration both of the augmentation and of the diminution of its light is subject to great fluctuations, and the former is at all times slower than the latter ; as, for instance, in the year 1840, when the star took sixty-two days to arrive at its greatest bright- ness, and then in forty-nine days became visible to the naked eye. The shortest period of increase that has as yet been observed took place in 1679, and lasted only thirty days; the longest (of sixty-seven days) oc- curred in 1709. The decrease of light lasted the longest in 1839, being then ninety-one days ; the shortest in the year 1660, when it was com- pleted in nearly fifty-two days. Occasionally, the star, at the period of its greatest brightness, exhibits for a whole month together scarcely any perceptible variation; at others, a difference may be observed within a very fe w days. On some occasions, after the star had decreased in bright- ness for several weeks, there was a period of perfect cessation, or, at least, a scarcely perceptible diminution of light during several days ; this was the case in 1678 and in 1847. The maximum brightness, as already remarked, is by no means al- ways the same. If we indicate the brightness of the faintest star that is visible to the naked eye by 0, and that of Aldebaran (a Tauri), a star of the first magnitude, by fifty, then the maximum of light of Mira fluc- tuates between 20 and 47, i. e., between the brightness of a star of the fourth, and of the first or second magnitude : the mean brightness is 28 174 COSMOS. or that of the star y Ceti. But the duration of its periods is still more irregular: its mean is 33 Id. 20h., while its fluctuations have extended to a month ; for the shortest time that ever elapsed from one maximum to the next was only 306 days, the longest, on the other hand, 367 days. These irregularities become the more remarkable when we compare the several occurrences of greatest brightness with those which would take place if we were to calculate these maxima on the hypothesis of a uni- form period. The difference between calculation and observation then amounts to 50 days, and it appears that, for several years in succession, those differences are nearly the same, and in the same direction. This evidently indicates that the disturbance in the phenomena of light is one of a very long period. More accurate calculations, however, have prov- ed that the supposition of one disturbance is not sufficient, and that sev- eral must be assumed, which may, however, all arise from the same cause ; one of these recurs after 1 1 single periods ; a second after 88 ; a third after 176 ; and a fourth after 264. From hence arises the form- ula of sines (given at p. 169, note *), with which, indeed, the several maxima very nearly accord, although deviations still exist which can not be explained by errors of observation. (2) (3 Persei, Algol ; R. A. 44 36', Decl. 4-40 22'. Although Gemi- niano Montanari observed the variability or this star in 1667, and Ma- raldi likewise noticed it, it was Goodricke that first, in 1782, discovered the regularity of the variability. The cause of this is probably that this star does not, like most other variable ones, gradually increase and di- minish in brightness, but for 2d. 13h. shines uniformly as a star of the 2-3d magnitude, and only appears less bright for seven or eight hours, when it sinks to the fourth magnitude. The augmentation and dimi- nution of its brightness are not quite regular; but when near to the minimum, they proceed with greater rapidity; whence the time of least brightness may be accurately calculated to within ten to fifteen minutes. It is moreover remarkable that this star, after having increased in light for about an hour, remains for nearly the same period at the same brightness, and then begins once more perceptibly to increase Till very recently the duration of the period was held to be perfectly- uniform, and Wurm was able to present all observations pretty closely by assuming it to be 2d. 21h. 48m. 58is. However, a more ai curate cal- culation, in which was comprehended a space of time nearly- twice as long as that at Wurm's command, has shown that the period becomes gradually shorter. In the year 1784 it was 2d. 20h. 48m. 59-4s., and in the year 1842 only 2d. 20h. 48m. 55-2s. Moreover, from the most re- cent observations, it becomes very probable that this diminution of the period is at present proceeding more rapidly than before, so that for this star also a formula of sines for the disturbance of its period will in time be obtained. Besides, this diminution will be accounted for if we as- sume that Algol comes nearer to us by about 2000 miles every year, or recedes from us thus far less each succeeding year ; for in that case his light would reach us as much sooner every year as the decrease of the period requires; *. e., about the twelve thousandth of a second. If this be the true cause, a formula of sines must eventually be deduced. (3) X Cygni, R. A. 296 12', Decl. +32 32'. This star also exhibits nearly the same irregularities as Mira. The deviations of the observed maxima from those calculated for a uniform period amount to forty days, but are considerably diminished by the introduction of a disturbance of 8J^ single periods, and of another of 100 such periods. In its maxi- mum this star reaches the mean brightness of a faint fifth magnitude, or VARIABLE STARS. 175 one gradat/on brighter than the star 17 Cygni. The fluctuations, how ever, are in this case also very considerable, and have been observed from thirteen gradations below the mean to ten above it. At this low- est maximum the star would be perfectly invisible to the naked eye, whereas, on the contrary, in the year 1847, it could be seen without the aid of a telescope for fully ninety-seven days ; its mean visibility extends to fifty-two days, of which, on the mean, it is twenty days on the increase, and thirty-two on the decrease. (4) 30 Hydras Hevetii, E. A. 200 23', Decl. 22 30'. Of this star, which, from its position in the heavens, is only visible for a short time during every year, all that can be said is, that both its period and its maximum brightness are subject to very great irregularities. (5) Leonis R. =420 Mayeri; R. A. 144 52', Decl. -f 12 7'. This star is often confounded with 18 and 19 Leonis, which are close to it, and, in consequence, has been very little observed ; sufficiently, how- ever, to show that the period is somewhat irregular. Its brightness at the maximum seems also to fluctuate through some gradations. (6) n Aquilss, called also y Antinoi ; R. A. 296 12', Decl. +0 37'. The period of this star is tolerably uniform, 7d. 4h. 13m. 53s. ; observa- tions, however, prove that at long intervals of time trifling fluctuations occur in it, not amounting to more than 20 seconds. The variation of light proceeds so regularly, that up to the present time no deviations have been discovered which could not be accounted for by errors of ob- servation. In its minimum, this star is one gradation fainter than i Aquilis ; at first it increases slowly, then more rapidly, and afterward again more slowly ; and in 2d. 9h. from its minimum, attains to its great- est brightness, in which it is nearly three gradations brighter than /?, but two fainter than 6 Aquilse. From the maximum its brightness does not diminish quite so regularly; for when the star has reached the bright- ness of (3 (*. e., in Id. lOh. after the maximum), it changes more slowly than either before or afterward. (7) /3 Lyra;, R. A. 281 8', Decl. -f-33 11'; a star remarkable from the fact of its having two maxima and two minima. When it has been at its faintest light, one third of a gradation fainter than f Lyrse, it rises in 3d. 5h. to its first maximum, in which it remains three fourths of a gradation fainter than y Lyrse. It then sinks in 3d. 3h. to its second minimum, in which its light is about five gradations greater than that of f. After 3d. 2h. more, it again reaches, in its second maximum, to the brightness of the first ; and afterward, in 3d. 12h., declines once more to its greatest faintness; so that in 12d. 21h. 46m. 40s. it runs through all its variations of light. This duration of the period, however, only applies to the years 1840 to 1844; previously it had been shorter in the year 1784, by about 2^h ; in 1817 and 1818, by more than an hour ; and at present, a shortening of it is again clearly perceptible. There is, therefore, no doubt that in the case of this star the disturbance of its period may be expressed by a formula of sines. (8) 6 Cephei, R. A. 335 54', Decl. +57 39'. Of all the known va- riable stars, this exhibits in every respect the greatest regularity. The period of 5d. 8h. 47m. 39is. is given by all the observations from 1784 to the present day, allowing for errors of observation, which will ac- count for all the slight differences exhibited in the course of the altern ations of light. This star is in its minimum three quarters of a gradation brighter than e ; in its maximum it resembles i of the same constellation (Cepheus). It takes Id. 15h. to pass from the former to the latter ; but, on the other hand, more than double that time, viz., 3d. 18h., to change 176 COSMOS. again to its minimum during eight hours of the latter period, however it scarcely changes at all, and very inconsiderably for a whole day. (9) a Herculis, R. A. 256 57', Decl. +14 34'; an extremely red double star, the variation of whose light is in every respect very irreg- ular. Frequently, its light scarcely changes for months together; at other times, in the maximum, it is nearly five gradations brighter than in the minimum ; consequently, the period also is still very uncertain. The discoverer of the star's variation had assumed it to be sixty-three days. I at first set it down at ninety-five, until a careful reduction of all iny observations, made during seven years, at length gave me the peri- od assigned in the text. Heis believes that he can represent all the ob- servations by assuming a period of 184-9 days, with two maxima and (10) Corona; R., R. A. 235 36', Decl. +28 37'. This star is varia- ble only at times ; the period set down has been calculated by Koch from his own observations, which unfortunately have been lost. (11) Scuti R., R. A. 279 52', Decl. 5 51'. The variations of bright- ness of this star are at times confined within a very few gradations, whereas at others it diminishes from the fifth to the ninth magnitude. It has been too little observed to determine when any fixed rule prevails in these deviations. The duration of the period is also subject to con- siderable fluctuations. (12) Virginis R., R. A. 187 43', Decl. +7 49'. It maintains its pe- riod and its maximum brightness with tolerable regularity ; some devi- ations, however, do occur, which appear to me too considerable to be ascribed merely to errors of observation. (13) Aquarii R., R. A. 354 11', Decl. 16 6'. (14) Serpentis R., R. A. 235 57', Decl. -f 15 3.T. (15) Serpentis S., R. A. 228 40', Decl. -f 14 o^'. (16) Cancri R., R. A. 122 6', Decl. 4-12 9'. Of these four stars, which have been but very slightly observed, little more can be said than what is given in the table. (17) a Cassiopeia, R. A. 8 0', Decl. +55 43'. This star is very diffi- cult to observe. The difference between its maximum and minimum only amounts to a few gradations, and is, moreover, as variable as the duration of the period. This circumstance explains the varying state- ments on this head. That which I have given, which satisfactorily rep- resents the observations from 1782 to 1849, appears to me the most prob- able one. (18) a Orionis, R. A. 86 46', Decl. +7 22'. The variation in the light of this star likewise amounts to only four gradations from the min- imum to the maximum. For 91^ days it increases in brightness, while its diminution extends over 104-fc, and is imperceptible from the twen- tieth to the seventieth day after the maximum. Occasionally its varia bility is scarcely noticeable. It is a very red star. (19) a Hydras, R. A. 140 3', Decl. 8 1'. Of all the variable stars, this is the most difficult to observe, and its period is still altogether un- certain. Sir John Herschel sets it down at from twenty-nine to thirty 1) e Aurigae, R. A. 72 48', Decl. -f 43 36'. The alternation of in this star is either extremely irregular, or else, in a period of sev- eral years, there are several maxima and minima a question which can not be decided for many years. (21) f Geminorum, R. A. 103 48', Decl. +20 47'. This star has hitherto exhibited a perfectly regular course in the variations of its ligbt VARIABLE STARS. 177 Ita brightness at its minimum keeps the mean between v and v of the same constellation ; in the maximum it does not quite reach that of A. It takes 4d. 21h. to attain its full brightness, and 5d. 6h. for its diminu- tion. (22) Pegasi, R. A. 344 7', Decl. -j-27 16'. Its period is pretty well ascertained, but as to the course of its variation of light nothing can as yet be asserted. (23) Pegasi R., R. A. 344 47'. Decl. +9 43'. (24) Cancri 8., R. A. 128 50', Decl. +19 34'. Of these two stars nothing at present can be said. FK. ARGELANDBR. Bonn, August, 1850. VARIATION OF LIGHT IN STARS WHOSE PERIODICITY is UNASCERTAINED. In the scientific investigation of important natural phenomena, either in the terrestrial or in the sidereal sphere of the Cosmos, it is imprudent to connect together, without due consideration, subjects which, as regards their proximate causes, are still involved in obscurity. On this account we are careful to distinguish stars which have ap- pe^red and again totally disappeared (as in the star in Cas- siopeia, 1572) ; stars which have newly appeared and not again disappeared (as that in Cygnus, 1600) ; variable stars with ascertained periods (Mira Ceti, Algol) ; and stars whose intensity of light varies, of whose variation, however, the pe- riodicity is as yet unascertained (as i\ Argus). It is by no means improbable, but still does not necessarily follow, that these four kinds of phenomena* have perfectly similar causes in the photospheres of those remote suns, or in the nature of their surfaces. As we commenced our account of new stars with the most remarkable of this class of celestial phenomena the sudden appearance of Tycho Brahe's star so, influenced by similar considerations, we shall begin our statements concerning the variable stars whose periods have not yet been ascertained, with the unperiodical fluctuations in the light of 77 Argus, which to the present day are still observable. This star is situated in the great and magnificent constellation of the * Newton (Philos. Nat. Principia Mathem., ed. Le Seur et Jacquier, 1760, torn, iii., p. 671) distinguishes only two kinds of these sidereal phenomena. " Stella? fixse quae per vices apparent et evanescunt, quae- quo paulatim crescunt, videntur revolvendo partem lucidam et partem obscuram per vices ostendere." The fixed stars, which alternately ap. pear and vanish, and which gradually increase, appear by turns to show an illuminated and a dark side. This explanation of the variation of light had been still earlier advanced by Riccioli. With respect to the caution necessary in predicating periodicity, see the valuable remarks of Sir John Herschel, in his Observations at the Cape, $ 261. K2 178 COSMOS. Ship, " the glory of the southern skies." Halley, as long ago as 1677, on his return from his voyage to St. Helena, expressed strong doubts concerning the alternation of light in the stars of Argo, especially on the shield of the prow and on the deck (damdiaKT] and KardoTpufid), whose relative or- ders of magnitude had been given by Ptolemy.* However, in consequence of the little reliance that can be placed on the positions of the stars as set down by the ancients, of the various readings in the several MSS. of the Almagest, and of the vague estimates of intensity of light, these doubts failed to lead to any result. According to Halley's observation in 1677, r) Argus was of the fourth magnitude ; and by 1751 it was already of the second, as observed by Lacaille. The star must have afterward returned to its fainter light, for Burchell, during his residence in Southern Africa, from 1811 to 1815, found it of the fourth magnitude ; from 1822 to 1826 it was of the second, as seen by Fallows and Brisbane ; in February, 1827, Burchell, who happened at that time to be at San Paolo, in Brazil, found it of the first magnitude, per- fectly equal to a Crucis. After a year the star returned to the second magnitude. It was of this magnitude when Bur- chell saw it on the 29th of February, 1828, in the Brazilian town of Goyaz ; and it is thus set down by- Johnson and Tay- lor, in their catalogues for the period between 1829 and 1833. Sir John Herschel also, at the Cape of Good Hope, estimated it as being between the second and first magnitude, from 1834 to 1837. When, on the 16th of December, 1837, this famous astron- omer was preparing to take the photometric measurements of the innumerable telescopic stars, between the eleventh and sixteenth magnitudes, which compose the splendid neb- ula around 77 Argus, he was astonished to find this star, which had so often before been observed, increase to such intensity of light that it almost equaled the brightness of a Centauri, and exceeded that of all other stars of the first magnitude, except Canopus and Sirius. By the 2d of January, 1838, it had for that time reached the maximum of its brightness. It soon became fainter than Arcturus ; but in the middle of April, 1838, it still surpassed Aldebaran. Up to March, 1843, it continued to diminish, but was even then a star of the first magnitude ; after that time, and especially in April, 1843, it began to increase so much in light, that, according * Delambre, Hist, de VAstron. Ancicnne, torn, ii., p. 280, arid Hist, de I'Attron. au IScme Siecle, p. 119. VARIABLE STARS. 179 to the observations of Mackay at Calcutta, and Maclear at the Cape, 77 Argus became more brilliant than Canopus, and almost equal to Sirius.* This intensity of light was contin- ued almost up to the beginning of the present year (1850). A distinguished observer, Lieutenant Gilliss, who commands the astronomical expedition sent by the government of the United States to the coast of Chili, writes from Santiago, in February, 1850 : " rj Argus, with its yellowish-red light, which is darker than that of Mars, is at present next in brill- iancy to Canopus, and is brighter than the united light of a Centauri."t Since the appearance of the new stars in Ophiuchus in 1604, no fixed star has attained to such an in- tensity of light, and for so long a period now nearly seven years. In the 173 years (from 1677 to 1850) during which we have reports of the magnitude of this beautiful star in Argo, it has undergone from eight to nine oscillations in the augmentation and diminution of its light. As an incitement to astronomers to continue their observations on the phenom- enon of a great but unperiodical variability in rj Argus, it was fortunate that its appearance was coincident with the famous five years' expedition of Sir John Herschel to the Cape. In the case of several other stars, both isolated and double, observed by Struve (Stellarum compos. Mensurce Microm., p. Ixxi.-lxxiii.), similar variations of light have been no- ticed, which have not as yet been ascertained to be period- ical. The instances which we shall content ourselves with adducing are founded on actual photometrical estimations and calculations made by the same astronomer at different times, and not on the alphabetical series of Bayer's Uranom- etry. In his treatise De fide Uranometricz Bayeriante, 1842 (p. 15), Argelander has satisfactorily shown that Bayer did not by any means follow the plan of designating the brightest stars by the first letters of the alphabet ; but that, on the contrary, he arranged the letters by which he desig- nated stars of equal magnitude according to the positions of * Compare Sir John Herschel's Observations at the Cape, $ 71-78; and Outlines of Astron., $ 830 (Cosmos, vol. i., p. 153). t Letter of Lieutenant Gilliss, astronomer of the Observatory at Wash- ington, to Dr. Fid gel, consul of the United States of North America at Leipsic (in manuscript). The cloudless purity and transparency of the atmosphere, which last for eight months, at Santiago, in Chili, are so great, that Lieutenant Gilliss (with the first great telescope ever con- structed in America, having a diameter ol seven inches, constructed by Henry Fitz, of New York, and William Young, of Philadelphia) wa able clearly to recognize the sixth star in the trapezium of Orion. 180 COSMOS. the stars in a constellation, beginning usually at the head, and proceeding, in regular order, down to the feet. The or- der of letters in Bayer's Uranometria has long led to a be- lief that a change of light has taken place in a Aquilae, in Castor Geminorum, and in Alphard of Hydra. Struve, in 1838, and Sir John Hersohel, observed Capella increase in light. The latter now finds Capella much bright- er than Vega, though he had always before considered it fainter.* Galle and Heis come to the same conclusion, from their present comparison of Capella and Vega. The latter finds Vega between five and six gradations, consequently more than half a magnitude, the fainter of the two. The variations in the light of some stars in the constella- tions of the Greater and of the Lesser Bear are deserving of especial notice. " The star 77 Ursae majoris," says Sir John Herschel, "is at present certainly the most brilliant of the seven bright stars in the Great Bear, although, in 1837, e unquestionably held the first place among them." This re- mark induced me to consult Heis, who so zealously and care- fully occupies himself with the variability of stellar light. " The following," he writes, " is the order of magnitude which results from my observations, carried on at Aix-la-Chapelle between 1842 and 1850 : 1. Ursse majoris, or Alioth ; 2. a, or Dubhe ; 3. r\, or Benetnasch ; 4. 6, or Mizar ; 5. /3 ; 6. ; 7. 6. The three stars, e, a, and rj, of this group, are near- y equal in brightness, so that the slightest want of clearness in the atmosphere might render their order doubtful ; is de- cidedly fainter than the three before mentioned. The two stars ft and y (both of which are decidedly duller than ) are nearly equal to each other ; lastly, 6, which in ancient maps is usually set down as of the same magnitude with ft and y, is by more than a magnitude fainter than these ; e is decided- ly variable. Although in general this star is brighter, I have nevertheless, in three years, observed it on five occasions to be undoubtedly fainter than a. I also consider ft Ursre ma- joris to be variable, though I am unable to give any fixed periods. In the years 1840 and 1841, Sir John Herschel found ft UrszB minoris much brighter than the Polar star ; whereas still earlier, in May, 1846, the contrary was ob- * Sir John Herschel ( Observations at the Cape, p. 334, 350, note 1, and 440). For older observations of Capella and Vega, see William Her- schel, in the Philos. Transact., 1797, p. 307, 1799, p. 121 ; and Bode's Jahrbuchfur 1810, B. 148. Argelander, on the other hand, advances many doubts as to the variation of Capella and of the stars of the Bear. r, VARIABLE STARS. 181 served by him. He also conjectures (3 to be variable.* Since 1843, I have, as a rule, found Polaris fainter than ft Urssa minoris ; but from October, 1843, to July, 1849, Polaris was, according to my registers, fourteen times brighter than (3. I have had frequent opportunities of convincing myself that the color of the last-named star is not always equally red ; it is at times more or less yellow, at others most decidedly red."f All the pains and labor spent in determining the relative brightness of the stars will never attain any certain result until the arrangement of their magnitudes from mere esti- mation shall have given place to methods of measurement founded on the progress of modern optical science.J The possibility of attaining such an object need not be despaired of by astronomers and physicists. The probably great physical similarity in the process of light in all self-luminous stars (in the central body of our own planetary system, and in the distant suns or fixed stars) has long and justly directed attention to the importance and significance which attach to the periodical or non-periodical variation in the light of the stars in reference to climatology generally ; to the history of the atmosphere, or the varying temperature which our planet has derived in the course of thousands of years from the radiation of the sun ; with the condition of organic life, and its forms of development in dif- ferent degrees of latitude. The variable star in the neck of the Whale (Mira Ceti) changes from the second magnitude to the eleventh, and sometimes vanishes altogether ; we have seen that t\ Argus has increased from the fourth to the first magnitude, and among the stars of this class has attained to the brilliancy of Canopus, and almost to that of Sirius. Sup- posing that our own sun has passed through only a very few of these variations in intensity of light and heat, either in an increasing or decreasing ratio (and why should it differ from other suns ?), such a change, such a weakening or augment- * Observations at the Cape, $ 259, note 260. t Heis, in his Manuscript Notices of May, 1 850 ; also Observations at the Cape, p. 325 ; and P. von Boguslawski, Uranus for 1848, p. 186. The asserted variation of n, a, and 6 Ursse majoris is also confirmed in Outlines, p. 559. See Madler, Astr., p. 432. On the succession of the stars which, from their proximity, will in time mark the north pole, until, after the lapse of 12,000 years, Vega, the brightest of all possible polar stars, will take their place. ; Vide supra, p. 96 $ William Herschel, On the Changes that happen to the Fixed Stars, in the Philos. Transact, for 1796, p. 186. Sir John Herschel, in the Observations at the Cape, p. 350-352 ; as also in Mrs. Somerviue's ex- cellent work, Connection of the Physical Sciences, 1846, p. 407. 182 COSMOS. ation of its light-process, may account for far greater and more fearful results for our own planet than any required for the explanation of all geognostic relations and ancient telluric revolutions. William Herschel and Laplace were the first to agitate these views. If I have dwelt upon them some- what at length, it is not because I would seek exclusively in these the solution of the great problem of the changes of temperature in our earth. The primitive high temperature of this planet at its formation, and the solidification of con- glomerating matter ; the radiation of heat from the deeper strata of the earth through open fissures and through unfilled veins ; the greater power of electric currents ; a very differ- ent distribution of sea and land, may also, in the earliest epochs of the earth's existence, have rendered the diffusion of heat independent of latitude ; that is to say, of position relatively to a central body. Cosmical considerations must not be limited merely to astrognostic relations. V. PROPER MOTION OF THE FIXED STARS. PROBLEMATICAL EXIST- ENCE OF DARK COSMICAL BODIES. PARALLAX MEASURED DIS- TANCES OF SOME OF THE FIXED STARS. DOUBTS AS TO THE AS- SUMPTION OF A CENTRAL BODY FOR THE WHOLE SIDEREAL HEAV- ENS. THE heaven of the fixed stars, in contradiction to its very name, exhibits not only changes in the intensity of light, but also further variation from the perpetual motion of the indi- vidual stars. Allusion has already been made to the fact that, without disturbing the equilibrium of the star-systems, no fixed point is to be found in the whole heavens, and that of all the bright stars observed by the earliest of the Greek astronomers, not one has kept its place unchanged. In the case of Arcturus, of/z Cassiopeise, and of a double star in Cyg- nus, this change of position has, by the accumulation of their annual proper motion during 2000 years, amounted respect- ively to 2|, 3i, and 6 moon's diameters. In the course of 3000 years about twenty fixed stars will have changed their places by 1 and upward.* Since the proper motions of the fixed stars rise from $th of a second to 7 - 7 seconds (and * Encke, Betrachtungen fiber die Anordnung des Stern-systems, B. 12. Vidctupra, p. 27. Madler, Astr., s. 445. PROPER MOTION OF THE STARS. 183 consequently differ, at the least, in the ratio of 1 : 154), the relative distances also of the fixed stars from each other, and the configuration of the constellations themselves, can not in long periods remain the same. The Southern Cross will not always shine in the heavens exactly in its present form, for the four stars of which it consists move with unequal veloc- ity in different paths. How many thousand years will elapse before its total dissolution can not be calculated. In the re- lations of space and the duration of time, no absolute idea can be attached to the terms great and small. In order to comprehend under one general point of view the changes that take place in the heavens, and all the mod- ifications which in the course of centuries occur in the phys- iognomic character of the vault of heaven, or in the aspect of the firmament from any particular spot, we must reckon as the active causes of this change: (1), the precession of the equinoxes and the mutation of the earth's axis, by the combined operation of which new stars appear above the horizon, and others become invisible ; (2), the periodical and non-periodical variations in the brightness of many of the fixed stars ; (3), the sudden appearance of new stars, of which a few have continued to shine in the heavens ; (4), the revolution of telescopic double stars round a common center of gravity. Among these so-called fixed stars, which change slowly and unequally both in the intensity of their light and in their position, twenty principal planets move in a more rapid course, five of them being accompanied by twenty satellites. Besides the innumerable, but undoubt- edly rotatory fixed stars, forty moving planetary bodies have up to this time (October, 1850) been discovered. In the time of Copernicus and of Tycho Brahe, the great improver of the science of observation, only seven were known. Near- ly two hundred comets, five of which have short periods of revolution and are interior, or, in other words, are inclosed within those of the principal planets, still remain to be men- tioned in our list of planetary bodies. Next to the actual planets and the new cosmical bodies which shine forth sud- denly as stars of the first magnitude, the comets, when, dur- ing their usually brief appearance they are visible to the na ked eye, contribute the most vivid animation to the rich^'c- ture I had almost said the impressive landscape of the starry heavens. The knowledge of the proper motion of the fixed stars is closely connected historically with the progress of the sci- 184 COSMOS. ence of observation through the improvement of instruments and methods. The discovery of this motion was first ren- dered practicable when the telescope was combined with graduated instruments ; when, from the accuracy of within a minute of an arc (which after much pains Tycho Brahe first succeeded in giving to his observations on the island of Hven), astronomers gradually advanced to the accuracy of a second and the parts of a second ; and when it became possible to compare with one another results separated by a long series of years. Such a comparison was made by Hal- ley with respect to the positions of Sirius, Arcturus, and Al- debaran, as determined by Ptolemy in his Hipparchian cat- alogue, 1844 years before. By this comparison he consid- ered himself justified (1717) in announcing the fact of a proper motion in the three above-named fixed stars.* The high and well-merited attention which, long subsequent even to the observations of Flamstead and Bradley, was paid to the table of right ascensions contained in the Triduum of Romer, stimulated Tobias Mayer (1756), Maskelyne (1770), and P;azzi (1800) to compare Homer's observations with more recent ones.f The proper motion of the stars was in some degree recognized as a general fact, even in the mid die of the last century ; but for the more precise and numer- ical determination of this class of phenomena we are in- debted to the great work of William Herschel in 1783, found- ed on the observations of Flamstead, $ and still more to Bes- sel and Argelander's successful comparison of Bradley's "Po- sitions of the Stars for 1755" with recent catalogues. The discovery of the proper motion of the fixed stars has proved of so much the greater importance to physical astron- omy, as it has led to a knowledge of the motion of our own solar system through the star-filled realms of space, and, in- deed, to an accurate knowledge of the direction of this mo- tion. We should never have become acquainted with this fact if the proper progressive motion of the fixed stars were so small as to elude all our measurements. The zealous at- tempts to investigate this motion, both in its quantity and its direction, to determine the parallax of the fixed stars, and * Halley, in the Philos. Transact, for 1717-1719, vol. xxx.. p. 736. The essay, however, referred solely to variations in latitude. Jacques Cassini was the first to add variations in longitude. (Arago, ii the An- Huairepour 1842, p. 387.) t Delambre, Hist, de V Aslron. Moderne, t. ii., p. 658. Als , f& de VAstron. au IMme Slide, p. 448. t Philos. Transact., vol. Ixxiii., p. 138. PROPER MOTION OP THE STARS. 185 their distances, have, by leading to the improvement and perfection of arc-graduation and optical instruments in con- nection with micrometric appliances, contributed more than any thing else to raise the science of observation to the height which, by the ingenious employment of great merid- ian-circles, refractors, and heliometers, it has attained, espe- cially since the year 1830. The quantity of the measured proper motions of the stars varies, as we intimated at the commencement of the present section, from the twentieth part of a second almost to eight seconds. The more luminous stars have in general a slower motion than stars from the fifth to the sixth and seventh mag- nitudes.* Seven stars have revealed an unusually great motion, namely : Arcturus, first magnitude (2"- 25) ; a Cen- tauri, first magnitude (3 //- 58) ;t ft Cassiopeia?, sixth magni- tude (3"-74) ; the double star, 6 Eridani, 5'4 magnitude (4"-08) ; the double star 61 Cygni, 5'6 magnitude (5"'123), discovered by Bessel in 1812, by means of a comparison with Bradley 's observations ; a star in the confines of the Canes Venatici,J and the Great Bear, No. 1830 of the catalogue of the circumpolar stars by Groombridge, seventh magnitude (according to Argelander, 6"-974) ; e Indi (7"'74, according to D'Ariest) ; 2151 Puppis, sixth magnitude (7"-&7l). The uilihrn-oiicalll mean of the several proper motions of the fixed stars in all the zones into which the sidereal sphere has been divided by Madler would scarcely exceed 0"'102. An important inquiry into the " Variability of the proper motions of Procyon and Sirius," in the year 1844, a short * Bessel, in the Jahrbuch wn Schumacher fur 1839, s. 38. Arago Annuaire pour 1842, p. 389. t a Centauri, see Henderson and Maclear, in the Memoirs of the Astron. Soc., vol. xi., p. 61 ; and Piazzi Smyth, in the Edinburgh Transact., vol. xvi., p. 447. The proper motion of Arcturus, 2"-25 (Daily, in the same Memoirs, vol. v., p. 165), considered as that of a very bright star, may be called very large in comparison with Aldeba ran, 0"-185 (Madler, CentraUonne, a. 11), and o Lyra, 0"-400. Among the stars of the first magnitude, a Centauri, with its great proper motion of 3"-58, firms a very remarkable exception. The proper motion of the binary system of Cygnus amounts, according to Bessel (Schum Astr. Nochr., bd. xvi., s. 93), to 5"-123. { Schumacher's Astr. Nochr., No. 455. $ Op. cit., No. 618, s. 276. D'Arest founds this result on comparisons of LacaiDe (1750) with Brisbane (1825), and of Brisbane with Taylor (1835). The star 2151, Puppis, has a proper motion of 7"-871, and ia of the sixth magnitude. (Maclear, in Madler's Unlert. uber die Fix- ttern-Sytteme, th. ii., s. 5.) || Schum., Aslr Nochr., No. 661, s. 201 186 COSMOS. time, therefore, before the beginning of his last and painful illness, led Bessel, the greatest astronomer of our time, to the conviction " that stars whose variable motion becomes appar- ent by means of the most perfect instruments, are parts of systems confined to very limited spaces in proportion to their great distances from one another." This belief in the exist- ence of double stars, one of which is devoid of light, was so firmly fixed in Bessel's mind, as my long correspondence with him testifies, that it excited the most universal attention, partly on his account, and partly from the great interest which independently attaches itself to every enlargement of our knowledge of the physical constitution of the sidereal heavens. " The attracting body," this celebrated observer remarked, " must be very near either to the fixed star which reveals the observed change of position, or to the sun. As, however, the presence of no attracting body of considerable mass at a very small distance from the sun has yet been per- ceived in the motions of our own planetary system, we are brought back to the supposition of its very small distance from a star, as the only tenable explanation of that change in the proper motion which, in the course of a century, be- comes appreciable."* In a letter (dated July, 1844) in an- swer to one in which I had jocularly expressed my anxiety regarding the spectral world of dark stars, he writes : "At all events, I continue in the belief that Procyon and Sirius are true double stars, consisting of a visible and an invisible star. No reason exists for considering luminosity an essen- tial property of these bodies. The fact that numberless stars are visible is evidently no proof against the existence of an equally incalculable number of invisible ones. The physical difficulty of a change in the proper motion is satisfactorily set aside by the hypothesis of dark stars. No blame attaches to the simple supposition that the change of velocity only takes place in consequence of the action of a force, and that forces act in obedience to the Newtonian laws." A year after Bessel's death, Fuss, at Struve's suggestion, renewed the investigation of the anomalies of Procyon and Sirius, partly with new observations with Ertel's meridian- telescope at Pulkowa, and partly with reductions of, and com- parisons with, earlier observations. The result, in the opin- ion of Struve and Fuss,t proved adverse to Bessel's assertion. * Schuin., Attr. Nachr., Nos. 514-516. t Struve, Etudes cCAstr. Stellaire, Texte, p. 47, Notes, p. 26, and 51- 57 ; Sir John Herschel, Outl., $ 859 and 860. PROPER MOTION OP THE STARS. 187 A. laborious investigation which Peters has now completed at Konigsberg, on the other hand, justifies it ; as does also a similar one advanced by Schubert, the calculator for the North American Nautical Almanac. The belief in the existence of non-luminous stars was dif- fused even among the ancient Greeks, and especially in the earliest ages of Christianity. It was assumed that among the fiery stars which are nourished by the celestial vapcrs, there revolve certain other earth-like bodies, which, however, remain invisible to us."* The total extinction of new stars, especially of those so carefully observed by Tycho Brahe and Kepler in Cassiopeia and Ophiuchus, appears to corroborate this opinion. Since it was at the time conjectured that the first of these stars had already twice appeared, and that, too, at intervals of nearly 300 years, the idea of annihilation and total extinction naturally gained little or no credit. The immortal author of the Mecanique Celeste bases his convic- tion of the existence of non-luminous masses in the universe on these same phenomena of 1572 and 1604 : " These stars, that have become invisible after having surpassed the brill- iancy of Jupiter, have not changed their place during the time of their being visible." (The luminous process in them has simply ceased.) " There exist, therefore, in celestial space dark bodies of equal magnitudes, and probably in as great numbers as the stars, "t So also Madler, in his Un- tersuchungen uber die Fixstern-Systeme, says :J "A dark body might be a central body ; it might, like our own sun, be surrounded in its immediate neighborhood only by dark bodies like our planets. The motions of Sirius and Procyon, pointed out by Bessel, force us to the assumption that there are cases where luminous bodies form the satellites of dark masses. " It has been already remarked that the advocates of the emanation theory consider these masses as both invis- ible, and also as radiating light : invisible, since they are of such huge dimensions that the rays of light emitted by them (the molecules of light), being impeded by the force of at- traction, are unable to pass beyond a certain limit. H If, as * Origen, in Gronov. Thesaur., t. x., p. 271. t Laplace, Expos, du Syst. du Monde, 1824, p. 395. Lambert, in his Kosmologische Brief e, shows remarkable tendency to adopt the hypoth- esis of large dark bodies. \ Madler, Untersitch. tober die Fixstern-Systeme, th. ii. (1848), s. 3; and his Astronomy, B. 416. $ Vide note t, p. 186 II Vide supra, p. 88, and note ; Laplace, in Zach's Alia. Geogr Epkcm., bd. iv., B. 1 ; Madler, Astr., B. 393. 188 COSMOS. may well be assumed, there exist, in the regions of space, dark invisible bodies in which the process of light-producing vibration does not take place, these dark bodies can not fall within the sphere of our own planetary and cometary system, or, at all events, their mass can only be very small, since their existence is not revealed to us by any appreciable dis- turbances. The inquiry into the quality and direction of the motion of the fixed stars (both of the true motion proper to them, and also of their apparent motion, produced by the change in the place of observation, as the earth moves in its orbit), the determination of the distances of the fixed stars from the sun by ascertaining their parallax, and the conjecture as to the part in universal space toward which our planetary system is moving, are three problems in astronomy which, through the means of observation already successfully em- ployed in their partial solution, are closely connected with each other. Every improvement in the instruments and methods which have been used for the furtherance of any one of these difficult and complicated problems has been beneficial to the others. I prefer commencing with the par- allaxes and the determination of the distances of certain fixed stars, to complete that which especially relates to our pres ent knowledge of isolated fixed stars. As early as the beginning of the seventeenth century, Galileo had suggested the idea of measuring the " certainly very unequal distances of the fixed stars from the solar sys- tem," and, indeed, with great ingenuity, was the first to point out the means of discovering the parallax ; not by de- termining the star's distance from the zenith or the pole, "but by the careful comparison of one star with another very near it." He gives, in very general terms, an account of the mi- crometrical method which William Herschel (1781), Struve, and Bessel subsequently made use of. " Perche io non credo," says Galileo,* in his third dialogue (Giornata terza), " che tutte le stella siano sparse in una sferica superficie egual- tnente distanti da un centra; ma stimo, che le loro lonta- nanze da noi siano talmente varie, che alcune ve ne possano esser 2 e 3 volte piu remote di alcune altre ; talche quando si trovasse col telescopic qualche picciolissima Stella vici- * Opere di Galileo Galilei, vol. xii., Milano, 1811, p. 206. This re- markable passage, which expresses the possibility and the project of a measurement, was pointed out by Arago ; see his Annuaire pour 1842 p. 382. DISTANCES OF THE STARS. 189 nissima ad alcuna delle maggiori, e che pero quella fussc al- tissima, potrebbe accadere che qualche sensibil mutazione succedes&e tra di loro." " \\Ther6fore I do not believe." says Galileo, in liis third discourse (Giornata terza), <: that all the stars are scattered over a spherical superficies at equal dis- tances from a common center ; but I am of opinion that their distances from us are so various that some of them may be two or three times as remote as others, so that when some minute star is discovered by the telescope close to one of the larger, and yet the former is highest, it may be that some sensible change might take place among them." The in- troduction of the Copernican system imposed, as it were, the necessity of numerically determining, by means of measure- ment, the change of direction occasioned in the position of the fixed stars by the earth's semi-annual change of place in its course round the sun. Tycho Brahe's angular determina- tions, of which Kepler so successfully availed himself, do not manifest any perceptible change arising from parallax in the apparent positions of the fixed stars, although, as I have already stated, they are accurate to a minute of the arc. For this the Copernicans long consoled themselves with the reflection that the diameter of the earth's orbit (165 mill- ions of geographical miles) was insignificant when compared to the immense distance of the fixed stars. The hope of being able to determine the existence of par- allax must accordingly have been regarded as dependent on the perfection of optical and measuring instruments, and on the possibility of accurately measuring -very small angles. As long as such accuracy was only secure within a minute, the non-observance of parallax merely testified to the fact that the distance of the fixed stars must be more than 3438 times the earth's mean distance from the sun, or semi-di- ameter of its orbit.* This lower limit of distances rose to 206,265 semi-diameters when certainty to a second was at- tained in the observations of the great astronomer, James Bradley ; and in the brilliant period of Frauenhofer's instru- ments (by the direct measurement of about the tenth part of a second of arc), it rose still higher, to 2,062,648 mean distances of the earth. The labors and the ingeniously con- trived zenith apparatus of Newton's great cotemporary , Rob- ert Hooke (1669), did not lead to the desired end. Picard, Horrebow (who worked out Romer's rescued observations) * Bessel, in Schumacher's Jahrb. fur 1839, s. 511. 190 COSMOS and Flamstead believed that they had discovered parallaxes of several seconds, whereas they had confounded the proper motions of the stars with the true changes from parallax. On the other hand, the ingenious John Michell (Phil. Trans. 1767, vol. Ivii., p. 234-264) was of opinion that the paral- laxes of the nearest fixed stars must be less than 0"'02, and in that case could only "become perceptible when magnified 12,000 times." In consequence of the widely-diffused opin- ion, that the superior brilliancy of a star must invariably in- dicate a greater proximity, stars of the first magnitude, as, for instance, Vega, Aldebaran, Sirius, and Procyon, were, with little success, selected for observation by Calandrelli and the meritorious Piazzi (1805). These observations must be classed with Jhose which Brinkley published in Dublin (1815), and which, ten years afterward, were refuted by Pond, and especially by Airy. An accurate and satisfactory knowledge of parallaxes, founded on micrometric measure- ments, dates only from between the years 1832 and 1838 Although Peters,* in his valuable work on the distance? of the fixed stars (1846), estimates the number of parallaxes hitherto discovered at 33, we shall content ourselves with re ferring to 9, which deserve greater, although very different, degrees of confidence, and which we shall consider in the probable order of their determinations. The first place is due to the star 61 Cygni, which Bessel has rendered so celebrated. The astronomer of Kbnigsberg determined, in 1812, the large proper motion of this double star (below the sixth magnitude), but it was not until 1838 that, by means of the heliometer, he discovered its parallax. Between the months of August, 1812, and November, 1813, my friends Arago and Mathieu instituted a series of numer- ous observations for the purpose of finding the parallax of the star 61 Cygni, by measuring its distance from the zenith. In the course of their labors they arrived at the very correct conclusion that the parallax of this star was less than half a second.f So late as 1815 and 1816, Bessel, to use his own * Struve, Astr. Slell., p. 104. t Arago, in the Connaissance des Temps pour 1834, p. 281 : " Nous observames avec beaucoap de soin, M. Mathieu et moi, pendant le mois d'Aout, 1812. et pendant le mois de Novembre suivant, In hauteur angulaire de l'4toile audessus de 1'horizon de Paris. Cette hauteur, & la seconde <poque, ne surpasse la hauteur angulaire a la premiere quo de 0"-66. Une parallaxe absolue d'une seule seconde aurait n&sessairo- raent amend entre ces deux hauteurs une difference de l"-2. Nos ob- servations u'indiqueut douc pas quo le rayon de 1'orbite terreste, que DISTANCES OF THE STARS. 191 words, " had arrived at no available result."* The observa- tions taken from August, 1837, to October, 1838, by means of the great heliometer erected in 1829, first led him to the parallax of 0"-3483, which corresponds with a distance of 592,200 mean distances of the earth, and a period of 9| years for the transmission of its light. Peters confirmed this result in 1842 by finding 0"'3490, but subsequently changed Bessel's result into 0"-3744 by a correction for temperature. 1 The parallax of the finest double star of the southern hem- isphere (a Centauri) has been calculated at 0"'9128 by the observations of Henderson, at the Cape of Good Hope, in 39 millions de lieues soient vus de la 61" du Cygne sous un angle de plus d'une demi-seconde. Mais une base vue perpendiculairement sou- tend un angle d'une demi-seconde quand on est eloigne de 412 mille fois sa longueur. Done la 61 e du Cygne est au moins a une distance de la terre egale a 412 mille fois 39 millions de lieues." " During the month of August, 1812, and also during the following November, Mr. Mathieu and myself very carefully observed the altitude of the star above the horizon, at Paris. At the latter period its altitude only ex- ceeded that of the former by 0"-66. An absolute parallax of only a single second would necessarily have occasioned a difference of l"-2 between these heights. Our observations do not, therefore, show that a semi-diameter of the earth's orbit, or thirty-nine millions of leagues, are seeu from the star 61 of Cygnus, at an angle of more than 0"-5. But a base viewed perpendicularly subtends an angle of 0"'5 only when it is observed at a distance of 412,000 times its length. Therefore the star 6 1 Cygni is situated at a distance from our earth at least equal to four hundred and twelve thousand times thirty-nine millions of leagues." * Bessel, in Schum., Jahrb. 1839, s. 39-49, and in the Astr. Nachr., No. 366, gave the result 0"-3136 as a first approximation. His later and final result was 0"-3483. (Astr. Nachr., No. 402, in bd. xvii., s. 274.) Peters obtained by his own observations the following, almost identical, result of 0"-3490. (Struve, Astr. Slell, p. 99.) The alteration which, after Bessel's death, was made by Peters in Bessel's calculations of the angular measurements, obtained by the Konigsberg heliometer, arises from the circumstance that Bessel expressed his intention (Astr. Nachr., bd. xvii., s. 267) of investigating further the influence of temperature on the results exhibited by the heliometer. This purpose he had, in fact, partially fulfilled in the first volume of his Astronomische Untersuch- ungen, but he had not applied the corrections of temperature to the ob- servations of parallax. This application was made by the eminent as- tronomer Peters (Ergdnzungscheft zu den Astr. Nachr., 1849, s. 56), and the result obtained, owing to the corrections of temperature, was 0"-3744 instead of 0"-3483. t This result of 0"-3744 gives, according to Argelander, as the dis- tance of the double star 61 Cygni from the sun, 550,900 mean distances of the earth from the sun, or 45,576,000 miles, a distance which light traverses in 3177 mean days. To judge from the three consecutive statements of parallax given by Bessel, 0"-3136, 0"-3483, and 0"-3744, this celebrated double star has apparently come gradually nearer to us in light passages amounting respectively to 10, 9\, and 8 T 7 ff yeara 192 COSMOS. 1832, and by those of Maclear in 1839.* According to this statement, it is the nearest of all the fixed stars that have yet been measured, being three times nearer than 61 Cygni. The parallax of a Lyrse has long been the object of Struve's observations. The earlier observations (1836) gavet between 0"-07 and 0"-18 ; later ones gave 0"-2613, and a distance of 771,400 mean distances of the earth, with a period of twelve years for the transmission of its light. t But Peters found the distance of this brilliant star to be much greater, since he gives only 0"'103 as the parallax. This result contrasts with another star of the first magni- tude (a Centauri), and one of the sixth (61 Cygni). The parallax of the Polar Star has been fixed by Peters at 0"*106, after many comparisons of observations made be- tween the years 1818 and 1838 ; and this is the more sat- isfactory, as the same comparisons give the aberration at 20"-455. The parallax of Arcturus, according to Peters, is 0"-127. Riimker's earlier observations with the Hamburg meridian circle had made it considerably larger. The parallax of an- other star of the first magnitude, Capella, is still less, being, according to Peters, 0"'046. The star No. 1830 in Groombridge's Catalogue, which, according to Argelander, showed the largest proper motion of all the stars that hitherto have been observed in the firm- ament, has a parallax of 0"-226, according to 48 zenith distances which were taken with much accuracy by Peters during the years 1842 and 1843. Faye had believed it to be five times greater, 1"-08, and therefore greater than the parallax of a Centauri. || * Sir John Herschel, Outlines, p. 545 and 551. Madler (Astr., s. 425) gives in the case of a Centauri the parallax 0"-9213 instead of 0"-9128. t Struve Stell. compos. Mens. Microm., p. clxix.-clxxii. Airy makes the parallax of a Lyrse, which Peters had previously reduced to 0"-1, still lower; indeed, too small to be measurable by our present instru- ments. (Mem. of the Royal Astr. Soc., vol. x., p. 270.) J Struve, On the Micrometrical Admeasurements by the Great Refract" or at Dorpat (Oct., 1839), in Schum., Astr. Nachr., No. 396, s. 178. $ Peters, in Struve, Aitr. Stell., p. 100. II Id., p. 101. DISTANCES OF THE STARS. 19 j Filed Star. Parallax. ProtabEi Error. Name of Observer. a Centauri 0"- 913 0"-070 Henderson and Maclear. 61 Cygni 0"3744 0"-020 Bessel. Sirius . ....... 0"- 230 Henderson 1830, Groombridge. i Ursse Maj. 0"- 226 0"- 133 0"-141 0"-106 Peters. Peters. Arcturus 0"- 127 0"073 Peters. a Lyras 0"- 207 0"-038 Peters. Polaris 0"- 106 0"-012 Peters. Capella 0"- 046 0"-200 Peters. It does not in general follow from the results hitherto ob- tained that the brightest stars are likewise the nearest to us. Although the parallax of a Centauri is the greatest of all at present known, on the other hand, Vega Lyrte, Arcturus, and especially Capella, have parallaxes from three to eight times less than a star of the sixth magnitude in Cygnus. More- over, the two stars which after 2151 Puppis and e Indi show the most rapid proper motion, viz., the star just mentioned in the Swan (with an annual motion of 5"- 123), and No. 1830 of Groombridge, which in France is called Argelander's star (with an annual motion of 6"- 974), are three and four times more distant from the sun than a Centauri, which has a proper motion of 3" -58. Their volume, mass, intensity of light,* proper motion, and distance from our solar system, stand in various complicated relations to each other. Al- though, therefore, generally speaking, it may be probable that the brightest stars are nearest to us, still there may be cer- tain special very remote stars, whose photospheres and sur- faces, from the nature of their physical constitution, maintain a very intense luminous process. Stars which from their brilliancy we reckon to be of the first magnitude, may be further distant from us than others of the fourth, or even of the sixth magnitude. When we pass by degrees from the consideration of the great starry stratum of which our solar system is a part, to the particular subordinate systems of our planetary world, or to the still lower systems of Jupiter's and Saturn's moons, we perceive central bodies surrounded by masses in which the successive order of magnitude and of in- tensity of the reflected light does not seem to depend on dis- tance. The immediate connection subsisting between our still imperfect knowledge of parallaxes, and our knowledge of * On the proportion of the amount of proper motion to the proximity of the brighter stars, see Struve, Stell, compot. Mensuree Aficrom., p clxi?. VOL. Ill -I 194 COSMOS. the whole structural configuration of the universe, lends a pe- culiar charm to those investigations which relate to the dis- tances of the fixed stars. Human ingenuity has invented for this class of investiga- tions methods totally different from the usual ones, and which, being based on the velocity of light, deserve a brief mention in this place. Savary, whose early death proved such a loss to the physical sciences, had pointed out how the aberration of light in double stars might be used for determining the parallaxes. If, for instance, the plane of the orbit which the secondary star describes around the central body is not at right angles to the line of vision from the earth to the double star, but coincides nearly with this line of vision itself, then the secondary star in its orbit will likewise appear to describe nearly a straight line, and the points in that portion of its orbit which is turned toward the earth will all be nearer to the observer than the corresponding points of the second half, which is turned away from the earth. Such a division into two halves produces not a real, but an apparent unequal velocity, with which the satellite in its orbit recedes from, or approaches, the observer. If the semi-diameter of this orbit were so great that light would require several days or weeks to traverse it, then the time of tlio half revolution through its more remote side will prove to be longer than the time in the side turned toward the observer. The sum of the two unequal times will always be equal to the true pe- riodic time ; for the inequalities caused by the velocity of light reciprocally destroy each other. From these relations of du- ration, it is possible, according to Savary's ingenious method of changing days and parts of days into a standard of length (on the assumption that light traverses 14,356 millions of geographical miles in twenty-four hours), to arrive at the absolute magnitude of a semi-diameter of the earth's orbit , and the distance of the central body and its parallax may be then deduced from a simple determination of the angle under which the radius appears to the observer.* In the same way that the determination of the parallaxes instructs us as to the distances of a small number of the fixed stars, and as to the place which is to be assigned to them in the regions of space, so the knowledge of the measure and duration of proper motion, that is to say, of the changes which take place in the positions of self-luminous stars,, throws some * Savary, in the Connaissance des Temps pour 1830, p. 56-69, and p. 163-171; and Struve, ibid., D. clxiv. PROPER MOTION OF THE STARS. 195 light on two mutually dependent problems ; namely, the mo- tion of the solar system,* and the position of the center of gravity in the heaven of the fixed stars. That which can only be reduced in so very incomplete a manner to numerical relations, must for that very reason be ill calculated to throw any clear light on such causal connection. Of the two prob- lems just mentioned, the first alone (especially since Arge- lander's admirable investigation) admits of being solved with a certain degree of satisfactory precision ; the latter has been considered with much acuteness by Madler, but, according to the confession of this astronomer himself, t his attempted solution is, in consequence of the many mutually compensa- ting forces which enter into it, devoid " of any thing like evi- dence amounting to a complete and scientifically certain proof." After carefully allowing for all that is due to the preces- sion of the equinoxes, the nutation of the earth's axis, the aberration of light, and the change of parallax caused by the earth's revolution round the sun, the remaining annual mo- tion of the fixed stars comprises at once that which is the consequence of the translation in space of the whole sola? system, and that also which is the result of the actual propel motion -of the fixed stars. In Bradley's masterly labors on nutation, contained in his great treatise of the year 1748, we meet with the first hint of a translation of the solar system, and in a certain sense, also, with suggestions for the most desirable methods of observing it.J " For if our own solar system be conceived to change its place with respect to ab- solute space, this might, in process of time, occasion an ap- parent change in the angular distances of the fixed stars ; and in such a case, the places of the nearest stars being more affected than of those that are very remote, their relative positions might seem to alter, though the stars themselves were really immovable. And, on the other hand, if our own system be at rest, and any of the stars really in motion, this might likewise vary their apparent positions, and the more so, the nearer they are to us, or the swifter their motions are, or the more proper the direction of the motion is, to be ren- dered perceptible by us. Since, then, the relative places of Cosmot, vol. i., p. 146. t Madler, Astronomic, B. 414. t Arago, in his Annuaire pour 1842, p. 383, was the first to call at- tention to this remarkable passage of Bradley's. See, in the same An- nuaire, the section on the translation of the entire solar system, p. 389- 399. i96 COSMOS. the stars may be changed from such a variety of causes, con- sidering that amazing distance at which it is certain some of them are placed, it may require the observations of many ages to determine the laws of the apparent changes even of a single star ; much more difficult, therefore, it must be to settle the laws relating to all the most remarkable stars." After the time of Bradley, the mere possibility, and the greater or less probability, of the movement of the solar sys- tem, were in turn advanced in the writings of Tobias Mayer, Lambert, and Lalande ; but William Herschel had the great merit of being the first to verify the conj ecture by actual ob- servations (1783, 1805, and 1806). He found (what has been confirmed, and more precisely determined by many later and more accurate inquiries) that our solar system moves to- ward a point near to the constellation of Hercules, in R. A. 260 44', and N. Decl. 26 16' (reduced to the year 1800). Argelander, by a comparison of 3 1 9 stars, and with a refer- ence to Lundahl's investigations, found it for 1800: R.A. 257 54'-l, Decl. +28 49'-2 ; for 1850, R.A. 258 23'-5, Decl. +28 45'-6. Otto Struve (from 392 stars) made it to be for 1800 : R. A. 261 26'-9, Decl. +37 35'-5 ; for 1850, 261 52'-6, Decl. 37 33'-0. According to Gauss,* the point in question falls within a quadrangle, whose extremes are, R. A. 258 40', and Decl. 30 40' ; R. A. 258 42', Decl. +30 57'; R.A. 259 13', Decl. +31 9'; R.A. 260 4', Decl. +30 32'. It still remained to inquire what the result would be if the observations were directed only to those stars of the south- ern hemisphere which never appear above the horizon in Eu- rope. To this inquiry Galloway has devoted his especial attention. He has compared the very recent calculations (1830) of Johnson at St. Helena, and of Henderson at the Cape of Good Hope, with the earlier ones of Lacaille and Bradley (1750 and 1757). The result! for 1790 was R. A. 260 0', Decl. 34 23' ; therefore, for 1800 and 1850, 260 5', + 34 22', and 260 33', + 34 20'. This agreement with the results obtained from the northern stars is extremely sat- isfactory. If, then, the progressive motion of our solar system may be considered as determined within moderate limits, the * In a letter addressed to me. See Sebum., Astr. Nachr., No. 622, 6. 348. t Galloway, on the Motion of the Solar System, in the Phdot. Trant- act. for 1847, p. 98. MOTION OF THE STARS. 197 question naturally arises, Is the world of the fixed stars com- oosed merely of a number of neighboring partial systems di- vided into groups, or must we assume the existence of a uni- versal relation, a rotation of all self-luminous celestial bodies (suns) around one common center of gravity which is either filed u-ith matter or void ? We here, however, enter the domain of mere conjecture, to which, indeed, it is not im- possible to give a scientific form, but which, owing to the incompleteness of the materials of observation and analogy which are at present before us, can by no means lead to the degree of evidence attained by the other parts of astronomy The fact that we are ignorant of the proper motion of an in- finite number of very small stars from the tenth to the four- teenth magnitude, which appear to be scattered among the brighter ones, especially in the important part of the starry stratum to which we belong, the annuli of the Milky Way, is extremely prejudicial to the profound mathematical treat- ment of problems so difficult of solution. The contempla- tion of our own planetary sphere, whence we ascend, from the small partial systems of the moons of Jupiter, Saturn, and Uranus, to the higher and general solar system, has naturally led to the belief that the fixed stars might in a similar manner be divided into several individual groups, and separated by immense intervals of space, which again (in a higher relation of these systems one to another) may be subject to the overwhelming attractive force of a great central body (one sole sun of the whole universe).* The in- ference here advanced, and founded on the analogy of our own solar system, is, however, refuted by the facts hitherto observed. In the multiple stars, two or more self-luminous stars (suns) revolve, not round one another, but round an external and distant center of gravity. No doubt something similar takes place in our own planetary system, inasmuch as the planets do not properly move round the center of the solar body, but around the common center of gravity of all the masses in the system. But this common center of grav- ity falls, according to the relative positions of the great plan- ets Jupiter and Saturn, sometimes within the circumference of the sun's body, but oftener out of it.f The center of gravity, which in the case of the double stars is a void is * Toe value or worthlessness of such views has been discussed by Argelanderin his essay, " Ueber die eigene Bewegung des Sonnensysteott kergelettet avs der eigenen Bewegwng der Sterne, 1837, s. 39. t See Cosmot, vol. i., p. 145. (Madler, Attr., p. 400.) accordingly, in the solar system, at one time void, at another occupied by matter. All that has been advanced with re- gard to the existence of a dark central body in the center of gravity of double stars, or at least of one originally dark, but faintly illuminated by the borrowed light of the planets which revolve round it, belongs to the ever-enlarging realm of mythical hypotheses. It is a more important consideration, and one more de- serving of thorough investigation, that, on the supposition of a revolving movement, not only of the whole of our planet- ary system which changes its place, but also for the proper motion of the fixed ^stars at their various distances, the cen- ter of this revolving motion must be 90 distant* from the point toward which our solar system is moving. In this con- nection of ideas, the position of stars possessing a great or very small proper motion becomes of considerable moment. Argelander has examined, with his usual caution and acute- ness, the degree of probability with which we may seek for a general center of attraction for our starry stratum in the constellation of Perseus. t Madler, rejecting the hypothesis of the existence of a central body preponderating in mass, as the universal center of gravity, seeks the center of grav- ity in the Pleiades, in the very center of this group, in or nearf to the bright star TJ Tauri (Alcyone). The present is * Argelander, ibid., p. 42 ; Madler, CentraJsonne, s. 9, and Astr., s. 403. t Argelauder, ibid., p. 43 ; and in Sebum., Astr. Nachr., No. 566. Guided by no numerical investigations, but following the suggestions of fancy, Kant long ago fixed upon Sirius, and Lambert upon the nebula in the belt of Orion, as the central body of our starry stratum. (Struve, Astr. Stell., p. 17, No. 19.) t Madler, Astr., s. 380, 400, 407, and 414 ; in his Centraltonne, 1846, p. 44-47 ; in Untersiickungen uber die Fixstern-Systeme, th. ii., s. 183- 185. Alcyone is in R. A. 54 30', Decl. 23 36', for the year 1840. If Alcyone's parallax were really 0"-0065, its distance would be equal to 3H million semi-diameters of the earth's orbit, and thus it would be fifty times further distant from us than the distance of the double star 61 Cygni, according to Bessel's earliest calculation. The light which comes to the earth from the sun in 8' 18"-2, would in that case take 500 years to pass from Alcyone to the earth. The fancy of the Greeks de- lighted itself in wild visions of the height of falls. In Hesiod's Theo- gonia, v. 722-725, it is said, speaking of the fall of the Titans into Tar- tarus: " If a brazen anvil were to fall from heaven nine days and nine nights long, it would reach the earth on the tenth." This descent of the anvil in 777,600 seconds of time gives an equivalent in distance of 309,424 geographical miles (allowance being made, according to Galle's calculation, for the considerable diminution in the force of attraction at planetary distances), therefore 1 i times the distance of the moon from DOUBLE STARS. 199 not the place to discuss the probability or improbability* of such an hypothesis. Praise is, however, due to the eminent- ly active director of the Observatory at Dorpat for having, by his diligent labors, determined the positions and proper motions of more than 800 stars, and at the same time ex- cited investigations which, if they do not lead to the satis- factory solution of the great problem itself, are nevertheless calculated to throw light on kindred questions of physical astronomy. VI. MULTIPLE OR DOUBLE STARS. THEIR NUMBERS AND RECIPROCAL DISTANCES. PERIOD OF REVOLUTION OF TWO SUNS ROUND A COM- MON CENTER OF GRAVITY. WHEN, in contemplating the systems of the fixed stars, we descend from hypothetical, higher, and more general consid- erations to those of a special and restricted nature, we enter a domain more clearly determined, and better calculated for direct observation. Among the multiple stars, to which be- long the binary or double stars, several self-luminous cosmic- al bodies (suns) are connected by mutual attraction, which necessarily gives rise to motions in closed curved lines. Be- fore actual observation had established the fact of the revo- lution of the double stars.t such movements in closed curves were only known to exist in our own planetary solar system. On this apparent analogy inferences were hastily drawn, which for a long time gave rise to many errors. As the term " double stars" was indiscriminately applied to every the earth. But, according to the Iliad, i., v. 592, Hephaestus fell down to Lemnos in one day, "when but a little breath was still in him." The length of the chain hanging down from Olympus to the earth, by which all the gods were challenged to try and pull down Jupiter (Il- iad, viii., v. 18), is not given. The image is not intended to convey an idea of the height of heaven, but of Jupiter's strength and omnipo- tence. * Compare the doubts of Peters, in Schum., Astr. Nachr., 1849, s. 661, and Sir John Herschel, in the Outl. of Astr., p. 589 : " In the pres- ent defective state of our knowledge respecting the proper motion of the smaller stars, we can not but regard all attempts of the kind as to a certain extent premature, though by no means to be discouraged as forerunners of something more decisive." t Compare Cosmos, vol. i., p. 146-149. (Struve, Ueber Dopplesternc *ach Dorpater Micnmeter-Messungen von 1824 bis 1837, s. 11.) 200 COSMOS. pair of stars, the close proximity of which precluded their separation by the naked eye (as in the case of Castor, a Lyrse, (3 Orionis, and a Centauri), this designation naturally comprised two classes of multiple stars : firstly, those which, from their incidental position in reference to the observer, appear in close proximity, though in reality widely distant and belonging to totally different strata ; and, secondly, those which, from their actual proximity, are mutually dependent upon each other in mutual attraction and reciprocal action, and thus constitute a particular, isolated, sidereal system. The former have long been called optically, the latter phys- ically, double stars. By reason of their great distance, and the slowness of their elliptical motion, many of the latter are frequently confounded with the former. As an illustration of this fact, Alcor (a star which had engaged the attention of many of the Arabian astronomers, because, when the air is very clear, and the organs of vision peculiarly sharp, this small star is visible to the naked eye together with in the tail of Ursa Major) forms, in the fullest sense of the term, one of these optical combinations, without any closer physical con- nection.* In sections II. and III. I have already treated of the difficulty of separating by the naked eye adjacent stars, with the very unequal intensity of light, of the influence of the higher brilliancy and the star's tails, as well as of the organic defects which produce indistinct vision. Galileo, without making the double stars an especial ob- ject of his telescopic observations (to which his low magni- fying powers would have proved a serious obstacle), men- tions (in a famous passage of the Giornata terza of his Dis- courses, which has already been pointed out by Arago) the use which astronomers might make of optically double stars (quando si trovasse nel telescopic qualche picciolissima stella vicinissima ad alcuna delle maggiori) for determining the parallax of the fixed stars, t As late as the middle of the last century, scarcely twenty double stars were set down in the stellar catalogues, if we exclude all those at a greater * Vide tupra. As a remarkable instance of acuteness of vision, we may further mention that MSstlin, Kepler's teacher, discovered with the naked eye fourteen, and some of the ancients nine, of the stars in the Pleiades. (Madler, Untersuch. fiber die Fixslern-Systeme, th. ii., s. 36.) t Vide supra. Dr. Gregory, of Edinburgh, also, in 1675 (consequent- ly thirty-three years after Galileo's decease), recommended the samo parallactic method. See Thomas Birch, Hist, of the Royal Soe., vol. iii., 1757, p. 225. Bradley (1748) alludes to this method at the conclu- sion of his celebrated treatise on Nutation. DOUBLE STARS. 201 distance from each other than 32" ; at present, a hundred yeais later (thanks chiefly to the great labors of Sir Will- iam Herschel, Sir John Herschel, and Struve), about 60UO have been discovered in the two hemispheres. To the ear- liest described double stars* belong Ursae maj. (7th Sep- tember, 1700, by Gottfried Kirch), a Centauri (1709, by Feu- illee), y Virginis (1718), a Geminorum (1719), 61 Cygni (1753) (which, with the two preceding, was observed by Bradley, both in relation to distance and angle of direction), p Ophiuclii and Cancri. The number of the double stars recorded has gradually increased from the time of Flamstead, who employed a micrometer, down to the star-catalogue of Tobias Mayer, which appeared in 1756. Two acutely spec- ulative thinkers, endowed with great powers of combination, Lambert (Photometria, 1760 ; Kosmologische Briefe uber die Einrichtung des Weltbaues, 1761) and John Michell, 1767, though they did not themselves observe double stars, were the first to diffuse correct views upon the relations of their attraction in partial binary systems. Lambert, like Kepler, hazarded the conjecture that the remote suns (fixed stars) are, like our own sun, surrounded with dark bodies, planets, and comets ; but of the fixed stars proximate to each other,! he believed, however much, on the other hand, he may appear inclined to admit the existence of dark central bodies, " that within a not very long period they completed a revolution round their common center of gravity." Michell,$ who was not acquainted with the ideas of Kant and Lam- bert, was the first who applied the calculus of probabilities to small groups of stars, which he did with great ingenuity, especially to multiple stars, both binary and quaternary. He showed that it was 500,000 chances to 1 that the colloca- tion of the six principal stars in the Pleiades did not result from accident, but that, on the contrary, they owed their grouping to some internal and reciprocal relation. He was so thoroughly convinced of the existence of luminous stars revolving round each other, that he ingeniously proposed to employ these partial star-systems to the solution of certain astronomical problems. $ * Miicller, Astr., s. 477. t Arago, in the Annuairepour 1842, p. 400. t An Inquiry into the probable parallax and magnitude of the fixed stars, from the quantity of light which they afford us, and the particu- lar circumstances of their situation, by the Rev. John Michell; in the Pkilos. Transact., vol. Ivii., p. 234-261. $ John Michell, ibid., p. 238. " If it should hereafter be found that any of the stars have others revolving about them (for 110 satellites bv 202 COSMOS. Christian Mayer, the Manheim astronomer, has the great merit of having first (1778) made the fixed stars a special object of research, by the sure method of actual observations. The unfortunate choice of the term satellites of the fixed stars, and the relations which he supposed to exist among the stars between 2 30' and 2 55' distant from Arcturus, exposed him to bitter attacks from his cotemporaries, and among these to the censure of the eminent mathematician, Nicolaus Fuss. That dark planetary bodies should become visible by reflected light, at such an immense distance, was certainly improbable. No value was set upon the results of his carefully-conducted observations, because his theory of the phenomena was rejected ; and yet Christian Mayer, in his rejoinder to the attack of Father Maximilian Hell, Di- rector of the Imperial Observatory at Vienna, expressly as- serts " that the smaller stars, which are so near the larger, are either illuminated, naturally dark planets, or that both of these cosmical bodies the principal star and its compan- ion are self-luminous suns revolving round each other." a borrowed light could possibly be visible"), we should then have the means of discovering " Throughout the whole discussion he de- nies that one of the two revolving stars can be a dark planet shining with a reflected light, because both of them, notwithstanding their dis- tance, are visible to us. Calling the larger of the two the " central star," he compares the density of both with the density of our sun, and merely uses the word " satellite" relatively to the idea of revolution or of reciprocal motion ; he speaks of the " greatest apparent elongation of those stars that revolve about others as satellites." He further says, at p. 243 and 249 : " We may conclude with the highest probability (the odds against the contrary opinion being many million millions to one) that stars form a kind of system by mutual gravitation. It is high- ly probable in particular, and next to a certainty in general, that such double stars as appear to consist of two or more stars placed near to- gether are under the influence of some general law, such, perhaps, as gravity " (Consult also Arago, in the Annuaire pour 1834, p. 308, and Ann. 1842, p. 400.) No great reliance can be placed on the indi- vidual numerical results of the calculus of probabilities given by Michell, as the hypotheses that there are 230 stars ir the heavens which, in in- tensity of light, are equal to (3 Capricorn!, KJid 1500 equal to the six greater stars of the Pleiades, are manifestly incorrect. The ingenious cosmological treatise of John Michell ends with a very bold attempt to explain the scintillation of the fixed stars by a kind of " pulsation in material effluxes of light" an elucidation not more happy than that which Simon Marius, one of the discoverers of Jupiter's satellites (see Cosmos, vol. ii., p. 320) has given at the end of his Mundus Jovialis (1614). But Michell has the merit of having called attention to the fact (p. 263) that the scintillation of stars is always accompanied by a change of color. " Besides their brightness, there is in the scintillation of the fixed stars a change of color." ( Vide supra.) DOUBLE STARS. 203 The importance of Christian Mayer's labors has, long after his death, been thankfully and publicly acknowledged by Struve and Madler. In his two treatises, Vertheidigung neuer Beobachtungen von Fixstern-trabanten (1778), and Dissertatio de novis in Ccdo sidereo Phcenomenis (1779), eighty double stars are described as observed by him, of which sixty-seven are less than 32" distant from each other. Most of these were first discovered by Christian Mayer him- self, by means of the excellent eight-feet telescope of the Man heim Mural Gluadrant ; " many even now constitute very difficult objects of observation, which none but very power- ful instruments are capable of representing, such as p and 71 Herculis, 5 Lyrse, and GJ Piscium." Mayer, it is true (as was the practice long after his time), only measured dis- tances in right ascension and declination by meridian instru- ments, and pointed out, from his own observations, as well as from those of earlier astronomers, changes of position ; but from the numerical value of these, he omitted to deduct what (in particular cases) was due to the proper motion of the stars.* These feeble but praiseworthy beginnings were followed by Sir William Herschel's colossal work on the multiple stars, which comprises a period of more than twenty-five years ; for although Herschel's first catalogue of double stars was published four years after Christian Mayer's treatise on the same subject, yet the observations of the former go back as far as 1779 indeed, even to 1776, if we take into consider- ation the investigations on the trapezium in the great nebula of Orion. Almost all we at present know of the manifold formation of the double stars has its origin in Sir William Herschel's work. In the catalogues of 1782, 1783, and 1804, he has not only set down and determined the position and distance of 846 double stars,! for the most part first dis- covered by himself, but, what is far more important than any augmentation of number, he applied his sagacity and power of observation to all those points which have any bearing on their orbits, their conjectured periodic times, their brightness, contrasts of colors, and classification according to the amount * Struve, in the Recueil des Actes de la Stance publique de VAcad. Imp. des Sciences de St. Petersbourg, le 29 Dec., 1832, p. 48-50. Mad- ler, A*tr., s. 478. t Philos. Transact, for the Year 1782, p. 40-126; for 1783, p. 112- 124 ; for 1804, p. 87. Regarding the observations on which Sir Will- iam Herschel founded his views respecting the 846 double stars, see Madler, in Schumacher's Jahrbuchfur 1839, s. 59, and his Untertuchun- gen itler die Fixstern-Systeme, th. i., 18 17. s. 7. 204 COSMOS. of their mutual distances. Full of imagination, yet alwayg proceeding with great caution, it was not till the year 1794, while distinguishing between optically and physically double Btars, that he threw out his preliminary suggestions as to the nature of the relation of the larger star to its smaller com- panion. Nine years afterward, he first explained his views of the whole system of these phenomena, in the 93d volume of the Philosophical Transactions. The idea of partial star-systems, in which several suns revolve round a common center of gravity, was then firmly established. The stupen- dous influence of attractive forces, which in our solar system extends to Neptune, a distance 30 times that of the earth (or 2488 millions of geographical miles), and which com- pelled the great comet of 1680 to return in its orbit, at the distance of 28 of Neptune's semi-diameters (853 mean dis- tances of the earth, or 70,800 millions of geographical miles), is also manifested in the motion of the double star 61 Cygni, which, with a parallax of 0"-3744, is distant from the sun 18,240 semi-diameters of Neptune's orbit (i. e., 550,900 earth's mean distances, or 45,576,000 millions of geograph- ical miles). But although Sir William Herschel so clearly discerned the causes and general connection of the phenome- na, still, in the first few years of the nineteenth century, the angles of position derived from his own observations, owing to a want of due care in the use of the earlier catalogues, were confined to epochs too near together to admit of perfect certainty in determining the several numerical relations of the periodic times, or the elements of their orbits. Sir John Herschel himself alludes to the doubts regarding the accu- racy of the assigned periods of revolution of a Geminorum (334 years instead of 520, according to Madler),* of y Vir- ginis (708 instead of 169), and of y Leonis (1424 of Struve's great catalogue), a splendid golden and reddish-green double star (1200 years). After William Herschel, the elder Struve (from 1813 to 1842) and Sir John Herschel (from 1819 to 1838), availing themselves of the great improvements in astronomical in- struments, and especially in micrometrical applications, have, with praiseworthy diligence, laid the proper and special foun- * Madler, ibid., th. i., s. 255. For Castor we have two old observa- tions of Bradley, 1719 and 1759 (the former taken in conjunction with Pond, the latter with Maskelyne), and two of the elder Herschel, taken in the years 1779 and 1803. For the period of revolution of y Virginia, ee Madler, Fixslern-Syst., th. ii., s. 234-40, 1848. DOUBLE STARS 205 dation of this important branch of astronomy. In 1820, Struve published his first Dorpat Table of double stars, 796 in number. This was followed in 1824 by a second, con- taining 3112 double stars, down to the ninth magnitude, in distances under 32", of which only about one sixth had been before observed. To accomplish this work, nearly 120,000 fixed stars were examined by means of the great Fraun- hofer refractor. Struve's third table of multiple stars ap- peared in the year 1837, and forms the important work Stel- larum compositarum Mensurcs Micrometrices.* It contains 2787 double stars, several imperfectly observed objects being carefully excluded. Sir John Herschel's unwearied diligence, during his four years' residence in Feldhausen, at the Cape of Good Hope, which, by contributing to an accurate topographical knowl- edge of the southern hemisphere, constitutes an epoch in astronomy,t has been the means of enriching this numbei by the addition of more than 2100 double stars (which, with few exceptions, had never before been observed). All these African observations were taken by a twenty-feet reflecting telescope ; they were reduced for the year 1830, and are included in the six catalogues which contain 3346 double stars, and were transmitted by Sir John Herschel to the As- tronomical Society for the sixth and ninth parts of their val- uable Memoirs.^. In these European catalogues are laid down the 380 double stars which the above celebrated as- tronomer had observed in 1825, conjointly with Sir James South. We trace in this historical sketch the gradual advance made by the science of astronomy toward a thorough knowl- edge of partial, and especially of binary systems. The num- ber of double stars (those both optically and physically double) may at present be estimated with some certainty at about 6000, if we include in our calculation those observed by Bes- sel with the excellent Fraunhofer heliometer, by Argelan- der at Abo (1827-1835), by Encke and G-alle at Berlin * Struve, Mensuree Microm., p. 40 and 234-248. On the whole, 26414-146, . e., 2787 double stars have been observed. (Madler, in Sebum., Jakrb., 1839, s. 64.) t Sir John Herschel, Attron. Observ. at the Cape of Good Hope, p. 16.5-303. t Ibid., p. 167 and 242. $ Argelander, in order carefully to investigate their proper motion, examined a great number of fixed stars. See his essay, entitled "DLX. Stellarum fixarum positionet media, ineunte anno 1830, ex observ. Aboa habitit (Heltingfortia, 1825)." Madler (Astr., a. 625) estimates the 206 COSMOS. (1836 and 1839), by Preuss and Otto Struve in Pulkowa (since the catalogue of 1837), by Madler in Dorpat, and by Mitchell in Cincinnati (Ohio), with a seventeen-feet Munich refractor. How many of these 6000 stars, which appear to the naked eye as if close together, may stand in an imme- diate relation of attraction to each other, forming systems of their own, and revolving in closed orbits or, in other words, how many are so-called physical (revolving} double stars is an important problem, and difficult of solution. More re- volving companions are gradually but constantly being dis- covered. Extreme slowness of motion, or the direction of the plane of the orbit as presented to the eye, being such as to render the position of the revolving star unfavorable for ob- servation, may long cause us to class physically double stars among those which are only optically so ; that is, stars of which the proximity is merely apparent. But a distinctly- ascertained appreciable motion is not the only criterion. The perfectly uniform motion in the realms of space (i. e., a com- mon progressive movement, like that of our solar system, in- cluding the earth and moon, Jupiter, Saturn, Uranus, and Neptune, with their satellites), which in the case of a con- siderable number of multiple stars has been proved by Arge- lander and Bessel, bears evidence that the principal stars and their companions stand in undoubted relation to each other in separate partial systems. Madler has made the in- teresting remark, that whereas, previous to 1836, among 2640 double stars that had been catalogued, there were only 58 in which a difference of position had been observed with certainty, and 105 in which it might be regarded as more or less probable ; at present, the proportion of physically double stars to optically double stars has changed so greatly in favor of the former, that among the 6000 double stars, according to a table published in 1849, 650 are known in which a change of relative position can be incontestably proved.* The earliest comparison gave one sixteenth, the number of multiple stars in the northern hemisphere, discovered at Pulkowa since 1837, at not less than 600. * The number of fixed stars in which proper motion has been un- doubtedly discovered (though it may be conjectured in the case of all) is slightly greater than the number of double stars in which change of position has been observed. (Madler, Astr., s. 394, 490, and 520-540.) Results obtained by the application of the Calculus of Probabilities, ac- cording as the several reciprocal distances of the double stars are be- tween 0" and 1", 2" and 8 , or 16" and 32", are given by Struve, in his Mens. Microm., p. xciv. Distances less than 0"-8 have been taken, and DOUBLE STARS. 207 most recent gives one ninth, as the proportion of the cosmic- al bodies which, by an observed motion both of the primary star and the companion, are manifestly proved to be phys- ically double stars. Very little has as yet been numerically determined re garding the relative distribution of the binary star-systems throughout space, not only in the celestial regions, but even on the apparent vault of heaven. In the northern hemi- sphere, the double stars most frequently occur in the direc- tion of certain constellations (Andromeda, Bootes, the Great Bear, the Lynx, and Orion). For the southern hemisphere Sir John Herschel has obtained the unexpected result, "that in the extra-tropical regions of this hemisphere the number of multiple stars is far smaller than that in the correspond- ing portion of the northern." And yet these beautiful south- ern regions have been explored, under the most favorable circumstances, by one of the most experienced of observers, with a brilliant twenty-feet reflecting telescope, which sep- arated stars of the eighth magnitude at distances even of three quarters of a second.* The frequent occurrence of contrasted colors constitutes an extremely remarkable peculiarity of multiple stars. Struve, in his great workf published in 1837, gave the following re- sults with regard to the colors presented by six hundred of the brighter double stars. In 375 of these, the color of both principal star and companion was the same and equally in- tense. In 101, a mere difference of intensity could be dis- cerned. The stars with perfectly different colors were 120 in number, or one fifth of the whole ; and in the remaining four fifths the principal and companion stars were uniform in color. In nearly one half of these six hundred, the princi- pal star and its companion were white. Among those of different colors, combinations of yellow with blue (as in i Cancri), and of orange with green (as in the ternary star y Andromedae),t are of frequent occurrence. Arago was the first to call attention to the fact that the diversity of color in the binary systems principally, or at least, in very many cases, has reference to the complementary col- experiments with very complicated systems have confirmed the astron- omer in the hope that these estimates are mostly correct within 0"'l (Strnve, iiber Doppelsterne nach Dorpater Beob., B. 29.) * Sir John Herschel, Observations al tht Cape, p. 166. t Struve, Mensurte Microm., p. Ixxvii. to Ixxxiv. t Sir John Herschel, Outlines of Aslr., p. 579. 208 COSMOS. ors the subjective colors, which, when united, form white.* It is a well known optical phenomenon that a faint white light appears green when a strong red light is brought near it, and that a white light becomes blue when the stronger surrounding light is yellowish. Arago, however, with his usual caution, has reminded us of the fact that even though the green or blue tint of the companion star is sometimes the result of contrast, still, on the whole, it is impossible to deny the actual existence of green or blue stars. t There are in- * Two glasses, which exhibit complementary colors when placed one upon the other, are used to exhibit white images of the sun. During my long residence at the Observatory at Paris, my friend very success- fully availed himself of this contrivance, instead of using shade glasses to observe the sun's disk. The colors to be chosen are red and green, yellow and blue, or green and violet. " Lorsqu'une lumiere forte ue trouve aupres d'une lumiere faible, la derniere prend la teinte comple- mentaire de la premiere. C'est la le contrast*; mais comme le rouge n'est presque jamais pur, on peut tout aussi bien dire que le rouge est complementaire du bleu. Les couleurs voisines du spectre solaire so substitueut." " When a strong light is brought into contact with a feeble one, the latter assumes the complementary color of the former. This is the effect of contrast ; but as red is scarcely ever pure, it may as correctly be said that red is the complementary of blue : the colors nearest to the solar spectrum reciprocally change." (Arago, MS. of 1847.) t Arago, in the Connaisance des Temps pour Van 1 828, p. 299-300 ; and in the Annuaire pour 1834, p. 246-250; pour 1842, p. 347-350: " Les exceptions que je cite, prouvent que j'avais bien raison en 1825 de n'introduire la notion physique du contraste dans la question des etoi- les doubles qu'avec la plus grande reserve. Le bleu est la couleur re- elle de certaines etoiles. II resulte des observations recueillies jusqu'ici que le firmament est non seulement parseme de soleils rouges etjaunes, comme le savaient les auciens, ma isencore de soleils Ileus et verts. C'est au terns et a des observations futures & nous apprendre si les etoi- les vertes et bleues ne sont pas des soleils deja en voie de decroissance ; si les differentes nuances de ces astres n'indiquent pas que la combustion s'y ope re 4 differens degres ; si la teinte, avec exces de rayons les plus refrangibles, que presente souvent la petite etoile, ne tiendrait pas a la force absorbante d'une atmosphere que developperait Faction de *' etoile, ordinairement beaucoup plus brillante, qu'elle accompagne." " The exceptions I have named proved that in 1825 I was quite right in the cautious reservations with which I introduced the physical notion of contrast in connection with double stars. Blue is the real color of cer tain stars. The result of the observations hitherto made proves that the firmament is studded not only with red and yellow suns (as was known long ago to the ancients), but also with blue and green suns. Time and future observations must determine whether red and blue stars are not suns, the brightness of which is already on the wane; whether the varied appearances of these orbs do not indicate the de- gree of combustion at work within them ; whether the color and the excess of the most refrangible rays often presented by the smaller of two stars be not owing to the absorbing force of an atmosphere devel DOUBLE STARS. 209 stances in which a brilliant white star (1527 Leonis, 1768 Can. ven.) is accompanied by a small blue star ; others, where in a double star (8 Serp.) both the principal and its companion are blue.* In order to determine whether the contrast of colors is merely subjective, he proposes (when the distance allows) to cover the principal star in the telescope by a thread or diaphragm. Commonly it is only the smaller star that is blue : this, however, is not the case in the double star 23 Orionis (696 in Struve's Catalogue, p. Ixxx.), where the prin- cipal star is bluish, and the companion pure white. If, in the multiple stars, the differently colored suns are frequently surrounded by planets invisible to us, the latter, being differ- ently illuminated, must have their white, blue, red, and green days.f As the periodical variability^, of the stars is, as we have already pointed out, by no means necessarily connected with their red or reddish color, so also coloring in general, or a contrasting difference of the tones of color between the prin- cipal star and its companion, is far from being peculiar to the multiple stars. Circumstances which we find to be fre- quent are not, on that account, necessary conditions of the phenomena, whether relating to a periodical change of light, or to the revolution in partial systems round a common cen- ter of gravity. A careful examination of the bright double stars (and color can be determined even in those of the ninth magnitude) teaches that, besides white, all the colors of the solar spectrum are to be found in the double stars, but that the principal star, whenever it is not white, approximates in general to the red extreme (that of the least refrangible rays), but the companion to the violet extreme (the limit of the most refrangible rays). The reddish stars are twice as fre- quent as the blue and bluish ; the white are about 21 times as numerous as the red and reddish. It is moreover remark- able that a great difference of color is usually associated with oped by the action of the accompanying star, -which is generally much the more brilliant of the two." (Arago, in the Annuairepour 1834, p. 295-301.) * Struve, Ueber Doppdtterne nach Dorpater Beobachtungen, 1837, a. 33-36, and Mensurce Microm., p. Ixxxiii., enumerates sixty-three double Btarsin which both the principal and companion are blue or bluish, and in which, therefore, the colcrs can not be the effect of contrast. When \ve are forced to compare together the colors of double stars, as report- ed by several astronomers, it is particularly striking to observe how fre- quently the companion of a red or orange-colored star is reported by some observers as blue, and by others as green. t Arago, Annuain pour 1834, p. 302. t Vide tupra, p. 130-136. 210 COSMOS. a corresponding difference in brightness. In two cases in Bootis and y Leonis which, from their great brightness, can easily be measured by powerful telescopes, even in the daytime, the former consists of two white stars of the third and fourth magnitudes, and the latter of a principal star of the second, and of a companion of the 3 -5th magnitude. This is usually called the brightest double star of the north- ern hemisphere, whereas a Centauri* and a Crucis, in the southern hemisphere, surpass all the other double stars in brilliancy. As in Bootis, so also in a Centauri and y Leonis, we observe the rare combination of two great stars with only a slightly different intensity of light. No unanimity of opinion yet prevails respecting the vari- able brightness in multiple stars, and especially in that of companions. We have already t several times made mention of the somewhat irregular variability of luster in the orange- colored principal star in a Herculis. Moreover, the fluctua- tion in the brightness of the nearly equal yellowish stars (of the third magnitude) constituting the double star y Virginis and Anon. 2718, observed by Struve (1831-1833), probably indicates a very slow rotation of both suns upon their axes.J Whether any actual change of color has ever taken place in double stars (as, for instance, in y Leonis and y Delphini) ; whether their white light becomes colored, and, on the other hand, whether the colored light of the isolated Sirius has be- come white, still remain undecided questions. Where the disputed differences refer only to faint tones of color, we should take into consideration the power of vision of the observer, and, if refractors have not been employed, the frequently red- dening influence of the metallic speculum. Among the multiple systems we may cite as ternaries, Librae, Cancri, 12 Lyncis, 11 Monoc. ; as quaternaries, 102 and 2681 of Struve's Catalogue, a Andromedae, e Lyrae : in 6 Orionis, the famous trapezium of the greater nebula of * " This superb double star (a Cent.) is beyond all comparison the most striking object of the kind in the heavens, and consists of two in- dividuals, both of a high ruddy or orange color, though that of the smaller is of a somewhat more somber and brownish cast." (Sir John Herschel, Observations at the Cape of Good Hope, p. 300.) And, ac- cording to the important observations taken by Captain Jacob, of the Bombay Engineers, between the years 1846 and 1848, the principal star is estimated of the first magnitude, and the satellite from the 2'5th to the third magnitude. ( Transact, of tht Royal Soc. of Edinb., vol. X'vn 1849, p. 451.) t Videtupra, p. 165, 166, and note. t Struve, Ueber Doppeltt. nach Dorp Beob., s. 33. $ Ibid., s. 36 DOUBLE STARS. 211 Orion, we have a combination of six probably a system sub- ject to peculiar physical attraction, since the five smaller stars (6'3m. ; 7m.; 8m.; ll'Sm. ; and 12m.) follow the prop- er motion of the principal star, 4-7m. No change in their relative positions has yet been observed.* In the ternary combinations of Librae and Cancri, the periodical move- ment of the two companions has been recognized with great certainty. The latter system consists of three stars of the third magnitude, differing very little in brightness, and the nearer companion appears to have a motion ten times more rapid than the remoter one. Tho number of the double stars, the elements of whose orbits it has been found possible to determine, is at present stated at from fourteen to sixteen. t Of these, Herculis has twice completed its orbit since the epoch of its first dis- covery, and during this period has twice (1802 and 1831) presented the phenomenon of the apparent occultation of one fixed star by another. For the earliest measurements of the orbits of double stars, we are indebted to the industry of Savary ( Ursae Maj.), Encke (70 Ophiuchi), and Sir John Herschel. These have been subsequently followed by Bes- sel, Struve, Madler, Hind, Smyth, and Captain Jacob. Sa- vary's and Encke's methods require four complete observa- tions, taken at sufficient intervals from each other. The shortest periods of revolution are thirty, forty- two, fifty-eight, and seventy-seven years ; consequently, intermediate be- tween the periods of Saturn and Uranus ; the longest that have been determined with any degree of certainty exceed five hundred years, that is to say, are nearly equal to three times the period of Le Verrier's Neptune. The eccentricity of the elliptical orbits of the double stars, according to the investigations hitherto made, is extremely considerable, re- sembling that of comets, increasing from 0'62 (a Coronae) up to 0'95 (a Centauri). The least eccentric interior comet- that of Faye has an eccentricity of 0-55, or less than that of the orbits of the two double stars just mentioned. Ac- cording to Midler's and Hind's calculations, i\ Coronas and Castor exhibit much less eccentricity, which in the former is 0-29, and in the latter 0-22 or 0-24. In these double stars the two suns describe ellipses which come very near to those of * Madler, Astr., s. 517. Sir John Herschel, Ovtl., p. 568. t Compare Madler, Untertuch. uber die Fixstcm-Systeme, th. i., s. 225-273 ; th. ii., s. 235-240 ; and his Astr., a. 541 Sir John HerscheL Outl., p. 573. 212 COSMOS. two of the smaller principal planets in our solar system, the eccentricity of the orhit of Pallas being 0-24, and that of Juno, 0-25. If, with Encke, we consider one of the two stars in a bi- nary system, the brighter, to be at rest, and on this supposi- tion refer to it the motion of the companion, then it follows from the observations hitherto made that the companion de- scribes round the principal star a conic section, of which the latter is the focus ; namely, an ellipse in which the radius vector of the revolving cosmical body passes over equal su- perficial areas in equal times. Accurate measurements of the angles of position and of distances, adapted to the determ- ination of orbits, have already shown, in a considerable num- ber of double stars, that the companion revolves round the principal star considered as stationary, impelled by the same gravitating forces which prevail in our own solar system. This firm conviction, which has only been thoroughly attain- ed within the last quarter of a century, marks a great epoch in the history of the development of higher cosmical knowl- edge. Cosmical bodies, to which long use has still preserved the name of fixed stars, although they are neither riveted to the vault of heaven nor motionless, have been observed to occult each other. The knowledge of the existence of partial systems of independent motion tends the more to en- large our view, by showing that these movements are them- selves subordinate to more general movements animating th regions of space. DOUBLE STARS. ELEMENTS OF THE ORBITS OF DOUBLE STARS. 213 _t Semi-Major Axis. Eccentricity. Period of Revolution in yean. Calculator. (1) f UrsseMaj.... 3"-857 0-4164 58-262 Savary, 1830. 3"-278 2"-295 0-3777 04037 60-720 61-300 John Herschel. Tables of 1849. Madler, 1847. (2) pOphiuchi 4"-328 04300 73-862 Encke, 1832. (3) fHerculis (4) Castor 1"-208 8"-086 04320 0-7582 3022 252-66 Madler, 1847 John Herschel 5"-692 0-2194 519-77 Tables of 1849. Madler, 1847. 6"-300 0-2405 632-27 Hind, 1849. (5) y Virginia 3"-580 3"-863 0-8795 0-8806 182-12 169-44 John Herschel. Tables of 1849. Madler, 1847. (6) oCentauri 15"-500 09500 7700 Captain Jacob, 1848. INDEX TO VOL. III. ACHROMATIC telescopes, 63. Adalbert, Prince, of Prussia, his observa- tions on the undulation of the stars, 59. Alcor, a star of the constellation Ursa Ma- jor, employed by the Persians as a test of vision, 49, 50, 200. Alcyone, one of the Pleiades, imagined the center of gravity of the solar sys- tem by Madler, 198. Alphonsine Tables, date of their construc- tion, 151. Anaxagoras of Clazomense, his theory of the world-arranging intelligence, 11 ; origin of the modern theories of rota- tory motion, 12. Andromeda's girdle, nebula in, 142. Arago, M., letters and communications of, to M. Humboldt, 46, 49, 67, 68, 73, 96, 207-209 ; on the effect of telescopes on the visibility of the stars, 69 ; on the velocity of light, 80, 84 ; on photometry, 92, 90 ; his cyanometer, 97. Aratus, a fragment of the work of Hip- parchas preserved in, 109. Archimedes, his " Arenarius," 30. Avcturus, true diameter of, 89. Argulander, his view of the number of the fixed stars, 105, 106 ; his additions to Bessel's Catalogue, 115 ; on period- ically variable stars, 166. ij Argus, changes in color and brilliancy of, 135, 178, 179. Aristotle, his distinct apprehension of the unity of nature, 13-15; his defective solution of the problem, 15; doubts the infinity of space, 29, 30 ; his idea of the generation of heat by the movement of the spheres, 124. aosy, the domain of the fixed stars, Astronomy, the observation of groups of fixed stars, the first step in, 118 ; very bright single stars, the first named, 89. Atmosphere, limits of the, 40, 41 ; effects of an untransparent, 104. Augustine, St., cosmical views of, 124. Autolycus of Pitane, era of, 89, 90. Auzout's object-glasses, 62. Bacon, Lord, the earliest views on the ve- locity of light found in his "Novum Organum," 79. Baily , Francis, his revision of De Lalande's Catalogue, 115. Bayer's lettering of the stars of any con- stellation not an evidence of their rel- ative brightness, 98. Berard, Captain, on the change of color of the star y Crucis, 135. Berlin Academy, star maps of the, 11B. Bessel, on repulsive force, 34, 35 ; his star maps have been the principal means of the recognition of seven new planets, 116 ; calculation of the orbits of double stars by, 211. Binary stars, 199. Blue stars, 136 ; less frequent than red, 209. Blue and green suns, the probable cause of their color, 208. Bond, of the Cambridge Observatory, United States, his resolution of the neb- ula in Andromeda's girdle into small stars, 142. Brewster, Sir David, on the dark lines of the prismatic spectra, 44. British Association, their edition of La- lande's Catalogue, 115. Bruno, Giordano, his cosmical views, 17 ; his martyrdom, 17. Busch, Dr., his estimate of the velocity of light incorrect, 82. Catalogues, astronomical, their great im- portance, 113, 114 ; future discoveries of planetary bodies mainly dependent on their completeness, 114 ; list of, 114, 115 ; Halley's, Flamstead's, and others, 114 ; Lalande's, Harding's, Bessel's, 115 Catasterisma of Eratosthenes, 89, 90. a Centauri, Piazzi Smyth on, 146, 147, 185; the nearest of the fixed stars that have yet been measured, 191, 192. Central body for the whole sidereal heav- ens, existence of, doubtful, 197. Chinese record of extraordinary stars (of Ma-tuan-lin), 109, 155-159; deserving of confidence, 162. Clusters of stars, or stellar swarms, 140 ; list of the principal, 141-143. Coal-sacks, a portion of the Milky Way in the southern hemisphere so called, 137. Colored rings afford a direct measure of the intensity of light, 96. Colored stars, 130; evidence of change of color in some, 131, 132; Sir John Herschel's hypothesis, 131 ; difference of color usually accompanied by differ ence of brightness, 209. Comets, information regarding celestial space, derived from observation on, 31, 39 ; number of visible ones, 151. Concentric rings of stars, a view favored by recent observation, 149. Constellations, arrangement of stars into, very gradual, 119. Contrasted colors of double stars, 207. Cosmical contemplation, extension of, la the Middle Ages, 16. Cosmical vapor, question as to condensa- tion of, 37 ; Tycho Brahe's and Sir Will- iam Herschel's theories, 154. " Cosmos," a pseudo-Aristotelian work, 16. Crystal vault of heaven, date of the desig- nation, 133 ; its signification according to Empedocles, 123 ; the idea favored by the Fathers of the Church, 125. Cyanometer, Arago's, 97. Dark cosmical bodies, question of, 164, 187. Dolambrc on the velocity of light, 82. Descartes, his cosmical views, 19, 20 ; sup- presses his work from deference to the Inquisition, 20. Dioptric tubes, the precursors of the tele- scope, 43. Direct and reflected light, 45. Distribution of the fixed stars, according to right ascension, 140. Dorpat Table (Struve's) of multiple stars, 205. Double stars, the name too indiscrimin- ately applied, 199, 200 ; distribution into optical and physical. 200 ; pointed out by Galileo as useful in determining the parallax, 200 ; vast increase in their ob- served number, 201, 205; those earliest described, 201; number in which a change of position has been proved, 206 ; greater number of double stars in the northern than in the southern hem- isphere, 207 ; occurrence of contrasted colors, 207 ; calculation of their orbits, 211 ; table of the elements, 213. Earth-animal, Kepler and Fludd's fancies regarding the, 19. Edda-Songs, allusion to, 8. Egypt, zodiacal constellations of, their date, 121. Egyptian calendar, period of the complete arrangement of the, 133. Ehrenberg on the incalculable number of animal organisms, 30. Electrical light, velocity of transmission of, 86. Electricity, transmission of, through the Elements, Indian origin of the hypothesis of four or five, 11. Emanations from the head of some com- ets, 39. Encke, his accurate calculation of the equivalent of an equatorial degree, 81 ; on the star-maps of the Berlin Academy, 116 ; an early calculator of the orbits of double stars, 2C \ ; his theory of their motion, 212. Encke's comet, considerations on space, derived from periods of revolution of, 27; a resisting medium proved from observation on, 39. Ether, different meanings of, in the East and the West, 31, 32. Ether (Ak so, in Sanscrit), one of the In- dian five elements, 31. Ether, the, fiery, 35. Euler's comparative estimate of the light f the sun and moon, 95. Fixed stars, the term erroneous, 27, 122 ; scintillation of the, 73 ; variations in its intensity, 76 ; our sun one of the fainter fixed stars, 95; photometric arrange- ment of, 99 ; their number, 105 ; num- ber visible at Berlin with the naked eye, 107; at Alexandria, 107; Struve and Herschel's estimates, 116; grouping of the, 117 ; distribution of the, 140 ; prop- er motion of the, 182 ; parallax, 188 ; number of, in which proper motion has been discovered, greater than of those in which change of position has been observed, 206, 207. Fizeau, M., his experiments on the veloc- ity of light, 80, 83. Formula for computing variation of light of a star, by Argelander, : Galactic circle, average number of stars in, and beyond the, 139. Galileo indicates the means of discover- ing the parallax, 188. Galle, Dr., on Jupiter's satellites, 50 ; on the photometric arrangement of the fixed stars, 99. Garnet star, the, a star in Cepheus, so called by William Hcrschel, 1G6. Gascoigne applies micrometer threads to the telescope, 42. Gauging the heavens, by Sir William Her- schel, 138, 139 ; length of time neces- sary to complete the process, 139. Gauss, on the point of translation in space of the whole solar system, 196. Gilliss, Lieutenant, on the change of color of the star ij Argus, 135. Gravitation, not an essential property of bodies, but the result of some higher and still unknown power, 22, 23. Greek sphere, date of the, 119, 121. Green and blue suns, 208. Groups of fixed stars, recognized even by the rudest nations, 117 ; usually the same groups, as the Pleiades, the Great Bear, the Southern Cross, &c., 117, 118. Halley asserted the motion of Sinus and other fixed stars, 26, 27. Hassenfratz, his description of the rays of stars as caustics on the crystalline lens, 52, 127. Heat, radiating, 35. Hepidannus, monk of Saint Gall, a new star recorded by, 157, 162. Herschel, Sir William, on the vivifying action of the sun's rays, 34 ; his estimate of the number of the fixed stars, 116, 117 ; his " gauging the heavens," and its result, 138, 139. Herschel, Sir John, on the transmission of light, 30 ; on the influence of the sun'g rays, 34 ; compares the sun to a per- petual northern light, 34 ; on the at- mosphere, 37 ; on the blackness of the ground of the. heavens, 39 ; on stars seen in daylight^ 57; on photometry. 217 93 ; photometric arrangement of the fixed stars, 99 ; on the number of stars Lassell's telescope, discoveries made by means of, 65. actually registered. 106 ; on the cause of the red color of Sirius, 131, 132 ; on Lepsius, on the Egyptian name (Sothis) of Sinus, 134. fhe Milky Way, 145; on the sun's place, 150; on the determined periods of vari- Leslie's photometer, defects of, 96. Libra, the constellation, date of its intro- able stars, 166; number of double stars duction into the Greek sphere, 120. the elements of whose orbits have been Light, always refracted, 44 ; prismatic determined, 211. spectra differ in number of dark lines Hieroglyphical signification of a star, ac- cording to Horapollo, 128. according to their source, 44, 45 ; polar- ization of, 45 ; velocity of, 79 ; ratio of Hind's discovery of a new reddish-yellow solar, lunar, and stellar, 95 ; variation star of the fifth magnitude, in Ophiu- of, in stars of ascertained and unascer- magnitude, 160 ; calculation of the or- tained periodicity, 168, 177. Light of the sun and moon, Euler's and bits of double stars by, 211. Hipparchus. on the number of the Plei- Michelo's estimates of the comparative, 95. ades, 48 ; his catalogue contains the Limited transparency of the celestial re- earliest determination of the classes of gions. 38. magnitude of the stars, 90 ; a fragment of his work preserved to us in Aratus, IftO Macrobius, " Sphsera aplanes" of, 27. Madler, on Jupiter's satellites. 52 ; on the ioy. Holtzmann, on the Indian zodiacs, 121. Homer, not an authority on the state of Greek astronomy in his day, 119, 123. Humboldt, Alexander von, works of, quoted in various notes: Ansichten der Natur, 79. Asie Centrale, 111, 112. Essai sur la Geogr. des Plantes, 58. Examen Critique de 1'Histoire de la determined periods of variable stars, 166; on future polar stars, 181 ; on non- luminous stars, 187 ; on the center of gravity of the solar system, 198. Magellanic clouds, known to the Arabs, 91. Magnitude of the stars, classes of, 90, 91. Mafus, his discoveries regarding light, 45. ' Mappa ccelestis" of Schwinck, 140. Ma-tuan-lin, a Chinese astronomical rec- ord of 109 G6ographie, 49, 112, 137. Lettre a M. Schumacher, 93. Recueil d'Observations Astrono- miques, 43. 47, 93. Relation Historique du Voyage aux Regions Equinoxiales, 56, 58. 79, 93. Vue des Cordilleres et Monumens des Peuples Indigenes de 1'Amer- Mayer, Christian, the first special observer of the fixed stars, 202. Melville Island, temperature of, 36. Michell, John, 95 ; applies the calculus of probabilities to small groups of stars, 201 ; little reliance to be placed in its individual numerical result?, 202. Michelo's comparative estimate of the ique, 121, 136. Humboldt, Wiihelm von, quoted, 25. Huygens, Christian, his ambitious but un- satisfactory Cosmotheoros, 20; exam- ined the Milky Way, 144. light of the sun and moon, 95. Milky Way, average number of stars in, and beyond the, according to Struve, 139 ; intensity of its light in the vicinity of the Southern Cross, 147 ; its course Huygens, Constantin, his improvements and direction, 147; most of the new in the telescope, 02. Hvergelmir, the caldron-spring of the Ed- stars have appeared in its neighbor- hood, 162. da-Songs, 8. Morin proposes the application of the tel- Indian fiction regarding the stars of the Southern hemisphere, 138. escope to the discovery of the stars in daylight, 41, 66. Indian theory of the five elements (Pant- Motion, proper, of the fixed stars, 182; echota*), 31. variability of, 185, 186. Indian zodiacs, their high antiquity doubt- ful, 121. Multiple stars, 130, 199 ; variable bright- ness of, difference of opinion regarding, 210. Jacob, Capt, on the intensity of light in the Milky Way. 146; calculation of the Nebulas, probably closely crowded stellar orbits of double stars, by, 211. Joannes Philoponus, on gravitation, 18. Jupiter's satellites, estimate of the magni- tudes of, 50; case in which they were visible by the naked eye, 52 ; occulta- tious of, observed by daylight, 62. swarms, 37. Neptune, the planet, its orbit used as a measure of distance of 61 Cygni, 204. New stars, 151 ; their small number, 151 ; Tycho Brahe's description of one, 152 ; its disappearance, 153 ; speculations as to their origin, 161 : most have appear- Kepler, his approach to the mathematical application of the theory of gravitation. 18; rejects the idea of solid orbs, 126. ed near the Milky Way, 162. Newton, embraces by his theory of gravi- tation the whole uranological portion of the Cosmos, 21. Lalande, his Catalogue, revised by Baily, 115. Non-luminous stars, problematical exist- ence of, 187. VOL III. K 218 INI Numerical results exceeding the grasp of the comprehension, furnished alike by the minutest organisms and the so- called fixed stars, 30 ; encouraging views on the subject, 31. Optical and physical double stars, 200; often confounded, 200. Orbits of double stars, calculation of the, 211 ; their great eccentricity, 211 ; hy- pothesis, that the brighter of the two stars is at rest, and its companion re- volves about it, probably correct, and a great epoch in cosmical knowledge, 212. Orion, the six stars of the trapezium of the nebula of, probably subject to pe- culiar physical attraction, 210, 211. Pantschata or Pantschatra, the Indian the- ory of the five elements, 31. Parallax, means of discovering the, point- ed out by Galileo, 183 ; number of par- allaxes hitherto discovered, 190 ; detail of nine of the best ascertained, 190. Penetrating power of the telescope, 145, 146. Periodically changeable stars, 164. Periods within periods of variable stars, 168 ; Argelander on, 168. Peru, climate of, unfavorable to astronom- ical observations, 103. Peters on parallax, 192. Photometric relations of self-luminous bodies, 89 ; scale, 99. Photometry yet in its infancy, 94 ; first numerical scale of, 94 ; Arago's meth- od, 96. Plato on ultimate principles, 12, 13. Pleiades, one of the, invisible to the naked eye of ordinary visual power, 48 ; de- scribed, 141. Pliny estimates the number of stars vis- ible in Italy at only 1600, 108. Poisson, his view of the consolidation of the earth's strata, 36, 37. Polarization of light, 45, 47. Poles of greatest cold, 36. Pouillet's estimate of the temperature of space, 36. Prismatic spectra, 44 ; difference of the dark lines of, 45. Ptolemy, his classification of the stars, 90 ; southern constellations known to, 137. Pulkowa, number of multiple stars dis- covered at, 205, 206. Pythagoreans, mathematical symbolism of tno, 12. Quaternary systems of stars, 210. Radiating heat, 35. Ratio of various colors among the mul- tiple and double stars, 209. Rays of ftars, 52, 126-128 ; number of, in- dicate distances, 128; disappear when the star is viewed through a very sma aperture, 138, 129. Red stars, 131 ; varia j Reflecting sextants applied to the determ- I ination of the intensity of stellar light. 1. variable stars mostly red, Reflecting and refracting telescopes, 63. i Regal stars of the ancients, 136. j Resisting medium, proved by observa- tions on Kncke's and other comets, 39. Right ascension, distribution of stars ac- cording to, by Schwinck, 140. Rings, colored, measurement of the in- tensity of light by, 96. Rings, concentric, of stars, the hypothesis of, favored by the most recent observa- ! tions, 149. i Rosse's, Lord, his great telescope, 65 ; it* services to astronomy, 66. I Ruby-colored stars, 135. Saint Gall, the monk of, observed a new j star distant from the Milky Way, 162. Saussure asserts that stars may be seen in daylight on the Alps, 57 ; the asser- tion not supported by other travelers' j experience, 58. Savary, on the application of the aberra- tion of light to the determination of the parallaxes, 194 ; an early calculator of the orbits of double stars, 211. Schlegel, A. W. von, probably mistaken as to the high antiquity of the Indian zodiacs, 121 Schwinck, distribution of the fixed stars in his " Mappa coslestis," 140. Scintillation of the stars, 73 ; variations in its intensity, 76 ; mentioned in the Chinese records, 77 ; little observed in tropical regions, 77, 78 ; always accom- panied by a change of color, 202. Seidel, his attempt to determine the quan- tities of light of certain stars of the first magnitude, 93. Self-luminous cosmical bodies, or suns, 199. Seneca, on discovering new planets, 28. Simplicius, the Eclectic, contrasts the cen- tripetal and centrifugal forces, 12 ; his vague view of gravitation, 18. Sirius, its absolute intensity of light, 95 ; historically proved to have changed its color, 131 ; its association with the ear- liest development of civilization in the valley of the Nile, 133 ; etymological re- searches concerning, 133, 134. Smyth, Capt W. H., calculations of the orbits of double stars by, 211. Smyth, Piazzi, on the Milky Way, 146, 147 ; on a Centauri, 185. Sothis, the Egyptian name of Sirius, 133, South, Sir James, observation of 380 dou- ble stars by, in conjunction with Sir John Herschel, 205. Southern constellations known to Ptol- emy, 137. Southern Cross, formerly visible on the shores of the Baltic-, 138. Southern hemisphere, in parts remark- ably deficient in constellations, 112; dis- tances of its stars, first measured about the end of the sixteenth century, 138. 219 Space, conjectures regarding, 29 ; com- | its influence pared to the mythic period of history, I 37. 29; fallacy of attempts at measurement , Temporary stars, the climate of the earth, t of, 155 ; notoe to, of, 30 ; po'rtions between cosmical bod- I 155-160." jes not void, 31 ; its probable low tem- Ternary stars, 210. perature, 35. Spectra, the prismatic, 44 ; difference of the dark lines of, according to their sources, 45. " Sphtera aplanes" of Macrobiue, 27. Spurious diameter of stars, 130. Star of the Magi, Ideler's explanation of the, 154. Star of St. Catharine, 137. Star systems, partial, in which several suns revolve about a common center of gravity, 204. Stars, division into wandering and non- wandering, dates at least from the early Greek period, 27 ; magnitude and visi- Timur Ulugh Beg, improvements in prac- tical astronomy in the time of, 91. Translation in space of the whole solar system, 195; first hinted by Bradley, 195 ; verified by actual observation by William Herschel, 196; Argelander. Strove, and Gauss's views, 196. Trapezium in the great nebula of Orion, investigated by Sir Wm. Herschel, 203. Tycho Brahe, his vivid description of the appearance of a new star, 152 ; his the- ory of the formation of such, 154. Ultimate mechanical cause" of all mo- tion, unknown, 24, 25. bility of the, 48 ; seen through shafts I Undulation of the stars, 58, 59. of chimneys, 57 ; undulation of the, 58, ' Undulations of rays of light, various 59 ; observation of, by daylight, 66 ; ! lengths of, 84. scintillation of the, 73 ; variations in its Unity of nature distinctly taught by Aris- intensity, 76 ; the brightest the earliest totle, 13-15. named, 89; rays of, 52, 127, 128 ; color Uranological and telluric domain of the Cosmos, 26. of, 130 ; distribution of, 140 ; concentric rings of, 149 ; variable, 161 ; vanished, 163 ; periodically changeable, 164 ; non- luminous, of doubtful existence, 187 ; ratio of colored stars, 209. Steinheil's experiments on the velocity of the transmission of electricity, 87 ; his photometer, 93. Stellar clusters or swarms, 140. Struve on the velocity of light, 82 ; his estimate of the number of tine fixed stars, 1 17 ; on the Milky Way, 139 ; his Dorpat Tables, 205 ; on the contrasted colors of multiple stars, 207 ; calcula- tion of the orbits of double stars by, 211. Sun, the, described as " a perpetual north- ern light" by Sir William Herschel, 34 ; in intensity of light merely one of the fainter fixed stars, 95 ; its place prob- ably in a comparatively desert region of the starry stratum, and eccentric, 150. Suns, self-luminous cosmical bodies, 199. Uranus observed as a star by Flamstead and others, 114. Vanished stars, 163; statements about such to be received with great caution, 163. Variable brightness of multiple and dou- ble stars, 209. Variable stars, 160-161 ; mostly of a red color, 165; irregularity of their periods, 167 ; table of, 172. Velocity of light, 79 ; methods of determ- ining, 80 ; applied to the determination of the parallax, 195. Visibility of objects, 55 ; how modified, 56. Vision, natural and telescopic. 41 ; aver- age natural, 47, 48; remarkable in- stances of acute natural, 52, 55. Wheatstone's experiments with revolv- mirrors,45; velocity of electrical ing IT light determined by, 86." Table of photometric arrangement of 190 White Ox, name given to the nebula now fixed stars, 100 ; of 17 stars of first mag- known as one of the Magellanic clouds, nitude, 102; of the variable stars, by 91. Argelander, 172, and explanatory re- Wollaston's photometric researches, 95. marks, 172-177; of ascertained paral- Wright, of Durham, his view of the origin laxes, 193 : of the elements of the or- of the form of the Milky Way, 149. bits of double stars, 213. Telescope, the principle of, known to the Yggdrasil, the World-tree of the Edda- Arabs, and probably to the Greeks and Songs, 8. Romans, 42, 43 ; di :overies by its means, 61 ; successive improvements ' Zodiac, period of its introduction into the Greek sphere, 119; its origin among the Chaldeans, 120 ; the Greeks borrowed from them only the idea of the division, and filled its signs with their own cataa- toriems, 120; great antiquity of the In- of the, 62; enormous focal lensrth of som.', 63; Lord Rosse's, 65; Bacon's comparison of, to discovery ships, 130; penetrating power of the, 145, 146. Telesio, Bernardino, of Cosenza, his v: of the iews phenomena of inert matter, 16. Temperature, low, of cclcstiiil uncertainty of results "el obta space, 35; ained, 36 ; dian ver 130; great y doubtful, -ID. THE END. Zodiacal light, Sir John Herschel on the, A if 3 THE LIBRARY UNIVERSITY OF CALIFORNIA Santa Barbara THIS BOOK IS DUE ON THE LAST DATE STAMPED BELOW. 000 657 424