Telescopic Work FOR W. F. DENNING. OF THE UNIVERSITY OF ASTRONOMY LIBRARV Jr %f bris ^ k\if>J\^^ v5 ' S 0. CD L } D R A R Y OF THS AST57 r^cJLIiCAL SOCIETY 304 j TELESCOPIC WORK FOR STARLIGHT EVENINGS. BY WILLIAM F. DENNING, F.R.A.S. (FORMERLY PRESIDENT OP THE LIVERPOOL ASTRONOMICAL SOCIETY). " To ask or search I blame thee not, for heaven Is as the book of God before thee set, Wherein to read his wondrous works." MILTON. LONDON: TAYLOR AND FRANCIS, RED LION COURT, FLEET STREET. 1891. [All rights reserved.] FLAMMAM. PRINTED BY TAYLOR AND FRANCIS, BED LION COURT, FLEET STEEET. ASTRONOMY LIBRARY PKEEACE. IT having been suggested by some kind friends that a series of articles on "Telescopes and Telescopic Work," which I wrote for the 'Journal of the Liverpool Astronomical Society' in 1887-8, should be reprinted, I have undertaken the revision and re- arrangement of the papers alluded to. Certain other contributions on " Large and Small Telescopes," " Planetary Observations," and kindred subjects, which I furnished to ' The Observatory ' and other scientific serials from time to time, have also been included, and the material so much altered and extended that it may be regarded as virtually new matter. The work has outgrown my original inten- tion, but it proved so engrossing that it was found difficult to ensure greater brevity. The combination of different papers has possibly had the effect of rendering the book more popular in some parts than in others. This is not altogether unintentional, for the aim has been to make the work intelligible to general readers, while also containing M677191 iv PREFACE. facts and figures useful to amateur astronomers. It is merely intended as a contribution to popular astro- nomy, and asserts no rivalry with existing works, many of which are essentially different in plan. If any excuse were, however, needed for the issue of this volume it might be found in the rapid progress of astronomy, which requires that new or revised works should be published at short intervals in order to represent existing knowledge. The methods explained are approximate, and tech- nical points have been avoided with the view to engage the interest of beginners who may find it the stepping-stone to more advanced works and to more precise methods. The object will be realized if observers derive any encouragement from its descrip- tions or value from its references, and the author sincerely hopes that not a few of his readers will experience the same degree of pleasure in obser- vation as he has done during many years. No matter how humble the observer, or how paltry the telescope, astronomy is capable of furnish- ing an endless store of delight to its adherents. Its influences are elevating, and many of its features possess the charms of novelty as well as mystery. Whoever contemplates the heavens wjth the right spirit reaps both pleasure and profit, and many amateurs find a welcome relaxation to the cares of PREFACE. v business in the companionship of their telescopes on " starlight evenings." The title chosen is not, perhaps, a comprehensive one, but it covers most of the ground, and no apology need be offered for dealing with one or two important objects not strictly within its scope. For many of the illustrations I must express my indebtedness to the Editors of the ' Observatory,' to the Council of the R.A.S., to the proprietors of ' Nature,' to Messrs. Browning, Calver, Cooke & Sons, Elger, Gore, Home Thornthwaite and Wood, Klein, and other friends. The markings on Venus and Jupiter as represented on pages 150 and 180 have come out much darker than was intended, but these illustrations may have some value as showing the position and form of the features delineated. It is difficult to reproduce delicate planetary markings in precisely the same characters as they are displayed in a good telescope. The apparent orbits of the satellites of the planets, delineated in figs. 41, 44, &c., are liable to changes depending on their variable position relatively to the Earth, and the diagrams are merely intended to give a good idea of these satellite systems. W. F. D. Bishopston, Bristol, 1891. PLATES I. and II. are views of the Observatory and Instruments recently erected by Mr. Klein at Stanmore, Middlesex, lat. 51 36' 57" N.,"long. 18' 22" W. The height above sea-level is 262 feet. The telescope is a 20-inch reflector by Oalver, of 92 inches focus ; the tube is, however, 152 inches long so as to cut off all extraneous rays. It is mounted equatoreally, and is provided with a finder of 6 inches aperture one of Tulley's famous instruments a century ago. The large telescope is fixed on a pillar of masonry 37 feet high, and weighing 115 tons. Mr. Klein proposes to devote the resources of his establishment to astronomical photography, and it has been provided with all the best appli- ances for this purpose. The observatory is connected by telephone with Mr. Klein's private residence, and the time- pieces and recording instruments are all electrically con- nected with a centre of observation in his study. CONTENTS, CHAPTER I. Page THE TELESCOPE, ITS INVENTION AND THE DEVELOPMENT OF ITS POWERS 1 Omission, p. 220. A column giving the periods of the satellites of Uranus should be added to the table as follows : d h m 2 12 29 4 3 27 8 16 57 13 11 7 CHAPTER Y. THESTJN ................. 87 CHAPTER VI. THE MOON .... ............. 113 CHAPTER VII. MERCURY CHAPTER VIII. VENUS , .......... 145 viii CONTENTS. CHAPTEE IX. Page MAES 155 CHAPTEE X. THE PLANETOIDS 167 CHAPTEE XI. JUPITEE . 170 CHAPTEE XII. SATURN . ... . . 195 CHAPTEE XIII. UBANUS AND NEPTUNE 215 i CHAPTEE XIV. COMETS AND COMET-SEEKING 227 CHAPTEE XV. METEOES AND METEOEIC OBSEEVATIONS 260 CHAPTEE XVI. THE STAES . . 286 CHAPTEE XVII. NEBULAE AND CLUSTEES OF STAES . 324 NOTES AND ADDITIONS . ..... ...... . 347 INDEX 353 ILLTJSTBATIONS, PLATE I. Interior of Mr. Klein's Observatory Frontispiece II. View of Mr. Klein's Grounds and Observatory. . To face p. 82 FIG. PAGE 1. The Galilean Telescope 7 2. Royal Observatory, Greenwich, in Flamsteed's time 8 3. Sir Isaac Newton , 1^ 4. Gregorian Telescope 10 5. Cassegrainian Telescope 11 6. Newtonian Telescope 11 7. Common Refracting-Telescope 12 8. Le Mairean or Herschelian Telescope 13 9. 10-inch Reflecting-Telescope on a German Equatoreal, by Oalver 17 10. Lord Rosse's 6-foot Reflecting-Telescope 22 11. Refracting-Telescope, by Browning 32 12. " The Popular Reflector," by Calver 40 13. 3-inch Refracting-Telescope, by Newton & Co 41 14. Huygens's Negative Eyepiece 46 15. Ramsden's Positive Eyepiece 47 16. Berthon's Dynamometer 50 17. Cooke and Sons' Educational Telescope 52 18. Refracting-Telescope on a German Equatoreal 67 19. The Author's Telescope : a 10-inch With-Browning Reflector . 77 20. Sun-spot of June 19, 1889 95 x ILLUSTRATIONS. FIG. PAGE 21. Solar Eclipses visible in England, 1891 to 1922 98 22. Total Solar Eclipse of August 19, 1887 98 23. Belts of Sun-spots, visible Oct. 29, 1868 104 24. Shadows cast by Faculse 109 25. Light-spots and streaks on Plato, 1879-82. (A. Stanley Williams.) 126 26. Petavius and Wrottesley at Sunset. (T. Gwyn Elger.) 129 27. Birt, Birt A, and the Straight Wall. (T. Gwyn Elger.) 130 28. Aristarchus and Herodotus at Sunrise. (T. Gwyn Elger.) .... 132 29. Mercury as a Morning Star 143 ,30. Venus as an Evening Star 150 31. Mars, 1886, April 13, 9 h 50^ 157 32. Orbits of the Satellites of Mars 159 33. Jupiter, as drawn by Dawes and others 178 34. Jupiter, 1886, April 9, 10 h 12 m 180 35. Occultation of Jupiter, Aug. 7, 1889 186 36. Jupiter and Satellites seen in a small glass 187 37. Shadows of Jupiter's Satellites II. and HI 192 38. Saturn as observed by Cassini in August 1676 198 39. Saturn, 1885, Dec. 23, 7 h 54^ 201 40. Saturn as observed by F. Terby, February 1887 203 41. Apparent orbits of the Five Inner Satellites of Saturn 212 42. Transit of the Shadow of Titan 213 43. Uranus and his belts 218 44. Apparent orbits of the Satellites of Uranus 221 45. Apparent orbit of the Satellite of Neptune 224 46. Mars, Saturn, and Regulus in same field, Sept. 20, 1889 226 47. Comet 1862 III. (Aug. 19, 1862). . .' 237 48. Sawerthal's Comet, 1888 I. (March 25, Brooks) 237 49. Brooks's Double Comet, Sept. 17, 1889 239 50. Pons's Comet (1812). Telescopic view, 1884, Jan. 6 242 51. Ditto. Ditto, 1884, Jan. 21 242 52. Radiation of Meteors. (Shower of early Perseids, 1878) 263 ILLUSTRATIONS. xi FIG. PAGE 53. Double Meteor. Curved Meteor. Fireball 265 54. Meteorite found in Chili in 1866 265 55. Meteorite which fell at Orgueil in 1864 265 56. Fireball of Nov. 23, 1877, 8" 24. ( J. Plant.) 269 57. Flight of Telescopic Meteors seen by W. R. Brooks 272 58. Meteor of Dec. 28, 1888, 6 h 17 m 277 69. Large Meteor and streak seen at Jask 278 60. The Constellation Orion 289 61. Diagram illustrating the Measurement of Angles of Position . . 291 62. Double Stars 301 63. Trapezium in Orion as seen with the 36-inch refractor 319 64. Nebulae and a Star-cluster 336 65. Nebula within a semicircle of stars . . . 342 TELESCOPIC WORK FOE STARLIGHT EVENINGS. CHAPTER I. THE TELESCOPE, ITS INVENTION AND THE DEVE- LOPMENT OF ITS POWERS. THE instrument which has so vastly extended our knowledge of the Universe, which has enabled us to acquire obser- vations of remarkable precision, and supplied the materials for many sublime speculations in Astronomy, was invented early in the seventeenth century. Apart from its special application as a means of exploring the heavens with a capacity that is truly marvellous, it is a construction which has also been utilized in certain other departments with signal success. It provided mankind with a medium through which to penetrate far beyond the reach of natural vision, and to grasp objects and phenomena which had either eluded detection altogether or had only been seen in dim and uncertain characters. It has also proved a very efficient instrument for various minor purposes of instruction and recreation. The invention of the telescope formed a new era in astronomy; and though, with a few exceptions, men were slow at first in availing themselves of its far-seeing resources, scepticism was soon swept aside and its value became widely acknowledged. 2 THE TELESCOPE, But though the telescope was destined to effect work of the utmost import, and to reach a very high degree of excellence in after times, the result was achieved gradually. Step by step its powers were enlarged and its qualities perfected, and thus the stream of astronomical discovery has been enabled to flow on, stimulated by every increase in its capacity. There is some question as to whom may be justly cre- dited with the discovery of its principles of construction. Huygens, in his i Dioptrics,' remarks : " I should have no hesitation in placing above all the rest of mankind the individual who, solely by his own reflections, without the aid of any fortuitous circumstances, should have achieved the invention of the telescope."" There is reason to conclude, however, that its discovery resulted from accident rather than from theory. It is commonly supposed that Galileo Galilei is entitled to precedence ; but there is strong evidence to show that he had been anticipated. In any case it must be admitted that Galilei* had priority in successfully utilizing its resources as a means of observational discovery ; for he it was who, first of all men, saw Jupiter's satellites, the crescent form of Venus, the mountains and craters on the Moon, and announced them to an incredible world. It has been supposed, and not without some basis of pro- bability, that a similar instrument to the telescope had been employed by the ancients ; for certain statements contained in old historical records would suggest that the Greek phi- losophers had some means of extending their knowledge further than that permitted by the naked eye. Democritus remarked that the Galaxy or " Milky Way " was nothing but an assemblage of minute stars ; and it has been asked, How could he have derived this information but by instru- mental aid ? It is very probable he gained the knowledge by inferences having their source in close observation ; for anyone who attentively studies the face of the sky must be naturally led to conclude that the appearance of the u Milky Way 5 ' is induced by immense and irregular clus- * Galileo Galilei is very generally called by his Christian name, but I depart from this practice here. ITS INVENTION AND DEVELOPMENT. 3 terings of small stars. In certain regions of the heavens there are clear indications of this : the eye is enabled to glimpse some of the individual star-points, and to observe how they blend and associate with the denser aggregations which give rise to the milky whiteness of the Galaxy. Refracting lenses, or " burning-glasses," were known at a very early period. A lens, roughly figured into a convex shape and obviously intended for magnifying objects, has been recovered from the ruins of Herculaneum, buried in the ejections from Vesuvius in the year 79 A.D. Pliny and others refer to lenses that burnt by refraction, and describe globules of glass or crystal which, when exposed in the sun, transmit sufficient heat to ignite combustible material. The ancients undoubtedly used tubes in the conduct of their observations, but no lenses seem to have been employed with them, and their only utility consisted in the fact of their shutting out the extraneous rays of light. But spectacles were certainly known at an early period. Concave emeralds are said to. have been employed by Nero in witnessing the combats of the gladiators, and they appear to have been the same in effect as the spectacles worn by short-sighted people in our own times. But the ancients supposed that the emerald possessed inherent qualities specially helpful to vision, rather than that its utility resulted simply from its concavity of figure. In the 13th century spectacles were more generally worn, and the theory of their construction understood. It is remarkable that the telescope did not come into use until so long afterwards. Vague references were made to such an instrument, or rather suggestions as to the possi- bilities of its construction, which show that, although the principle had perhaps been conceived, the idea was not successfully put into practice. Roger Bacon, who flourished in the 13th century, wrote in his ' Opus Majiis ' : " Greater things may be performed by refracted light, for, from the foregoing principles, follows easily that the greatest objects may be seen very small, the remote very near, and vice versa. For we can give transparent bodies such form and position with respect to the eye and the object that the rays are refracted and bent to where we like, so that we, under any B2 4 THE TELESCOPE, an'gle, see the objects near or far, and in that manner we can, at a great distance, read the smallest letters, and we can count atoms and sand-grains, on account of the greatness of the angle under which they are seen/ 5 Fracastor, in a work published at Venice in 1538, states : " If we look through two eye-lenses, placed the one upon the other, everything will appear larger and nearer." He also says : " There are made certain eye-lenses of such a thickness that if the moon or any other celestial body is viewed through them they appear to be so near that their distance does not exceed that of the steeples of public buildings." In other writings will also be found intimations as to the important action of lenses ; and it is hardly accountable that a matter so valuable in its bearings was allowed to remain without practical issues. The progressive tendency and the faculty of invention must indeed have been in an incipient stage, and contrasts strongly with the singular avidity with which ideas are seized upon and realized in our own day. Many important discoveries have resulted from pure acci- dent ; and it has been stated that the first bond fide telescope had its origin in the following incident : The children of a spectacle-maker, Zachariah Jansen, of Middleberg-, in Zea- land, were playing with some lenses, and it chanced that they arranged two of them in such manner that, to their astonishment, the weathercock of an adjoining church appeared much enlarged and more distinct. Having men- tioned the curious fact to their father, he immediately turned it to account, and, by fixing two lenses on a board, produced the first telescope ! This view of the case is, however, a very doubtful one, and the invention may with far greater probability be attributed to Hans Lippersheim in 1 608. Galilei has little claim to be considered in this relation ; for he admitted that in 1609 the news reached him that a Dutchman had devised an appliance capable of showing distant objects with remarkable clearness. He thereupon set to work and experimented with so much aptitude on the principles involved that he very soon pro- duced a telescope for himself. With this instrument he detected the four satellites of Jupiter in 1610, and other ITS INVENTION AND DEVELOPMENT. 5 successes shortly followed. Being naturally gratified with the improvements he had effected in its construction, and with the wonderful discoveries he had made by its use, we can almost excuse the enthusiasm which prompted him to attribute the invention to his own ingenuity. But while according him the honour due to his sagacity in devoting this instrument to such excellent work, we must not overlook the fact that his claim to priority cannot be justified. Indeed, that Galilei had usurped the title of inventor is mentioned- in letters which passed between the scientific men of that time. Fuccari, writing to Kepler, says : " Galileo wants to be considered the inventor of the telescope, though he, as well as I and others, first saw the telescope which a certain Dutch- man first brought with him to Venice, and although he has only improved it very little." In a critical article by Dr. Doberck*, in which this letter is quoted and the whole question reviewed with considerable care, it is stated that Hans Lippersheim (also known as Jan Lapprey), who was born in Wesel, but afterwards settled at Middleberg, in the Netherlands, as a spectacle-maker, was really the first to make a telescope, and the following facts are quoted in confirmation : " He solicited the States, as early as the 2nd October, 1608, for a patent for thirty years, or an annual pension for life, for the instrument he had invented, promising then only to construct such instru- ments for the Government. After inviting the inventor to improve the instrument and alter it so that they could look through it with both eyes at the same time, the States deter- mined, on the 4th October, that from every province one deputy should be elected to try the apparatus and make terms with him concerning the price. This committee declared on the 6th October that it found the invention useful for the country, and had offered the inventor 900 florins for the instrument. He had at first asked 3000 florins for three instruments of rock-crystal. He was then ordered to deliver the instrument within a certain time, and the patent was promised him on condition that he kept the invention secret. Lapprey delivered the instrument in due time. He had * ' Observatory,' vol. ii. p. 364. (5 THE TELESCOPE, arranged it for both eyes, and it was found satisfactory ; but they forced him, against the agreement, to deliver two other telescopes for the same money, and refused the patent because it was evident that already several others had learned about the invention." The material from which the glasses were figured appears to have been quartz ; and efforts were made to keep the invention a profound secret, as it was thought it would prove valuable for " strategetical purposes." The cost of these primitive binoculars was about 75 each. It is singular that, after being allowed to rest so long, the idea of telescopic construction should have been carried into effect by several persons almost simultaneously, and that doubts and disputes arose as to precedence. The probable explanation is that to one individual only priority was really due. but that, owing to the delays, the secret could not be altogether concealed from two or three others who recog- nized the importance of the discovery and at once entered into competition with the original inventor. Each of these fashioned his instrument in a slightly different manner, though the principle was similar in all ; and having in a great measure to rely upon his individual faculties in com- pleting the task, he considered himself in the light of an inventor and put forth claims accordingly. Not only were attempts made to assume the position of inventor, but there arose fraudulent claimants to some of the discoveries which the instrument effected in the hands of Galilei. Simon Marius, himself one of the very first to construct a telescope and apply it to the examination of the heavenly bodies, asserted that he had seen the satellites of Jupiter on December 29, 1609, a few days before Galilei, who first glimpsed them on January 7, 1610. Humboldt, in his i Physical Description of the Heavens/ definitely ascribes the discovery of these moons to Marius ; but other autho- rities uniformly reject the statement, and accord to Galilei the full credit. It is stated that Galilei's first instrument magnified only three times, but he so far managed to amplify its resources that he was ultimately enabled to apply a power of 30. The ITS INVENTION AND DEVELOPMENT. 7 lenses consisted of a double-convex object-glass, and a small double-concave eye-glass placed in front of the focal image formed by the object-glass. The ordinary opera-glass is constructed on a similar principle. Fig. 1. The Galilean Telescope. The discoveries which Galilei effected with this crude and defective instrument caused a great sensation at the time. He made them known through the medium of a publication which he issued under the title of ' Nuncius SiderusJ or ' The Messenger of the Stars/ In that superstitious age great ignorance prevailed, bigotry was dominant, and erroneous views of the solar system were upheld and taught by authority. We can therefore readily conceive that Galilei 's discoveries, and the direct inferences he put upon them, being held anta- gonistic to the ruling doctrines, would be received with incredulity and opposition. His views were regarded as heretical. In consequence of upholding the Copernican system he suffered persecution, and had to resort to artifice in the publication of his works. But the marvels revealed by his telescope, though discredited at first, could not fail to meet with final acceptance, for undeniable testimony to their reality was soon forthcoming. They were not, however, regarded until Ions: afterwards as affirming: the views enun- ^ O O ciated by their clever author. Ultimately the new astronomy, based on the irrepressible evidence of the telescope, and clad in all the habiliments of truth, took the place of the old falla- cious beliefs, to form an enduring monument to Copernicus and Galilei, who spent their lives in advancing its cause. No special developments in the construction of the tele- scope appear to have taken place until nearly half a century subsequent to its invention. Kepler suggested an instrument formed of two convex lenses, and Scheiner and Huygens 8 THE TELESCOPE, made telescopes on this principle in the middle of the 17th century. Huygens found great advantage in the employment of a compound eyepiece consisting of two convex lenses, which corrected the spherical aberration, and, besides being achromatic, gave a much larger field than the single lens. This eyepiece, known as the (t Huygenian," still finds favour with the makers of telescopes. Royal Observatory, Greenwich, in Flamsteed's time*. Huygens may be said to have inaugurated the era of long telescopes. He erected instruments of 12 and 23 feet, having an aperture of 2^ inches and powers of 48, 50, and 92. He afterwards produced one 123 feet in focal length and 6 inches in aperture. Chief among his dis- coveries were the largest satellite of Saturn (Titan) and the true form of Saturn's ring. Hevelius of Dantzic built an instrument 150 feet long, which he fixed to a mast 90 feet in height, and regulated by ropes and pulleys. Cassini, at the Observatory at Paris, had telescopes by Campani of 86, * Reproduced, by permission, from Cassell's 'New Popular Educator/ ITS INVENTION AND DEVELOPMENT. 9 100, and 136 French feet in length ; but the highest powers he used on these instruments do not appear to have exceeded 150 times. He made such good use of them as to discover three of the satellites of Saturn and the black division in the ring of that planet. The largest object-glasses employed by Hevelius and Cassini were of 6, 7, and 8 inches diameter. This was during the latter half of the 17th century. In 1712 Bradley made observations of Venus, and obtained measures of the planet's diameter, with a telescope no less than 212 feet in focal length. The instruments alluded to were manipulated with extreme difficulty, and observations had to be conducted in a manner very trying to the observer. Tubes were sometimes dispensed with, the object- glass being fixed to a pole and its position controlled by various contrivances the observer being so far off, however, that he required the services of a good lantern in order to distinguish it ! The immoderate lengths of refracting-telescopes were necessary, as partially avoiding the effects of chromatic aberration occasioned by the different refrangibility of the seven coloured rays which collectively make white light. In other words, the coloured rays having -various indices of refrac- tion cannot be brought to a coincident focus by transmission through a single lens. Thus the red rays have a longer focus than the violet rays, and the immediate effect of the different refractions becomes apparent in the telescopic images, which are fringed with colour and not sharply defined. High magnifying powers serve to intensify the obstacle alluded to, and thus the old observers found it imperative to employ eye-glasses not beyond a certain degree of convexity. The great focal lengths of their object-lenses enabled moderate power to be obtained, though the eye-glass itself had a focus of several inches and magnified very little. Sir Isaac Newton made many experiments upon colours, and endeavoured to obviate the difficulties of chromatic aber- ration, but erroneously concluded that it was not feasible. He could devise no means to correct that dispersion of colour which, in the telescopes of his day, so greatly detracted from their effectiveness. His failure seems to have had a pre- 10 THE TELESCOPE, judicial effect in delaying the solution of the difficulty, which was not accomplished until many years afterwards. Fig. 3. Sir Isaac Newton *. The idea of reflecting-telescopes received mention as early as 1639 ; but it was not until 1663 that Gregory described the instrument, formed of concave mirrors, which still bears his name. He was not, however, proficient in mechanics, Fig. 4. Gregorian Telescope. and after some futile attempts to carry his theory into effect the exertion was relinquished. In 1673 Cassegraiu revived the subject, and proposed a modification of the form pre- * Reproduced, by permission, from Cassell's 'New Popular Educator.' ITS INVENTION AND DEVELOPMENT. 11 viously indicated by Gregory. Instead of the small concave mirror, he substituted a convex mirror placed nearer the speculum ; and this arrangement, though it made the tele- scope shorter, had the disadvantage of displaying objects Fig. 5. Cassegrainian Telescope. in an inverted position. But the utility of these instruments was not demonstrated in a practical form until 1674, when Hooke, the clever mechanician, gave his attention to the sub- ject and constructed the first one that was made of the kind. In the meantime (1672) Sir Isaac Newton had completed with his own hands a reflecting-telescope of another pattern. In this the rays from the large concave speculum were re- ceived by a small plane mirror fixed centrally at the other Fig. 6. Newtonian Telescope. end of the tube, and inclined at an angle of 45 ; so that the image was directed at right angles through an opening in the side, and there magnified by the eye-lens. But for a long period little progress was effected in regard to reflecting- telescopes, owing to the difficulty of procuring metal well adapted for the making of specula. In 1729 Mr. Chester Moor Hall applied himself to the study of refracting-telescopes and discovered that, by a combination of different glasses, the colouring of the images might be eliminated. It is stated that Mr. Hall made several achromatic glasses in 1733. A quarter of a century after this 10 THE TELESCOPE, judicial effect in delaying the solution of the difficulty, which was not accomplished until many years afterwards. Fig. 3. Sir Isaac Newton *. The idea of reflecting-telescopes received mention as early as 1639 ; but it was not until 1663 that Gregory described the instrument, formed of concave mirrors, which still bears his name. He was not, however, proficient in mechanics. Fig. 4. Jt.v---- ; Gregorian Telescope. and after some futile attempts to carry his theory into effect the exertion was relinquished. In 1673 Cassegraiu revived the subject, and proposed a modification of the form pre- * Reproduced, by permission, from Cassell's { New Popular Educator.' ITS INVENTION AND DEVELOPMENT. 11 viously indicated by Gregory. Instead of the small concave mirror, he substituted a convex mirror placed nearer the speculum ; and this arrangement, though it made the tele- scope shorter, had the disadvantage of displaying objects Fig. 5. Cassegrainian Telescope. in an inverted position. But the utility of these instruments was not demonstrated in a practical form until 1674, when Hooke, the clever mechanician, gave his attention to the sub- ject and constructed the first one that was made of the kind. In the meantime (1672) Sir Isaac Newton had completed with his own hands a reflecting-telescope of another pattern. In this the rays from the large concave speculum were re- ceived by a small plane mirror fixed centrally at the other Fig. 6. Newtonian Telescope. end of the tube, and inclined at an angle of 45 ; so that the image was directed at right angles through an opening in the side, and there magnified by the eye-lens. But for a long- period little progress was effected in regard to reflecting- telescopes, owing to the difficulty of procuring metal well adapted for the making of specula. In 1729 Mr. Chester Moor Hall applied himself to the study of refracting-telescopes and discovered that, by a combination of different glasses, the colouring of the images might be eliminated. It is stated that Mr. Hall made several achromatic glasses in 1733. A quarter of a century after this 12 THE TELESCOPE, John Dollond independently arrived at the same result, and took out a patent for achromatic telescopes. He found, by experiments with prisms, that crown and flint glass operated unequally in regard to the divergency of colours induced by refraction \ and, applying the principle further, he obtained a virtually colourless telescope by assorting a convex crown lens with a concave flint lens as the object- glass. Dollond also made many instruments having triple Fig. 7. \ Common Refracting-Telescope. object-lenses, and in these it was supposed that previous defects were altogether obliterated. Two convex lenses of crown glass were combined with a concave lens of flint glass placed between them. Whether we regard Hall or Dollond as entitled to the most praise in connection with this important advance, it is certain that it was one the value of which could hardly be overestimated. It may be said to have formed a new era in practical astronomy. Instruments only 4 or 5 feet long could now be made equally if not more effective than those of 123 and 150 feet previously used by Huygens and Hevelius. All the troubles incidental to these long un- manageable machines now disappeared, and astronomers were at once provided with a handy little telescope capable of the finest performances. Reflecting-telescopes also underwent marked improvements in the eighteenth century. Short, the optician, who died in 1768, was deservedly celebrated for the excellent instruments he made of the Gregorian form. Towards the latter part of the century William Herschel, by indomitable perseverance, figured a considerable number of specula. Some of these ITS INVENTION AND DEVELOPMENT. 13 were mounted as Newtonians ; others were employed in the form known as the " Front view," in which a second mirror is dispensed with altogether, and the rays from the large concave speculum are thrown to the side of the tube and Fig. 8. The Le Mairean or Herschelian Telescope. direct to the eyepiece. This construction is often mentioned as the " Herschelian," but the idea had long before been detailed by Le Maire. In 1728 he presented a paper to the Academie des Sciences, giving his plans for a new reflecting- telescope. He proposed to suppress the small flat speculum in Newtonians, and " by giving the large concave speculum a little inclination, he threw the image, formed in its focus, to one side of the tube, where, an eyeglass magnifying it, the observer viewed it, his back at the time being turned towards the object in the heavens ; thus the light lost in the New- tonian telescope by the second reflexion was saved." After making several instruments of from 18 to 24 inches aperture, Herschel began one of larger calibre, and it was finished on August 28, 1789. The occasion was rendered historical by the discovery of one of the faintest interior satellites of Saturn, Bnceladus. The large telescope had a speculum 48 inches in diameter ; the tube was made of rolled or sheet iron, and it was 39 ft. 4 in. long and 4 ft. 10 in. in diameter. It was by far the largest instrument the world had seen up to that time ; but it cannot be said to have realized the expectations formed of its powers, for its defining properties were evidently not on a par with its space-penetrating power. Many of HerscheFs best obser- vations were made with much smaller instruments. The large telescope, which was mounted in HerscheFs garden at Slough, soon fell into comparative disuse, and, regarding it as incapable of further usefulness, Sir John Herschel sealed it up on January 1, 1840. 16 THE TELESCOPE, 5-foot telescope is undoubtedly of much greater capacity than the colossal reflector of Lord Rosse, though it is not so large. Mr. Calver has recently figured a 50-inch mirror for Sir H. Bessemer, but the mounting is not completed ; and he is expecting to make other large reflectors, viz. one over 5 feet in diameter and another over 3 feet. The late Mr. Nasmyth also erected some fine instruments, and adopted a combination of the Cassegrainian and Newtonian forms to ensure greater convenience for the observer. Instead of per- mitting the rays from the small convex mirror to return through the large mirror, he diverted them through the side of the tube by means of a flat mirror, as in Newtonians. But this construction is not to be commended, because much light is lost and defects increased by the additional mirror. Smaller telescopes of the kind we have been referring to have become extremely popular : and deservedly so. They are likely to maintain their character in future years ; for the Newtonian form of instrument, besides being thoroughly effective in critical work, is moderate in price and gives images absolutely achromatic. Moreover, it is used with a facility and ease which an experienced observer knows how to appreciate. Whatever may be the altitude of the objects under scrutiny, he is enabled to retain a perfectly convenient and natural posture, and may pursue his work during long intervals without any of the fatigue or discomfort incidental to the use of certain other forms of instrument. Returning now to refractors : many years elapsed after Dollond patented his achromatic object-glass before it was found feasible to construct these instruments of a size sufficient to grasp faint and delicate objects. Opticians were thwarted in their efforts to obtain glass of the requisite purity for lenses, unless in small disks very few inches in diameter. It is related that Dollond met with a pot of uncommonly pure flint glass in 1760, but even with this advantage of material he admitted that, after numerous attempts, he could not provide really excellent object-glasses of more than 3j-inches diameter. It may therefore be readily imagined that a refractor of 4J or 5-inches aperture was an instrument ITS INVENTION AND DEVELOPMENT. 17 Fig. 9. 10-inch Reflecting-Telescope on a German Equatoreal, by Calver. 18 THE TELESCOPE, of great rarity and expense. Towards the latter part of the 18th century Tulley's price was 275 for a 5-inch equatoreally mounted. In later years marked improvements were effected in the manufacture of glass. A sign of this is apparent in the fact that, in 1829, Sir James South was enabled to purchase a 12-inch lens. Four years before this the Dorpat telescope, having an objective of 9^ inches, had created quite a sen- sation. As time went on, still larger glasses were made. In 1862 Alvan Clark & Sons, of New York, U.S.A., finished an instrument of 18J-inches aperture, at a cost of 3700 ; and in 1869 Cooke & Sons mounted a 24' 6-inch object-glass for the late Mr. Newall, of Grateshead. The latter instrument was much larger than any other refractor hitherto made, but it was not long to maintain supremacy. One of 25*8 inches and 29-feet focus was finished in 1872 by Alvan Clark & Sons for the Naval Observatory, Washington, at a cost of 9000. Another, of similar size, was supplied by the same firm to Mr. McCormick, U.S.A. Several important disco- veries, including the satellites of Mars, were effected with the great Washington telescope. A few years later a 27-inch was completed by Grubb for the Vienna Observatory, and quite recently the four largest refractors ever made have been placed in position and are actively employed in various departments of work. These include a 29-inch by Martin for the Paris Observatory, a 30-inch by Henry Bros, for Nice, a 30-inch by A. Clark & Sons, for Pulkowa, and a 36-inch, also by A. Clark & Sons, for the Lick Observatory on Mount Hamilton in California. The latter has no rival in point of size, though rumours are current that still larger lenses are in contemplation. The tube of the 36-inch is 56 feet long and 3^ feet in diameter at the ends, but the dia- meter is greater in the middle. It is placed within a great dome 75 feet in diameter. The expense of the entire appa- ratus is given as follows : Cost of the dome, $5 6,850 ; of the visual objective, $53,000 ; of the photographic objective, $13,000; of the mounting, $42,000. Total, $164,850. This noble instrument due to the munificence of one indi- vidual, the late Mr. James Lick, of Chicago, who bequeathed ITS INVENTION AND DEVELOPMENT. 19 $700,000 for the purpose may be regarded as the king of refracting-telescopes. Placed on the summit of Mount Hamilton, where the atmosphere is exceptionally favourable for celestial observations, and utilized as its resources are by some of the best observers in America, we may confidently expect it to largely augment our knowledge of the heavenly bodies. The great development in the powers of both refracting and reflecting-telescopes, as a means of astronomical dis- covery, exemplifies in a remarkable degree the ever- increasing resources and refinements of mechanical art. In 1610 Galilei, from his window at Padua, first viewed the moon and planets with his crude instrument having a power of 3, and he achieved much during the remaining years he lived, by increasing it tenfold, so that at last he could magnify an object 30 times. Huygens laboured well in the same field ; and others who succeeded him formed links in the chain of progress which has almost uninterruptedly run through all the years separating Galilei's time from our own. The primitive efforts of the Florentine philosopher appear to have had their sequel in the magnificent telescope which has lately been erected under the pure sky of Mount Hamilton. The capacity of this instrument relatively to that of earlier ones may be judged from the fact that a power of about 3300 times has lately been employed with success in the measurement of a close and difficult double star. Could Galilei but stand for a few moments at the eyepiece of this great refractor, and contemplate the same objects which he saw, nearly three centuries ago, through his imperfect little glasses at Padua, he would be appalled at the splendid achievements of modern science. C 2 RELATIVE MERITS OF CHAPTER II. RELATIVE MERITS OF LARGE AND SMALL TELESCOPES. THE number of large telescopes having so greatly increased in recent rears, and there being every prospect that the demand for such instruments will continue, it may be well to consider their advantages as compared with those of much inferior size. Object-glasses and specula will probably soon be made of a diameter not hitherto attained ; for it is palpably one of the ambitions of the age to surpass all previous efforts in the way of telescopic construction. There are some who doubt that such enormous instruments are really necessary, and question whether the results obtained with them are sufficient return for the great expense involved in their erection. Large instruments require large obser- vatories ; and the latter must be at some distance from a town, and in a locality where the atmosphere is favourable. Nothing can be done with great aperture in the presence of smoke and other vapours, which, as they cross the field, become ruinous to definition. Moreover, a big instrument is not to be manipulated with the same facility as a small one : and when anything goes wrong with it, ifs rectification may be a serious matter, owing to the size. Such telescopes need constant attention if they would be kept in thorough working order. On the other hand, small instruments involve little outlay, they are very portable, and require little space. They may be employed in or out of doors, according to the inclina- tion and convenience of the observer. They are controlled with the greatest ease, and seldom get out of adjustment. They are less susceptible to atmospheric influences than larger instruments, and hence may be used more frequently with success and at places by no means favourably situated in this respect. Finally, their defining powers are of such LARGE AND SMALL TELESCOPES. 21 excellent character as to compensate in a measure for feeble illumination. In discussing this question it will be advisable to glance at the performances of certain instruments of considerable size. The introduction of really large glasses dates from a century ago, when Sir W. Herschel mounted his reflector, 4 feet in aperture, at Slough. He discovered two of the inner satellites of Saturn very soon after it was completed ; but apart from this the instrument seems to have achieved little. Herschel remarked that on August 28, 178^, when he brought the great instrument to the parallel of Saturn, he saw the spots upon the planet better than he had ever seen them before. The night was probably an exceptionally good one, for we do not find this praise reiterated. Indeed, Herschel appears to have practically discarded his large instrument for others of less size. He found that with his small specula of 7-ft. focus and 6'3-in. aperture he had " light sufficient to see the belts of Saturn completely well, and that here the maximum of distinctness might be much easier obtained than where large apertures are concerned.-"' Even in his sweeps for nebulae he employed a speculum of 20-ft. focus and 18J-in. aperture in preference to his 4-ft. instrument, though on objects of this nature light-grasping power is essentially necessary. The labour and loss of time involved in controlling the large telescope probably led to its being laid aside for more ready means, though Herschel was not the man to spare trouble when an object was to be gained. His life was spent in gleaning new facts from the sky ; and had the 4 -foot served his purpose better than smaller instruments, no trifling obstacle would have deterred him from its constant employment. But his aim was to accomplish as much as possible in every available hour when the stars were shining, and experience doubtless taught him to rely chiefly upon his smaller appliances as being the most serviceable. The Le Mairean form, or u Front view," which Herschel adopted for the large instrument may quite possibly have been in some degree responsible for its bad definition. Lord Rosse's (>-ft. reflector has now been used for nearly half a century, and its results ought to furnish us with good 22 RELATIVE MERITS OF evidence as to the value of such instruments. It has done important work on the nebulae, especially in the reobservation of the objects in Sir J. Herschel's Catalogues of 1833 and 1864. To this instrument is due the discovery of spiral Fig. 10. Lord Rosse's 6-foot Reflecting-Telescope. nebulae ; and perhaps this achievement is its best. But when we reflect on the length of its service, we are led to wonder that so little has been accomplished. For thirty years the satellites of Mars eluded its grasp, and then fell a prize to one of the large American telescopes. The bright planets * have been sometimes submitted to its powers, and careful drawings * Such objects show considerable glare in a very large instrument. The advent of Jupiter into the field of the 6-foot has been compared to the brightness of a coach-lamp. The outer satellite of Mars was seen twice with this instrument in 1877, "but the glare of the planet was found too strong to allow of good measures being taken." LARGE AND SMALL TELESCOPES. 23 executed by good observers ; but they show no extent of detail beyond what may be discerned in a small telescope. This does not necessarily impugn the figure of the large speculum, the performance of which is entirely dependent upon the condition of the air. The late Dr. Robinson, of Armagh, who had the direction of the instrument for some time, wrote in 1871: "A stream of heated air passing before the telescope, the agitation and hygrometric state of the atmosphere, and any differences of temperature between the speculum and the air in the tube are all capable of injuring or even destroying definition, though the speculum were absolutely perfect. The effect of these disturbances is, in reflectors, as the cube of their apertures ; and hence there are few hours in the year when the 6-foot can display its full powers." Another of the regular observers, Mr. G. J. Stoney, wrote in 1878 : " The usual appearance [of the double star y 2 Andromedse] with the best mirrors was a single bright mass of blue light some seconds in diameter, and boiling violently/' On the best nights, however, " the disturbance of the air would seem now and then suddenly to cease for perhaps half a second, and the star would then instantly become two very minute round specks of white light, with an interval between which, from recollection, I would estimate as equal to the diameter of either of them, or perhaps slightly less. The instrument would have furnished this appearance uninterruptedly if the state of the air had permitted." The present observer in charge, Dr. Boeddicker, wrote the author in 1889 : " There can be no doubt that on favourable nights the definition of the 6-foot is equal to that of any instrument, as is fully shown by Dr. Copeland's drawings of Jupiter published in the ' Monthly Notices ' for March 1874. It appears to me, however, that the advantage in going from the 3-foot to the 6-foot is not so great in the case of planets as in the case of nebulae ; yet, as to the Moon, the detail re\ 7 ealed by the 6-foot on a first-class night is simply astounding. The large telescope is a Newtonian mounted on a universal joint. For the outlying portions of the great drawing of the Orion nebula it was used as a Herschelian. As to powers profitably to be used, I find 24 RELATIVE MERITS OF no advantage in going beyond 600 ; yet formerly on short occasions (not longer than perhaps 1 hour a night) very much higher powers (over 1000) have been successfully employed by my predecessors." Mr. Lassell's 4-foot reflector was taken to Malta, and while there its owner, assisted by Mr. Marth, discovered a large number of nebulae with it, but it appears to have done nothing- else. His 2-foot reflector, which he had employed in previous years, seems to have been his most effective instru- ment ; for with this he discovered Ariel and Umbriel, the two inner satellites of Uranus, Hyperion, the faintest satellite of Saturn, and the only known satellite of Neptune. He also was one of the first to distinguish the crape ring of Saturn. Mr. Lassell had many years of experience in the use of large reflectors ; and in 1871 he wrote : " There are formidable and, I fear, insurmountable difficulties attending the con- struction of telescopes of large size. . . . These are, primarily, the errors and disturbances of the atmosphere and the flexure of the object-glasses or specula. The visible errors of the atmosphere are, I believe, generally in proportion to the aperture of the telescope. . . . Up to the size [referring to an 8-in. O.-Gr.] in question, seasons of tranquil sky may be found when its errors are scarcely appreciable ; but when we go much beyond this limit (say to 2 *feet and upwards), both these difficulties become truly formidable. It is true that the defect of flexure may be in some degree eliminated, but that of atmospheric disturbance is quite unassailable. These cir- cumstances will always make large telescopes proportionately less powerful than smaller ones ; but notwithstanding these disadvantages they will, on some heavenly objects, reveal more than any small ones can." Mr. Lassell's last sentence refers to " delineations of the forms of the fainter nebulae," to " seeing the inner satellites of Uranus, the satellite of Neptune, and the seventh satellite of Saturn." He mentions that, when at Malta, he " saw, in the 2-foot equatoreal, with a power of 1027, the two components of y 2 Andromedse distinctly separated to the distance of a neat diameter of the smaller one. Now, no telescope of anything like 8-inches diameter could exhibit the star in this style."- LARGE AM) SMALL TELESCOPES. 25 The large Cooke refractor of 24'8-inches aperture, which has been mounted for about twenty years at Gatesliead, has a singularly barren record. Its atmospheric surroundings appear to have rendered it impotent. The owner of this fine and costly instrument wrote the author in 1885: "Atmo- sphere has an immense deal to do with definition. I have only had one fine night since 1870 ! I then saw what I have never seen since." The Melbourne reflector of 4-feet aperture performed very indifferently for some years, and little work was accomplished with it. Latterly its performance has been more satis- factory ; excellent photographs of the Moon have been taken, and it has been much employed in observations of nebulae. The speculum having recently become tarnished, it has been dismounted for the purpose of being repolished. The silver-on-glass reflector of 47'2-in. diameter, at the Paris Observatory, was used for some years by M. Wolf, who has also had the control of smaller telescopes. He was in a favourable position to judge of their relative effectiveness. In a lecture delivered at the Sardonne on March 6, 1886, he said : " During the years I have observed with the great Parisian telescope I have found but one solitary night when the mirror was perfect.''' Further on, he adds : " 1 have observed a great deal with the two instruments [both re- flectors] of 15' 7 inches and 47*2 inches. I have rarely found any advantage in using the larger one when the object was sufficiently luminous." M. Wolf also avers that a refractor of 15 inches or reflector of 15*7 inches will show everything -in the heavens that can be discovered by instruments of very large aperture. He always found a telescope of 15' 7-inch aperture surpass one of 7*9 inches, but expresses himself con- fidently that beyond about 15 inches increased aperture is no gain. The Washington refractor of 25'8 inches effected a splendid success in Prof. Hall's hands in 1877, when it revealed the two satellites of Mars. But immediately afterwards these minute bodies were shown in much smaller instruments ; whence it became obvious that their original discovery was not entirely due to the grasp of the 25 "8-inch telescope, but 26 RELATIVE MERITS OF in a measure to the astuteness displayed by Prof. Hall in the search. A good observer had been associated with a good telescope ; and an inviting research having been undertaken, it produced the natural result an important success. The same instrument, in the same hands, enabled the rotation- period of Saturn to be accurately determined by means of a white spot visible in December 1876 on the disk of the planet, and which was subsequently seen by other observers with smaller glasses. Good work in other directions has also been accomplished at Washington, especially in observations of double stars and faint satellites. But notwithstanding these excellent performances, Prof. Hall expressed himself in rather disparaging terms of his appliances, saying " the large telescope does not show enough detail." He gave a more favourable report in 1888 ; for we find it stated that " the objective retains its figure and polish well. By comparison with several other objectives which Prof. Hall has had an opportunity of seeing during recent years, he finds that the glass is an excellent one." Prof. Young, who has charge of the 23-inch refractor at Princeton, has also commented on the subject of the definition of large telescopes. He says : " The greater susceptibility of large instruments to atmospheric distur- bances is most sadly true ; and yet, on the w r hole, I find also true what Mr. Clark told me would be the case on first mounting our 23-inch instrument, that / can almost always see witli the 23-inch everything I see with the $\-incli under the same atmospheric conditions, and see it better, if the seeing is bad only a little better, if good immensely better." Prof. Young also mentioned that a power of 1200 on the 23-inch " worked perfectly on Jupiter on two different evenings in the spring of 1885 in bringing out fine details relating to the red spot and showing the true forms of certain white dots on, the S. polar belt." The 26-inch refractor at the Leander McCormick Obser- vatory, U. S. A., is successfully engaged in observations of nebulae, and many new objects of this character have been found. It does not appear that the telescope is much used for other purposes ; so that we can attach no significance to LARGE AND SMALL TELESCOPES. 27 the fact that important discoveries have not been made with it in other departments. The great Vienna refractor of 2 7 -inches aperture "does not seem to accomplish quite what was expected of it," according to Mr. Sawerthal, who recently visited the Observatory at Wahring, Vienna. The Director, Dr. Weiss, states in his last report that " the 27-inch Grubb refractor has only been occasionally used, when the objects were too faint for the handier instruments." The still larger telescopes erected at the Observatories at Pulkowa and Nice have so recently come into employment that it would be premature to judge of their performance. In the Annual Report from Pulkowa (1887) it is stated that Dr. H. Struve was using the 30-inch refractor " in mea- suring those of Burnham's double stars which are only seldom measurable with the ' old 15-inch,' together with other stars of which measures are scarce. He made 460 measures in eight or nine months, as well as 166 micro- metric observations of the fainter satellites of Saturn and 15 of that of Neptune." At Nice the 30-inch refractor was employed by M. Perrotin in physical observations of Mars in May and June 1888. The canal-shaped markings of Schiaparelli were confirmed, and some of them were traced " from the ocean of the southern hemisphere right across both continents and seas up to the north polar ice-cap." The 30-inch also showed some remarkable changes in the markings ; but these were not confirmed at other obser- vatories. The telescope evidently revealed a considerable amount of detail on this planet ; whence we may infer that its defining power is highly satisfactory. The great Lick refractor, which appears to have been " first directed to the heavens from its permanent home on Mount Hamilton on the evening of January 3, 1888," has been found ample work by the zealous astronomers who have it in charge. Prof. Holden, in speaking of it, says : " It needs peculiar conditions, but when all the conditions are favourable its performance is superb." Mr. Keeler, one of the observers, writes that, on January 7, 1888, when Saturn was examined, " he not only shone with the brilliancy due to the great size of 28 RELATIVE MERITS OF the objective, but the minutest details of his surface were visible with wonderful distinctness. The outlines of the rings were very sharply defined with a power of 1000." Mr. Keeler adds : " According to my experience, there is a direct gain in power with increase of aperture. The 12-inch equatoreal brings to view objects entirely beyond the reach of the (3^- inch telescope, and details almost beyond perception with the 12-inch are visible at a glance with the 36-inch equatoreal. The great telescope is equal in defining power to the smaller ones." This is no small praise, and it must have been extremely gratifying, not only to those who were immediately associated with the construction of the telescope, but to astro- nomers everywhere who were hoping to hear a satisfactory report. In its practical results this instrument has not yet, it is true, given us a discovery of any magnitude. It has dis- closed several very small stars in the trapezium of the Orion nebula, some difficult double stars have been found and mea- sured, and some interesting work has been done on the planets and nebulae. Physical details have been observed in the ring nebula, between /3 and 7 Lyras, which no other telescope has ever reached before. Mr. Common's 5-foot reflector has been employed on several objects. In the spring of 1889 Uranus was fre- quently observed with it, and several minute points of light, suspected to be new satellites, were picked up. Evidence was obtained of a new satellite between Titania and Umbriel ; but bad weather and haze, combined with the low altitude of Uranus, interfered with the complete success of the observa- tions. " With only moderate powers, Uranus does not show a perfectly sharp disk. No markings are visible on it, and nothing like a ring has been seen round it." Mr. Common, in a letter to the writer, dated Kovember 9, 1889, says: " The 5-foot has only been tried in an unfinished state as yet, the mirror not being quite finished when put into the tube 'last year. This was in order to gain experience and save the season, it performed much better than I had hoped, and is greatly superior to the 3-foot. I took some very fine photo- graphs with it last year. It has been refigured, or rather completed, this summer, and has just been resilvered." From LARGE AND SMALL TELESCOPES. 29 this it is evident that Mr. Common's large instrument has not yet been fully tested ; but it clearly gives promise of suc- cessful results, and encourages the hope that it will exert an influence on the progress of astronomy. Owing to the highly reflective quality of silvered glass, the 5-foot speculum has a far greater command of light (space-penetrating power) than the great objective mounted at the Lick Observatory. Mr. Common's mirror may therefore be expected to grasp nebulas, stars, satellites, and comets which are of the last degree of faintness and quite invisible in the Lick refractor. But we must not forget that the latter instrument is certainly placed in a better atmosphere, and that its action is not therefore arrested in nearly the same degree by haze and undulations of the air. With equal conditions, the great reflector at Baling would probably far surpass the large refractor we have referred to, the latter having less tjian one third of the light-grasping power of the former. This rapid sketch of the performances of some of our finest telescopes must suffice for the present in assisting us to estimate their value as instruments of discovery. And it must be admitted that, on the whole, these appliances have been disappointing. The record of their successes is by no means an extended one, and in some individual cases absolute failure is unmistakable. We must judge of large glasses by their revelations ; their capacity must be estimated by results. We often meet with glowing descriptions of colossal tele- scopes : their advantages are specified and their performances extolled to such a degree that expectation is raised to the highest pitch. But it is not always that such praise is justified by facts. The fruit of their employment is rarely prolific to the extent anticipated, because the observers have been defeated in their efforts by . impediments which in- separably attend the use of such huge constructions. Our atmosphere is always in a state of unrest. Its con- dition is subject to many variations. Heat, radiated or evolved from terrestrial objects, rises in waves and floats along with the wind. These vapours exercise a property of refraction, with the result that, as they pass in front of celestial objects, the latter at once become subject to a rapid 30 RELATIVE MERITS OF series of contortions in detail. Their outlines appear tre- mulous, and all the features are involved in a rippling effect that seriously compromises the definition. Delicate markings are quite effaced on a disk which is thus in a state of ebulli- tion ; and on such occasions observers are rarely able to attain their ends. Telescopic work is, in fact, best deferred until a time when the air has become more tranquil. In large instruments these disturbances are very troublesome, as they increase proportionately with aperture. They are so pronounced and so persistent as to practically annul the advantage of considerable light-grasping power ; for unless the images are fairly well defined, mere brightness counts for nothing. Keflectors are peculiarly susceptible to this obstacle ; moreover, the open tube, the fact that rays from an object pass twice through its length, and that a certain amount of heat radiated from the observer must travel across the mouth of the tube all serve to impair the definition. A speculum, to act well, must be of coincident temperature in every part. This is not always the case, owing to the variableness of the weather or to unequal exposure of the speculum. Large refractors, though decidedly less liable to atmospheric influences, are yet so much at the mercy of them that one of the first and most important things discussed in regard to a new instrument is that of a desirable site for it. The great weight of large objectives and specula tends to endanger the perfect consistency and durableness of their figure, and imposes a severe strain upon their cellular mount- ing. The glasses must obviously assume a variety of bearings during active employment. This introduces a possible cause of defective performance ; for in some instances definition has been found unequal, according to the position of the glass. Specula are very likely to be affected in this manner, as they are loosely deposited in their cells to allow of expan- sion, and the adjustment is easily deranged. The slightest flaw in the mounting of objectives immediately makes itself apparent in faulty images. Special precautions are of course taken to prevent flexure and other errors of the kind alluded to, and modern adaptations may be said to have nearly elimi- nated them ; but there is always a little outstanding danger, LARGE AND SMALL TELESCOPES. 31 from the ease with which glasses may be distorted or their adjustment become unsettled. Another difficulty formerly urged against telescopes of great size was the trouble of managing them; but this objection can scarcely be applied to the fine instruments of the present day, which are so contrived as to be nearly as tractable as small ones. A century ago, glass of the requisite purity for large objectives could not be obtained ; but this difficulty appears also to have quite disappeared. And the process of figuring lenses of considerable diameter is now effected with the same confidence and success as that of greatly inferior sizes. Let us now turn for a moment to the consideration of small instruments, premising that in this category are included all those up to about 12-inches aperture. Modern advances have quite altered our ideas as to what may be regarded as large and small telescopes. Sixty-five years ago the Dorpat refractor, with a 9J-inch objective by Fraun- hofer, was considered a prodigy of its class ; now it occupies a very minor place relatively to the 30-inch and 36-inch objectives at Nice, Pulkowa, and Mount Hamilton. Prof. Hall remarked, in 1885 : " There is too much scepticism on the part of those who are observing with large instruments in regard to what can be seen with small ones." This is undoubtedly true ; but a mere prejudice or opinion of this sort cannot affect the question we are discussing, as it is one essentially relying upon facts. Small instruments have done a vast amount of useful work in every field of astronomical observation. Even in the realm of nebulae, which, more than any other, requires great penetrating power, D'Arrest showed what could be effected with small aperture. Burnham, with only a 6-inch refractor, has equally distinguished himself in another branch ; for he has discovered more double stars than any previous observer. Dawes was one of the most successful amateurs of his day, though his instrumental means never exceeded an 8-inch glass. But we need not particularize further. It will be best to get a general result from the collective evidence of past years. We find that nearly all the comets, planetoids, RELATIVE MERITS OF Fiff. 11. Befracting-Telescope, by Browning. LARGE AND SMALL TELESCOPES. 33 double stars, &c. owe their first detection to comparatively small instruments. Our knowledge of sun-spots, lunar and planetary features is also very largely derived from similar sources. There is no department but what is inlebted more or less to the services of small telescopes : the good work they have done is due to their excellent defining powers and to the facility with which they may be used. We have already said that the record of discoveries made with really large instruments is limited ; but it should also be remarked that until quite recently the number of such instruments has been very small. And not always^ perhaps, have the best men had the control of them. Virtually the observer himself constitutes the most important part of his telescope : it is useless having a glass of great capacity at one end of a tube, and a man of small capacity at the other. Two different observers essentially alter the character of an instrument, according to their individual skill in utilizing its powers. Large telescopes are invariably constructed for the special purpose of discovering unknown orbs and gleaning new facts from the firmament. But in attempting to carry out this design, obstacles of a grave nature confront the observer. The comparatively tranquil and sharply definite images seen in small instruments disappear, and in their places forms are presented much more brilliant and expansive, it is true, but involved in glare and subject to constant agitation, which serve to obliterate most of the details. The observer becomes conscious that what he has gained in light has been lost in definition. At times perhaps on one occasion in fifty this experience is different ; the atmosphere has apparently assumed a state of quiescence, and objects are seen in a great telescope with the same clearness of detail as in smaller ones. It is then the observer fully realizes that his instrument, though generally ineffective, is not itself in fault, and that it would do valuable work were the normal condition of the air suitable to the exercise of its capacity. Those who have effected discoveries with large instruments have done so in spite of the impediment alluded to. Relying 1) 34 RELATIVE MERITS OF mainly upon great illuminating power, bad or indifferent definition has been tolerated ; and they have succeeded in detecting minute satellites, faint nebulas, clusters, and small companions to double stars. Telescopes of great aperture are at home in this kind of work. But when we come to con- sider discoveries on the surfaces of the Sun, Moon, and planets, the case is entirely different ; the diligent use of small appliances appears to have left little for the larger constructions to do. There are some thousands of drawings of the objects named, made by observers employing telescopes from 3 up to 72 inches in diameter ; and a careful inspection shows that the smaller instruments have not been outdone in this interesting field of observation. In point of fact they rather appear to have had the advantage, and the reason of this is perhaps sufficiently palpable. The details on a bright planetary object are apt to become obliterated in the glare of a large instrument. Even with a small telescope objects like Venus and Jupiter are best seen at about the time of sunset, and before their excessive brilliancy on the dark sky is enabled to act prejudicially in effacing the delicate markings. Probably this is one of the causes which, in combination with the undulations of the atmosphere, have restricted the discoveries of large instruments chiefly to faint satellites, stars, and nebube. Prof. Young ascribes many of the successes of small instru- ments to exceptional cuteness of vision on the part of certain observers, and to the fact that such instruments are so very numerous and so diligently used that it is fair to conclude they must reap the main harvest of discoveries. We must remember that for every observer working with an aperture of 18 inches and more, there are more than a hundred employing objectives or specula of from 5 to 12 inches ; hence we may expect some notable instances of keen sight amongst the latter. The success of men like Dawes and others, who outstrip their contemporaries, and with small glasses achieve phenomenal results, is to be ascribed partly to good vision and partly to that natural aptitude and per- tinacity uniformly characteristic of the best observers. These circumstances go far to explain the unproductiveness of large LARGE AND SMALL TELESCOPES. 35 telescopes : in the race for distinction they are often distanced by their more numerous and agile competitors. The objections which applied to the large reflecting instru- ments of Herschel, Lassell, and Rosse scarcely operate with the same force in regard to the great refractors of the present day, and for these reasons : Refractors are somewhat less sensitive to atmospheric disturbances than reflectors. The modern instruments are mounted in much improved style, and placed in localities selected for their reception. In fact, all that the optician's art can do to perfect such appliances has been done, and Nature herself has been consulted as to essentials ; for we find the most powerful refractor of all erected on the summit of Mount Hamilton, where the skies are clear and Urania ever smiles invitingly. Some observers who have obtained experience both with large and small telescopes aver that, even on a bright planet, they can see more, and often see it much better, with the larger glasses. But we rarely, if ever, find them saying they can discern anything which is absolutely beyond the reach of small instruments. It would be much more satisfactory evidence of the super-excellence of the former if definite features could be detected which are quite beyond the reach of telescopes of inferior size ; but we seldom meet with experiences of this kind, and the inference is obvious. There is undoubtedly a certain aperture which combines in itself sufficient light-grasping power with excellent definition. It takes a position midway between great illuminating power and bad definition on the one hand, and feeble illuminating power and sharp definition on the other. Such an aperture must form the best working instrument in an average situation upon ordinary nights and ordinary objects. M. Wolf fixes this aperture at about 15 inches, and he is probably near the truth. The quaint Dr. Kitchener, who, early in the present century, made a number of trials with fifty-one telescopes, entertained a very poor opinion of big instruments. In his book on l Telescopes,' he says : " Immense telescopes are only about as useful as the enormous spectacles suspended over the doors of opticians.". . .''Astronomical amateurs should D 2 36 RELATIVE MERITS OF rather seek for perfect instruments than large ones. What good can a great deal of bad light do ? " We shall be in a better position a few years hence to estimate the value of great telescopes ; for the principal instruments of this class have only been completed a short time. Judging from the statements of some of the observers, who are men of the utmost probity and ability, certain of the large instruments are capable of work far in advance of anything hitherto done. Definition, they say, is excellent, notwithstanding the great increase of aperture. The old stumbling-block appears, therefore, to have been removed, and astronomy is to be congratulated on the acquirement of such vastly improved implements of research. Even should the large telescopes continue to prove disappointing in certain branches, they may certainly be expected to maintain their advantage in others. They will always be valuable as a cor- rective to smaller and handier instruments. For special lines of work in which very small or very faint objects are con- cerned, considerable light-grasping power is absolutely required ; and it is chiefly in these departments that large instruments may be further expected to augment our know- ledge. In photographic and spectroscopic work they also have a special value, which late researches have brought pro- minently to the fore. The telescopes of the future will probably surpass in dimensions those of our own day. The University of Los Angelos, in California, propose to erect a 42-inch refractor on the summit of Wilson's Peak of the Sierra Madre mountains, which is 6000 feet high and about 25 miles from Los Angelos. In reference to this con- templated extension of size, it may be opportune to mention that large objectives do not transmit light proportionately with their increased diameter, owing to greater thickness of the lenses, which increases the absorption. The Washington objective of 25*S-inch aperture is 2*87 inches in thickness, and more than half the light which falls upon it is lost by absorption. On the other hand, specula, with every enlarge- ment of aperture, give proportionately more light-grasping power, and their diameters might be greatly increased but for LARGE AND SMALL TELESCOPES. 37 the mechanical obstacles in the way of their construction. Mr. Ranyard expresses the opinion that " with the refractor we are fast approaching the practical limit of size." Ai'ter referring to the Washington object-glass as above, he says: " If we double the thickness, more than three quarters of the light would be absorbed and less than one quarter would be transmitted. The greatest loss of light is only for the centre of the object-glass; but in all parts the. absorption is qua- drupled for a lens of double aperture." If, therefore, future years see any great development in the sizes of telescopes, it will probably be in connection with reflectors ; for the loss of light by absorption in the thick lenses of large refractors must ultimately determine their limits. Mr. Calver says : " The light of reflectors exceeding 18 inches in diameter is certainly greater than that of refractors of equal size, and for anything like 3 feet very much greater." He nearly obtained the order for a monster reflector for the Lick Observatory, the Americans admitting that the reflector must be the instru- ment of the future for power and light because there were practically no limits to its size. But the reflector has not been much used in America, and therefore is little known. For this reason the authorities decided to erect a large refractor, and they appear to have been justified in their selection, for the 36-inch objective has proved excellent* 38 NOTES ON TELESCOPES AND CHAPTER III. NOTES ON TELESCOPES AND THEIR ACCESSORIES. Choice of Telescopes. Refractors and Reflectors. Observer's Aims. Testing Telescopes. Mounting. Eyepieces. Requisite Powers. Over- stating Powers. Method of finding the Power. Field of Eyepiece. Limited Means no obstacle. Observing-Seats. Advantage of Equatoreals. Test-Objects. Cheapness and increasing number of Telescopes. Utility of Stops. Cleaning Lenses. Opera-Glasses. Dewing of Mirrors. Celestial Globe. Observatories. C/ioice of Telescopes. The subject of the choice of telescopes has exercised every astronomer more or less, and the question as to the best form of instrument is one which has occasioned endless controversy. The decision is an important one to amateurs, who at the outset of their observing careers require the most efficient instruments obtainable at reasonable cost. It is useless applying to scientific friends who, influenced by different tastes, will give an amount of contradictory advice that will be very perplexing. Some invariably recommend a small refractor and unjustly disparage reflectors, as not only unfitted for very delicate work, but as constantly needing readjustment and resilvering *. Others will advise a moderate-sized reflector as affording wonderfully fine views of the Moon and planets. The question of cost is greatly in favour of the latter construction, and, all things considered, it may claim an unquestionable advantage. A man who has decided to spend a small sum for the purpose not merely of gratifying his curiosity but of doing really serviceable work, must adopt the reflector, because refractors of, say, 5 inches and upwards are far too costly, and become enormously expensive as the diameter increases. This is not the case with reflectors ; they come * My 10-inch reflector by With-Browning was persistently used for four yeais without being resilvered or once getting out of adj ustmeut. THEIR ACCESSORIES. 39 within the reach of all, and may indeed be constructed by the observer himself with a little patience and ingenuity. Refractors and Reflectors. The relative merits of refractors and reflectors* have been so frequently compared and dis- cussed that we have no desire to re-open the question here. These comparisons have been rarely free from bias, or suffi- ciently complete to afford really conclusive evidence either way. There is no doubt that each form of instrument possesses its special advantages : aperture for aperture the refractor is acknowledged to be superior in light-grasping power, but the ratio given by different observers is not quite concordant. A silver-oii-glass mirror of 8-inches aperture is certainly equal to a 7-inch objective in this respect, while as regards dividing power and the definition of planetary markings, the reflector is equal to a refractor of the same aperture. The much shorter focal length of the reflector is an advantage not to be overlooked. A century ago Sir W. Herschel figured his specula to foci of more than a foot to every inch of aperture, except in the case of his largest instruments. Thus he made specula of 18 ^-inches and 24-inches diameter, the former of which had a focal length of 20 feet and the latter of 25 feet. The glass mirrors of the present time are much shorter, and the change has not proved incompatible with excellent performance. Calver has made two good mirrors of 17^- inches aperture, and only 8 ft. 4 in. focus. Mr. Common's 5-foot mirror is only 27^ feet, so that in these instances the length of the tube is less than six times the diameter. It has long been proved that refractors and reflectors alike are, in good hands, capable of producing equally good results ; and we may depend upon it that, in spite of all argument and experiment, both kinds of telescope will continue to hold their own until superseded by a new combination, which hardly seems likely. If the observer is free from prejudice, he will have no cause to deplore the character of his instru- ment, always supposing it to be by a good maker. Be it * In this and future references to reflectors the Newtonian form is alluded to. The direct-vision reflectors of Gregory and Cassegrain have gone out of use, and the present popularity of Newtonians may be regarded as a case of the " survival of the fittest." 40 NOTES ON TELESCOPES AND object-glass or speculum, he will rarely find it lacking in effectiveness. It happens only too often that the telescope or Fig. 12. " The Popular Eeflector " by Calver. the atmosphere is hastily blamed when the fault rests with the observer himself. Let him be persistent in waiting oppor- tunities, and let the instrument be nicely adjusted and in good THEIR ACCESSORIES. 41 condition, and in the great majority of cases it will perform all that can reasonably be expected of it. In choosing appliances for observational purposes, the observer will of course be guided by his means and require- ments. If his inclination lead him to enter a particular department of research, he will take care to provide himself with such instruments as are specially applicable to the work in hand. Modern opticians have effected so many improve- ments, and brought out so many special aids to smooth the way of an observer, that it matters little in which direction he advances ; he will scarcely find his progress impeded by want of suitable apparatus. In size, as also in character, the observer should be careful to discriminate as to what is really essential. Large instruments and high powers are not necessary to show what can be sufficiently well seen in a small telescope with moderate power. Of course there is nothing like experience in such matters, and practice soon renders one more or less proficient in applying the best available means. An amateur who really wants a competent instrument and has to consider cost, will do well to purchase a Newtonian reflector. A 4-J-inch refractor will cost about as much as a Fig. 13. 3-inch Refracting-Telescope, by Newton & Co. 10-inch reflector, but, as a working tool, the latter will possess a great advantage. A small refractor, if a good one, will do wonders, and is a very handy appliance, but it will not have sufficient grasp of light for it to be thoroughly serviceable on 42 NOTES ON TELESCOPES AND faint objects. Anyone who is hesitating in his choice should look at the cluster about ^ Persei through instruments such as alluded to, and he will be astonished at the vast difference in favour of the reflector. For viewing sun-spots and certain lunar objects small refractors are very effective, and star-images are usually better seen than in reflectors, but the latter are much preferable for general work on account of three important advantages, viz., cheapness, illuminating power, and con- venience of observation. When high magnifiers are employed on a refractor of small aperture, the images of planets become very faint and dusky, so that details are lost. Observer's Aims. If the intending observer merely requires a telescope to exhibit glimpses of the wonders which he has seen portrayed in books, and has no intention of pursuing the subject further than as an occasional hobby, he will do well to purchase a small refractor between 3 and 4 inches in aperture. Such instruments are extremely effective on the Sun and Moon, which are naturally the chief objects to attract attention, and, apart from this, appliances of the size alluded to may be conveniently used from an open window. The latter is an important consideration to many persons; more- over, a small telescope of this kind will reveal an astonishing- number of interesting objects in connection with the planets, comets, &c., and it may be employed by way of diversion upon terrestrial landscape, as such instruments are almost invariably provided with non-inverting eyepieces. Out-of-door ob_ serving is inconvenient in many respects, and those who procure a telescope merely to find a little recreation will soon acknowledge a small refractor to be eminently adapted to their purposes and conveniences. Those who meditate going farther afield, and taking up observation habitually as a means of acquiring practical know- ledge, and possibly of doing original work, will essentially need different means. They will require reflectors of about 8 or 10 inches aperture ; and, if mounted in the open on solid ground, so much the better, as there will be a more expansive view, and a freedom from heated currents, which renders an apartment unsuited to observations, unless with small aper- tures where the effects are scarcely appreciable. A reflector THEIR ACCESSORIES. 43 of the diameter mentioned will command sufficient grasp to exhibit the more delicate features of planetary markings, and will show many other difficult objects in which the sky abounds. If the observer be specially interested in the surface configuration of Mars and Jupiter he will find a reflector a remarkably efficient instrument. On the Moon and planets it is admitted that its performance is, if not superior, equal to that of refractors. If, however, the inclination of the observer leads him in the direction of double stars, their discovery and measurement, he will perhaps find a refractor more to be depended upon, though there is no reason why a well-mounted reflector should not be successfully employed in this branch ; and the cost of a refractor of the size to be really useful as an instrument of discovery must be something very considerable perhaps ten times as great as that of a reflector of equal capacity. As far as my own experience goes the refractor gives decidedly the best image of a star. In the reflector, a bright star under moderately high power is seen with rays extending right across the field, and these appear to be caused by the supports of the flat. Testing Telescopes. No amateur should buy an instrument, especially a second-hand one, without testing it, and this is a delicate process involving many points to be duly weighed. Experience is of great service in such matters, and is, in fact, absolutely necessary. Even old observers are sometimes misled as to the real worth of a glass. In such cases, there is nothing like having a reliable means of comparison, i. e. another telescope of acknowledged excellence with which to test the doubtful instrument. In the absence of such a stan- dard judgment will be more difficult, but with care a satisfactory decision may be arrived at. The Moon is too easy an object for the purpose of such trials ; the observer should rather select Venus or Jupiter. The former is, however, so brilliant on a dark sky, and so much affected with glare, that the image will almost sure to be faulty even if the glass is a good one. Let the hour be either near sunrise or sunset, and if the planet has a tolerably high altitude, her disk ought to be seen beautifully sharp and white. Various powers should be tried, increasing them each time, and it should be 44 NOTES ON TELESCOPES AND noticed particularly whether the greater expansion of the image ruins the definition or simply enfeebles the light. In a thoroughly good glass faintness will come on without seri- ously impairing the definite contour of the object viewed, and the observer will realize that the indistinctness is merely occasioned by the power being relatively in excess of the light-grasp. But in a defective telescope, a press of magni- fying power at once brings out a mistiness and confuses the details of the image in a very palpable manner. Try how he will, the observer will find it impossible to get rid of this, except, perhaps, by a " stop " which cuts off so much light that the instrument is ineffective for the work required of it. The blurred image is thought, at the moment of its first per- ception, to be caused by the object being out of focus, and the observer vainly endeavours to get a sharper image until he finds the source of error lies elsewhere. A well-figured glass ought to come very sharply to a focus. The slightest turn of the adjusting-screw should make a sensible difference. On the other hand, an inferior lens will permit a slight alter- ation of focusing without affecting the distinctness, because the rays from the image are not accurately thrown to a point. Jupiter is also a good test. The limbs of the planet, if shown clean and hard, and the belts, if they are pictured like the finely cut details of an engraving, will at once stamp a tele- scope as one of superior quality. Saturn can also be examined though not, perhaps, so severe a test. The belts, crape ring, Cassini's division, ought to be revealed in any telescope of moderate aperture. If, with regard to any of these objects, the details apparently run into each other and there is a u fuzzy" or woolly aspect about them which cannot be eliminated by careful focusing, then either the atmosphere or the telescope is in fault. If the former, another opportunity must be awaited. An observer of experience will see at a glance whether the cause lies in the air or the instrument. The images will be agitated by obnoxious currents, if the defects are due to the atmosphere, but if the glass itself is in error, then the objects will be comparatively tranquil but merged in hazy outlines, and a general lack of distinctness will be apparent. Perhaps the best test of all as to the efficiency of THEIR ACCESSORIES. 45 a telescope is that of a moderately bright star, say of the 2nd or 3rd magnitude. "With a high power the image should be very small, circular, and surrounded by two or three rings of light lying perfectly concentric with each other. No rays, wings, or extraneous appearance other than the diffraction rings should appear. This, however, specially applies to refractors, for in reflec- tors the arms of the flat occasion rays from any bright star ; I have also seen them from Mars, but of course this does not indicate an imperfect mirror. If there is any distortion on one side of the image, then the lenses are inaccurately centred though the* instrument may be otherwise good, and a little attention may soon set matters right. When testing a glass the observer should choose objects at fairly high altitudes, and not condemn a telescope from a single night's work unless the evidence is of unusually convincing character. If false colour is seen in a silver-on-glass reflector it is originated by the eyepiece, though not necessarily so in a refractor. The object-glass of the latter will be sure to show some uncorrected colour fringing a bright object. A good lens, when exactly focused, exhibits a claret tint, but within the focus purple is seen and beyond the focus green comes out. In certain cases the secondary spectrum of an object-glass is so inadequately corrected that the vivid colouring of the images is sometimes attributed by inexperienced observers to a real effect. A friend 'who used a 3-inch refractor once called on me to have a glimpse of Jupiter through my 10-inch With- reflector. On looking at the planet he at once exclaimed " But where are the beautiful colours, Mr. Denning ? " I replied to his question by asking another, viz., " What colours ? " he answered, " Why, the bright colours I see round Jupiter in my refractor ? " I said, " Oh, they exist in your telescope only ! " He looked incredulous, and when he left me that night did not seem altogether pleased with the appearance of Jupiter shorn of his false hues ! Mounting. Too much care cannot be given to the mounting of telescopes, for the most perfectly figured glass will be rendered useless by an inefficient stand ; a faulty lens, if thoroughly well mounted, will do more than a really good one on a shaky 46 NOTES ON TELESCOPES AND or unmanageable mounting. Whatever form is adopted, the arrangement should ensure the utmost steadiness, combined with every facility for readily following objects. A man who has every now and then to undergo a great physical exertion in bodily shifting the instrument is rendered unfit for delicate work. The telescope should be provided with every requisite for carrying on prolonged work with slight exertion on the part of the observer. Unless the stand is firm there will be persistent vibrations, especially if the instrument is erected in the open, for there are very few nights in the year when the air is quite calm. These contingencies should be provided against with scrupulous attention if the observer would render his telescope most effective for the display of its powers, and avoid the constant annoyance that must otherwise follow. Eyepieces. Good eyepieces are absolutely essential. Many object-glasses and specula have been deprecated for errors really originated by the eyepiece. Again, telescopes have not unfrequently been blamed for failures through want of discrimination in applying suitable powers. A consistent adaptation of powers according to the aperture of the tele- scope, the character of the object, the nature of the observa- tion, and the atmospheric conditions prevailing at the time, is necessary to ensure the best results. If it is required to exhibit a general view of Jupiter and his satellites to a friend, we must utilize a low power with a large field ; if, on the other hand, we desire to show the red spot and its configura- Fig. 14. Huygens's negative eyepiece. tion in detail, we must apply the highest power that is satisfactorily available. The negative or Huygenian eyepiece is the one commonly used, and it forms good colourless images, though the field is rather small. The positive or Ramsden THEIR ACCESSORIES. 47 eyepiece gives a flatter and larger field, but it is not often achromatic. A Kellner eyepiece, the feature of which is a very large field, is often serviceable in observations of nebulae, clusters, and comets. Telescopes are sometimes stated to bear 100 to the inch on planets, but this is far beyond their Fisr. 15. Ramsden's positive eyepiece. capacities even in the very best condition of air. Amateurs soon find from experience that it is best to employ those powers which afford the clearest and most comprehensive views of the particular objects under scrutiny. Of course when abnormally high powers are mentioned in connection with an observation, they have an impressive sound, but this is all, for they are practically useless for ordinary work. I find that 40, or at the utmost 50 to the inch, is ample, and generally beyond the capacities of my 10-inch reflector. A Barlow lens used in front of the eyepiece raises the power about one third, and thus a whole set of eyepieces may be increased by its insertion. It is said to improve the definition, while the loss of light is very trifling. I formerly used a Barlow lens in all planetary observations, but finally dispensed with it, as I concluded the improved distinctness did not compensate for the fainter image. A great advantage, both in light and definition, results from the employment of a single lens as eyepiece. True, the field is very limited, and, owing to the spherical aberration, the object so greatly distorted near the edges that it must be kept near the centre, but, on the whole, the superiority is most evident. By many careful trials I find it possible to glimpse far more detail in planetary markings than with the ordinary eyepiece. Dawes, and other able observers, also found a great advantage in the single lens, and Sir W. Herschel, more than a century ago, 48 NOTES ON TELESCOPES AND expressed himself thus : " I have tried both the double and single lens eye-glass of equal powers, and always found that the single eye-glass had much the superiority in light and distinctness." Requisite Powers. For general purposes I believe three eyepieces are all that is absolutely requisite, viz., a low power with large field for sweeping up nebulae and comets ; a moderate power for viewing the Moon and planets ; and a high power for double stars and the more delicate forms on the planets. For a 3-inch refractor, eyepieces of about 15, 75, and 150 would be best, and for a 10-inch reflector 40, 150, and 300. For very difficult double stars a still higher power will be occasionally useful, say 250 for the refractor, and 500 for the reflector. The definition usually suffers so much under high powers, and the tremors of the atmosphere are brought out so conspicuously, that the greater expansion of the image of a planet does not necessarily enable it to present more observable detail. The features appear diluted and merged in hazy outlines, and there is a lack of the bright, sharply determinate forms so steadily recognized under lower magnifiers. In special cases great power may become essential, and, under certain favourable circumstances, will prove really serviceable, but, in a general way, it is admitted that the lowest power which shows an object well is always the best. I have occasionally obtained very fair views of Saturn with a power of 865, but find that I can perceive more of the detail with 252. Some daylight observations of Venus were also effected under very high power, and, though the definition remained tolerably good, I found as the result of careful comparison that less power answered more satisfactorily. But it would be absurd to lay down in- violable rules in such cases. Special instruments, objects, and circumstances require special powers, and observers may always determine with a little care and experience the most eligible means to support their endeavours. One thing should be particularly remembered, that the power used must not be beyond the illuminating capacity of the instrument, for planetary features appear so faint and shady under exces- sive magnifiers that nothing is gained. To grasp details THEIR ACCESSORIES. 49 there must be a fair amount of light. I have seen more with 252 on my 10-inch reflector than with 350 on a 5^-inch refractor, because of the advantage from the brighter image in the former case. Overstating Powers. It seems to be a fashionable impo- sition on the part of opticians to overstate magnifying powers. Eyepieces are usually advertised at double their true strength. My own 10-inch reflector was catalogued as having four eyepieces, 100 to 600, but on trial I found the highest was no more than 330. This custom of exaggerating powers seems to have long been a privileged deception, and persons buying telescopes ought to be guarded against it. Dr. Kitchiner says it originated with the celebrated maker of reflectors, James Short, and justly condemns it as a practice which should be discontinued. I suppose it is thought that high powers advertised in connection with a telescope have an exalted sound and are calculated to attract the unwary purchaser ; but good instruments need no insidious trade artifices to make them saleable. The practice does not affect observers of experience, because it is well understood, and they take good care to test their eyepieces directly they get them. But the case is different with young and inexperienced amateurs, who naturally enough accept the words of respect- able opticians, only to find, in many cases, that they have been misleading and a source of considerable annoyance. Method of finding the Power. The magnifying power of a telescope may be determined by dividing the focal length of the object-glass or mirror by the focal length of the eye-lens. Thus, if the large glass has a focus of 70 inches and the eye- lens a focus of one inch, then the power is 70. If the latter is only ^-inch focus, the resulting power will be 280. But this method is only applicable to single lens eyepieces. "We may, however, resort to several other means of finding the powers of the compound eyepieces of Huygens or Ramsden. Let the observer fix a slip of white cardboard, say 1 inch wide, to a door or post some distance off, and then (with a refractor) view it, while keeping the disengaged eye open, and note the exact space covered by the telescopic image of the card as projected on the door seen by the other eye. The E 50 NOTES ON TELESCOPES AND number of inches included in the space alluded to will repre- sent the linear magnifying power. A brick wall or any surface with distinct, regularly marked divisions will answer the same purpose, the number of bricks or divisions covered by the telescopic image of one of them being equivalent to the power. But it should not be forgotten that a telescope magnifies slightly less upon a celestial object than upon a near terrestrial one owing to the shorter focus, and a trifling- allowance will have to be made for this. Another plan may be mentioned. When the telescope is directed to any fairly bright object or to the sky, and the observer removes his eye about 10 inches from the eyepiece, a sharply defined, bright little disk will be perceived in the eye-lens. If the diameter of this disk is ascertained and the clear aperture of the object- glass or mirror is divided by it, the quotient will be the magnifying power. Thus, if the small circle of light is '2 inch diameter and the effective aperture of the large glass 5 inches, then the power is 25. If the former is '02 inch dia- meter and the latter 7*5 inches, the power will be 375. The dynamometer is a little instrument specially designed to facili- tate this means of fixing the magnifying power. It enables the diameter of the small luminous circle in the eye-lens to be very accurately measured, and this is a most important factor in deriving the power by this method. Fig. 16. .02 -04 -06 . -08 . -10 . -12 , -14 , -16 , "If , ; 2 , 1111 BERTHON'S DYNAMOMETER. HORNZÞTHWAITE LONDON. Field of Eyepiece. Observers often require to know the diameter of the fields of their eyepieces. Those engaged in sweeping up comets, nebulae, or other objects requiring large fields and low powers, find it quite important to have this information. They may acquire it for themselves by simple methods. A planet, or star such as S Orionis, 97 or 7 Yirginis, or ?; Aquilse, close to the equator, should be allowed to run THEIR ACCESSORIES. 51 exactly through the centre of the field, and the interval occupied in its complete transit from ingress to egress noted several times. The mean result in min. and sec. of time must then he multiplied by 15, and this will represent the diameter required in min. and sec. of arc on the equator. A planet or star near the meridian is the best for the purpose. If the object occupies 1 min. 27 sec. of time in passing from the E. to the W. limit of the field, then 87 sec. x 15 = 1305", or 21' 45". A more accurate method of deriving the angle subtended by the field is to let a star, say Regulus, pass through the centre, and fix the time which lapses in its entire passage by a sidereal clock ; then the interval so found x 15 x cosine of the declination of Regulus will indicate the diameter of the field. Suppose for instance, that the star named occupies 2 min. 14 sec. = 134 sec. in its passage right across the whole and central part of the field : then 134 log 2-127105 15 log 1-176091 Dec. of Regulus 12 30' log cos 9-989581 1962" log 13-292777 so that the diameter of the field of the eyepiece must be 32' 42", nearly corresponding with the diameter of the Moon. Limited Means no Obstacle. There are many observers who, having limited means, are apt to consider themselves practi- cally unable to effect good work. This is a great illusion. There are several branches of astronomy in which the diligent use of a small instrument may be turned to excellent account. Perseverance will often compensate for lack of powerful appliances. Many of the large and expensive telescopes, now becoming so common, are engaged in work which could be as well performed with smaller aperture, and when the manifold advantages of moderate instruments are considered, amateurs may well cease to deplore the apparent insufficiency of their apparatus. It is_, however, true that refractors have now attained dimensions and a degree of proficiency never contem- plated in former times, and that the modern ingenuity of art has given birth to innumerable devices to facilitate the work of those engaged in observation. In many of our best E 2 52 NOTES ON TELESCOPES AND Fig. 17. THEIR ACCESSORIES. 53 appointed observatories the arrangements are so very replete with conveniences, and so sedatory in their influences, that the observer has every inducement to fall asleep, though we do not find instances of " nodding " recorded in their annals. Further progress in the same direction leads us to joyfully anticipate the time when, instead of standing out in the frost, we may comfortably make our observations in bed. This will admirably suit all those who, like Bristol people, are re- ported to sleep with one eye open ! But, to be more serious, the work of amateurs is much hindered by lack of means to construct observatories wherein they may conduct researches without suffering from all the rigours of an unfavourable climate. Many of them have, like William Herschel a century ago, to pursue their labours under no canopy but the heavens above, and are exposed to all the trying severity of frost and keen winds, which keep them shivering for hours together, and very much awake ! Observing-Seats. As to observing-seats, many useful con- trivances have been described from time to time in the Astronomical Register ' and ' English Mechanic.' Some of .these answer their design admirably, but I believe a good chair, embodying all the many little requirements of the observer,' yet awaits construction. Those I have seen, while supplying certain acknowledged wants, are yet deficient in some points which need provision. With my reflector I find an ordinary step-ladder answers the purpose very well. It is at once light, simple, and durable, and enables observations to be secured at any altitude. It may be readily placed so that the observer can work in a sitting posture, and the upper shelves, while convenient to lean upon, may be so arranged as to hold eyepieces, and are to be further utilized when making draw- ings at the telescope. I find it possible to obtain very steady views of celestial objects in this way. Everyone knows that during a critical observation it is* as essential for the observer to be perfectly still as it is for the instrument to be free from vibration. A person who stands looking through a telescope feels a desire to ensure a convenient stability by catching hold of it. The impression is no doubt correctly conveyed to his mind that he may obtain a better view in this way ; and so he 54 NOTES ON TELESCOPES AND would, were it not for the dancing of the image which in- stantly follows the handling of the instrument. For this reason it is absolutely necessary that no part of the observer touch the telescope while in use. He must ensure the desired steadiness, which is really a most important consideration, by other means ; and an observer who provides for this contin- gency will have taken a useful step in the way of achieving delicate work. Advantage of Equatoreals. Those who employ equatoreal mounting and clock-work will manifestly command an advantage in tracing features on a planet or other object requiring critical scrutiny. Common stands, though often good make-shifts, require constant application on the part of the observer, when his undivided attention should be concen- trated on the object. With an alt-azimuth stand nearly one half the observer's time is occupied in keeping the object near the middle of the field. Though good views are obtainable, they are very fugitive. Just as the delicate features are being impressed on the retina they are lost in the ill-defined margin of the field, or from the necessity of suddenly shifting the object back. A succession of hurried views of this kind, during which the observer is frantically endeavouring to grasp details which only require a steady view to be well displayed, are often tantalizing and seldom satisfactory in their issues. This is especially the case when a single lens and high powers are used, and if the night is windy the difficulty is intensified. It is, therefore, evident that a clock-driven telescope possesses marked advantages in delicate work on faint objects, because the prolonged view better enables the eye to gather in the details which are all but lost in the elusive glimpses afforded by inferior means. Still we must not forget that rough appliances do not present an effectual barrier to success. The very finest definition comes only in momentary glimpses. The sharply-cut out- lines of planetary configuration cannot steadily be held for long together. Only now and then the image acquires the distinctness of an engraving, when the air and the focus of eye and telescope severally combine to produce a perfect picture. Observers, therefore, whose instruments are simply, THEIR ACCESSORIES. 55 though perhaps substantially mounted in handy fashion, must profit by these moments of fine seeing, and, when drawing, will find it expedient to fill in, little by little, the delicate forms which reach the eye. This will take much time owing to the drawbacks alluded to, but the outcome will more than justify its expenditure, and the observer will gain patience and perseverance which will prove a useful experience in the future. Lenses out of centre or misplaced are, like other defects, calculated to give rise to errors as numerous as they are various. But the most striking of these apparently belong to a period when telescopes were far less perfect and popular than at the present day. Indeed, it is surprising that so very few false or imaginary discoveries are announced when we consider the vast array of instruments that are now em- ployed. It is true we occasionally hear that a comet has been discovered close to Jupiter, that several companions have been seen to Polaris, or that some other extraordinary " find " has been effected, but the age is dead when such announcements were accepted without suitable investigation. The satellite of Venus has long since ceased to exist. The active volcanoes on the Moon have become extinct. Even Vulcan will have to be set aside, and, like many another sensation which caused quite a furore in its day, must soon be altogether expunged from the category of " suspects." Test-objects. Opticians sometimes advertise lists of objects generally double stars which may be seen with their instru- ments, but it does not appear to be sufficiently understood that the character of a telescope is dependent in a great degree upon the ability of the observer, who can either make or mar it, according to the skill he displays in its manage- ment. Some men will undoubtedly see more with 5 inches of aperture than others will wdth 10. Certain observers appear to excel in detecting delicate planetary markings, while others possess special aptitude for glimpsing minute objects such as faint satellites, or comites to double stars, and the explanation seems to be that partly by experience and partly by differences in the sensitiveness of vision, exceptional powers are sometimes acquired in each of these departments. 06 NOTES ON TELESCOPES AND The various test-objects which have been given by reliable authorities, though representing average attainments, are not applicable to the abnormal powers of vision possessed by certain observers. In fact, the capacity of a telescope cannot be correctly assigned and its powers circumscribed by ar- bitrary rules, because, as already stated, the character of the observer himself becomes a most important factor in this relation. Climatic influences have also considerable weight, though less so than the personal variations referred to, for one man will succeed, where another meets with utter failure. This is unquestionably due to differences in eyesight, method, and experience. But whatever the primary causes may be, everyone knows they induce widely discordant results, and occasion many of the contradictions which become the subjects of controversy. And, as a rule, amateurs should avoid con- troversy, because it very rarely clears up a contested point. There is argument and reiteration, but no mutual under- standing or settlement of the question at issue. It wastes time, and often destroys that good feeling which should sub- sist amongst astronomers of every class and nationality. In cases where an important principle is involved, and discussion promises to throw light upon it, the circumstances are quite different. But paltry quibblings. fault-finding, or the constant expression of negative views, peculiar to sceptics, should be abandoned, as hindering rather than accelerating the progress of science. Let observers continually exercise care and discretion and satisfy themselves in every legitimate way as to the accuracy of their results, and they may fearlessly give them expression and overcome any objections made to their acceptance. They should accord one another an equal desire for the promotion of truth. Competition and rivalry in good spirit increase enthusiasm, but there is little occasion for the bitterness and spleen sometimes exhibited in scientific journals. There are some men whose reputations do not rest upon good or original work performed by themselves, but rather upon the alacrity with which they discover grievances and upon the care they will bestow in exposing trifling errors in the writings of their not-infallible contemporaries. Such critics would earn a more honourable title to regard were they to THEIR ACCESSORIES. 57 devote their time to some better method of serving the cause of science. Cheapness and increasing number of Telescopes. A marked feature of optical instruments is their increasing cheapness. Little more than half a century ago Tulley charged 315 for a 10-inch Newtonian reflector. At the present time Calver asks 50 for an instrument of the same aperture, and some- times one may be picked up, second-hand, for half of that amount. Not only have telescopes become cheaper, but they have greatly improved in performance since silvered glass superseded the metallic speculum. Hence we find mode- rately-powerful instruments in the hands of a very large number of observers. Astronomical publications have pro- portionately increased, so that amateurs of to-day can boast of facilities, both of making and recording observations, which were scarcely dreamt of a century ago. It must be admitted, however, that the results hardly do justice to the means avail- able. Such an enormous number of telescopes are variously employed that one cannot avoid a feeling of surprise at the comparative rarity of new discoveries, and, indeed, of pub- lished observations generally. It is certain that the majority of existing telescopes are either lying idle or applied in such a desultory fashion as to virtually negative the value of the results. Others, again, are indiscriminately employed upon every diversity of object without special aim or method, and with a mere desire to satisfy curiosity. Now it is to be greatly deplored that so much observing strength is either latent or misdirected. The circumstances obviously demand that an earnest effort should be made to utilize and attract it into suitable channels. To do this effectually, the value of col- lective effort should be forcibly explained, the interest and enthusiasm of observers must be aroused in a permanent manner, and they must be banded together according to their choice of subjects. An effort in this direction has been made by the Liverpool Astronomical Society, and the results have proved distinctly favourable ; a considerable amount of useful work has been effected in several branches and it forms the subject of some valuable reports which have been annually published in the * Journal.' 58 NOTES ON TELESCOPES AND Utility of Stops. There are a good many details connected with observation which, though advice may be tendered in a general way, are best left to the discrimination of observers, who will very soon discover their influences by practical trial and treat them accordingly. The employment of stops or diaphragms to contract the aperture of telescopes is a question on which a diversity of opinion has been expressed. It is often found, on nights of indifferent seeing, that the whole aperture, especially of a faulty instrument, gives bad images, and that, by reducing it, definition becomes im- mensely improved. But Mr. Burnham, the double star observer, records his opinion that a good glass needs no con- traction, and that the whole aperture shows more than a part unless there is defective figuring at the outer zone of the lens, which will be cut off by the stop and its performance thereby greatly improved. He seems to think that a glass requiring contraction is essentially defective, but this is totally opposed to the conclusions of other observers. It is almost universally admitted that, on bad nights, the advan- tages of a large aperture are neutralized by unsteady de- finition, and that, by reducing the diameter, the character of the images is enhanced. As regards instruments of moderate calibre the necessity is less urgent. With my 10-inch reflector I rarely, if ever, employ stops, for by- reducing the aperture to 8 inches the gain in definition does not sufficiently repay for the serious loss of light. But in the case of large telescopes the conservation of light is not so important, and a 14-inch or 16-inch stop may be frequently employed on an 18-inch glass with striking advantage. The theory that only defective lenses improve with contraction is fallacious, for in certain cases where stops are regularly employed it is found that, under circumstances of really good seeing, the whole aperture gives images wdn'ch are as nearly perfect as possible. It is clear from this that the fault lies with the atmosphere, and that under bad conditions it becomes imperative to limit its interference consistently with the retention of sufficient light to distinguish the object well. In large reflectors, particularly, the undulations of the air are very active in destroying definition, and the fact w r ill be THEIR ACCESSORIES. 59 patent enough to anyone who compares the images given in widely different apertures. The hard, cleanly cut disks shown by a small speculum or ohject-glass offer an attractive contrast to the flaring, indefinite forms often seen in big telescopes. Cleaning Lenses. As to wiping objectives or mirrors, this should be performed not more often than absolute necessity requires ; and in any case the touches should be delicate and made with materials of very soft texture. The owner of a good objective should never take the handkerchief out of his pocket and, in order to remove a little dust or dew, rub the glass until the offensive deposit is thought to be removed. Yet this is sometimes done, though frequent repetition of such a process must ultimately ruin the best telescope notwith- standing the hardness of the crown glass forming the outer lens of the objectiA-e. It will not bear such "rough and ready " usage and in time must show some ugly scratches w r hich will greatly affect its value though they may not seriously detract from its practical utility. Good tools deserve better treatment. When the glass really wants cleaning, remove it from the tube and sw r eep its whole surface gently with a dry camel's-hair brush, or when this is not at hand get a piece of linen and " flick " off the dust particles. Then wipe the lens, as soon as these have been dislodged, with an old silk, or soft cambric handkerchief ; fine chamois leather is also a good material, and soft tissue paper, aided by the breath, has been recommended. But whatever sub- stance may be adopted it must be perfectly clean and free from dust. When not in use it should be corked up in a wide-necked bottle where it w r ill be safe from contact with foreign particles. In the case of mirrors there is an obvious need that, when being repolished, the material used should be perfectly dry and that the mirror also should be in the same state. It is unnecessary to say here that in no case must the silver film be touched when it is clouded over with moisture. This must first be allowed to evaporate in a free current of air or before a fire ; the former is to be preferred. A suitable polishing-pad may be made with a square piece of washleather or chamois in which cotton-wool is placed and then tied into a bag. This may be dipped into a little of the 60 NOTES ON TELESCOPES AND finest rouge, and its employment will often restore a bright surface to the mirror. But the latter should be left " severely alone " unless there is urgent occasion to repolish it, as every application of the rouged pad wears the film and may take off minute parts of it, especially when dust has not been alto- gether excluded. The precarious nature of the silvered sur- face undoubtedly constitutes the greatest disadvantage of modern reflectors. The polish on the old metallic mirrors was far more durable. Some of Short's, figured 150 years ago, still exist and are apparently as bright as when they were turned out of the workshop ! I have a 4-inch Gregorian by Watson which must be quite a century old, and both large and small specula seem to have retained their pristine con- dition. With regard to the duration of the silver-on-glass films, much of course depends upon the care and means taken to preserve them. Calver says that sometimes the deposit does not last so long as expected, though he has known the same films in use for ten years. A mirror that looks badly tar- nished and fit for nothing will often perform wonderfully well. With my 10-inch in a sadly deteriorated state I have obtained views of the Moon, Venus, and Jupiter that could hardly be surpassed. The moderate reflection from a tar- nished mirror evidently improves the image of a bright object by eliminating the glare and allowing the fainter details to be readily seen. When not in use a tight-fitting cap should always be placed over the mirror, and if a pad of cotton wadding of the same diameter is made to inlay this cap it tends to preserve the film by absorbing much of the moisture that otherwise condenses on its surface. The i Hints on Reflecting-Telescopes,' by W. H. Thornthwaite and by G. Calver, and the ' Plea for Reflectors/ by J. Browning, may be instructively consulted by all those who use this form of instrument. The latter work is now, however, out of print, and Mr. Browning tells me that he has quite relinquished the manufacture of reflecting-telescopes. Mr. G. With of Here- ford, who formerly supplied the mirrors for his instruments, has recently disposed of his reserve stock and entered an entirely different sphere of labour. In the publications above THEIR ACCESSORIES. 61 alluded to amateurs will find a large amount of practical information on the value and treatment of glass mirrors. Opera- Glass. A very useful adjunct, and often a really valuable one to the astronomical amateur, is the Opera-Grlass. or rather the larger form of this instrument generally known as the Field- Glass. Of certain objects it gives views which cannot he surpassed, and it is especially useful in observations of variable stars and large comets. Whenever the horizon is being scanned for a glimpse of -the fugitive Mercury, or when it is desired to have a very early peep at the narrow crescent of the young Moon, or to pick up Venus at midday, or Jupiter before sunset, all one has to do is to sweep over the region where the object is situated, when it is pretty sure to be caught, and the unaided eye will probably reach it soon afterwards. The opera-glass has the dignity of being the first telescope invented, for even its binocular form is not new ; it is virtually the same pattern of instrument that was introduced at Middleburg in 1609, though its compound object-glasses are of more modern date. Anyone who enter- tains any doubts as to the efficacy of the opera-glass or has had little experience in its use will do well to look at the Pleiades and compare the splendid aspect of that cluster, as it is there presented, with the view obtained by the naked eye, and he will acknowledge at once that it constitutes a tool without which the observer's equipment is by no means perfect. The object-glasses should have diameters of 2 or 2J- inches, and the magnifying power lie between 4 and 6. There is a large field of view and the images are very bright. The observer is enabled to enjoy the luxury of using both his eyes, and when he directs the instrument upon a terrestrial landscape he will be gratified that it does not turn the world upside down ! It is not surprising that an appliance, with recommendations so significant, is coming more into favour every day, and for those branches suitable to its means it is doing much useful work. A volume has been recently published dealing ex- pressly with the use of the opera-glass in Astronomy ; and in the i Journal of the L.A.S.' vol. vii. p. 120, there is an excellent paper by Major Markwick on the same subject. This instrument will never, of course, by the nature of its 62 NOTES ON TELESCOPES AND construction, be comparable to a modern telescope in regard to power, for Galilei, when lie augmented his magnifiers to 30, appears to have practically exhausted the resources of this appliance. But in all those departments requiring an expansive field and little power with a brilliant and distinct image, the larger form of opera-glass is a great desideratum, and its portability is not one of the least of its advantages. Dewing of Mirrors. The disposition of mirrors to become clouded over upon rises of temperature is a point meriting comment. When permanently left in a telescope, fully exposed out of doors, the speculum undergoes daily transitions. The heat generated in the interior of the tube by the sun's action causes a thick film of moisture to form upon the silvered surface of the mirror, which remains in this state for a considerable time, though the moisture evaporates before the evening. The flat is similarly affected, and the result of these frequent changes is that the coating of silver becomes impaired and presents a crackly appearance all over the surface. Sometimes when a marked increase of tem- perature occurs towards evening the speculum is rendered totally unserviceable until it has been submitted to what Dr. Kitchiner terms a process of " roasting/' The vapour will soon disappear when the mirror is brought indoors and placed before a fire ; but it is not till some time after it has been remounted in the tube that it will perform satisfactorily. Those who keep their mirrors in more equable temperatures will not experience these inconveniences, which may also in some measure be obviated by regularly placing a tight-fitting cap, inlaid with cotton-wool, over the speculum at the con- clusion of work. This also protects the silver from the yellow sulphurous deposit which soon collects upon it if used in a town. All sudden variations of temperature act prejudicially on the performance of specula, and their best work is only accomplished when free from such disturbing elements. I have rarely found the flat to become dewed in a natural way during the progress of observation. If on a cold night the observer puts his hand upon its supports in order to alter its adjustment it instantly becomes dewed, or if he stands looking down the tube it is almost sure to be THEIR ACCESSORIES. 63 similarly affected ; but in the ordinary course of work the flat is little liable to become dewed in sensible degree. With refractors dew-caps are very necessary, though they do not always prevent the deposition of moisture on the object-glass, and this occasions frequent wiping or drying, which in either case is very objectionable. Celestial Globe. This forms another extremely useful adden- dum to the appliances of the amateur. It enables a great many problems to be solved in a very simple manner, and helps the young student to a lucid comprehension of the apparent motions and positions of the fixed stars. With < Keith on the Globes ' as a reference-book he may soon acquire the method of determining the times of rising, southing, and setting of any celestial object the place of which is known. He can also readily find the height (altitude) and bearing (azimuth) at any time. The distance in degrees between any two stars or between a star and the Moon, a planet, or a comet may be found at a glance by laying the quadrant of altitude on the pair of objects and reading off the number of degrees separating them. If a new comet has been dis- covered, its position should be marked in pencil upon the globe ; and the observer, after having noted its exact place relatively to neighbouring stars, may proceed to identify the object with his telescope. If a large meteor is seen, its apparent path amongst the constellations should be projected on the globe and the points, in R.A. and Dec., of beginning and ending of the flight read off and entered in a book. In many other practical branches of astronomy this instrument will prove highly serviceable, and is far preferable to a star- atlas. But the latter is the most useful to the beginner who is just learning the names of the stars and the configuration of the chief groups, because on the globe the positions are all reversed east and west. The surface of the globe represents the entire star-sphere reduced to a common distance from the earth, and as seen from outside that sphere. The observer, therefore, must imagine his eye to be situated in the centre of the globe, if he would see the stars in the same relative places as he sees them in the heavens. The reversion of the star-positions to which we have been alluding is very con- 64 NOTES ON TELESCOPES AND fusing at first, and no doubt it provokes mistakes, but a little experience will practically remove this objection. The one great recommendation to a star-atlas is that it displays the stars in the natural positions in which they are discerned by the eye, thus enabling the student to become readily acquainted with them, whereas the celestial globe affords no such facility. But in other respects the latter possesses some valuable functions, and the amateur who devotes some of his leisure to mastering the really useful problems will attain a knowledge that will be of great benefit to him in after years. A globe of 12-inches diameter will be large enough for many purposes, but one of 18-inches will be the most effective size. It should be mounted on a tall stand with single body and tripod base. The stands, fitted with three parallel legs, in which the globe is supported in the middle by weak connections from them, are not nearly so durable. I have used several 18-inch globes mounted in this manner, and the supports have quite given way under the pressure of constant use ; but this is impossible with the strong single body, which is capable of withstanding any strain. Globes are frequently to be obtained second-hand, and at trifling cost ; but the observer must allow for precession if he uses an old article. Many of the stars will be 1 or 2 east of the positions in which they are marked on the globe ; and it will be necessary to remember this if the appliance is to be employed for exact results. Observatories.- Massive and lofty buildings have long gone out of fashion, and lighter, drier structures have properly supplanted them. Instruments of size are generally placed on or near the ground and solidly supported to ensure stability, while the other erections are made consistent with the necessity for pretty equable temperature and freedom from damp. Amateurs will ordinarily find that a simple wooden enclosure for the telescope, with suitable arrangements for opening the top in any direction, is sufficient for their purpose and very inexpensive. Some observers have, in- deed, secured the desired shelter for themselves and their telescopes by means of a canvas tent provided with ready means for obtaining sky-room. Berthon has given a good THEIR ACCESSORIES. 65 description of an amateur's observing-hut in ' The English Mechanic' for October 13th and 20th, 1871; and Chambers supplies some information about amateur observatories in < Nature ' for November 19th, 1885 *. Mr. Thomthwaite's ( Hints on Telescopes ' may be usefully consulted for details of the Romsey Observatory, which, like the Berthon model, seems peculiarly adapted to the necessities of the amateur. The great requirements in such structures are that they should be dry and not obstruct any region of the firmament. They should also be large enough to allow the observer perfect freedom in his movements and during the progress of his observations. They are then decided advantages, and will materially add to that comfort and convenience without which it is rarely possible to accomplish really good work. When an observatory is to be dispensed with it becomes necessary to erect a small wooden house near the instrument, especially if placed at the far end of a garden, in which the observer may keep certain appliances, such as a lantern, celestial globe, step-ladder or observing-seat, oil, &c. Here also he may record his seeings, complete his sketches, and consult his working-list, star-charts, and ephemerides. A shelter of this sort, apart from its practical helpfulness, avoids any necessity for the observer to go in and out of doors, up and down stairs, &c., to the annoyance of the rest of his family, who, on a frosty night, are decidedly not of an astronomic turn, and vastly prefer house-warming to star- gazing ! * Chambers's l Descriptive Astronomy/ 4th ed. vol. ii;, also contains some useful references and diagrams. NOTES ON TELESCOPIC WORK. CHAPTER IV. ix* NOTES ON TELESCOPIC WORK. Preparation. Working-Lists. Wind. Vision. Records. Drawing. Friendly Indulgences. Open-Air Observing. Method. Perseverance. Definition in Towns. Photography. Publications. Past and Future. Attractions of Telescopic Work. Preparation. An observer in commencing work in any department of astronomy will find it a very great assistance to his progress if he carefully reads and digests all that has been previously effected in the same line. He will see many of the chief difficulties and their remedies explained. He will farther learn the best methods and be in the position of a man who has already gained considerable experience. If he enter upon a research of which he has acquired no foreknowledge he will be merely groping in the dark, and must encounter many obstacles which, though they may not effectually turn him from his purpose, will at least involve a consider- able expenditure of time and labour. On the other hand, a person who relies upon guidance from prior experimentalists will probably make rapid headway. He will be fortified to meet contingencies and to avoid complications as they arise. He will be better enabled to discriminate as to the most eligible means and will confidently endeavour to push them to the furthest extent. By adopting existing instructions for his direction and familiarizing himself with the latest infor- mation from the best authorities he will in a great measure ensure his own success or at least bring it within measure- able distance. The want of this foreknowledge has often been the main cause of failure, and it has sometimes led to misconceptions and imaginary discoveries ; for after much thought and labour a man will overcome an impediment or achieve an end in a way for which he claims credit, only to find that he has been anticipated years before and that NOTES ON TELESCOPIC WORK. Fig. 18. 67 Refracting-Telescope on a German Equatoreal,, F2 08 NOTES ON TELESCOPIC WORK. had he consulted past records, his difficulties would have been avoided and he might have pressed much nearer the goal. Too much importance cannot be attached to the acquisition of foreknowledge of the character referred to, though we do not mean that former methods or results are to be implicitly trusted. Let every observer judge for himself to a certain extent and let him follow original plans whenever he regards them as feasible ; let him test preceding results whenever he doubts their accuracy. We recommend past experiences as a guide, not as an infallible precept. It would be as much a mistake to follow the old groove with a sort of credulous infatuation as it would be to enter upon it in utter ignorance of theoretical knowledge. An observer should take the direction of his labours from previous workers, but be prepared to diverge from acknowledged rules should he feel justified in doing so from his new experiences. Working-Lists. Full advantage should be taken of good observing weather. Sir John Herschel most aptly said that no time occupied in the preparation of working-lists is ill-spent. In our climate the value of this maxim cannot be overrated. If the 100 hours of exceptionally good seeing, available in the course of the year, are to be profitably employed, we must be continually prepared with a scheme of systematic work. The observer should compile lists of objects it is intended to examine, and their places must be marked upon the globe or chart so as to avoid all troublesome refer- ences during the actual progress of observation. If he has to consult ephemerides and otherwise withdraw attention from the telescope he loses valuable time : moreover the positions hurriedly assigned in such cases are frequently wrong and entail duplicate references, involving additional waste of time ; all this may be avoided by careful preparation beforehand. If he has a series of double or variable stars to observe he must tabulate their places in convenient order so as to facilitate the work. If he intend hunting up nebula? or telescopic comets he must carefully mark their positions relatively to adjoining stars. In the case of selenographical objects or planetary markings he may equally prepare him- self by previous study. Adopting these precautions, NOTES ON TELESCOPIC WORK. 69 objects may be readily identified and the work expedited. When no such preparation is made much confusion and loss of time result. On a cloudy , wet day observers often con- sider it unnecessary to make such provision and they are taken at a great disadvantage when the sky suddenly clears. A good observer, like a good general, ought to provide, by the proper disposition of his means, against any emergency. In stormy weather valuable observations are often permissible if the observer is prompt, for the definition is occasionally suitable under such circumstances. The most tantalizing weather of all is that experienced during an anti-cyclone in winter. For a week or two the barometer is very steady at a high reading, the air is calm, and the sky is obscured with an impenetrable mass of clouds. Wind. The influence of wind on definition has been much discussed in its various aspects, but it is scarcely feasible to lay down definite rules on the subject. The east wind is rarely favourable to good seeing, but the law is far from absolute. We must remember that several distinct currents sometimes prevail, and the air strata at various elevations are of different degrees of humidity and therefore exercise different effects upon telescopic definition. A mere surface breeze from the east may underlie an extensive and moist current from the south-west, and telescopic definition may prove very fair under the combination. Calm nights when there is a little haze and fog, making the stars look somewhat dim, frequently afford wonderfully good seeing. As a rule, when the stars are sparkling and brilliant, the definition is bad ; planetary disks are unsteady and the details obliterated in glare. But this is not always so. I have sometimes found in windy weather after storms from the west quarter, when the air has become very transparent, that exceptionally sharp views may be obtained ; but unfortunately they are not with- out drawbacks, for the telescope vibrates violently with every gust of wind and the images cannot be held long enough for anything satisfactory to be seen. The tenuous patches of white cirrous cloud which float at high altitudes will often improve definition in a surprising manner, especially on the Moon and planets. Of coarse this does not apply to nebulae 70 NOTES ON TELESCOPIC WORK. or comets, which are objects of totally different character and essentially require a dark night rather than good definition before they may be seen under the best conditions. As a rule, a steady, humid atmosphere is highly conducive to good seeing, and it is rather improved than impaired by a little fog or thin, white cloud. Some unique effects of peculiar definition, such as oval or triangular star disks, have been occasionally recorded, but we must content ourselves with a bare reference to these phenomena. With regard to the general question it may, however, be added that the character of the seeing often varies at very short intervals in this climate. In the course of a night's work the definition will sometimes fluctuate in a most remarkable manner. An observer who comes to the telescope and finds it impossible to obtain satisfactory images should not entirely relinquish work at the first trial. After an interval he should again test its performance, for it frequently happens that a night ushered in by turbulent vapours, improves greatly at a later period, and in the morning part becomes so fine that it is worthy to be included in the select 100 hours assigned by Sir W. Herschel as the annual limit. Those who reside in towns will usually get the best definition after midnight, because there is less interference then from smoke and heated vapours. It would greatly conduce to our knowledge of atmospheric vagaries as affecting definition, if observers, especially those employing large aperture, preserved records as to the quality of the seeing, also direction of wind and readings of the barometer and thermometer. Visio7i. There are perhaps differences quite as considerable in powers of vision as in quality of definition. It is not meant by this that the same person is subject to great individual variations, though some people are certainly liable to fluctua- tions, according to state of health and other conditions. Some eyes, as already stated, are less effective in defining planetary markings than in detecting minute stars or faint satellites of distant planets. Of course the natural capacity is greatly enhanced by constant practice, for the human eye has proved itself competent to attain a surprising degree of excellence by habitual training. Frequent efforts, if not NOTES ON TELESCOPIC WORK. 71 overpressed so as to unduly strain the optic nerves, are found to intensify rather than weaken the powers of sight. Thus a distinguishing trait among astronomers has been their keen- ness of vision, which, in many cases, they have retained to an advanced age. It is true Dr. Kitchiner said his " eye at the age of forty-seven became as much impaired by the extreme exertion it had been put to in the prosecution of telescope trials, as an eye which has been employed only in ordinary occupations usually is at sixty years of age I to cultivate a little acquaintance with the particular and com- parative powers of telescopes requires many extremely eye- teasing experiments." But the Doctor's opinion is not gene- rally confirmed by other testimony, the fact being that the eye is usually strengthened by special service of this character. To unduly tax or press its powers must result in injury ; but it is well known that the capacities of our sight and other senses are enhanced by their healthy exercise, and that com- parative disuse is a great source of declining efficiency. Before the observer may hope to excel as a telescopist it is clear that a certain degree of training is requisite. Many men exhibit very keen sight under ordinary circumstances, but when they come to the telescope are hopelessly beaten by a man who has a practised eye. On several occasions the writer was much impressed with evidences of extraordinary sight in certain individuals, but upon being tested at the telescope they were found very deficient, both as regards planetary detail and faint satellites. Objects which were quite conspicuous to an experienced eye were totally invisible to them. I believe it is a good plan for habitual observers to employ method in exercising their sight. In my own case I invariably use the right eye on the markings of planets and the left on minute stars and satellites. Practice has given each eye a superiority over the other in the special work to which it has been devoted, and I fancy the practice might be more generally followed with success. It is an advantage to keep both eyes open when in the act of observing, especially when surrounding objects are perfectly dark and there is no distracting light from neigh- bouring windows or lamps. The slight effort required to 72 NOTES ON TELESCOPIC WORK. keep the disengaged eye closed interferes with the action of the other, and though this is but trivial, critical work is not O ' efficiently performed under such conditions. Whenever light interferes the observer may exclude it by a shade so arranged as to afford complete protection to the unoccupied eye. Jf faint objects are to be examined the observer should remain in a dark situation for some little time previously, so that the pupil of the eye may be dilated to the utmost extent and in a state most suitable for such work. After coming from a brilliantly lit apartment, or after viewing the Moon or a conspicuous planet, the eye is totally unfit to receive im- pressions from a difficult object, such as a minute star or faint nebula or comet ; some time must be allowed to elapse so that the eye may recover its sensitiveness. As a rule amateurs will find it best to confine their attention to one class of objects only on the same evening, for if the Moon is first examined and then immediately afterwards the telescope is directed upon double stars and nebulae, the latter objects are little likely to be seen with good effect. If faint objects generally are persistently studied night after night and the observer refrains from solar and lunar work, his eye will acquire greater sensitiveness and he will readily pick up minute forms which are utterly beyond the reach of a man who indiscriminately employs his eye and telescope upon bright and faint objects. Records. With regard to records, every observer should make a note of what he sees, and at the earliest possible instant after the observation has been effected. If the duty is relegated to a subsequent occasion it is either not done at all or done very imperfectly. The most salient features of whatever is observed should be jotted down in systematic form, so as to permit of ready reference afterwards. It is useful to preserve these records in a paged book, with an index, so that the matter can be regularly posted up. The negligence of certain observers in this respect has resulted in the total loss of valuable observations. Even if the details appear to possess no significance, they should be faithfully registered in a convenient, legible form, because many facts deemed of no moment at the time may become of considerable XOTES ON TELESCOPIC WORK. 73 importance. The observer should never refrain from such descriptions because he attributes little value to them. Some men keep voluminous diaries in which there is scarcely anything worth record ; but this is going to the other extreme. All that is wanted is a concise and brief statement of facts. Some persons have omitted references to features or objects observed because they could not understand them, and rather distrusted the evidence of their eyes ; but these are the very experiences which require careful record and reinvestigation. Drawing. Few observers are good draughtsmen ; but it is astonishing how seldom we meet with real endeavours to excel in this respect. Every amateur should practise drawing, however indifferent his efforts may be. Delinea- tions, even if roughly executed, are often more effective than whole pages of description. Pictorial representations form the leading attraction of astronomical literature, and are capable of rendering it more, interesting to the popular mind than any other influence. They induce a more apt con- ception of what celestial objects are really like than any amount of verbal matter can possibly do. For this reason it becomes the obvious duty of every observer to cultivate sketching and drawing, at least in a rudimentary way. He will frequently find it essential to illustrate his descriptions, so as to ensure their ready comprehension. In fact, a thoroughly efficient observer must of necessity become a draughtsman. It should, however, be his invariable aim to depict just what he sees and in precisely the form in which it impresses his eye. Mere pictorial embellishments must be disregarded, and he should be careful not to include doubtful features, possibly existing in the imagination alone, unless he intends them simply for his own guidance in future investi- gations. If he sees but little, and it is faithfully delineated, it will be of more real value than a most elaborate drawing in which the eye and imagination have each played a part. It is an undoubted fact that some of the most striking illus- trations in astronomical handbooks are disfigured by features either wrongly depicted or having no existence whatever. There is very great need for caution in representing such 74 NOTES ON TELESCOPIC WORK. markings only as are distinctly and unmistakably visible. In all cases where the object is new or doubtful the observer should await duplicate observations before announcing it. It is better that new features should evade discovery than that delusive representations should be handed down to posterity. As regards selenographical drawings I would refer the reader to what Mr. Elger advises on p. 21 and 22 of volume v. of the i Journal of the Liverpool Astronomical Society.' My own plan in sketching at the telescope is to first roughly delineate the features bit by bit as I successively glimpse them, assuring myself, as I proceed, as to general correctness in outline and position ; then, on completion, I go indoors to a better light and make copies while the details are still freshly impressed on the mind. To soften details a small piece of blotting- paper must be wrapped round the pointed end of the pencil, and the parts requiring to be smoothed gently touched or rubbed until the desired effect is attained. This simple method, properly applied, will enable delicate markings to be faithfully reproduced, and it certainly adds in no small degree to the merit of a drawing. Friendly Indulgences. Every man whose astronomical pre- dilections are known, and who has a telescope of any size, is pestered with applications from friends and others who wish to view some of the wonders of the heavens. Of course it is the duty of all of us to encourage a laudable interest in the science, especially when evinced by neighbours or acquaint- ances ; but the utility of an observer constituting himself a showman, and sacrificing many valuable hours which might be spent in useful observations, may be seriously questioned. The weather is so bad in this country that we can ill spare an hour from our scanty store. Is it therefore desirable to satisfy the idle curiosity of people who have no deep-seated regard for astronomy, and will certainly never exhibit their professed interest in a substantial manner ? Assuredly not. The time of our observers is altogether too valuable to be employed in this fashion. Yet it is an undisputed fact that some self-denying amateurs are unwearying in their efforts to accommodate their friends in the respect alluded to. My own impression is that, except in special cases, the NOTES ON TELESCOPIC 1VORK. 75 observer will best consult the interests of astronomy, as well as his own convenience and pleasure, by declining the cha- racter of showman ; for depend upon it a person who appre- ciates the science in the right fashion will find ways and means to procure a telescope and gratify his tastes to the fullest capacity. Some years ago I took considerable trouble on several evenings in showing a variety of objects to a clerical friend, who expressed an intention to buy a telescope and devote his leisure to the science. I spent many hours in explanations &c. ; but some weeks later my pupil informed me his expenses were so heavy that he really could not afford to purchase instruments. Yet I found soon after that he afforded 30 in a useless embellishment of the front of his residence, and it so disgusted me that I resolved to waste no more precious time in a similar way. Open- Air Observing. Night air is generally thought to be pernicious to health ; but the longevity of astronomers is certainly opposed to this idea. Those observers who are unusually susceptible to affections of the respiratory organs must of course exercise extreme care, and will hardly be wise in pursuing astronomical work out of doors on keen, wintry nights. But others, less liable to climatic influences, may conduct operations with impunity and safety during the most severe weather. Precautions should always be taken to maintain a convenient degree of warmth ; and, for the rest, the observer's enthusiasm must sustain him. A "wadded dressing-gown " has been mentioned as an effective pro- tection from cold. I have found that a long, thick overcoat, substantially lined with flannel, and under this a stout cardigan jacket, will resist the inroads of cold for a long time. On very trying nights a rug may also be thrown over the shoulders and strapped round the body. During intense frosts, however, the cold will penetrate (as I have found while engaged in prolonged watches for shooting-stars) through almost any covering. As soon as the observer becomes uncomfortably chilly he should go indoors and thoroughly warm his things before a fire. He may then return fortified to his work and pursue it for another period before the frost again makes its presence disagreeably felt. On windy nights 76 NOTES ON TELESCOPIC WORK. a knitted woollen helmet to cover the head, and reaching to the shoulders, is an excellent protection ; but an observer had better not wear it more often than is imperative, or it becomes a necessity on ordinary nights. It is a great mistake to suppose that " a glass of something hot " before going into the night air is a good preventive to catching cold. It acts rather in the contrary way. The reaction after the system has been unduly heated only renders the observer more sensitive, and the inhalation of cold air is then very liable to induce affections of the throat. A telescope permanently erected in the open, and exposed to all weathers, must soon lose its smart and bright appearance, but it need lose none of its efficiency, which is of far more importance ; for it is intended for service, not for show. The instrument should be kept well painted and oiled. I find vaseline an excellent application for the screws and parts controlling the motions, as it is not congelative like common oils. The observer, before a night's work and before darkness sets in, will do well to examine his instru- ment and see that it is in the best condition to facilitate work. Whole tribes of insects take up their habitation in the base or framework, and even in the telescope itself if they can effect a lodgment ; and I have sometimes had to sweep away a perfect labyrinth of spiders' webs from the interior of the main tube. On one occasion I could not see anything through the finder, try how I would. I afterwards discovered that a mason-wasp ( Odynerus murarius) had adopted the vacuity in front of the eye-lens as a suitable site for her nest ; and here she had formed her cells, deposited her eggs, and enclosed the caterpillars necessary for the support of the young when hatched. On another night I came hurriedly to the telescope to observe Jupiter with my single-lens eyepiece, power 252, but could make nothing out of it but a confused glare, subject to sudden extinctions and other extraordinary vagaries. I supposed that the branches of a tree, waving in the wind, must be interposed in the line of sight, but soon saw this could not possibly be the explanation. Looking again into the eyepiece, I caught a momentary glimpse of \\hat I interpreted for the legs of an insect magnified into NOTES ON TELESCOPIC WORK. Fig. 19. 77 The Author's Telescope : a 10-inch With-Browning Reflector. 78 NOTES ON TELESCOPIC WORK. gigantic proportions and very distinct on the bright back- ground formed by Jupiter much out of focus. On detaching the eyepiece and carrying it indoors to a light, an innocent- looking sample of the common earwig crawled out of it. The gyrations of the insect in its endeavours to find a place of egress from its confinement had clearly caused the effects alluded to. Telescopic observers are thus liable to become microscopic observers before they are conscious of the fact, and perhaps also in opposition to their intention. Other experiences might be narrated, especially as regards noc- turnal observing in country or suburban districts, where the " serious student of the skies " may, like myself, find diversion to his protracted vigils by the occasional capture of a too-inquisitive hedgehog or some other marauding quadruped. Metlwd. Nearly all the most successful observers have been men of method. The work they took in hand has been followed persistently and \vith certain definite ends in view. They recognized that there should be a purpose in every observation. Some amateurs take an incredible amount of pains to look up an object for the simple satis- faction of seeing it. But seeing an object is not observing it. The mere view counts for nothing from a scientific standpoint, though it may doubtless afford some satisfaction to the person obtaining it. A practical astronomer, with his own credit at stake and the interests of the science at heart, will require something more. In observing a comet he will either fix its position by careful measure- ment with reference to stars near, or critically examine its physical peculiarities, or perhaps both. In securing these data he will have accomplished useful work, which may quite possibly have an enduring value. In other branches of observation his aim will be similar, namely to acquire new materials with regard to place or to physical pheno- mena, according to the nature of the research upon which he happens to be engaged. Such results as he gathers are neatly tabulated in a form convenient for after comparisons. There have been instances, we know, where sheer carelessness has resulted in the loss of important discoveries. Lalande NOTES ON TELESCOPIC WORK. 79 must have found Neptune (and mathematical astronomy would have been robbed of its greatest triumph) half a century before it was identified in Galle's telescope, but Jus want of care enabled it to elude him just when he was hovering on the very verge of its discovery. Numerous other instances might be mentioned. Failure may either arise from imperfect or inaccurate records, from a want of discrimination, from neglect in tracing an apparent dis- cordance to its true source, or from hesitation. I may be pardoned for mentioning a. case within my own experience. On July 11, 1881, just before daylight, I stood contem- plating Auriga, and the idea occurred to me to sweep the region with my comet eyepiece, but I hesitated, thinking the prospect not sufficiently inviting. Three nights later Schasberle at Ann Arbor, U.S.A., discovered a bright tele- scopic comet in Auriga ! Before sunrise on October 4 of the same year I had been observing Jupiter, and again hesitated as to the utility of comet-seeking, but, remem- bering the little episode in my past experience, I instantly set to work, and at almost the first sweep alighted upon a suspicious object which afterwards proved itself a comet of short period. These facts teach one to value his oppor- tunities. They cannot be lightly neglected, coming as they do all too rarely. The observer should never hesitate. He must endeavour to at least effect a little whenever an occasion offers ; for it is just that little which may yield a marked success greater, perhaps, than months of arduous labour may achieve at another time. Perseverance. Persistency in observation, apart from the value derived from cumulative results, increases the powers of an observer to a considerable degree. This is especially the case when the same objects are subjected to repeated scrutiny. A first view, though it may seem perfectly satis- factory in its conditions and results, does not represent what the observer is capable of doing with renewed effort. Let us suppose that a lunar object with complicated detail is to be thoroughly surveyed. The observer delineates at the first view everything that appears to be visible. But a sub- sequent effort reveals other features which eluded him before, 80 NOTES ON TELESCOPIC WORK. and many additional details are gradually reached during later observations. Ultimately the observer finds that his first drawing is scarcely more than a mere outline of the formation as he sees it at his latest efforts. Details which he regarded as difficult at first have become comparatively conspicuous, and a number of delicate structures have heeii exhibited which were quite beyond his reach at the outset. The eye has become familiarized with the object, and its powers fairly brought out by training and experience. This training is very serviceable, but is seldom appreciated in the degree of its influence. Many a tyro has abandoned a pro- jected series of observations on finding that his initiatory view falls wofully short of published drawings or descrip- tions. He considers himself hopelessly distanced, and regards it as impossible to attain much less excel the results achieved by his predecessors. He does not realize that their work is the issue of years of close application, and that it represents the collective outcome of many successive nights. I need hardly say that it is a great mistake to anticipate failure in this way. No telescopic work has been done in the past that will not be done better in the future. No observer can rate his capacity until he has rigorously tested it by experience. The eye must become accustomed to an object before it is able to do itself justice. Those who have been sedulously engaged in a certain research will, as a rule, see far more than others who are but just entering upon it not from a natural superiority of vision, but because of the aptitude and power acquired by practice. No matter how meagre an observer's primary attempts may be, he should by no means relax his efforts, but rather feel that his want of success must be remedied by experience. It is a common fault with observers that they leave too much to their instruments, and rely upon them for the results which really depend entirely upon their personal endeavours. A skilled workman will do good work with indifferent tools ; for after all it is the character of the man that is evident in his results, and not so much the resources which art places in his hands. Much also depends upon the feelings by which the amateur is actuated when he commences work. A few enter into it NOTES ON TELESCOPIC WORK. 81 with a degree of energy and determination that knows no wearying and will accept no defeat. Others display a half- hearted enthusiasm, and are constantly doubting either their personal ability or their instrumental means. Many others, again, when the circumstances appear a little against them regard failure as inevitable. It need hardly be said, how- ever, that every difficulty may be surmounted by perse- verance, and that a man's enthusiasm is often the measure of his success, and success is rarely denied to him whose heart is in his work. Definition in Towns. The astronomical journals contain some interesting references to the definition of telescopes in large towns. Of course the purer the air the better for obser- vational purposes. But observers who reside in populous districts need not despair of doing really useful work. The vapours hanging over a large city are by no means so objec- tional as is commonly supposed. When they are circulating rapidly across the observer's field of view they will prove very troublesome at times ; but in a comparatively tranquil state of the air definition is excellent. I have frequently found planetary markings very sharp and steady through the smoke and fog of Bristol. The interposing vapours have the effect of moderating the bright images and improving their quality. When there is a driving wind, and these heated vapours from the city are rolling rapidly past, objects at once appear in a state of ebullition, and the work of observation may as well be postponed. Smoke from neighbouring chimneys is utterly ruinous to definition : a bright star is transformed into a seething, cometary mass, and the planets undergo contortions of the most astonishing character. Large instruments being more susceptible to such influences and, indeed, to atmo- spherical vagaries of all kinds are chiefly affected by the drawbacks we have alluded to ; but there are many oppor- tunities when their powers may be fully utilized. In sweeping for faint comets, or in other work (such as the observation of nebulae) where a dark sky is the first essential, a town station has a manifest disadvantage because of the artificial- illu- mination of the atmosphere. But for general telescopic work the conditions do not offer a serious impediment, espe- 82 NOTES ON TELESCOPIC WORK. cially if the observer is careful to seize the many suitable occasions that must occur. The direction of the wind rela- tively to his position and the central part of the city, will occasion considerable differences to an observer who uses a telescope in a suburban locality. Photography. Upon this branch of practical astronomy not much will be said in this volume, as it is rather beyond its scope, and possibly also beyond the resources of ordinary amateurs, so far as really valuable work is concerned. A reference must, however, be made to an innovation which has deservedly assumed, a very prominent place, and is clearly destined to exert an accelerating influence on the progress of exact astronomy. At present it is impossible to foretell how far it may be employed and extended, but judging from recent developments its applications will be as manifold as they will be valuable. Photographic records possess a great advantage over others, because they are more accurate and therefore more reliable. The}' are pictures from Nature taken by means free from the bias and error inseparable from mere eye-estimations or. hand-drawings. The latter are full of discordances when compared one with another, and can seldom be implicitly trusted ; but in the photograph a different state of things prevails. Here we have a faithful portrayal or reproduction of the object impressed by itself upon the plate. Hence it can be depended upon, because there has been no intermediate meddling either with its position or features by what may be termed artistic mis- representation. True, there may be imperfections in the process ; trifling flaws and obstructions will invariably creep in wherever comparatively new and novel work is attempted, but these will but little detract from the value of its results. Photography is obviously a means of discovery as well as a means of accurate record ; for nebulse and faint stars quite invisible to the eye have been distinguished for the first time upon the negatives. Those of our amateurs who intend working in this branch will find it a productive one, and not decaying in interest ; but the necessary outfit will be expensive if thoroughly capable instruments are to be employed in the service. NOTES ON TELESCOPIC WORK. 83 Publications. The observer of to-day may esteem himself particularly fortunate in regard to the number and quality of the astronomical journals within his reach. Discoveries and current events receive prompt notice in these, and readers are fully informed upon the leading topics. Among the best of the periodicals alluded to are ' The Observatory ' (Taylor & Francis, London), l The Sidereal Messenger ' (Northfield, Minn., U.S.A.), and L'Astronomie (Gautier- Yillars, Paris). The Astronomische Nachrichten (Kiel, Ger- many) is a very old and valued serial, and ' The Astronomical Journal ' (Cambridge, Mass., U.S.A.) may also be favourably mentioned. The ' Monthly Notices ' of the Royal Astro- nomical Society and the ' Journals ' of the Liverpool and British Astronomical Societies contain many interesting materials. ' Nature/ * The English Mechanic,' and i Know- ledge ' are among the English journals which devote part of their space to the science ; and the beautiful illustrations in the latter entitle it to special recognition. It is evident, from this short summary, the amateur will find that his literary appetite may be amply satisfied, and should he desire a channel for recording his own work or ideas the publications referred to offer him every facility and encouragement. As to almanacks, the ' Nautical/ which has been termed - of the surface, and that the remaining f~ are permanently beyond our reach. JEarths/tine. A few mornings before new moon, and on a few evenings after it, the whole outline of the dark portion of the lunar globe may be distinctly perceived. A feeble illumi- nation like twilight pervades the opaque part, and this is really earthlight thrown upon our t satellite, for near the times of new moon the Earth appears at her brightest (her disk being fully illuminated) as seen from the Moon. The French term for this light is la lumiere cendree, or " the ashy light/' The appearance is often popularly referred to in our own country as " the old Moon in the new Moon's arms." Some of the old observers remarked that the waning Moon showed this earthlight more strongly than the new Moon. Telescopic Observations of the Lunar Surface. Our tele- scopes give by far the most pleasing view of the Moon when she is in a crescent shape. At such a period the craters and mountains, with their dark shadows, are splendidly displayed. A good view is also obtainable with the Moon at first or last quarter, or when the disk is gibbous. But the full Moon is decidedly less attractive ; for the shadows have all dis- appeared, and the various formations have quite lost their distinctive character. The disk is enveloped in a flood of light, causing glare, and though there is a large amount of detail, including systems of bright rays, many differences THE MOON. 117 of tint, and bright spots, yet the effect is altogether less satis- factory than at the time of a crescent phase. The nature of the work undertaken by the amateur must largely depend upon his opportunities and the capacity of his appliances. It is evident that in the investigation of lunar details it is essential to be very particular in recording obser- vations ; for unless the conditions of illumination are nearly the same, lunar objects will present little resemblance. He should therefore examine the formations at intervals of 59 d l h 28 m , when the terminator is resting on nearly identical parts of the surface. In periods of 442 d 23 h ( = 15 lunations) there is another repetition of similar phase ; also in periods of 502 d O h 28 m ( = 17 lunations). The observer, in entering results into his note-book, should state the Moon's age to the nearest minute, and give aperture and power of telescope and state of sky. Those objects which he has recorded at one lunation should be reobserved after an intervening lunation, or at intervals of 59 d l h 28 m . He will then find bis notes and drawings are comparable. By the persistent scrutiny of special structures he will discern more and more of their details ; in other words, he will find his eye soon acquires power with experience and familiarity with the object. Comparisons of his own work with the charts and records of previous observers will be sure to interest him greatly, and the differences which he will almost certainly detect may exert a useful influence in inciting him to ascertain the source of them. He must not be premature in attributing such discordances to actual changes on the Moon ; for he must remember that perfect harmony is rarely to be found in the experiences of different observers. But whenever his own results are inconsistent with those of others, the fact should be carefully noted and the observations repeated and rediscussed with a view to reconcile them. The charts and descriptions of former selenographers are excellent in their way, and the outcome of much zealous labour ; but they contain omissions and inaccuracies which it has been impracticable to avoid. The amateur who discovers a mountain, craterlet, or rill not depicted on his lunar maps must therefore neither regard it as a new formation or as a new discovery ; for it may have been 118 THE MOON. overlooked by some of the previous observers, and is possibly drawn or described in a work whicb he does not happen to have consulted. Such differences should, however, always be announced, as they clear the way for others working in the same field. A small instrument, with an object-glass of about 2^ inches, will reveal a large amount of intricate detail on the surface of our satellite, and will afford the young student many evenings of interesting recreation. But for a more advanced survey of the formations, with the view to discover unknown objects or traces of physical change in known features, a telescope of at least 8 or 10 inches aperture is probably necessary, and powers of 300, 350, and more. Eclipses of the Moon. These phenomena comprise a variety of interesting aspects. They are less numerous, in actual occurrence, than solar eclipses in the proportion of about 2 to 3 ; but they are more frequently visible, because they may be witnessed from any part of an entire hemisphere, whereas eclipses of the Sun are only observable from a tract of the Earth's surface not exceeding 180 miles in breadth. The Moon may remain totally eclipsed for a period of 2 hours 4 minutes, and the whole duration, including the penumbral obscuration from its first to its last projection, is about 6 hours. Sometimes the Moon suffers total eclipse twice in the same year, and both may be visible, as in 1844, 1877, 1964, &c. It is possible for three such eclipses to occur within a single year, as in 1544. In 1917 there will be three total lunar eclipses, but not all visible in England. In the latter vear there will O , be no less than seven eclipses, as in 1935. On the last two occasions Oct. 4, 1884, and Jan. 28, 1888 when the Moon was totally immersed in the Earth's shadow, the atmosphere was very clear ; and it is hoped equally favourable conditions will attend the similar phe- nomena of Nov. 15, 1891, Sept. 4, 1895, and Dec. 27, 1898. One of the most interesting features during these temporary obscurations of our satellite is the occupation of small stars. Prof. Struve compiled a list of no less than 116 of these objects that would pass behind the Moon's shadowed limb during the eclipse of Oct. 4, 1884. THE MOON. 119 Another important effect is the variable colouring on the Moon. This differs considerably in relative intensity as seen during successive eclipses, and the cause is not perhaps fully accounted for. Kepler thought it due to differences in humidity of those parts of the Earth's atmosphere through which the solar rays pass and are refracted to the eclipsed Moon. The intense red hue which envelopes the lunar surface on such occasions is due to the absorption of the blue rays of light by our atmosphere. The sky at sunset is often observed to be similarly coloured, and from the operation of similar causes. Sometimes the Moon entirely disappears when eclipsed, but on other occasions remains distinctly obvious, like a bright red ball suspended in the firmament. On May 5, 1110, Dec. 9, 16i1), May 18, 1761, and June 10, 1816, our satellite is said to have become absolutely imper- ceptible during eclipse. Wargentin, who described the appearance in 1761, remarks : " The Moon's body dis- appeared so completely that not the slightest trace of any portion of the lunar disk could be discerned, either with the naked eye or with the telescope." On Oct. 4, 1884, I noticed that the opacity was much greater than usual ; at the middle period of the eclipse the Moon's diameter was appa- rently so much reduced that she looked like a dull, faint, nebulous mass, without sharply determinate outlines. The effect was similar to that of a star or planet struggling through dense haze. Yet, on March 19, 1848, the Moon " presented a luminosity quite unusual. The light and dark places on the face of our satellite could be almost as well made out as on an ordinary dull moonlight night/' On July 12, 1870, Feb. 27 and Aug. 23, 1877, and Jan. 28, 1888, the Moon, as observed at Bristol, was also fairly bright when totally immersed in the Earth's shadow. In expla- nation of these singular differences, Dr. Burder has suggested that Kepler's views seem inadequate, and that the solar corona is probably implicated in producing light and dark eclipses. He concludes that, as the corona sometimes extends to considerable distances from the Sun, and is very variable in brightness, it may have sufficient influence to occasion the effects alluded to. 120 THE MOON. Lunar Changes. The question as to whether physical changes are occurring in the surface-formations of our satellite is one which offers attractive inducements to tele- scopic observers. Though the Moon appears to have passed the active state, it is very possible that trivial alterations continue to affect some of her features. In April 1787 Sir W. Herschel wrote : " I perceive three volcanoes in different places of the dark part of the new Moon. Two of them are already nearly extinct, or otherwise in a state of going to break out ; the third shows an eruption of fire or luminous matter." Schroter, however, was correctly of opinion that these appearances were due to reflected light from the Earth falling upon elevated spots of the Moon having unusual capacity to return it. Schroter himself thought he detected sudden changes in 1791. He says that, on the 30th of December, at 5 h P.M., with a 7-foot reflector magnifying 161 times, he perceived the commence- ment of a small crater on the S.W. declivity of the volcanic mountain in the Mare Crisium, having a shadow of at least 2' 5". On the llth of January, 1792, at 5 h 20 m P.M., on looking at the place again he could see neither the new crater nor its shadow. In ' this case the disappearance was doubtless an apparent one, merely due to the reversed illu- mination under which the object was examined in the interval of 12 days. Many other observers besides Herschel have been struck with the brightness of certain spots situated in the opaque region of the lunar disk ; but there is no doubt the cause has been uniformly one and the same, viz. the highly reflective properties of some of the mountains (notably of one named A^ristarchus), which are distinctly visible as luminous spots amid the relatively dark regions surrounding them. They afford no certain evidence of existing volcanic energy, and in the light of modern researches such an idea cannot be entertained. On June 10, 1866, Temple noticed a remarkable light appearance, agreeing with the position of Aristarchus, upon the dark side of the Moon, faintly illuminated by earthshine. The object did not exhibit a faint white light analogous to THE MOON. 121 that of other craters in the dark side, but it was star-like, diffused, in colour reddish yellow, and evidently dissimilar to other bright spots. He wrote, in reference to this matter : " Of course 1 am far from surmising a still active chemical outbreak, as such an outbreak supposes water and an atmo^- sphere, both of which are universally allowed not to exist on the Moon, so that the crater-forming process can only be thought of as a dry, chemical, although warm one." On November 17, 18G6, Schmidt announced that the lunar crater Linne, about 5^ miles in diameter, and situated in the Mare Serenitatis, had disappeared ! He averred that he had been familiar with the object as a deep crater since 1841, but in October 1866 he found its place occupied by a whitish cloud. This cloud was always visible, but the crater itself appeared to have become filled up, and was certainly invisible under its former aspect. Such a definite state- ment, emanating as it did from a diligent and experienced student of selenography, naturally aroused keen interest, and Linne at once became the object of wide-spread observation. But a reference to Schroter's results, obtained in the latter part of the last century, threw some doubt upon the alleged change. This observer had figured Linne on November 5, 1788, as a round white spot, and there is nothing in his drawing indicating a crateriform aspect. His description of Linne was: "A flat, somewhat doubtful crater, which appears as a round white spot/' Mr. Huggins regarded Schroter's observations as correctly expressing the appearance of this object in 1867 under the same conditions of illu- mination. On the other hand, Lohrmann (1823) and Mlidler (1831) referred to Linne as a deep crater, and in terms inconsistent both with Schroter's drawing and with the present aspect of the object. The outcome of the many fresh observations that were collected was that Linne appeared as a white cloud, with a small black crater within a large shallow-ringed depression. But as usual in such cases, the observers were far from being unanimous as to the details of the formation; and certainly in regard to a lunar object this need occasion no surprise, for slight differences in the angle of illumination produce marked changes in the aspect of 122 THE MOON. lunar features. The fact of actual change could not be demonstrated, and the negative view appears to have sub- sequently gained weight. Another instance of alleged activity on the Moon was notified by Dr. Klein in the spring of 1877. He saw a deep black crater about 18 miles to the W.N.W. of Hyginus, and in a particular place where he had previously recognized no such object, though he had frequently examined the region and was perfectly familiar with it. Forthwith every tele- scope was directed to this part of the Moon. The maps of earlier observers were eagerly consulted, and lunar photo- graphs scanned for traces of the new object. Many drawings were made of the district near Hyginus and of the remarkable rill or cleft connected with it ; but amongst both old and new records some puzzling discordances were detected. Many of the observers, instead of finding Dr. Klein's new formation a sharply-cut, deep crater, saw it rather in the character of a saucer-like depression ; and I drew it under this aspect on several occasions with a 10-inch reflector. The fact, there- fore, of its being a new feature admitted of no valid and convincing proofs, and thus the same uncerrainty remains attached to this object as to Linne, nothing being absolutely proved*". The problem as to whether the Moon is still the seat of physical activity has yet to be solved. Many circumstances are antagonistic to the discovery of changes on the Moon. As the Sun's altitude is constantly varying with reference to lunar objects, they assume different aspects from hour to hour. In a short interval the same formations become very dissimilar. When the Sun is rising above the more minute craters they are often distinguished in their true characters ; but near the period of full Moon they are visible as bright spots, and it is impossible to tell whether they represent craters or conical hills. With a vertical Sun, as at the full, all the shadows have disappeared in fact, the entire configuration has been transformed, and many of the * In September 1889 Prof. Thury, of Geneva, reported a change in the centre of the crater Plinius. With a 6-inch refractor he saw, instead of the usual two hills in the interior, a circular chalk-like disk " with a dark spot in its centre like the orifice of a mud-volcano." THE MOON. 123 interesting lineaments displayed at the crescent phase are no longer seen. The Moon's libration also introduces slight differences in the appearance of objects. And these are not the only drawbacks ; for observations, in themselves, are seldom accordant, and it is found that drawings and descrip- tions are not always to be reconciled, though referring to identical and invariable features. The lunar landscape must be studied under the same conditions of illumination and libration, with the same instrument and power, and in a similar state of atmosphere, if results are to be strictly com- parable. But it is very rarely that observations can be effected under precisely equal conditions ; hence discordances are found amongst the records. The whole of the Moon's visible sphere exhibits striking imprints of convulsions and volcanic action in past times, though no such forces appear to operate now. The surface seems to have become quiescent, and to have assumed a rigidity inconsistent with the idea of present energy. But we cannot be absolutely certain that minute changes are not taking place, and, being minute, the prospect of their detection is somewhat remote. Students of lunar scenery will probably have to watch details with scrupulous care and for long periods before an instance of real activity can be demonstrated. Lunar Formations. The Moon abounds in objects of* very diversified character, and they have been classified according to peculiarities of structure. The names of eminent astro- nomers have been applied to many of the more definite features a plan of nomenclature which originated with Riccioli, who published a lunar map at the middle of the seventeenth century. The following brief summary comprises many of the principal formations : Mare. A name applied by Hevelius to denote the large and relatively level plains on the Moon, which present some similarity to terrestrial seas. They are visible to the naked eye as dusky spots, and in a telescope show many craters, hills, and mounds, and some extensive undulations of surface. Palus (Marsh) and Lacus (Lake) were titles given by Eiccioli to minor areas of a dark colour, and exhibiting greater variety of detail and tint than the Maria. 124 THE MOON. Sinus (Bay) has been applied to objects like deep bays on the borders of the Maria. Walled Plains extend from 40 to 150 miles in diameter, .and are commonly surrounded by a terraced wall or mountain- ranges. The interiors are tolerably level, though often marked with crater-pits, mounds, and ridges. Mountain-Rings. These represent rings of mountains and hills, enclosing irregularities, possibly furnished by the debris of the crumbling exterior walls, which, in certain instances, appear to have fallen inwards. Ring-Plains are more circular and regular in type than the walled plains, and consist of a moderately flat surface surrounded by a single wall. Crater-Plains are somewhat similar, and seldom exceed 20 miles in diameter. They " rise steeply from the mass of debris around the foot of their walls to a considerable height, and then fall precipitously to the interior in a rough curved slope, whilst on their walls, especially on the exterior, craterlets and crater-cones often exist in considerable numbers." , Craters, Craterlets, and Crater-Pits. Usually circular in form, and severally offering distinctions as to dimensions and shape. The craters are surrounded by walls, rising abruptly to tolerable heights, and pretty regular in their contour. When the Sun is rising the shadow of the walls falls upon the .interior of the craters, and many of these dark conspicuous objects are to be seen near the Moon's terminator. With a high Sun some of the craters are extremely bright. In proof of the large number of these objects, it may be noted here that in Mtidler's lunar map (1837) 7735 craters are figured, while in Schmidt's (1878) there are no less than 32,856 ! Crater-Cones. Conical hills or mountains, visible as small luminous spots about the period of full Moon. They are from ^ to 3 miles in diameter, and show deep central depressions. It is somewhat difficult to distinguish them from the ordinary mountain-peaks and white spots, and they are not unlike the cones of terrestrial volcanoes. Rills or Clefts. These are very curious objects. They were first discovered by Schroter in 1787, and some of them are to be traced over a considerable extent of the lunar surface, THE MOON. 125 their entire length being 200 or 300 miles. They have the appearance of cuttings or canals, and are sometimes straight, sometimes bent, and not unfrequently develop branches which intersect each other. They apparently run without interrup- tion through many varieties of lunar objects. The bottoms of these rills are nearly flat, and look not unlike dried river- beds. Some observers have regarded them as fractures or cracks in the Moon's surface ; but their appearance and cir- cumstances of arrangement are opposed to such a view. Our present knowledge includes more than 1000 of these rills. Mountain-Ranges are chains of lofty peaks and highlands, sometimes divided by rills and numerous ravines and cross valleys. Some of these ranges are of vast magnitude, and the summits of the mountains reach altitudes between 15,000 and 20,000 feet, and sometimes even more. Mountain-Ridges are to be found scattered in the greatest abundance in the most disturbed localities of the lunar surface. They sometimes connect several formations, or surmount ravines or depressions of large extent. Peaks attaining altitudes of more than 5000 feet rise from them, and they range in several cases over 100 miles. Ray-Centres. Systems of radiating light-streaks, having a mountain-ring as the centre of divergence, and stretching to distances of some hundreds of miles round. Tycho, Coper- nicus, Kepler, Anaxagoras, Aristarchus, and Gibers are pro- nounced examples of this class. In Beer and Mlidler's chart of the Moon the names are attached to the various formations, as they are also in Nei son's maps and in some other works. One of these will be absolutely necessary to the student in prosecuting his studies. He will then have a ready means of acquainting himself with the various formations, and making comparisons between his new results and the drawings of earlier seleno- graphers. I w^ould refer the reader to Neison's and Webb's books for many references in detail to lunar features, and must be content here with a brief description of a few leading objects : t : Plato is an extensive walled plain, 60 miles in diameter, aad situated on the N.E. boundary of the Mare Imbrium. 126 THE MOON. Nasmyth and Carpenter describe the wall as "serrated with noble peaks, which cast their black shadows across the plateau in a most picturesque manner, like the towers and spires of a great cathedral." It has received a large amount of attention, with a view to trace whether changes are occurring in the numerous white spots and streaks lying in its interior. In 186.) 71 Mr. Birt collected many observations, and on dis- cussing them was led to believe that " there is strong pro- bability that activity, of a character sufficient to render its Fig. 25. Light-spots and streaks on Plato, 1879-82. (A. Stanley Williams.) effects visible, had been manifested." The inquiry was renewed by (Stanley Williams in 1882-84, and he concluded that the results were strongly confirmatory ot actual change having occurred since 1869-71. The relative visibility of several of the bright spots had altered in the interim, and the curious intermingling bright streaks also exhibited traces of variation. At sunrise the interior of Plato is pure grey ; but with the sun at a considerable height above it, the plain becomes a dark steel-grey. The change is an abnormal one, and difficult to explain. South of Plato the*^ is a fine example of an isolated peak, named Pico, which is about 8000 feet high. THE MOON. 127 Great Alpine Valley. This object, supposed to have been discovered by Bianchini in 1727, and having a length, according to Madler, of 83 miles and a breadth varying from 3J to 6 miles, is a very conspicuous depression situated near Plato, and running from the Mare Frigoris to the Mare Imbrium. It exhibits at its southern extremity an oval formation, and a narrow gorge issues from it to the north- ward, opening out further on, and imparting to the whole appearance a shape which Webb likened to a Florence oil- flask. Elger has fully described this singular structure. l( It is only when for removed from the terminator that its Y-shaped outlet to the Mare Imbrium flanked on either side by the lofty Alps can be traced to advantage, or the flask-like expansion with the constricted gorge leading up to it from the N.W. satisfactorily observed. At other times these features are always more or less concealed by the shadows of neighbouring heights. The details of the upper or more attenuated end of the valley are, however, best seen under a setting sun, when many striking objects come to light, of which few traces appear at other times." Archimedes. One of the most definite and regular of the walled plains. It is 60 miles in diameter, with a wall rising about 4200 feet above the surface. Some small craters and various streaks diversify its centre. Tycho. A grand ring-plain, 54 miles in diameter and about 17.000 feet ( = nearly 3 miles) deep, and forming the centre of the chief ray-system of the Moon. The light-radiations stretch over one fourth of the visible hemisphere at the full, but they are imperceptible with the Sun's altitude below 20. These remarkable radiations from Tycho form a striking aspect of lunar scenery, and any small telescope reveals them. Webb has termed Tycho " the metropolitan formation of the Moon ; " and the idea embodied in the expression must strike observers as very apposite. This object is visible to the naked eye at the time of full. A fine hill rises from its centre to a height of 5500 feet. Copernicus. A magnificent ring-plain, 56 miles in dia- meter, and surrounded by a wall (in which there are terraces and lofty peaks, separated by ravines) attaining an elevation 128 THE MOON. of 11,000 feet. There is a central hill of nearly 2500 feet. From Copernicus light-streaks are plentifully extended on all sides, and apparently connect this object with the many others of similar character which are situated in this region. Neison says that near Copernicus the light-streaks unite and form a kind of nimbus or light-cloud about it. The streaks are most conspicuous towards the N., where they are from 5 to 14 miles in width. To the N.W. of Copernicus, about halfway in *the direction of the neighbouring ring-plain Eratosthenes (and N. of Stadius), there is a considerable number of crater-pits. Madler figured sixty-one of these, and regarded that number as certainly less than half the total number visible. They appear to be ranged in rows or streams, and are so close together in places as to nearly form crater-rills. Schmidt saw the ground hereabout pierced like a honeycomb, and managed to count about 300 little craters ; but they are so thickly strewn in this district that exact numbers or places cannot be assigned. They are best observable when the Sun is rising on the E. wall of Copernicus. The interior of this fine object shows six or seven peaks, which are often capped with sunshine, and very brilliant amid the black shadow thrown from the sur- rounding wall. Theophilus. Another ring- plain, and one of the deepest visible. Its terraced lofty wall, 64 miles in diameter, rises in a series of peaks to heights varying between 14,000 and 18,000 feet. There is a central mountain, broken by ravines ; but from one of the masses a peak ascends to a height of about 6000 feet, Petavius. A large walled plain, surrounded by a double wall or rampart, which rises to 11,000 feet 011 its E. side. There are hills and ridges in the interior, and a central peak, A, reaching to 5500 feet above the E. part of the floor, which is convex in form. A smaller peak, of nearly 4000 feet, lies W. of A. Several small craterlets have been seen in the interior. - JWewton'. The deepest walled plain known upon the Moon's surface. In form it is elliptical ; its length is 143 miles, while its breadth is only 69 miles. The walls show the THE MOON. 129 terracing so common in these objects, and one lofty peak reaches the unusual height of 24,000 feet above the floor. The interior includes some small craters, mountain protu- berances, and other irregularities, Neison says that, owing Fig. 2tf. Petavius and Wrottesley at sunset. 1885, Dec. 23, (T. Gwyn Elger.) to. 10 ' to " the immense height of the wall, a great part of the floor is entirely lost in shadow, neither Earth nor Sun being ever visible from it." Grimaldi. An immense walled plain, extending over 148 miles from N. to S. and about 130 miles from E. to W. Its interior is very dark. Clavius is another grand example K 180 THE MOON. of this class of object, and is rather larger than Grrimaldi, but unfavourably placed near the S. pole. Schickard may also be mentioned as a large formation of similar type, and situated near the S.E. limb of the Moon. Tip. 27. Birt, Birt A, and the Straight Wall. 1883, JFeb. 15, fit to 8^ 40m. (T. Gwyn Elger.) Rill or Cleft of Hyginus. A conspicuous example of the lunar rills, and one which yields to very moderate instru- ments. Neison notes that it is readily visible in a 2-inch telescope ; while Webb remarks that a power of only 40, in a good instrument, is enough to show it under any illumination. The rill is. about 150 nriles long. It cuts through a number THE MOON. 131 of crater-pits, and Madler found so many widenings in it that it appeared like a confluent train of craters. The rill traverses the large crater-pit Hyginus, which is 3f miles in diameter and moderately deep. Other fine examples of rill- systems will be found between Rheita and Metius and near Triesnecker and Ramsden. Straight Wall. A singular structure on the E. side of the ring-plain Thebit. It is a ridge or wall, which looks regular enough for a work of art, according to Webb. Its average height is 450 feet (Schroter), 1004 feet (Madler), or 880 feet (Schmidt). These several determinations are given to show the discordances sometimes found in the measures of good observers. This object is about 60 miles long ; at one extremity lies a small crater, at the other there is a branching mountain nearly 2000 feet high. Elger has drawn this object, under both a rising and a setting sun, in the Liverpool Astronomical Society's i Journal,' vol. v. p. 156, and remarks that it may be well observed at from 20 to 30 hours after the Moon's first quarter. Valley near Rheita. South of the ring-plain Rheita, on the S.W. limb, there is an enormous valley, which extends in its entire length over 187 miles, with a width ranging from 10 to 25 miles. There are several fine valleys in this particular region. Leibnitz Mountains. These are really situated on the further hemisphere of the Moon, but libration brings them into view, and they are sometimes grandly seen in profile on the S. margin. Four of the peaks ascend to elevations of 26,000 or 27,000 feet, and one mass, towering far above the others, is fully 30,000 feet in height, and is unques- tionably the most lofty mountain on the Moon. Dorfel Mountains. Visibleon the Moon's S.S.E. limb. They exhibit three peaks, which, on the authority of Schroter, rise to more than 26,000 feet above the average level of the limb. The loftiest mountains on the Earth are in the Himalayas a range of immense extent to the N. of India. The three highest peaks are Mount Everest (29,002 feet), Kunchinjinga (28,156 feet) , and Dhawalagiri (28,000 feet) . The only lunar mountain more elevated than these is that of the Leibnitz 132 THE MOON. range, which, as we have already stated, ascends to fully 30,000 feet. Apennines. A vast chain of mountains, extending over more than 450 miles of the lunar surface. Huygens is Fig. 28. Aristarchus and Herodotus at sunrise. 1884, Jan. 9, 8& 30 m to 10^ 3(K (T. Gwyn Elger.) the most elevated peak, rising to more than 18,000 feet, and on its summit it shows a small crater. There are several other very lofty peaks in this range. The Sun rises upon the westerly region of these mountains at the time of first quarter, and the peaks and ridges, with their contrasting shadows, create a gorgeous effect just within, and projecting into the THE MOON. 133 darkness beyond, the terminator. There is an immense amount of detail to be studied here, and much of it is within the reach of small instruments. As the lunar mountains and craters are best seen near the terminator, it may be useful to give a table of objects thus favourably placed between the times of new and full Moon. The summary may assist the student, though it does not aim at exactness, only even days being given. Moon's age Objects near the Terminator. in days. 2 Mare Crisium, Messala, Sunrise on the Mare Hum- boldtianum, Langrenus, Vendelinus, Condorcet, Hansen, Gauss ft, Hahn, Berosus. 3 a Craters in Mare Crisium, Taruntius, Picard, Fraun- hofer, Vega, Pontecoulant, Cleomedes v , Furnerius, Petavius, Endymion, Messier 5 , Vlacq. 4 Mare Nectaris, Macrobius e , Proclus, Sunrise on Fracastotius, Rheita and Metius with the inter- vening valley, Guttemberg, Colombo, Santbech, Mountainous region W. of Mare Serenitatis, Hercules, Atlas. 5 Palus Somnii, Plana, Capella, Isidorus, Polybius, Piccolomini, Vitruvius, Littrow, Fabricius, Posi- donius,LeMonnier,Theophilus, Cyrillus, Catharina, Hommel. 6 Tacitus, Maurolycus, Barocius, Dionysius^, Sosigenes, Abulfeda, Descartes, Alinamon, Gemma Frisius, Plinius, Ross, Arago, Delambre, Aristoteles, Eudoxus, Julius Cassar, Linne, Menelaus. The objects for observation when the Moon's age is from 2 to 4 days may be suitably re-examined a few days after the full. ft An extensive walled plain, 110 miles in length. v A large walled plain containing a small crater, Cleomedes A. 5 A curious double crater, with comet-like rays crossing the Mare Foecunditatis. e A circular ring-plain, 42 miles in diameter. The interior of this crater exhibits some interesting features as the Sun rises higher above it. 134 THE MOON. Moon's age. days. 7 Ptolemseus, Albategnius, Manillas'", Hyginus and its rill-system, Hipparchus, Autolycus, Aristillus, Cassini, Alpine Valley, W. C. Bond, Walter, Miller, LaCaille, Apennines, Triesnecker and the rills W. of it. 8 Mare Frigoris, Arzachel, Alphonsus, Alpetragius, Bode, Pallas, Archimedes, Plato, Maginus*, Mosting l , Thebit, Saussure ; Moretus, Straight Wall, Lalande, Kirch. 9 Tycho, Clavius, Eratosthenes", Stadius and the craters running to N.E., Timocharis, Pitatus, Grruem- berger, Teneriffe Mountains, Straight Range A ? Formation W. of Fontenelle^, Gambart. 10 Sinus Iridum, Copernicus, Hesiodus and the rill to E., Wilhelm I., Longomontanus v , Heinsius, Pytheas, Lambert, Helicon, Wurzelbauer. 11 Bullialdus, Cauipanus, Mercator, Reinhold, Riphaean Mountains, Hippalus, Capuanus, Blancanus, Tobias Mayer. 12 Mare Imbrium, Gassendi^, Aristarchus and sinuous valley to the KE., Herodotus, Marius, Flam- steed, Letronne, Schiller, Mersenius, Doppel- mayer. 13 Schickhard, Wargentin, Grimaldi, Byrgius, Pho- cylides, Hevelius, Seleucus, Criiger, Briggs, Segner, Sirsalis. *i A fine ring-plain, 25| miles in diameter. Madler says "the full Moon knows no Maginus, ' meaning that this object is invisible under a vertical Sun. < Mosting, Lalande, and Herschel form a fine triangle when the Su has attained a great altitude. Mosting is a ray-centre. K A. ring-plain 37| miles in diameter, with very irregular terraced walls. * A range of mountains, with intervening valleys. M Madler describes this as a square enclosure with rampart-like boundaries, which u throw the observer into the highest astonishment." " A great walled plain, 91 miles in diameter. f A walled plain, 55 miles in diameter, in which Schroter suspected changes. THE MOON. 135 * Moon's age. days. 14 Mare Smythii, Bailly, Inghirami, Bouvard, Riccioli, Olbers, Hercynian Mountains, Cardanus, Krafft, Cordilleras , Pythagoras 17 . Occultations of Stars. Among the various phenomena to which the lunar motions give rise none are more pleasing to the possessors of small telescopes than occupations of stars. Several of these occurrences are visible every month. If the amateur has the means of obtaining accurate time, he will en- gage himself usefully in noting the moments of disappearance and reappearance of the stars occulted. This work is efficiently done, it is true, at some of our observatories, and therefore little real necessity exists for amateurs to embark in routine work which can be conveniently undertaken at establishments where they have better appliances and trained observers to use them. The mere watching of an occultation, apart from the registry of exact results, is interesting ; and there are features connected with it which have proved exceedingly difficult to account for. The stars do not always disappear instantaneously. On coming up to the edge of the Moon they have not been suddenly blotted out, but have appeared to hang on the Moon's limb for several seconds. This must arise from an optical illusion, from the action of a lunar atmosphere, or the stars must be observed through fissures on the Moon's edge. The former explanation is probably correct ; for it has happened that two observers at the same place have received different impressions of the phe- nomenon. One has seen the star apparently projected on the Moon's limb for about 5 seconds, while the other has witnessed its sudden extinction, in the usual manner, as it met the Moon's edge. New observations, made with good instruments and reliable eyes, and fully described, will doubtless throw more light on the peculiar effects sometimes recorded. An extensive mountain-range on the E. by S. limb. * A walled plain, 95 miles in diameter, and probably the deepest in the N.E. quadrant, for the S.E. side of its wall rises to nearly 17,000 feet. After the full the same objects should be re-examined under the reversed illumination. 136 . THE MOON. Visibility of the new and old Moon. It is an interesting feature of observation to note how soon after conjunction the Moon's thin crescent is observable with the naked eye. A case has been mentioned in which the old Moon was seen one morning before sunrise and the new Moon just after sunset on the next day. At Bristol, on the evening of March 30, 1881, I saw the new Moon at 7 h 10 m , the horizon being very clear in the west. She was then only 20 h 38 m old. On June 4, 1875, I observed the Moon's crescent at 9 h 10 m , or 22 h 49 m after new Moon. Dr. De- groupet, of Belgium, saw the old Moon on the morning of Nov. 22, 1889, between 6 b 47 m and 7 h 22 m G.M.T., or within 18 h 22 m of the time of new Moon. MERCURY. 137 CHAPTER VII. MERCURY. Supposed planet, "Vulcan." Visibility of Mercury. Period &c. Elongations. Amateur's first view. Phases. Atmosphere of Mer- cury. Telescopic observations. Schiaparelli's results. Observations of Schroter and Sir W. Herschel. Transits of Mercury. Occultations of Mercury. " Come, let us view the glowing west, Not far from the fallen Sun ; For Mercury is sparkling there, And his race will soon be run. With aspect pale, and wav'ring beam, He is quick to steal away, And veils his face in curling mists, L et us watch him while we may.'* Supposed planet " Vulcan.'" Mercury is the nearest known planet to the Sun. It is true that a body, provisionally named Vulcan*, has been presumed to exist in the space interior to the orbit of Mercury ; but absolute proof is lacking, and every year the idea is losing strength in the absence of any confirmation of a reliable kind. Certain planetary spots, observed in motion on the solar disk, were reported to have been transits of this intra-Mercurial orb. Some eminent astronomers were thus drawn to take an affirmative view of the question, and went so far as to compute the orbital elements and predict a few ensuing transits of the suspected planet. But nothing was seen at the important times, and some of the earlier observations have been shown to possess no significance whatever, while grave doubts are attached to many of the others. Not one of the regular and best observers of the Sun has recently * Chambers, in his ' Descriptive Astronomy,' 4th edition, 1889, devotes a chapter to the discussion of facts having reference to Vulcan ; and the reader desiring full information will find it here. 138 MERCURY. detected any such body during its transits (which would be likely to occur pretty frequently), and there is other evidence of a negative character ; so that the ghost of Vulcan may be said to have been laid, and we may regard it as proven that no major planet revolves in the interval of 36,000,000 miles separating Mercury from the Sun. Visibility of Mercury. Copernicus, amid the fogs of the Vistula, looked for Mercury in vain, and complained in his last hours that he had never seen it. Tycho Brahe, in the Island of Hueen, appears to have been far more successful. The planet is extremely fugitive in his appearances, but is not nearly so difficult to find as many suppose. When- ever the horizon is very clear, and the planet well placed, a small sparkling object, looking more like a scintillating star than a planetary body, will be detected at a low altitude and may be followed to the horizon. Period fyc. Mercury revolves round the Sun in 87 d 23 h 15 m 44 s in an eccentric orbit, so that his distance from that luminary varies from 43,350,000 to 28,570,000 miles. When in superior conjunction the apparent diameter of the planet is 4"'5; at inferior conjunction it is 12"'9, and at elongation 7". His real diameter is 3000 miles. Elongations. Being situated so near to the Sun, it is obvious that to an observer on the Earth he must always remain in the same general region of the firmament as tbat body. His orbital motion enables him to successively assume positions to the E. and W. of the Sun, and these are known as his elongations, which vary in distance from 18 to 28. He becomes visible at these periods either in the morning or evening twilight, and under the best circumstances may remain above the horizon two hours in the absence of the Sun. The best times to observe the planet are at his E. elongations during the first half of the year, or at his W. elongations in the last half; for his position at such times being N. of the Sun's place, he remains a long while in view. It is unfortunate that when the elongation approaches its extreme limits of 28 the planet is situated S. of the Sun, and therefore not nearly so favourably visible as at an elongation of only 18 or 20, when his position is N. of the Sun. MERCURY. 139 I have seen Mercury on about sixty-five occasions with the naked eye. In May 1876 I noticed the planet on thirteen different evenings, and between April 22 and May 11, 1890, I succeeded on ten evenings. I believe that anyone who made it a practice to obtain naked-eye views of this object would succeed from about twelve to fifteen times in a year. In a finer climate, of course, Mercury may be distinguished more frequently. Occasionally he presents quite a con- spicuous aspect on the horizon, as in February 1868, when I thought his lustre vied with that of Jupiter, and in November 1882, when he shone brighter than Sirius. The planet is generally most conspicuous a few mornings after his W. elon- gations and a few evenings before his E. elongations. Amateur s First View. The first view of Mercury forms quite an event in the experience of many amateurs. The evasive planet is sought for with the same keen enthusiasm as though an important discovery were involved. For a few evenings efforts are vain, until at length a clearer sky and a closer watch enables the glittering little stranger to be caught amid the vapours of the horizon. The observer is delighted, and, proud of his success, he forthwith calls out the members of his family that they, too, may have a glimpse of the fugitive orb never seen by the eye of Copernicus. Phases. In the course of his orbital round Mercury exhibits all the phases of the Moon. Near his elongations the disk is about half illuminated, and similar in form to that of our satellite when in the first or third quarter. But the phase is not to be distinctly made out unless circumstances are propitious. Galilei's telescope failed to reveal it, and Hevelius, many years afterwards, found it difficult. This is explained by the small diameter of the planet and the rarity with which his disk appears sharply defined. The phase is sometimes noted to be less than theory indicates ; for the planet has been seen crescented when he should have presented the form of a semicircle. Several observers have also remarked that his surface displays a rosy tint, and that the terminator is more deeply shaded and indefinite than that of Venus. Atmosphere. The atmosphere of Mercury is probably far less dense than that of Venus. The latter being farthest from the 140 MERCURY Sun might be expected to shine relatively more faintly than the former, but the reverse is the case. Mercury has a dingy aspect in comparison with the bright white lustre of Venus. On May 12, 1890, when the two planets were visible as evening stars, and separated from each other by a distance of only 2, I examined them in a 1 0-inch reflector, power 145. The disk of Venus looked like newly-polished silver, while that of Mer- cury appeared of a dull leaden hue. A similar observation was made by Mr. Nasinyth on September 28, 1878. The expla- nation appears to be that the atmosphere of Mercury is of great rarity, and incapable of reflection in the same high degree as the dense atmosphere of Venus. Telescopic Observations. As this planet is comparatively seldom to be observed under satisfactory conditions, it is scarcely surprising that our knowledge of his appearance is very meagre, or that amateurs consider the planet an object practically inaccessible as regards the observation of physical peculiarities, and one upon which it is utterly useless to apply the telescope in the hope of effecting new discoveries. Former attempts have proved the extreme difficulty of obtaining good images of this planet. The smallness of the disk, and the fact that it is usually so much affected by the waves of vapour passing along the horizon as to be constantly flaring and moulding in a manner which scarcely enables the phase to be made out, are great drawbacks, which render it impossible to distinguish any delicate features that may be presented on the surface. These circumstances are well calculated to lead observers to abandon this object as one too unpromising for further study; but I think the view is partly induced by a misconception. The planet's diminutive size is a hindrance which cannot be overcome ; but the bad definition, resulting from low altitude, may be obviated by those who will select more suitable times for their observations and not be dismayed if their initiatory efforts prove futile. As a naked-eye object, Mercury must necessarily be looked for when near the horizon ; but there is no such need in regard to telescopic observation, which ought to be only attempted when the planet surmounts the dense lower vapours and is placed at a sufficient elevation to give MERCURY. 141 the instrument a fair chance of producing a steady image. The presence of sunshine need not seriously impair the definition or make the disk too faint for detail. I have occasionally seen Mercury, about two or three hours after his rising, with outlines of extreme sharpness and quite comparable with the excellent views obtained of Venus at the time of sunrise or sunset. Those who possess equatoreals should pick up the planet in the .afternoon and follow him until after sunset, when the horizontal vapours will interfere. Others who work with ordinary alt-azimuth stands will find it best to examine the planet at his western elongations during the last half of the year, when he may be found soon after rising by the naked eye or with an opera-glass, and retained in the telescope for several hours after sunrise if necessary. He may sometimes also be brought into the field before sunset (at the eastern elongations in the spring months), by careful sweeping with a comet-eyepiece, especially when either the Moon, Venus, or Jupiter happens to be near, and the observer has found the relative place of the planet from an ephemeris. Schia par ellis Results. Mercury was displayed under several advantages in the morning twilight of November 1882, and I made a series of observations with a 10-inch reflector, power 212. Several dark markings were perceived, and a con- spicuous white spot. The general appearance of the disk was similar to that of Mars, and I forwarded a summary of my results to Prof. Schiaparelli, of Milan, who favoured me with the following interesting reply : " I have myself been occupied with this planet during the past year (1882). You are right in saying that Mercury is much easier to observe than Venus, and that his aspect resembles Mars more than any other of the planets of the solar system. It has some spots which become partially obscured and sometimes completely so ; it has also some brilliant white spots in a variable position. As I observe the planet entirely by day and near the meridian I have been able to see its spots many times, but not always with the necessary distinctness to make drawings sufficiently reliable to serve as a base for a rigorous investigation. It is remarkable that the views taken near superior conjunction have been more 142 MERCURY. instructive for me than those taken when the disk is near dichotomy, the defect in diameter being compensated by the possibility of seeing nearly all the disk, which, under those conditions, is more strongly illuminated. I believe that by instrumental means, such as our 8^-inch refractor at Milan gives, it is possible to prove the rotation-period of Mercury and to gain a knowledge of the principal spots as regards the generality of their forms. But these spots are really very complicated, for, besides the difficulties attending their obser- vation, they are extremely variable." Prof. Schiaparelli used an 8f-inch refractor in this work, and was able, under some favourable conditions, to apply a power of 400. The outcome of his researches, encouraged since 1882 by the addition of an 18-inch refractor to the appliances of his Observatory, has been recently announced in the curious fact that the rotation of Mercury is performed in the same time that the planet revolves round the Sun ! If this conclusion is just, Mercury constantly presents one and the same hemisphere to the Sun, and the behaviour of the Moon relatively to the Earth has found an analogy. But these deductions of the eminent Italian astronomer require corroboration, and this is not likely to be soon forthcoming owing to the obstacles which stand in the way. Observations of Schroter and Sir W. Herschel. Schrb'ter observed Mercury with characteristic diligence between 1780 and 1815. In 1800 he several times remarked that the southern horn of the crescent was blunted, and fixed the planet's rotation-period at 24 h 4 m . He also inferred the existence of a mountain 12 miles in height. But elements of doubt are attached to some of Schroter's observations ; and Sir W. Herschel, whose telescopic surveys of both Mercury and Venus were singularly barren of interesting results, pointed out their improbability. But the great observer of Slough was not very amicably disposed towards his rival in Germany. His strictures appear, however, to have been not without justice if we consider them in the light of modern observations. Surface-markings. Spots or markings of any kind have rarely been distinguished on Mercury. On June 11, 1867, MERCURY. 143 Prince recorded a bright spot, with faiiit lines diverging from it N.E. and S. The spot was a little S. of the centre. Bir- mingham, on March 13, 1870, glimpsed a large white spot near the planet's E. limb, and Vogel, at Bothkamp, observed spots on April 14 and 22, 1871. These instances are quoted by Webb, and they, in combination with the markings seen by Ficr.29. 1882, Nov. 5, 18 h 4U m . 1882, Nov. 0, 18 h 55 m . Mercury as a morning star. (lO-inch Reflector ; power 212.) Schiaparelli at Milan and by the author at Bristol in 1882, sufficiently attest that this object deserves more attentive study. Amateurs with moderately large instruments would be usefully employed in following this planet at the most opportune periods and making careful drawings under the highest powers that can be successfully applied. Mercury has been persistently neglected by many in past years, and no doubt this " swift- winged messenger of the Gods" has eluded some of his would-be pursuers ; but there is every prospect that a patient observer, careful to utilize all available opportunities, would soon gather some profitable data relating to his appearance. Transits of Mercury. One of the most interesting phe- nomena, albeit a somewhat rare event, in connection with 144 MERCURY. Mercury, is that of a transit across the Sun. The planet then appears as a black circular spot. Observers have noticed one or two very small luminous points on the black disk, and an annulus has been visible round it. These features are pro- bably optical effects, and it will be worth while to remember them on the occasion of future transits, of which the subjoined is a list : 1891, May 9. 1894, Nov.- 10. 1907, Nov. 12. 1914, Nov. 6. 1924, May 7. 1927, Nov. 8. 1937, May 10. 1940, Nov. 12. 1953, Nov. 13. 1960, Nov. 6. 1970, May 9. 1973, Nov. 9. The first observer of a transit of Mercury appears to have been Gassendi, at Paris, on Nov. 7, 1631. Occupations of Mercury. There was an occupation of Mercury by the Moon on April 25, 1838. It occurred on the day of the planet's greatest elongation E., and at a time in the evening when it might have been most suitably witnessed, but cloudy skies appear to have frustrated the hopes of intending observers. There was a' repetition of the event on the morning of May 2, 1867, and it occurred, curiously enough, less than 24 hours after an occultation of Venus. VENUS. 145 CHAPTEK VIII. VENUS. Beauty of Venus. Brilliancy. Period &c. Venus as a telescopic object. Surface-markings on the planet. Rotation-period. Faintness of the markings. Twilight on Venus. Alleged Satellite. Further ob- servations required. Transits of Venus. Occultations of Venus. " Friend to mankind, she glitters from afar, Now the bright evening, now the morning star." Beauty of Venus. This planet has an expressive name, and it naturally leads us to expect that the object to which it is applied is a beautiful one. The observer will not be disappointed in this anticipation : he will find Venus the most attractive planet of our system. No such difficulties are encountered in finding Venus as in detecting Mercury ; for the former recedes to a distance of 47 from the Sun, and sometimes remains visible 4J hours after sunset, as in February 1889. But Venus owes her beauty not so much to favourable position as to surpassing lustre. None of the other planets can compare with her in respect to brilliancy. The giant planet Jupiter is pale beside her, and offers no parallel. Ruddy Mars looks faint in her presence, and does not assume to rivalry. This planet alternately adorns the morning and evening sky, as she reaches her W. and E. elongations from the Sun. The ancients styled her Lucifer (" the harbinger of day ") when a morning star and Hesperus when an evening star. Brilliancy. Her brightness is such as to lead her to occasionally become a conspicuous object to the naked eye in daytime, and at night she casts a perceptible shadow. This is specially the case near the epoch of her maximum brilliancy, which is attained when the planet is in a crescent form, with an apparent diameter of about 40", and situated some 5 weeks L 146 VENUS. from inferior conjunction. Though only a fourth part of the disk is then illuminated, it emits more lustre than a greater phase, because the latter occurs at a wider distance from the Earth and when the diameter is much less. Her appearance is sometimes so striking that it is not to be wondered at that people, not well informed as to celestial events, have attributed it to causes of unusual nature. When the planet was visible as a morning star in the autumn of 1887, an idea became prevalent in the popular mind that the " Star of Bethlehem " had returned, and there were many persons who submitted to the inconvenience of rising before daylight to gaze upon a spectacle of such phenomenal import. And they were not disappointed in the expectancy of beholding a star of extreme beauty, though altogether wrong in surrounding it with a halo of mystery and wonder. At intervals of eight years the elongations of Venus are repeated on nearly the same dates as before, and the planet is presented under very similar conditions. This is because five synodical periods (nearly =13 sidereal periods) of Venus are equal to eight terrestrial years. Thus very favourable E. elongations occurred on May 9, 1860, May 7, 1868, May 5, 1876, and May 2, 1884 ; and on April 30, 1892, there will be a similar elongation. Period 49. For a moment the star seemed 194 JUPITER. to disappear, but a moment later was plainly seen, as if through a well-defined notch in the otherwise continuously even margin. This notch lasted 46 8 '26, and at ll h 28 m 56 8 '91 it vanished, and the light of the star was entirely extinguished" The emersion of the star could not be observed, as clouds supervened. SATURN. 195 CHAPTEK XII. SATURN. Apparent lustre. Grand spectacle afforded by the Rings. Period &c. " Square-shouldered" aspect. Early Observations. Belts and Spots on the Planet. Rotation-Period. The Rings. Divisions in the outer Ring. The transparent or Crape-ring. Discordant Observations. Eccentric position of the Rings. Aspect. Further Observations required. Occul- tations of Saturn. The Satellites. Occultations of Stars by Saturn. <4 Muse, raise thy voice, mysterious truth to sing, How o'er the copious orb a lucid ring, Opaque and broad, is seen its arch to spread Round the big globe, at stated periods led." THIS planet shines brighter than an ordinary first-magnitude star, and is a pretty conspicuous object, though less luminous than either Venus, Jupiter, or Mars. He emits a dull yellowish light, steadier than the sparkling lustre of Mercury or Venus. The globe of Saturn is surrounded by a system of highly reflective rings, giving to the planet a character of form which finds no parallel among the other orbs of our system. His peculiar construction is well calculated to be attractive in the highest degree to all those who -take delight in viewing the wonders of the heavens. Saturn is justly considered one of the most charming pictures which the telescope unfolds. A person who for the first time beholds the planet, encircled in his rings and surrounded by his moons, can hardly subdue an exclamation of surprise and wonder at a spectacle as unique as it is magnificent. Even old observers, who again and again return to the contemplation of this remarkable orb, confess they do so unwearyingly, because they find no parallel elsewhere ; the beautifully curving outline of the symmetrical image always retains its interest, and refreshes them with thoughts of the Divine Architect who framed it ! o2 196 SATURN. The luminous system of rings attending this planet not only gratifies the eye but gives rise to entertaining speculations as to its origin, character, and purposes with regard to the globe of Saturn. Why, it has been asked, was this planet alone endowed with so novel an appendage ? and what particular design does it fulfil in the economy of Saturn ? It cannot be regarded as simply an ornament in the firmament, but must subserve important ends, though these may not yet have been revealed to the eye of our understanding. Period Sfc. Saturn revolves round the Sun in 10,759 days 5 hrs. 16 min., which is equal to nearly 29J years. His mean distance from the Sun is 886,000,000 miles, but this interval varies from 841 to 931 millions, owing to the eccentricity of his orbit. When in opposition his apparent diameter reaches 20"' 7, and declines to 15" at the time of conjunction. The planet's actual diameter is 75 7 000 miles, and his polar compression very considerable, viz. about fa which exceeds that of any other planet. His synodic period is equal to 378 days ; so that he comes into opposition with the Sun thirteen days later every year. The oblate figure of his disk is very noticeable when the rings are turned edgeways to the Earth and practically invisible ; but when they are inclined the complete contour of the globe is lost, and the polar flattening becomes scarcely obvious. " Square-shouldered " Aspect. Sir W. Herschel, from ob- servations in April 1805, said : " There is a singularity which distinguishes the figure of Saturn from that of all the other planets/' On April 19 of the year named he described the planet as " like a parallelogram with the four corners rounded off deeply, but not so much as to bring it to a spheroid/' This gave the globe a " square-shouldered " aspect. But this curious figure appears to have been very rarely observed in subsequent years ; and accurate measures with the micrometer were adduced in 1833-48 in proof that no such anomaly had a real existence. Dr. Kitchiner, com- menting on Herschel's remarks, said : " I have occasionally observed this planet during thirty years, and I do not remember to have seen the body of it of this singular form except for a few months about September 1818." But there SATURN. . .197 is no doubt that occasionally the planet does assume an apparent form similar to that attributed to it by Herschel. In the autumn of 1880 I studied the visible appearance of Saturn by means of a 10-inch reflector, and recorded as follows : " The S. pole, over which the dark belts lay, seemed compressed in the most remarkable manner ; but where a bright belt intervened, in about lat. 45, the contrary effect was produced. Here the limbs were apparently raised (by irradiation) above the spherical contour; so that the distorted image gave the planet that distinctly ' square-shouldered ' aspect sometimes mentioned in text-books/' The explanation appears to me very simple. The singular figure is due to the contrasting effects of the belts. While the bright belt in lat. 45 causes a very evident shouldering-out of the limbs at its extremities, the dark belts nearer the pole and the equator act with opposite effect, for they apparently compress the disk where they meet the limbs, and thus the eye discerns a figure to all appearance distorted into the " square-shouldered " form. Mr. J. L. McCance confirmed these remarks by independent observations at the same period with a 10-inch reflector by Calver ('Monthly Notices,' vol. xli. pp. 84,282). Early Observations. The appearance of Saturn offered a considerable difficulty to observers soon after the invention of the telescope. Galilei became greatly perplexed. He saw the planet, not as a circular globe like Jupiter, but distinctly elongated in shape, and conceived the appearance to be due to a central globe with smaller spheres hanging on the sides ! He continued his observations, without, however, arriving at the solution of the mystery, until the malformation began to dis- appear ; and in 1612 he was astonished to find the disk spherical. In his surprise, he asked " Were the appearances indeed illusion and fraud, with which the glasses have so long deceived me, as well as many others to whom I have shown them ? . . . The shortness of the time, the unexpected nature of the event, the weakness of my understanding, and the fear of being mistaken, have greatly confounded me." Gassendi, in 1633, also announced that Saturn appeared to him to be closely attended by two globes of the same colour as the planet. Eiccioli alleged that the planet was surrounded by a thin, 108 SATURN. plain, elliptic ring, connected with the sphere by two arms. None of Galilei's contemporaries possessed the instrumental means to extricate him from his doubts ; and it remained for Huygens, in 1654 (twelve years after the death of Galilei), to discover that Saturn " is surrounded by a slender flat ring, which in no part coheres with the body of the planet, and is inclined to the ecliptic " *. The same observer showed that the disappearance which had so puzzled Galilei arose from the varying inclination in the ring : at times it would become invisible, when presenting its narrow edge to the Earth, and this actually occurred again in 1671, as Huygens had pre- dicted. In 1676 Cassini detected a belt upon the planet, and also a dark division in the ring. Dr. Smith's ' Optics' (1738) thus alludes to these discoveries : " In the year 1676, after Saturn had emerged from the Sun's rays, Sig. Cassini saw him in the morning twilight Fig. 38. Saturn, as observed by Cassini in August 1676. with a darkish belt upon his globe, parallel to the long axis of his ring as usual. But what was most remarkable, the broad side of the ring was bisected right round by a dark elliptical line, dividing it, as it were, into two rings, of which the inner ring appeared brighter than the outer one, with nearly the like difference in brightness as between that of silver polished and unpolished which, though never ob- served before, was seen many times after with tubes of 34 and 20 feet, and more evidently in the twilight or moon- light than in a darker sky." From the time when Galilei's inadequate glass revealed the * Huygens appears to have used a refractor of 2^-inch aperture and 23-feet focal length, with a power of 100, in effecting this discovery. SA1URN. 199 " threefold " aspect of Saturn, and led up to Huygens's solution of the mystery in 1654, this planet has been succes- sively interrogated with the improved telescopes which every generation has produced. Cassini, W. Herschel, Encke, Bond, Lassell, Dawes, and Hall are names familiar to us as having materially advanced our knowledge of this unique orb, both as to his surface-configuration and as to his numerous retinue of satellites. Belts and Spots on the Planet. Parallel belts are seen on the surface of Saturn, but they are much fainter than those on Jupiter, and they seldom display the spots and other irregu- larities interspersed with the belts of the latter planet. Well- bounded spots have rarely been distinguished on the disk of Saturn ; the belts normally appear equal in tone, without breaks, condensations, abrupt curves, or branches, so that the rotation-period has only been accurately determined by Herschel and Hall. And in these cases the markings were certainly atmospheric, and probably affected by proper motions similar to those operating on Jupiter. Cassini and Fatio remarked two bright streaks on the planet as early as 1683. Sir W. Herschel, in 1790, observed a very dark spot near the margin of the limb, and a few modern observers have been successful in distinguishing either bright or dark spots or patches, though no continuous and useful observations appear to have been secured. In the winter of 1793 Herschel noticed a very distinct quintuple belt, which consisted of three dusky and two intervening light zones. The dark belts presented a dusky yellow hue, while the spaces separating them were white. He recognized the evidences of rotation in the quintuple belt ; for on the same nights, after a few hours' interval, it exhibited considerable variation. Though seen with great precision at first, it became indistinct at a later hour, and the individual belts were placed at unequal distances. Rotation-Period. Prof. A. Hall, at Washington, discovered on Dec. 7, 1876, a well-defined white spot, 2" or 3" in diameter, and situated just below the ring of Saturn. He watched this object till Jan. 2 following, when it had become faint and indistinct, and the planet baing low and the weather 200 SATURN. unfavourable no further observations were made. The spot had fortunately been seen at four other observatories in the United States, Prof. Hall having notified its existence to them; and on discussing the results, a rotation-period was found not differing largely from Herschel's value derived from the quintuple belt in 1793. These are, in fact, the only two determinations on which we may place confidence. They are as below : h m s Probable error. 1793. Sir W. Herschel , . 10 16 0'4 2 min. 1877. Prof. A. Hall ... 10 14 23*8 2'3 sec. Schroter, from different spots, computed periods of ll h 40 m 30 8 , ll h - 51 m , and more than 12 h ; but these are probably excessive. The difference of l m 37 s between the values of Herschel and Hall is relatively a trivial one, as the markings observed were doubtless atmospheric and subject to irregularities of motion. As to the rotation of the ring, Herschel, in 1789, detected some bright marks on it, and deduced the period as 10 h 32 m 15 8 '4*. Many astronomical works give the rotation-period of Saturn as 10 h 29 m 16 8< 8 ; and this is adopted in Chambers's ' Descriptive Astronomy,' 4th edit. vol. i. p. 653. The mistake has its origin in Laplace's Systeme du Monde, where it is stated that Saturn rotates in 0*428 of a day, and the ring in 0'437, which, reduced to hours, minutes, and seconds, give 10 h 16 m 17 s * 2 and 10 h 29 m 16 8 '8. The equator of Saturn is usually the brightest part of the disk. On its S. side, in recent years, it has been bounded by a very dark narrow belt. Further S. the whole disk seems involved in a faint shading, of a decidedly yellowish hue. * Schroter, Harding, Schwabe, and others hare observed luminous points on the rings, but they hare remained stationary, so that the period of rotation announced by Herschel has never been confirmed, but rather disproved by counter-evidence. Herschel wrote, in November 1789 : " 1 formerly supposed the surface of the ring to be rough, owing to luminous points like mountains seen on the ring, till one of these supposed luminous points was kind enough to venture off the edge of the ring and appear as a satellite. I have always found these appearances to be due to satellites." SATURN. 201 Sometimes a considerable number of belts are visible ; but they are evidently liable to changes, so that the same number and arrangement are not preserved from year to year. The Rings. As to the luminous rings, the extreme dia- meter of the outer one is about 40", or more than 170,000 miles; and the black division, separating it from the inner one, is 0"'4, or 1700 miles. The outer ring has a breadth of 2"-4, or 10,000 miles ; while the inner one measures 3"'9, or 17,000 miles. The outer ring is less luminous than the inner ; the Saturn, 1885, Dec. 23, 7 h 54m. (10-inch reflector, power 252.) latter, round its outer edges, is extremely brilliant, and has sometimes been described as the brightest part of the Saturnian system. The inner part of this ring is much shaded-off, and offers a strong contrast to the silvery whiteness of the other portion. Divisions inthe Outer Ring. In the middle of the eighteenth century Short, the optician, using one of his excellent reflectors, 202 SATURN. thought he saw the outer ring divided by several dark lines ; but no other observer confirmed his suspicion. In the third decade of the present century Quetelet and Capt. Kater appear to have observed Short's divisions, but Sir J. Herschel and Struve looked for them in vain. In 1837 Encke fully satis- fied himself, by several observations and measurements, as to the objective existence of the divided outer ring. The division was not central, cutting the ring into equal parts, but situated in the inner part of the ring, so that the wider part was outer- most. In subsequent years this division has been sometimes seen and placed nearest the outer edge of the ring. Certain observers, provided with ample means, have seen nothing of it ; others regard the division as variable. It is sometimes described as a narrow black line ; while others refer to it as a faint pencil-like shading, and not a real division at all. One observer occasionally sees it with considerable distinctness at the very same time that another observer, with a more powerful telescope, cannot glimpse it though looking specially for such an appearance ! It is difficult to reconcile such discordant experiences, and unsafe to accept results of such a contra- dictory nature. The " Crape "-Ring. A far more certain feature was dis- covered in the autumn of 1850 *, and one in reference to which there is unanimity of testimony. On Nov. 11 G. P. Bond, in America, and Dawes, in England, on JSTov. 25, saw a nebulosity or faint luminous appearance like twilight, fringing the interior margin of the inner ring. Later observations showed this to be occasioned by a transparent ring situated immediately within the inner luminous ring. Dawes considered the new ring to be divided into two parts ; but Lassell, with his large reflector, subsequently negatived this supposition. Both limbs * Galle, at Berlin, had, twelve years previously, made an observation which, if it had been interpreted correctly, would have given him priority. In June 1838 he remarked, on several nights, that the inner boundary of the inner ring was very indistinct and " gradually lost itself towards the body of the planet." The space between the ring and Saturn was half filled with a dim veil, extending inwards from the ring. These observations failed to attract the notice their importance deserved, and Galle himself did not appreciate their full significance until the announce- ments of Bond and Dawes in 1850. SATURN. 203 of Saturn may be readily perceived through the transparent ring where it crosses the globe of the planet. Some irregu- larities have been suspected in it at different times by various observers. In 1887 dark condensations were reported to disturb its normal aspect ; but these were not seen at many observatories where such features, if real, could hardly have escaped detection. It is strange to reflect that this transparent ring avoided discovery for so long a period. It forms a feature distinctly Fig. 40. Saturn, as observed by F. Terby, February 1887. to be recognized in relatively small telescopes in fact, Grover has. seen it, where it crosses the globe of Saturn, with only 2 inches of aperture. Yet, though ever on the alert to detect new formations, and exercising constant vigilance in their pursuit, Sir W. Herschel, Schroter, and many others allowed this ring to escape them ! There is no reason to suppose that it is variable, and that it was not so plain a century ago as now. It affords another instance of how easily an unknown object may elude recognition, though everyone sees it readily enough when attention is called to it. In March 1889 a white spot was detected on the rings by Dr. Terby, at Lou vain, and it was seen by other observers with comparatively small instruments. The spot was stationary, and placed near the apparent junction of the globe and rings, in the E. ansa. But with large telescopes nothing of this object could be detected : it was shown to be an optical effect. 2CI4 SATURN. Discordant Observations. It is curious that the details of Saturn have occasioned more dissension amongst observers than those of any other planet. This may have partly arisen from the great distance of Saturn, the comparative feebleness of his light, and complexity of his structure. The planet is usually better defined than either Mars or Jupiter; but with tolerably high powers on small instruments the image is faint, and the features so diluted that the impressions received cannot always be depended upon, especially when the air is unsteady. A fluttering condition of the object is sufficient in itself to cause deception. Prof. Hall, in speaking of the work done by the 25'8-inch refractor at Washington in 1883, says : " Saturn's ring has been observed, but many of the strange phenomena noted by other observers have not been seen even on the best nights/' The evidence afforded by this large instrument may not always be conclusive, but in this case there can be no doubt it properly failed to show " phenomena " which had no existence. Eccentric Position of the Rings. The rings are slightly eccentric with regard to the ball ; in other words, the ball is not situated in the centre of the rings. Differences have been observed denoting this, though the observations are not alto- gether satisfactory. It has been shown theoretically that the eccentricity referred to is necessary to maintain the stable equilibrium of the system ; for were the rings perfectly con- centric with the planet, they must coalesce with the ball. The preservation of so complicated a structure must evidently require judicious and nicely balanced conditions. With the great 23-inch refractor at Princeton, U.S.A., the ball of Saturn was seen through the division in the ring in November 1883 an observation which had previously been made by Lassell in 1852. Aspect of the Rings. In different years the rings present a varying outline, owing to the fact of their inclination (28 10') and to changes in the relative positions of the Earth and Saturn. At intervals of about fifteen years the rings are widely open, as they were in 1855, 1869, and 1885, and will be in 1899. At similar intervals they are rendered invisible, being turned edgeways to the Earth, as in 1848, 1862, 1877, SATURN. 205 and 1891. Since 1877 the S. side of the rings has been presented to terrestrial observers; but in 1893 the N. side will come under inspection, and remain in view until 1907. The S. side of the rings is obviously more favourably visible to observers in England and other N. latitudes, because the planet is always above the equator and attains a fair altitude when it is presented. The N. side of the rings is exposed when Saturn is in S. declination, and therefore more liable to our atmospheric disturbances owing to his comparatively low altitude. The extreme narrowness of the rings is apparent at the periods when the planet crosses the node and they are situated in the plane of the line of sight. In small telescopes they become invisible, and the finest instruments only exhibit them as thread-like extensions from the equator of the planet. Sir J. Herschel says that on April 29, 1833, the disappearance of the ring was complete when observed with a reflector of 18 inches aperture and 20 feet focal length. It remained visible in 1862 as a broken line of light. At such times the satellites are seen as bright beads, threading their way along the narrow wavering line of the belts. Inequalities have been observed at such times ; for the line of light into which the rings are then resolved is not uniform in breadth, but appears broken and undulatory, as though indicating a very rugged character of surface. Sir J. Herschel estimated the thickness of the rings as 250 miles, but Bond thought it far less about 40 miles. There are great obstacles in the way of ascertaining the exact proportions of a structure so distant and offering such an extremely slender form to our view. Further Observations required. The globe and rings of Saturn offer an encouraging prospect for additional dis- coveries. Though the more prominent details have already been descried, there remain other features, probably of more delicate outline and intermittent visibility, which will be glimpsed in future years. Small instruments will scarcely be competent to deal efficiently with this object : observers who can command at least a moderate grasp of light may, however, enter upon the work with every assurance of in- teresting results. In this, as in other sections of observational 206 SATURN. astronomy, the student will realize that in oft-repeated obser- vation and comparison of records and drawings he acquires a familiarity with the appearance of the object which will enable him to discern more and more of its configuration, until ultimately he feels confident he has progressed as far as the utmost capacity of his instrument will permit. It is in the sedulous application of his powers that the observer will find the key to success. Partial devotion to a subject offers a prospect far less encouraging ; for observations of a dis- connected character are seldom valuable. Changes are unquestionably occurring both in connection with the ball and rings of Saturn*. Some of the discre- pancies between the observations published from time to time are only to be explained on this assumption. It should therefore be the aim of observers to obtain further evidence of such variations, and this may be best accomplished by assi- duously watching the lineaments of the planet during the most favourable periods of each opposition. The collection of a number of reliable materials through a series of years would undoubtedly possess weight in removing some of the anomalies of past observation, and afford us a more thorough knowledge of the delicate markings. The rotation-period of Saturn is probably not much different from that given by the atmospheric markings seen by Herschel and Hall. But additional determinations are very desirable for many reasons. The spots which are so plentiful on Mars and Jupiter have furnished observers w r ith a valid and concise means of ascertaining the rate of axial motion of those planets. Saturn, however, has far more sparingly provided the data for such an investigation ; for if we disregard Schroter's uncertain figures, we have but two values for the rotation-period. These were fortunately effected by observers of exceptional ability, and the periods may be accepted without reservation; but other independent determinations are much required. By multiplying results of this nature, we have a prolific source * Struve wrote, in 1883 : " That changes do take place in the ring-- system is sufficiently proved." Trouvelot, Schiaparelli, and others have also remarked variations of a sufficiently decided character to be placed on record. SATURN. 207 of comparison ; and comparisons, apart from being interesting, are of importance in denoting erratic results and indicating those entitled to credence. Moreover, a reliable mean value may be sometimes deduced from multiple records ; hence it becomes advisable to secure as many as possible. The planet should be frequently examined during every opposition with the highest powers that are consistent with a perfectly distinct image ; and the observer should closely scan the various parts of the disk, with an endeavour to trace spots, breaks, or other irregularities in the belts. Certain inequalities of tone have been occasionally apparent in past years, and they will doubtless reappear. The recovery of these features will form a welcome addition to our know- ledge, and, if adequately observed, will enable the rotation- period of the planet to be rediscussed. In an enquiry of this kind many observations are needful, and the longer the interval over which they extend the more accurate the results derived from them are likely to be. If a broken belt should appear on Saturn, the time of its passing the planet's central meridian should be recorded, either by measurement or careful esti- mation, and an ephemeris computed based on a rotation- period of 10|; h , which is equal to a daily rate of nearly 843. Then it should be carefully looked for on subsequent evenings at the times given in the ephemeris, and on every occasion when re-observed its time of transit should be noted as at first. As long as the break continues visible, so long ought it to be kept in view and the times of its central passages tabulated. It would be advisable in such a case to secure cooperation from other observers, as more numerous observations would be sure to accrue, so that, on the appearance of a marking such as that alluded to, the discoverer will do well to announce it imme- diately to other amateurs who are engaged upon planetary work and most likely to assist him. A white or dark spot, or any condensation on the belts, would of course serve the same purpose as a broken belt. The nature of the object is not necessarily to be considered, the main requirement being that it is one of which the longitude admits of determination. Markings on the belts, if they are ever discernible, must be watched with corresponding assiduity for traces of motion ; 208 SATU&N. and if such motion should betray itself, the object of the observer will be to ascertain its rate. With reference to the narrow division in the outer ring, usually termed "Encke's division/' astronomers would regard it as a gratifying advance could the doubts overhanging this feature be removed. Is it a real division in the ring, or simply a pencil-line of shading on the flat surface? Is it constant in place and appearance, or does it frequently exhibit changes both as to intensity and position ? Judging from prior experiences, this particular object would appear to be extremely fugitive, and incapable of being assigned either a definite place or aspect. Yet the more pronounced and well- attested details of Saturn show no such vagaries : Cassini's division seems invariable. Are we therefore to surmise that the curious behaviour of Encke's division is to be referred to errors of observation arising from the effects of unsteady air upon a very delicate object ? It is for future observers to answer these questions, and this will entail no ordinary effort, for the same impediments will be encountered in the future as in the past. But fortunately our science is rapidly pro- gressive, and there is no doubt the mystery of Encke's division will find its solution before long. A powerful tele- scope, and a keen and continuous study of the outer ring, will enable some discriminating observer to tell us the true story of its phenomena. Many other points in the Saturnian system require renewed attention, but some of them appear to be so doubtful as to scarcely deserve mention. Possibly the student had better commence his review of the planet without any of the bias or prejudice which former observations might occasion. But it is as well to know the true state of the case ; for the judgment of a careful observer is not likely to be warped by pre- conception, and of course some of the doubtful observations may be amply verified at a future time. Several of these have already been briefly referred to, and a few others may here be noted. The form of the shadow thrown on the rings from the ball has been observed of a curious shape, and M. Trouvelot supposes it to be variable and occasioned by changes on the level surface of the rings. The same observer SATURN. 209 has noticed transverse notches in the edges of the inner bright ring. Evidence of variation is not entirely wanting in regard to the chief division, and observers should notice whether it appears uniformly black, as it has been suggested that a gauze ring fills the interval. Exterior to the outer ring a faint luminosity has also been suspected, as though the phenomenon of the inner ring had its counterpart here. The colour of the belts on the ball should be ascribed by careful estimates, as many such observations may give an insight into the variations occurring. Some observers have alleged that the transparent ring of Bond and Dawes is subject to very perceptible alterations. It must be remembered, however, that the visible aspect of this exceedingly delicate structure is much affected by the condition of the atmo- sphere, and that the inclination of the Saturnian system must obviously introduce changes. When the inclination is con- siderable, the globe of the planet may be discerned through this ring with greater effect than at other times, because we have to look through a thinner stratum of its material. The observer, in seeking to elucidate some of the anomalies of former researches, will possibly himself gain a knowledge of features not hitherto recognized. Of the real existence of these he should assure himself by many critical observations 1 before venturing to announce them. We have hinted that further discoveries upon Saturn may be considered as practically beyond the reach of small tele- scopes ; but the gratifying fact remains that some of the more noteworthy of the known features are visible in glasses of little pretention as regards size. With a 2-inch refractor, power about 90, not only are the rings splendidly visible, but Cassini's division is readily glimpsed, as well as the narrow dark belt on the body of the planet. This sufficiently proves that a very small and portable instrument is capable of affording some excellent views of one of the most wonderful objects in the heavens. Grrover has seen, with an aperture similar to that named, not only the belts and the shadow of the ball on the rings, but two of the satellites as well ; and others may be equally successful. Occultations of Saturn by the Moon. Phenomena of this p 210 SATURN. kind were well observed in England on May $, 1859, April 20 and Sept. 30, 1870. Those of 1859 and Sept. 30, 1870, were observed by the Rev. S. J. Johnson, who noted that " the dull hue of the planet contrasted strikingly with the brilliant yellow of the Moon." Dawes witnessed the occupation in 1859, and saw the opaque edge of our satellite sharply defined on the ball and rings of Saturn, without the slightest distortion of form. No dark shading was remarked by him contiguous to the Moon's bright edge at the reappearance, such as he and others had observed on Jupiter on the occasion of his occupation, Jan. 2, 1857. Saturn was described as of a pale greenish hue, and offered a strong contrast to the brilliant yellow lustre of the Moon. On the early morning of April 20, 1870, several observers were on the qui vive for this interesting occurrence ; and their experiences are reported in the ' Monthly Notices R. A. S/ vol. xxx. p. 175 et seq., from which the fol- lowing are brief extracts : Mr. Ellis : " The light of the planet, by contrast with the Moon, was very faint. 1 " Mr. Carpenter : " There was not the least alteration in the planet's form." Capt. Noble : " Saturn appeared of a richly-greenish yellow when compared with the brilliant white light of the Moon." Mr. Gr. C. Talmage : " The difference in colour between Saturn and the Moon was most marked, the planet appearing of a yellow tint." Mr. J. Carpenter : " At disappearance the planet was a very dull object when in contact with the Moon ; its light probably a twentieth as bright. At reappearance the planet was rather tremulous ; no distortion was noticed." On June 13, 1870, the Rev. J. Spear, of Bengal, watched the Moon pass " steadily over the planet without causing any change of form or giving any indication of the planet's light passing through an atmospheric medium. When near the Moon's limb Saturn assumed a sickly green hue/' 7 I observed the occultation of Sept. 30, 1870, at Bristol, with a 4J-inch refractor; but the event offered no novel traits, the most prominent feature being the difference of brightness in the Moon and Saturn. Mr. C. L. Prince observed this event with a Tulley refractor of 6'8 inches aperture, power 250. He says there was not the slightest SATURN. 211 distortion of either body, but he noticed that " the edge of the ring lingered somewhat upon the Moon's limb about the time of disappearance." Another occultation occurred soon after new Moon on April 9, 1883, and one of the observers, Mr. Loomis, de- scribed the disappearance of the rings as a spectacle of great interest, and said the impression was forcibly conveyed to his mind that the Moon was very much nearer to the eye than Saturn. The Satellites. The discovery of the eight moons of this planet ranged over the long period of 193 years. Five different observers share the honours between them. Our knowledge of the Saturnian satellites may almost be said to furnish us with a history of improvements in the telescope ; for they were severally detected at epochs corresponding to instrumental advances. The following are the periods, dis- tances, &c. of the satellites : Mean Distance. No. and Name. Sidereal Period. Eeal Diam. Date of Discovery. Discoverer. Diameters Milp of Saturn. lyjLiies. d h m miles. 7th. Mimas 1-53 115,000 22 37 1000 1789, Sept. 17. W. Herschel. 6th. Enceladus... 1-97 148,000 1 8 53 1789, Aug. 28. W. Herschel *. 5th. Tethys 2-44 183,000 1 21 18 500 1684, Mar. 21. J. D. Cassini. 4th. Dione 312 234,000 2 17 41 500 1684, Mar. 21. J. D. Cassini. 3rd Ehea 4-36 327,000 4 12 25 1200 1672, Dec. 23. T ~n n 1st. Titan 10-12 759,000 15 22 41 3300 1655, Mar. 25. J . j-f* Oassini. C. Huygens. 8th. Hyperion ... 12-23 917,000 21 7 7 1848, Sept. 19. Bond & Lassell. 2nd. lapetus 29-61 2,221,000 79 7 53 1800 1671, Oct. 25. J. D. Cassini. The numbers in the first column refer to the order of discovery. Titan is by far the largest satellite, being equal to a star of the 8th mag. and visible in any small telescope. lapetus * Herschel remarks that lie saw this satellite in his 20-foot speculum two years "before, viz. on Aug. 19, 1787, but he was then much engaged in observations of the satellites of Uranus. P2 212 SATURN. ranks next, ordinarily about 9th mag., but there are variations at different parts of the orbit similar to the variations which affect the satellites of Jupiter ; a variegated surface, and the effects of rotation, originate the changes observed and give strong support to the inference that this satellite rotates in the same period that it revolves round its primary. Tethys, Dione, and Rhea are fainter, and the difficulty of seeing them Fig. 41. s o'E X 'Apparent Orbits of the Five Inner Satellites of Saturn, as seen in an Inverting Telescope. (The arrow's in the diagram show the direction of the motion of the satellites. The figures indicate the interval, in hours, from the time of last East elongation.) is intensified by their proximity to the planet ; but a good 4-inch refractor will reveal them on a clear dark night. The others are objects for powerful instruments and pellucid skies ; but Enceladus is sometimes seen with moderate aperture. The planet being usually much inclined, his satellites are dispersed round about the rings, and are not easy of identification. Minute stars lying near the path of Saturn are very liable to be mistaken for them. But the ephemerides drawn up by Mr. Marth, and published annually by the Royal Astronomical Society, are of the utmost service to amateurs engaged in these observations. By simple SATURN. 213 reference they may readily identify the individual satellites on any night ; and these ephemerides are additionally useful as giving the times of conjunctions of some of the satellites with the ends of the ring and N. and S. points of the ball. When the thin side of the rings is presented to the Earth, transits and other phenomena may be observed in connection with the Saturnian moons ; but they appear to have been rarely recorded. Sir W. Herschel describes a " beautiful observation of the transit of the shadow of Titan over the disk in 1789, November 2." It was also seen in 1833 and 1862. The late Mr. Capron re-observed it on Dec. 10, 1877, with a 8J-inch reflector, power 144, and made the following sketch : Fig. 42. These shadow-transits admit of easy observation with appliances of very moderate capacity. Mr. Banks witnessed a phenomenon of the kind with a refractor of only 2f inches, and says it was watched with the same facility and ease as the shadow of Sat. I. on Jupiter. In looking for lapetus it must be remembered that it is commonly situated at a great distance from the planet. Titan is relatively much nearer, and will always be recognized without trouble. Enceladus, Tethys, Dione, and Rhea hover near the outskirts of the ring ; while Mimas is extremely close to it. Prof. Hall, with the great Washington refractor, has effected many valuable measures of this system in recent years. 214 SATURN. He finds the orbits of the five inner satellites are sensibly circular, and that they are situated in the plane of the rings. Hyperion revolves in a very eccentric orbit, and this satellite may approach very near to Titan. He obtained an observation on March 25, 1885, which seems pertinent to the question of variation in the light of the satellites. He says : " Mimas was remarkably bright, and could not be missed even when the full light of the planet was admitted to the eye. Gene- rally this satellite is a difficult object, and from the ease with which it is occasionally seen one might think it variable ; but I think the difference is due to the quality of the image. " There is no doubt that this is the main cause of many assumed changes in celestial objects, and especially in regard to those of a minute and delicate character. Occultations of Stars. Stars are rarely observed to be occulted by Saturn. Webb mentions that, in 1707 or 1708, Dr. Clark noticed a star in the interval between the ball and rings ; and Dawes once remarked a star of 8*5 mag. disappear behind the outer edge of the exterior ring. It would be extremely interesting to watch a tolerably conspicuous star pass centrally behind the Saturnian system, and to trace it through Cassini's division and the transparent ring, noting any changes in magnitude or appearance as they occurred. URANUS AND NEPTUNE. 215 CHAPTER XIII. URANUS AND NEPTUNE. Discovery of Uranus. Mistaken for a Comet. True character revealed. Period &c. Observations. Belts on Uranus. Further Observations required. The Satellites. Discovery of Neptune. The planet observed in 1795. Period &c. Observations. Supposed Ring. Satellite. A trans-Neptunian Planet. Planetary Conjunctions. Discovery. While Sir W. Herschel was a musician at Bath he formed the design of making a telescopic survey of the heavens. When engaged in this he accidentally effected a discovery of great importance, for on the night of March 13, 1781, an object entered the field of his 6 "3-inch reflector which ultimately proved to be a new major planet of our system. The acute eye of Herschel, directly it alighted upon the strange body, recognized it as one of unusual character, for it had a perceptible disk, and could be neither fixed star nor nebula. He afterwards found the object to be in motion, and its appearance being " hazy and ill-defined " with very high powers he was led to regard it as a comet, and communicated his discovery to the Royal Society at its meeting on April 26, 1781. His paper begins as follows : " On Tuesday, March 13, 1781, between 10 and 11 in the evening, while I was examining the small stars in the neigh- bourhood of H Geminorum, I perceived one that appeared visibly larger than the rest. Being struck with its uncommon magnitude, I compared it to H Geminorum and the small star in the quartile between Auriga and Gemini, arid finding it so much larger than either of them suspected it to be a comet. .... The power I had on when I first saw the comet was 227." The supposed " comet " soon came under the observation of others, including Maskelyne the Astronomer Royal, and Messier, the " Comet Ferret " of Paris. The latter, in a letter to Herschel, said : " Nothing was more difficult than 216 URANUS AND NEPTUNE. to catch it, and I cannot conceive how you could have hit this star or comet several times, for it was absolutely necessary for me to observe it for several days in succession before I could perceive that it was in motion." True character revealed. As observations began to accu- mulate it was seen that a parabolic orbit failed to accomo- date them. Ultimately the secret was revealed. The only orbit to represent the motion of the new body was found to be an approximately circular one situated far outside the path of Saturn, and the inference became irresistible that the supposed " comet " must in reality be a new primary planet revolving on the outskirts of the solar system. This con- clusion was justified by facts of a convincing nature, and its announcement created no small excitement in the scientific world. Every telescope was directed to that part of the firmament which contained the new orb, and its pale blue disk, wrapped in tiny proportions, was viewed again and again with all the delight that so great a novelty could inspire. From the earliest period of ancient history, no discovery of the same kind had been effected. The Chaldseans were acquainted with five major planets, in addition to the Earth, and the number had remained constant until the vigilant eye of Herschel enlarged our knowledge, and Saturn was relieved as the sentinel planet going his rounds on the distant frontiers of our system. When the elements of the new body had been computed a search was instituted amongst the records of previous observers, and it was found that HerscheFs planet had been seen on many occasions, but it had invariably been mistaken for a fixed star. Flamsteed observed it on six occasions between 1690 and 1715, while Le Monnier saw it on 12 nights in the years from 1750 to 1771, and it seems to have been pure carelessness on the part of the latter which prevented him from anticipating Herschel in one of the greatest discoveries of modern times. The name Uranus was applied to the new planet, though the discoverer himself called it the Georgium Sidus, and there were others who termed it " Herschel/' in honour of the man through whose sagacity it had been revealed. URANUS AND NEPTUNE. 217 Period fyc. Uranus revolves round the Sun in 30,687 days, which very slightly exceeds 84 terrestrial years. His mean distance from the Sun is 1,782,000,000 miles, but the interval varies between 1,699 and 1,865 millions of miles. The appa- rent diameter of the planet undergoes little variation ; the mean is 3"' 6, but observers differ. His real diameter is approximately 31,000 miles, and the polar compression about -T3? though this value is not that found by all authorities. Observations. The planet near opposition shines like a star of the 6th magnitude, and is observable with the naked eye. He emits a bluish light. While engaged in meteoric observa- tions, I have sometimes followed the planet with the naked eye during several months, and noted the changes in his position relatively to the stars near. It is clear from this that Uranus admitted of detection before the invention of the telescope. A luminous ring, similar to that of Saturn, was at first supposed to surround Uranus, and Herschel suspected the existence of such a feature on several occasions; but it scarcely survived his later researches, and modern observations have finally disposed of it. Lassell, when working with his 2 -foot speculum at Malta, thought he saw a spot near the centre of the planet' s disk, but he considered this might possibly be due to an optical illusion. In 1862, Jan. 29, he said : " I received an impres- sion which I am unable to render certain of an equatoreal dark belt." In the early months of 1870, Mr. Buffhani, using a 9-inch "With" mirror, powers 212 and 320, saw bright spots and zones on the planet, and inferred a rotation- period of about 12 hours. On Jan. 16, 1873, when definition was very good, no traces of any markings were visible in Lord Eosse's 6-foot reflector. In May and June 1883 Prof. Young, having the advantage of the fine 23-inch refractor at the Princeton Observatory, observed two faint belts, one on each side of the equator, and much like the belts of SaturH. On March 18, 1884, Messrs. Thollon and Perrotin, with the 14-inch equatoreal at Nice, remarked dark spots similar to those on Mars, towards the centre of the disk, and a white spot was seen on the limb. Two different tints were perceived, 218 URANUS AND NEPTUNE. the colour of the N.W. hemisphere being dark, and that of the S.E. a bluish-white colour. In April observations were continued, and the white spot was seen " rather as a luminous band than a simple spot," but it was most conspicuous near the limb. The observers thought the appearances indicated a rotation-period of about 10 hours. The brothers Henry at Paris, in 1884, invariably noticed two belts lying parallel to each other, and including between them the brighter equa- toreal zone of the planet. Their results apparently show that the angle between the plane of the Uranian equator and that of the satellite-orbits is about 41. M. Perrotin, with the great 30-inch equatoreal at Nice, re- observed the belts in May and June 1889. He wrote that dark parallel bands were noticed several times, and they were very similar to the belts of Jupiter. On May 31 and June 1 Fig. 43. Q O t* M J r.X^c < .*-$ ' &, ^ fc? Uranus and his Belts. 1884. and 7 the direction of the Uranian belts was measured, and the mean result showed [that the plane of the equator of Uranus differs little (about 10) from the common plane of the orbits of the satellites. This deduction is not, it will be observed, consistent with that of the Brothers Henry at Paris, who found a difference of 41. M. Perrotin notes that the bands of Uranus do not always present the same aspect. URANUS AND NEPTUNE. 219 They vary in size and number in different parts of their circumference. This unequal distribution raises the hope that by an attentive study of these bands it will be possible to determine the duration of the planet's rotation. Further Observations required. In the case of an object so faint and diminutive as Uranus, a powerful telescope is abso- lutely required to deal with it effectively. A small instru- ment will readily show the disk, and present the picture that caught the eye of Herschel more than a century ago, but considerable light and power must be at command if the observer would enter upon a study of the planet's surface- markings. With my 10-inch reflector I have suspected the existence of the belts, but under high powers the image is too feeble to exhibit delicate forms of this character. It is to be hoped that with the large telescopes now available at various observatories, some attention will be given to this planet, more especially with regard to the study of the belts and determination of the rotation-period. Amateurs will have little trouble in picking up Uranus ; his position can be learnt from an ephemeris and marked upon a star-map. A little careful sweeping with a low power in the region indicated will soon reveal the object sought for, and a higher power may then be applied to expand the disk and render identification certain. It may be mentioned as an interesting point that some fifty years after the discovery of Uranus by Sir W . Herschel the planet was accidentally rediscovered by his son Sir John Herschel, who mentioned the fact as follows in a letter to Admiral Smyth, written on Aug. 8, 1830 : " I have just completed two 20-foot reflectors, and have got some interest- ing observations of the satellites of Uranus. The first sweep I made with my new mirror I rediscovered this planet by its disk, having blundered upon it by the merest accident for 19 Capricorni." Had the father failed to detect this planet in 1781, the discovery might therefore have been made by the son half a century later. Some spectroscopic observations of Uranus made in 1889 with Mr. Common's 5-foot reflector, appear to show that the planet " is to a large extent self-luminous." But Mr. Huggins 220 URANUS AND NEPTUNE. on June 3 seems to have obtained a different result (see ' Monthly Notices,' xlix. p. 404 et seq.). The Satellites. For many years it was supposed that Uranus possessed six satellites, all of which were discovered by Sir W. Herschel, but later observations proved that four of these had no existence. They were small stars near the planet. But two of HerschePs satellites were fully corrobo- rated, and two new ones were discovered by Lassell and Struve. The number of known satellites attending Uranus is four, and it is probable that many others exist, though they are too minute to be distinguished in the most powerful in- struments hitherto constructed. The following are the periods, distances, &c., of the known satellites : Mean distance. Number and name. Max. Elonga- tion. Date of Discovery. Discoverer. Diameters of Uranus. Miles. 3rd. Ariel 4-03 125,000 12 1847, Sept. 14. W. Lassell. 4th. Umbriel... 5-61 174,000 15 1847, Oct. 8. O. Struve. 1st, Titania ... 9-19 285,000 33 1787, Jan. 11. W. Herschel. 2nd. Oberon ... 12-32 382,000 44 1787, Jan. 11. W. Herschel. Titania and Oberon are the two brightest satellites, but none of them can be seen except in large instruments. The two outer ones are said to have been glimpsed in a 4*3-inch refractor, but this feat is phenomenal, and certainly no crite- rion of ordinary capacity. Sir J. Herschel found them tolerably conspicuous in a reflector of 18 or 20 inches aper- ture, and mentioned a test-object by which observers might determine whether their telescopes were adequate to reveal them. This test is a minute double star lying between the stars ft and /3 2 Capricorni. The magnitudes are 15 and 16, and distance 3". Relatively to the satellites of Uranus this faint double is a " splendid object." From observations with large modern instruments it .appears highly probable that the four known satellites must URANUS AND NEPTUNE. 221 be considerably larger than any others which may be re- volving round the planet. A curious fact in connection with these satellites is that their motions are retrograde. Fig. 44. Apparent Orbits of the Satellites of Uranus, as seen in an Inverting Telescope. (The small circle in the above diagram represents the planet and is on the same scale as the orbits. The arrows show the direction of the motion of the satellites, and the figures indicate the number of days from the time of the last North elongation.) Discovery of Neptune. The leading incidents in the nar- rative of the discovery of Uranus and Neptune present a great dissimilarity Uranus was discovered by accident, Neptune by design. Telescopic power revealed the former, 222 URANUS AND NEPTUNE. while theory disclosed the latter. In one case optical appli- ances afforded the direct means of success, while in the other the unerring precision of mathematical analysis attained it. The telescope played but a secondary part in the discovery of Neptune, for this instrument was employed simply to realize or confirm what theory had proven. Certain irregularities in the motion of Uranus could not be explained but on the assumption of an undetected planet situated outside the known boundaries of the system. Two able geometers applied themselves to study the problem of these irregularities, and to deduce from them the place of the disturbing body. This was effected independently by Messrs. Le Verrier and Adams ; and Dr. Galle, of Berlin, having re- ceived from Le Verrier the leading results of his computations, and the intimation that the longitude of the suspected planet was then 326, found it with his telescope on the night of Sept. 23, 1846, in longitude 326 52'. The calculated place by Prof. Adams was 329 19' for the same date and less accurate than the prediction of Le Verrier. The former had priority both in attacking the problem and resolving it, though unfortunately his efforts were not backed up in a practical way. But for the supineness of certain officials, there is little doubt that the planet would have been telescopically discovered in the autumn of 1845, when it was within 1 49' of the place attributed to it by Prof. Adams. Delays occurred owing to the doubts prevailing, and in the meantime the planet was found elsewhere. This circumstance does not rob Prof. Adams of his hard-earned laurels, though it shows how seriously official negligence can mar the character of a discovery. Observations in 1795. The name given to the new planet was Neptune. When the elements were computed it was found that they presented rather large differences with those theoretically computed by Messrs. Le Verrier and Adams. It was also found that the planet had been previously observed by Lalande on May 8 and 10, 1795, but its true character escaped detection. This astronomer had observed a star of the 8th mag. on May 8 ; but on May 10, not finding the same star in the exact place noted on the former evening, URANUS AND NEPTUNE. 223 he rejected the first observation as inaccurate and adopted the second, marking it doubtful. Had Lalande exercised a little discretion, and confided in his work, he would hardly have allowed the matter to rest here. A subsequent observation would at once have exhibited the cause of the discrepancy, and the- mathematical triumph of Le Verrier and Adams, half a century later, would have been forestalled. Lalande, like Le Monnier, the unsuspecting observer of Uranus, let a valu- able discovery slip through his hands. Period $c. Neptune revolves round the Sun in 60,126 days, which is equal to rather more than 164^ of our years. His mean distance from the Sun is 2,792,000,000 miles, and his usual diameter 2"'7. He exceeds Uranus in dimensions,, his real diameter being 37,000 miles. Observations. Our knowledge of this distant orb is ex- tremely limited, owing to his apparently diminutive size and feebleness. No markings have ever been sighted on his miniature disk, and we can expect to learn nothing until one of the large telescopes is employed in the work. No doubt this planet exhibits the same belted appearance as that of Uranus, and there is every probability that he possesses a numerous retinue of satellites. In dealing with an object like this small instruments are useless ; they will display the disk, and enable us to identify the object and determine its position if necessary, but beyond this their powers are restricted by want of light. Supposed Ring. Directly the new planet was discovered, Mr. Lassell turned his large reflector upon it and sought to learn something of its appearance, and possibly detect one or more of its satellites. On October 3 and 10, 1846, he was struck with the appearance of the disk, which was obviously not perfectly spherical. He subsequently confirmed this impression, and concluded that a ring, inclined about 70, surrounded the planet. Prof. Chain's supported this view, but later observations in a purer sky led Mr. Lassell to abandon the idea. Thus the ring of Neptune, like the ring of Uranus, though apparently obvious at first, vanished in the light of more modern researches. The Satellite. But if Mr. Lassell quite failed to demon- 224 URANUS AND NEPTUNE. strate the existence of a ring, he nevertheless succeeded in discovering a satellite belonging to the planet. This was on Oct. 10, 1846. The new satellite was found to have a period of 5 d 21 h 3 m , and to be situated about 220,000 miles distant from the planet. Its apparent star mag. is 14, and at max. Fig. 45. Apparent Orbit of the Satellite of Neptune, as seen in an Inverting Telescope. (The small circle in the above diagram represents the planet, the arrows show the direction of motion, and the figures indicate the interval from the time of last North-east elongation.) elongation it extends its excursions to 18" on either side of its primary. Compared with the other satellites of our system the one attending Neptune must be excessive in regard to size, or it could not be discerned at the vast distance separating it from the Earth. A trans-Neptunian Planet. Is there a planet beyond Neptune? Prof. Forbes wrote a memoir in 1880 tending to prove that two such planets exist. From the influences exerted by these bodies on certain comets of long period, he approximately deduced the positions of the former, and they were searched for with the great Washington refractor, but without success. Flammarion and Todd have also arrived at conclusions affirming the existence of a planet outside Neptune ; but the idea has not yet been realized by its telescopic discovery. VRANUS AND NEPTUNE. 22o Planetary Conjunctions. Before concluding this chapter, an allusion should be made to a noteworthy class of events, viz., planetary conjunctions. THese include some of the most attractive aspects displayed by the heavenly bodies, and they are sometimes witnessed by ordinary persons with the same amount of gratification as by the astronomical amateur. In almanacks the times of such conjunctions are given, so that intending observers may always be prepared for these events. In a strict sense a conjunction occurs at the instant when two or more bodies have the same right ascension, but the term is here intended to have a more general reference, i. e., to denote the assembling together of two or more planets in the same region of the firmament. Historical records furnish us with a considerable number of planetary conjunctions, and some of them were attentively observed long before the telescope came into use. Thus in 2012 B.C., Feb. 26, the Moon, Mercury, Venus, Jupiter, and Saturn were in the same constellation, and within 14 of one another. In 1186 A.D., Sep. 14, the Sun, Moon, and all the known planets are said to have been situated in Libra. In 1524 Venus, Mars, Jupiter, and Saturn were near together. Many similar instances might be quoted, but this is unnecessary. Occasionally the conjunctions were so close that one planet appeared to occult another. Kepler refers to an occultation of Jupiter by Mars which he saw on January 9, 1591 ; but this would really be a transit of Mars across the disk of Jupiter, if contact actually occurred, for the apparent diameter of Jupiter always exceeds that of Mars. Moestlin seems to have witnessed an occultation of Mars by Venus on Oct. 3, 1590. It is probable, however, that these were near approaches only. A genuine occultation of Mer- cury by Venus was telescopically observed on May 17, 1737. On the evening of March 3, 1881, the new Moon, Venus, Jupiter, and Saturn formed a brilliant quartet in Pisces. On the morning of July 21, 1881, I saw the Moon, Venus, Mars, Jupiter, Saturn, and Aldebaran in the same region above the eastern horizon. There was a very close conjunction of Mars and Saturn on the morning of Sept. 20, 1889. Mr. Marth computed that the nearest approach would occur at 8 h 7 m A.M., Q 226 URANUS AND NEPTUNE. when the distance hetween the centres would be 54"' 8 and less than that (74") observed at the time of the close conjunction of the same planets on June' 30, 1879. The interest centred in the conjunction of Sept. 20, 1889, was enhanced by the fact that Regulus was only 47' distant, Fig, 46. Mars, Saturn, and Regulus in same field, Sept. 20 1889, 4h 45m A.M. while Yenus was also in the same region. I observed this phenomenon in my 10-inch reflector, and with the help of a comet-eyepiece made the above sketch of the positions of the objects as they were presented in the field. Perhaps there is not much scientific importance attached to the observation of these conjunctions, though comparisons of colour and surface-brilliancy are feasible at such epochs, and are not wholly without value. As spectacles merely, they possess a high degree of interest to everyone who " considers the heavens." COMETS AND COMET-SEEKING. 227 CHAPTER XIV. COMETS AND COMET-SEEKING. Ideas concerning Comets. Appearance. Large number visible. Nature of Apparition. Tenuity of Comets. Differences of Orbit. Dis- coveries of Comets. Large Comets. Periodical Comets. The Comets of Halley, Encke, Biela, Brorsen, Faye, D'Arrest, Pons-Winnecke, and Tuttle. Grouping. Further Observations required. Nomenclature of Comets. Curiosities of Comets. Naked-eye Comets. Comet-seeking. English weather. Aperture and Power required. Annual rate of Discovery. Telescopic Comets and Nebulae. Ascertaining Positions. Dr. Doberck's hints. Prizes. SUPERSTITIOUS ideas with regard to comets as the harbingers of disaster have long since been discarded for more rational opinions. They are no longer looked upon as ill-omened presages of evil, or as " From Saturnius sent, To fright the nations with a dire portent." Many references are to be found among old writings to the supposed evil influence of these bodies, and to the dread which their appearance formerly incited in the popular mind. Shake- speare makes an allusion to the common belief : *' Hung be the heavens with black, yield day to night I Comets, importing change of time and states, Brandish your crystal tresses in the sky j " and in relation to the habit of connecting historical events with their apparition, he further says : " When beggars die, there are no comets seen ; The heavens themselves blaze forth the death of princes." But, happily, the notions prevalent in former times have been superseded by the more enlightened views naturally resulting from the acquirement and diffusion of knowledge ; -so that comets, though still surrounded by a good deal of mystery, are now regarded with considerable interest, and welcomed, not Q2 223 COMETS AND COMET-SEEKING. only as objects devoid of malevolent character, but as furnishing many useful materials for study. Mere superstition has been put aside as an impediment to real progress, and a more intelligent age has recognized the necessity of dealing only with facts and explaining them according to the laws of nature ; for it is on facts, and their just interpretation, that all true searchers after knowledge must rely. Comets are properly regarded as bodies which, though far from being thoroughly understood in all the details of their physical structure and behaviour, have yet a wonderful history, and one which, could it be clearly elucidated, would unfold some new and marvellous facts. Under these circumstances we need evince no surprise that these visitors are invariably hailed with enthusiasm, not only by scientific men, who make them the special subjects of close observation, but by everyone who regards celestial "sights and signs" with occasional attention. Appearance. From whatever point of view a large comet is considered, it deserves all the interest manifested in it and all the labour expended in its investigation. Whilst its grand appearance in the firmament arrests the notice of all classes alike, and is the subject of much curious speculation amongst the uninformed, it merits, apart from other con- siderations, the most assiduous observation on account of the singular features it displays and the striking variations they undergo. Indeed, the visible deportment of a comet during its rapid career near perihelion is so extraordinary as to form a problem, the solution of which continues to defy the most ingenious theories. The remarkable changes in progress, the quickness and apparent irregularity of their development, are the immediate result of a combination of forces, the operations of which can neither be defined nor foreseen. Jets of flame and wreaths of vapour start from the brilliant nucleus; while, streaming away from the latter, in a direction opposite to the Sun, is the fan-shaped tail, often traceable over a large span of the heavens and commingling its extreme fainter limits with the star-dust in the background. Large number visible. The orbits of 400 comets have now been computed, and more than 500 others have been ob- COMETS AND COMET-SEEKING. 229 served ; so that these bodies are extremely plentiful. Kepler described them to be as numerous as the fishes in the sea, and no doubt the allegory is justly applied. Their vagaries of form, size, and place are equally noteworthy ; and those who enter upon the discussion of facts relating to these objects will find an endless store of interesting materials, opening up a wide field for conjecture. Nature of Apparition. The apparition of a comet may be either gradual or sudden. Usually the telescope gives us the earliest intimation that one of these bodies is approaching us*. It is first seen as a small round nebulosity, with probably a central condensation or stellar nucleus of the 10th or llth mag. The whole object brightens and expands as its distance grows less, and it assumes an elongated form preparatory to the formation of a tail. The latter varies greatly in different instances: it may either be a narrow ray, as shown in the southern comet of January 1887, or a fan-shaped extension like that of the great comet of 1744. Barnard's Comet of December 1886 exhibited a duple tail. Occasionally a fine comet bursts upon us suddenly, like that of 1843 or 1861. The forme-r was sufficiently bright to be discovered when only 4 from th Sun, and the latter presented itself quite unex- pectedly as a magnificent object even in the strong twilight of a June sky. Tenuity of Comets. Comets are noteworthy for the extreme thinness of their material. The smallest stars may be discerned through the denser portions of the head, without suffering any apparent diminution of light. Yet such stars would be quite obscured by the interposition of a minute speck of cloud or by a little fog or any vapour of trifling density. Comets are visible in the form of transparent nebulosities ; and their mass must be inconceivably small relatively to the enormous space over which they frequently extend. Sir J. Herschel has described the "all but spiritual texture" of comets ; and other autho- rities have referred to them as feeble wreatho of vapour, which, though obeying the laws of gravitation and suffering * Donati's Comet of 1858 and Coggia's Comet of 1874 may be men- tioned as good examples of the gradual approach and development of these visitors witnessed by means of the telescope. 230 COMETS AND COMET-SEEKING. much perturbation, are yet themselves incapable of exercising any disturbing influence upon the other bodies near which they pass. It has been asserted that comets would show phases were they rendered luminous by reflected sunlight, and that, such features being absent, these bodies must possess a phosphorescence of their own sufficient to cause the glow observed. This idea, however, is hardly consistent with our present knowledge. Comets are not compact and coherent masses of matter ; they more likely represent vast groups of planetary atoms, more or less loosely dispersed and sometimes forming streams. The effect of sunshine upon such assemblages will be that the whole mass becomes illu- mined according to density, and that no phase will be apparent, inasmuch as the light is enabled to penetrate through its entirety. Differences of Orbit. When three trustworthy observations of a comet's place have been made, its orbit may be computed. This may be either an ellipse, a parabola, or hyperbola. If an ellipse the comet is periodical, and the period depends upon the degree of eccentricity. If a parabola the comet will not be seen again, because this form of orbit does not reunite ; it consists of branches equally divergent and uniting at peri- helion, but extending outwards indefinitely in nearly parallel lines and without convergence. If a hyperbola the comet is also not returnable ; the branches of the orbit are widely divergent, and show no tendency to parallelism. These several forms of orbit are somewhat different as applied to various comets, but they are the same in effect. Thus Tempel's Comet of 1867 revolves in an ellipse having an eccentricity of about 0'4630, while that of Halley's Comet is 0*9674. No doubt some of the parabolic orbits applied to comets really represent very eccentric ellipses; but the parabola is a convenient form of orbit for computation, and unless ellipticity is very decided it indicates the path with sufficient accuracy. Discoveries of Comets. In the latter part of the last century Messier, Mechain, and Miss Herschel shared nearly all the cometary discoveries between them. Then Pons entered the COMETS AND CO MET- SEEKING. 231 field, and he may be said to have monopolized this branch during the period from 1802 to 1827, for he was the first to announce thirty comets. Pons died in 1831, but the search was actively continued by others. In about 1843 a great rise became apparent in the rate of these discoveries ; and we find Di Vico, Mauvais, and Brorsen very successful at this period. Later on, the work was sustained with the same prolific results by Klinkerfues, Bruhns, and Donati, and subsequently by Winnecke, Tempel, and Coggia. Swift and Borrelly also assisted materially to swell our knowledge ; while during the last few years Barnard and Brooks have exhibited a surprising amount of zeal in this department. Since 1881 no less than twenty-six comets are to be enu- merated as the fruits of their endeavours, and they are still engaged in nightly explorations of the sky with similar ends in view. Their diligent pursuit of these fugitive bodies will doubtless result in many further additions during ensuing years. It is a curious circumstance that Sir W. Herschel, during all his star-gaugings and sweeps for nebulae, never discovered a comet. He found a nebula on Dec. 18, 1783, near 8 Ceti, which he described as " small and cometic." In Sir J. HerschePs ' General Catalogue of Nebulae/ 1864, p. 17, this object is presumed to have been a comet, as it could not be identified; but at p. 45 the doubts are cleared up, and Sir W. Herschel's nebula, the position of which was only roughly given, is shown to be the same as another very near ; it is No. 1055 of the new ' General Catalogue ' published by the Royal Astro- nomical Society in January 1888. Quite possibly Sir W. Herschel's lists of nebulse contain several comets, as some of his objects are missing ; but errors of observation in ascribing positions may explain this. Herschel himself, in speaking of a comet visible in the winter of 1807-8, says : " If I had met the comet in one of my sweeps, as it appeared between Dec. 6 and Feb. 21, I should have put it down as a nebula. Perhaps my lists of nebulae, then, contain some comets." Large Comets. The most widely observed and attractive 232 (!(>MfiTft AND COMET-ftEEKING. class of comets includes those of large proportions, as they are not only visible to the naked eye, hut exhibit features having the lustre necessary to permit of their examination with high wngnifying powers. A brief summary of some of the finest cornets of modern times is subjoined ; but, to save space, n few only of the more salient facts concerning them are given : 1577, Nov. and Dec. Observed by Tycho Brahe. At the end of November it had a double tail ; the longest of the two branches was about 20. This comet was visible in the day- time. IT) 18 II., Nov. " The length of its tail equalled in extent one sixth part of the zodiac." On Nov. 18 it was estimated as 40. Longomontanus, however, described it as 104 long, and Cysatus estimated it as 75. Kepler referred to it as the largest comet that had appeared for a hundred and fifty years. 1(580, Dec. A fine comet, which on Dec. 12 had a narrow tail about 80 long. The nucleus was equal to a 1st mag. stir. Hooko remarked jets of flame issuing from the nucleus. At perihelion the comet approached very near the Sun's surface, similarly to the fine comets of 1843, 1880, and 1882. 1744, Jan.-Feb. Probably the largest comet of the 18th century. At one time it displayed six tails, each of which was 4 in breadth. The head was so bright that it was per- ceived with the naked eye in full sunshine. At the middle of February the tail was 24 long, and it was divided into two branches. 1769, Sept. Discovered on Aug. 8 by Messier. On Aug. 30 the comet had a trifid tail; there was a central ray of 24 and two outlying ones of 4 each. On Sept. 19 the tail had increased to 75, and a few nights later Pingre estimated U as 90 and 97. 1811 1., Sept.-Oct. A very fine comet. The tail was branched ; it did not, however, exceed 25 in length and about 6 in breadth. Sir W. Herschel found the nucleus to be 428 miles in diameter. This remarkable comet remained visible during a period extending over seventeen months, Its period is approximately 3000 years. COMETS AND CO MET- SEEKING. 233 1843, Mar. Visible in the daytime. On Mar. 4 its tail was 69 in length ; it was very narrow, being only 1J in breadth throughout. At perihelion this object passed very near to the Sun, like the great comet of 1680. It revolves in an elliptical orbit ; period about 376 years. This comet swept past perihelion with a velocity of 366 miles per second ! The real length of its tail was 200 millions of miles I 1858 VI., Sept.-Oct. Donati's Comet : one of the most brilliant comets of the 19th century. Early in October it displayed a tail about 40 long, and on the 5th it passed over the star Arcturus. Its period of revolution appears to be about 2000 years. 1861 II., June-July. Became suddenly visible at the end of June. In the opinion of Sir John Herschel this comet surpassed in grandeur the comets of 1811 and 1858. On June 30 the nucleus was equal to the brightness of Venus, and the tail was 80 long ; but early in July it increased to 90. One observer estimated its length as 100 on July 2. This comet remained visible during twelve months. It appears to have an elliptical orbit, with a period of 409 years. 1874, July. Coggia's Comet : a fine object in the northern sky. On July 14 the tail was 35 long, and it remained visible several days after the nucleus had disappeared below the horizon. The nucleus was about equal to a star of the 1st mag. Orbit probably elliptical, with a period of about 5711 years. 1880 I., Jan.-Feb. A southern comet, with a long narrow tail, variously estimated from 30 to 40 in length. It passed very near to the Sun, and presents an orbital resemblance to the fine comets of 1680 and 1843. 1881 III., June-July. This large comet appeared in the northern heavens on June 22, and became generally visible to observers in England. On the 27th it had a tail 15 long. Its period of visibility extended over nine months. 1881 IV., Aug.: rThis comet is scarcely entitled to rank as one of exceptional character ; but it was a conspicuous object for several weeks in August, and had a tail 6 long on the 19th. COMETS AND COMET-SEEKING. 1882 III., Oct. Well visible in the morning sky, with a tail 22 long. The nucleus underwent remarkable changes, and on Oct. 23 it showed four or five bright points or nuclei, looking like " a string of beads." The comet threw off several small condensations, which were observed as separate comets near the parent mass. At perihelion this comet passed very close to the Sun, like the comets of 1680, 1843, and 1880 ; and these bodies were suspected to have an intimate relation, if not an absolute identity. But subsequent inquiries dis- proved this startling supposition ; for the comet of 1882 was shown to have a period of about 718 years. 18871., Jan. A fine southern comet, presenting many points of resemblance to that of 1880 I. On Jan. 22, as observed at Adelaide, the comet had a long narrow tail of about 30, but no well-defined nucleus. On the same date, at the Cape of Good Hope, the tail appeared as a narrow ribbon of light, quite straight, and of nearly uniform brightness throughout its length. It was visible in the same region of the sky as the comet 1880 I., and came into view with equal suddenness. Periodical Comets. On page 235 is a list of the periodical comets as at present known. Some of these, marked with an asterisk, have only been observed at one return, and therefore await complete confirmation. Many other comets have shown indications of pursuing elliptical orbits. Amongst those of short period may be mentioned 17431,176611., 17831., 1819 IV., 1844 L, and 1873 VII. The following are examples of longer periods : Comet. Period. 1862 III 321 years. 1857 IV 234 18611 415 1860 III 1089 1889 IV 5100 1877II 8393 1847 III. . 13918 Comet. Period. 1877 III 28,000 years. 18501 29,000 17801 75,314 184411 102,050 1744 122,683 18491 382,801 18821 400,000 These figures are to be regarded as approximations only. COMETS AND COMET-SEEKING. 235 *3 GO GC GO GO 00 GO GO 4 s i^t < T^OOCOi-HOCOCC^COl-- Long, of Perihelion vO^i iOO^COC5-H' iOi^C' i T-I ri TT (M COCO(MGMiOCO 00 Or- (^C^GOCOC5GOCiO5t^'MOr-( >O OO'-it^cOTtiCNiOO'-i iCCO iiCOCOr- i. A stone weighing 262 Ib. fell at Ensisheirn, in Alsace. 1642. A stone of 4 Ib. fell near Woodbridge, in Suffolk. 1795, Dec. 13. A stone of 56 Ib. fell at Wold Cottage, Thwing, Yorkshire. 1860, July 14. A shower of aerolites fell at Dhurmsnla, in India. A tremendous detonation attended their descent, and the natives became greatly alarmed. They supposed the stones to have been thrown by some of their deities from the summit of the Hima- layas, and many of them were preserved as objects of religious veneration. 1864, May 14. A very large meteor was observed in France. At Montauban and the neighbourhood deafening explosions occurred, and showers of stones fell near the villages of Orgueil and Nohic. 1876, April 20. A piece of iron weighing 7| Ib. fell at Rowton, Shropshire. 1881, March 14. A stone weighing 3 Ib. 8J oz. fell at Middlesborough, Yorkshire, on a part of the North- Eastern Railway Company's branch line. The descent of the aerolite was witnessed by an inspector and three platelayers, who were working about fifty yards distant. At first they became aware of a whizzing or rushing noise in the air, immediately METEORIC OBSERVATIONS. 267 followed by the sudden blow of a body striking the ground near. The hole, 11 inches deep, which the stone made was found directly after, and the stone was extracted. Many other examples might be given, but the above will be sufficient for our purpose. Records of this nature were discredited in former times ; but more modern researches have long since placed their reality beyond all question. The fall of stones from the sky is no longer regarded as a mere legendary tale, but as one of the well-assured operations of nature. Meteoric stones and irons have been classified according to the ingredients of their composition. Those in which iron is found in considerable amount are termed siderites, those con- taining an admixture of iron and stone, siderolites, and those consisting almost entirely of stone are known as aerolites. The siderite which fell in Shropshire on April 20, 1876, forms only the seventh recorded instance where a mass of meteoric iron has been actually seen to fall. Fireballs. The table on p. 268 gives the dates, heights, &c. of fifteen fireballs observed during the last quarter of a century. Fireballs are sometimes detonating, though more often silent. The fireball of Nov. 23, 1877, gave a sound like salvoes of artillery, and doors and windows were shaken violently. At Chester the noise of its explosion was compared to loud but distant thunder. Lieut. -Col. Tupman says that u thunder, to be loud, must be within five miles ; hence it appears that the violence of the explosion must have been at least a hundred times greater than a peal of thunder, the intensity of sound-waves diminishing as the square of the distance." " The explosion of a 13-inch bomb-shell, consisting of some 200 Ib. of iron, would not have produced a sound of one hundredth part of the intensity of the meteor-explosion/' This fireball must therefore have been an object of considerable mass before its dissolution ; and it is fortunate that such bodies are usually destroyed by the effects of combustion before they reach the Earth's surface. These phenomena exhibit many varieties of appearance. METEORS AND i i * ^ s ii~o||-g|-s.;'" j3 "5 oj ^aJB^^^aj^ajpq^^^pq "*5 O - QOQOCOCO iii COOOGO iii METEORIC OBSERVATIONS. Ffc. 56. 269 Fireball of Nov. 23, 1877, 8h 24m, emerging from behind a cloud. (Prawn by J. Plant, Salford.) 270 METEORS AND Sometimes there is no visible explosion ; the bright nucleus slowly dies out until reduced to a faint spark before final disappearance. Several outbursts of light are often noted ; and a curious halting motion has been observed in regard to large slow-moving meteors. I have occasionally remarked a succession of four brilliant flashes given by individual fire- balls. These flashes, though sometimes of startling intensity, are somewhat different to the transient vividness of lightning ; they come more softly, and remind one forcibly of moonlight breaking suddenly from the clear intervals in passing clouds. Fireballs differ vastly from shooting-stars in point of size ; but their origin is identical. The August meteor-shower yields the smallest shooting-stars and the largest type of fire- balls. The great display of meteors on Nov. 27, 1885, not only presented us with large and small members, but it also furnished us with a siderite or piece of iron, presumably from Biela's Comet. This fell at Mazapil, Mexico ; and as con- siderable interest is attached to the case, I quote a part of the discoverer's statement : " It was at about 9 o'clock on the night of November 27th, when I went out to the corral to feed certain horses : suddenly I heard a, loud sizzing noise, exactly as though something red- hot was being plunged into cold water ; and almost instantly there followed a somewhat loud thud. At once the corral was covered with a phosphorescent light ; while suspended in the air were small luminous sparks, as though from a rocket. . . . A number of people came running towards me ; and when we had recovered from our fright we saw the light disappear, and bringing lanterns to look for the cause found a hole in the ground, and in it a ball of light. We retired to a distance, fearing it would explode and harm us. Looking up to the sky, we saw from time to time exhalations of stars, which soon went out without noise. We returned after a little, and found in the hole a hot stone which we could barely handle ; this, on the next day, we saw looked like a piece of iron. All night it rained stars ; but we saw none fall to the ground, as they all seemed to be extinguished while yet very high up." This is the first observed instance in which a meteorite has actually reached the Earth's surface during the progress of a METEORIC OBSERVATIONS. 271 star-shower. If its identity with the meteors of Biela's Comet is admitted, then all classes of meteoric phenomena would appear to have a community of origin. Differences of Motion. Great differences are observed in the velocity of meteors. An observer may notice all varieties on the same night of observation. Some will move very slowly, others shoot quickly across the sky. These differ- ences are occasioned by the astronomical conditions affecting the position of the meteor-orbit relatively to the motion of the Earth. Thus the meteors of Nov. 13 move with great velocity (44 miles per second), because they come directly from that part of the heavens towards which the Earth is moving ; hence the orbital speed of the Earth (18^ miles per second) and meteors (26 miles per second) is combined in the observed effects. But in the case of the meteor-shower of Nov. 27 the motions are extremely slow (about 10 miles per second), as the Earth and the meteors are travelling nearly parallel in the same direction, and the latter have to overtake the Earth. Nomenclature of Meteor-Systems. It is customary to name the showers after the constellation from which the meteors appear to diverge. Thus the meteors of April 20 are called Lyrids, the radiant being in Lyra ; the meteors of August 10 are termed Perseids, the point of emanation being in Perseus. The two great streams of November are known as the Leonids (13th) and Andromedes (27th). Several showers are often visible in the same constellation ; and when it is desired to name these according to the above system, it is necessary to add the approximate star to distinguish them. Thus, in August there are showers of //. Perseids, e Perseids, and a Perseids, in addition to the well-known Perseids of August 10. Meteor-Storms. On Nov. 12, 1799, Humboldt, at Cumana, in South America, saw " thousands of bolides and falling stars succeed each other during four hours." On Nov. 12, 1833, this shower recurred, and was witnessed with magnificent effect in America. One observer stated that between 4 and "6A.M. (Nov. 13) about 1000 meteors per minute might have been counted ! Another display occurred on Nov. 13, 272 METEORS AND 1866, and on this occnsion 8485 meteors were enumerated by several observers at Greenwich. A different system gave us a brilliant exhibition on Nov. 27, 1872, when 33,000 meteors were counted by Denza and his assistants at Moncalieri, in Italy, between the hours of 5 h 50 m and l() h 30 m P.M. A repetition of this phenomenon occurred on Nov. 27, 1885, when the same observers counted nearly 40 7 000 meteors between 6 h and 10 h P.M. Telescopic Meteors. Observers who are engaged in seeking for comets or studying variable stars employ low powers and large fields, and during the progress of their work notice a considerable number of small meteors. At some periods these bodies are more plentiful than at others, and appear in such rapid succession that the observer's attention is dis- Fig. 57. VERTICAL. Flight of Telescopic Meteors seen by W. K. Brooks, Nov. 28, 1883. tracted from the special work he is pursuing to watch them more narrowly and record their numbers. Schmidt saw 146 telescopic meteors during ten years. They ranged between tho 7th and llth mags. Winnecke in tlie year 1854 noticed 105 of these objects on thirty- two evenings of observation with METEORIC OBSERVATIONS. 273 a 3-inch finder, power 15, and field of 3. I have also remarked many of these objects when using the comet-eyepieces of my 10-inch reflector *, and find they are apparently more nume- rous than the ordinary naked-eye meteors in the proportion of 22 to 1. It would be supposed from the great rapidity with which the latter shoot across the firmament that the smaller telescopic meteors are scarcely distinguishable by their motion, as they must dart through the field instantaneously and only be perceptible as lines of light. But this impression is alto- gether inconsistent with the appearances observed. They possess no such velocity, but usually move with extreme slowness, and not unfrequently the whole of the path is com- prised within the same field of view. The eye is enabled to follow them as they leisurely traverse their courses, and to note peculiarities of aspect. Of course, there are considerable differences of speed observed, but as a rule the rate is decidedly slow and far less than that shown by naked-eye meteors. I believe that telescopic meteors are situated at great heights in the atmosphere, and that their diminutive size and slowness of movement are due to their remoteness. This conclusion will hardly be avoided by anyone who attentively studies the several classes of meteors in their various aspects. Unfortu- nately no attempt appears to have been hitherto made to de- termine the actual heights of telescopic meteors, owing to the difficulty of obtaining two reliable observations of the same object. The only way of securing such data would be for several observers to watch certain selected regions by pre- arrangement either with a low-power telescope or field-glass, and record the exact times and paths of the meteors seen. On a comparison of the results a good double observation of the same object might be found, in which case the real path could be readily computed. Future observers should note the different forms of tele- scopic meteors. Safarik has divided them into four classes. viz. : (1) Well-defined star-like objects of very small size ; (2) Large luminous bodies of some minutes of arc in dia- meter ; (3) Well-defined disks of a very perceptible diameter * During the seven months from May to November 1890 I noted ninety-five telescopic meteors while engaged in comet-seeking. T 274 METEORS AND brighter at the border than at the centre, which gives them the aspect of hollow transparent shells ; and (4) faint diffused masses of irregular shape, considerable size, and different colours. He has seen hundreds of meteors of every mag- nitude from the 2nd down to the 12th pass through the field of his 6^-inch reflector (ordinary power 32, field 54'). On Aug. 30, 1880, 9 h to 15 h he observed between 50 and 100 telescopic meteors, and many others were seen on the following night. Whenever a shower of these bodies, such as that wit- nessed by Brooks on Nov. 28, 1 883, occurs, observers should notice whether the objects participate in a common direction of motion ; because, if so, the radiant-point will admit of deter- mination. The horary rate of their apparition ought also to be ascertained. Those who habitually search for comets should invariably make a note of telescopic meteors, as such records would aid inquiries into the relative frequency of these phenomena. Meteor Showers. The following short list includes the principal displays of the year : Name of Shower. Duration. Date of Max. Kadiant- Point. Sun's Longitude. Quadrantids Dec 28-Jan 4 Jan. 2 a S O 229-8 +52v; 281-6 Lyrids April 16-22 April 20 2697+32-5 313 n Aquarids ... April 30- May 6 May 6 337-6- 2-1 46-3 S Aquarids July 23-Aug. 25 July 28 339.4-11.6 125-6 July 8-Aug. 22 Aue 10 45 -9 -f 56 -9 138-5 Orionids Oct. 9-29 Oct. 18 921 + 15-5 205-9 I/eonids . .. Nov 9-17 Nov 13 150-04-22-9 231-5 Androuiedes Geminids , Nov. 25-30 Dec 1-14 Nov. 27 Dec 10 25-3+43-8 108-1+32-6 245-8 259-5 Notes. Quadrantids. Heis was the first to determine this^ radiant accurately. It was subsequently observed by Masters and Prof. Herschel (1863-4). The radiant is circumpolar in this latitude, but low down during the greater part of the night, METEORIC OBSERVATIONS. 275 hence the display is usually seen to the best advantage on the morning of Jan. 2. Lyrids. Attention was first drawn to the April meteors by Herrick in the United States. Active displays occurred in 1863 and 1884. rj Aquarids. Further observations are urgently required of this stream. The radiant is only visible for a short time before sunrise. There is a considerable difference between my results and those secured by Lieut.-Col. Tupman, the discoverer of this system in 1870, whose observations place the radiant at 326 J 2 J April 29-May 3. These May Aquarids are inter- esting from the fact that they present an orbital resemblance to Halley's Comet, which makes a near approach to the Earth on May 4, twelve days before reaching the descending node. 8 Aquarids. The meteoric epoch, July 26-30, was first pointed out by Quetelet many years ago. Biot also found, from the oldest Chinese observations, a general maximum between July 18 and 27 (Humboldt). Showers of Aquarids were remarked by Schmidt, Tupman (1870), and others ; but it was not known until my observations in 1878 that the Aquarids formed the special display of the epoch, and that there were many early Perseids visible at the same time. Perseids. Muschenbroeck, in his work on f Natural Philo- sophy/ printed in 1762, mentions that he observed shooting- stars to be more numerous in August than in the other months of the year. Quetelet, in 1835, was, however, the first to attribute a definite maximum to the 9th-10th. This stream is remarkable for its extended duration, and for the obvious displacement which occurs from night to night in the place of its radiant. It furnishes an annual display of considerable strength, and is, perhaps, the best known system of all. Orionids. Profs. Schmidt and Herschel were the first to discover the Orionids as the most brilliant display of the October period, and accurately determined its radiant in 1863-4-5. Herrick recorded a shower at 99 + 26, Oct. 20- 26, 1839, and Zezioli in 1868 recorded many meteors which were ascribed to a radiant at 111 4-29; but there is no doubt that the Orionids were observed in both these cases, though 7 O the radiant was badly assigned. T2 276 METEORS AND The radiant of the Orionids shows no displacement like that of the Perseids. Leonids. Observed from the earliest times. Humboldt and Bonpland saw it well on the night of November 11-12, 1799, and the phenomenon at its magnificent return on November 12, 1833, was ably discussed by Olmsted. It furnished a splendid shower in 1866, November 13, and many meteors were seen at the few subsequent returns. I observed fairly conspicuous showers of Leonids in 1879 and 1888. There is no doubt the meteors form a complete ellipse, for the earth encounters a few of them at every passage through the node. Grand displays may be expected at the end of this century. Andromedes. Observed by Brand es, at Hamburg, Dec. 7, 1798. It also recurred in 1838 ; the very brilliant showers of November 27, 1872 and 1885, are still fresh in the memory. It is uncertain whether this group forms an un- broken stream ; if so, the regions far removed from the parent comet must be extremely attenuated. Some of the meteors were seen in 1877 and 1879. The radiant is diffuse to the extent of 7 or 10. Returns of the shower should be looked for in 1892 and 1898. Geminids. Mr. Greg first called attention to the importance of this shower. It was well observed by Prof. Herschel in 186134, and some later years. There are an enormous number of minor systems, but these are generally feeble, and interesting only to the regular ob- server of meteors. Many showers are .so slightly manifested that they yield but one visible meteor in 6 or 7 hours, and on the same night of observation there are often as many as 50 or 60 different systems in operation. I gave a list of 918 radiant-points of showers observed at Bristol in the ' Monthly Notices/ May 1890, and other catalogues will be found in the ' British Association Eeports ' for 1874 and 1878. Varieties of Meteors. The amateur who systematically watches for meteors will occasionally remark instances of anomalous character. I have sometimes observed meteors which are apparently very near, and move with enormous velocity. They are mere gleams of pale light, which have little analogy to ordinary shooting-stars, and suggest an elec- METEORIC OBSERVATIONS. 277 trie origin, though I do not know whether the marvellous quickness with which they flash upon the eye is not to be held responsible for the impression of nearness. They are some- what rare, and one may watch through several entire nights without a single example, but as far as my memory serves I must have witnessed some scores of these meteoric flashes. One of the most interesting class of meteors includes those which move so slowly that the eye is enabled to note the details of their appearance. Some of these objects are small when first seen, but enlarge considerably under the increasing temperature, and after a great slackening of speed (due to atmospheric resistance) their nuclei are finally spent in thick streams of luminous dust. Gn Dec. 28, 1888, I recorded a meteor which on its first apparition was tolerably bright^ Fi>. 58. Meteor of Dec. 28, 1888, & 17 m . small, and compact. It moved slowly, and I had an excellent view of its passage. The nucleus quickly expanded, though with no increase of brilliancy. Towards the end it assumed a sensible disk, and at the last phase the mass spread or de- ployed itself into a wide stream of fine ashes and disappeared. The whole phenomenon was so curious, and observed with such distinctness, that I made the above sketch of it directly afterwards. Heights of Meteors. Usually the height of meteors at their first appearance is less than 90 miles, and at disappearance more than 40 miles. From a comparison of a large number of computations I derived the following average values : Beginning height . . . 76'4 miles (683 meteors) End height ..... 50*8 (756 But if fireballs and the smaller shooting-stars are separated I find the usual heights at disappearance are : fireballs, 30 miles ; shooting-stars, 54 miles. Fireballs therefore approach 278 METEORS AND much nearer to the Earth's surface before disruption than the ordinary falling stars. A very slight acquaintance with trigonometry will enable anyone to compute the real path of a meteor if two or more observations, made at distant stations, are available for the purpose. The observed courses of the meteor should be marked upon a celestial globe, and extended backwards to the point where they mutually intersect ; this will be the Fig. 59. Large Meteor, and successive appearances of its streak, seen at Cape Jask, in the Persian Gulf, on June 8, 1883, 7 h 51"* to 8^ 33^. radiant-point. The globe having been set for the time and latitude, the apparent tracks should also be prolonged in a forward direction until they meet the horizon, this will indi- cate the Earth-points, or azimuths of the place where the meteor would have been precipitated on ihe Earth had it been enabled to continue its flight so far. The azimuths and alti- tudes of the beginning and end of the path, and the azimuths of the Earth-point should then be read off, and by means of a reliable map and a protractor their points of intersection over the Earth's surface may be readily found by lines METEORIC OBSERVATIONS. 279 drawn from the two places of observation. From the spot where the Earth-points intersect a straight line should also be drawn in the direction of the radiant, and it is along this line the meteor's motion was directed. The coordinates of the observed points of appearance and disappearance of the meteor, at the two stations, would intersect this line at iden- tical points were the observations perfectly accurate, but this is rarely the case. The distance between the observer's station and the places over which the meteor began and ended is easily derived from the map, and the height of the object may be found by adding the logarithm of the distance to the log. of the tangent of the altitude. Thus, if the end of a meteor is witnessed from London in azimuth 130 W. of S. (alt. 25), and from Bristol in azimuth 216 W. of S. (alt. 30) the place of intersection on the map will be at Warwick, so that the meteor must have disappeared when vertically over this city. London is distant from Warwick about 86 miles, and from Bristol 70 miles, and the resulting height of the meteor is: London. Bristol. 86 log. 1-93450 70 log. 1-84510 25 tan 9'66867 30 tan 9*76144 1-60317 = 40-1 1-60654 = 40-4 so that the observations accord very closely in fixing the height at a little exceeding 40 miles at disappearance, but a slight correction is necessary to allow for the Earth's cur- vature. There are other methods of computing the heights, one of which is explained by Prof. A. S. Herschel in a paper entitled " Height of a Meteor " (' Monthly Notices/ vol. xxv. p. 251). Meteoric Observations. A large number of meteor-showers still await discovery, and there are features even in connection with the best known streams which remain to be elucidated. Such doubts as now exist are only to be cleared away by assiduous observation made with the utmost accuracy possible both of the directions and durations of meteors. This attractive field of investigation has certainly been neglected in recent years, and the reason of this may perhaps 280 METEORS AND be found in the complications inseparable from it, in the need of great patience and scrupulous care in observation, and the necessity of gaining experience before the observer can feel a reliance on his work, and draw safe conclusions. Meteors are so fugitive, so diverse and erratic in their apparitions, as to be quite beyond the scope of instrumental refinements. They must necessarily be observed under many disadvantages, Positions have to be fixed from very hurried and often imperfect im- pressions. But these drawbacks, formidable as they at first appear, may be severally overcome by practice, by careful regard for the conditions under which meteors are displayed, and the marked differences of aspect induced by these condi- tions. When the observer has acquired a practical knowledge he will proceed with confidence in his work, and avoid many of the difficulties surrounding it. In recording meteor-tracks for the purpose of discovering the radiant-points, the chief feature in which precision is essential is the direction of flight. A perfectly straight wand, held in the hand for the purpose, should be projected upon the path of every meteor directly it is seen, and then when the eye has quickly noted the position and slope relatively to the fixed stars near, it should be reproduced on the chart or celes- tial globe. The time, mag., estimated duration, and details of appearance should be registered in a tabular form, with the E.A. and Dec. of the beginning-point and probable radiant. The end-point and length of path may be left until next day, in order to save valuable time. The wand is a great assist- ance to the eye in retaining the approximate directions and noting the places. If a meteor belongs to the slow, trained class, or if it belongs to the swift, streak-leaving order, the path may be very accurately noted, for the wand can be adjusted to its direction before the meteor or its visible ofFcome has died away. In the case of short, quick meteors, devoid of either streaks or trains, and generally shooting from radiants at high altitudes, they are more difficult to secure, as they vanish before one may turn, and the observer must rely upon the mere impression he received. But even these suc- rumb to experience, and will be found to resolve themselves into a number of sharply defined radiants scarcely less certain METEORIC OBSERVATIONS. 281 than the positions derived from the streaked or trained meteors. These positions are only to be fixed by the exercise of much cautious discrimination on the part of the observer, for the direction of the flight is not sufficient, alone, to indicate it. The visible aspect of the meteor has to be equally considered, for the place of its radiant imparts certain peculiarities to it which are rarely to be mistaken. First, the astronomical position of the radiant. If the radiant is at, or within 50 of, the Earth's apex (a point 90 preceding the Sun along the ecliptic, and towards which the Earth's motion is directed) the meteors generally leave streaks, especially the brighter ones, and move with great speed. They are usually white, exhibiting a high degree of incandescence. If the radiant is near the anti-apex or anywhere in the anti-apex half-sphere the meteors are streakless, they travel slowly or very slowly, and often leave trails of yellowish sparks. Bearing these facts in mind the region may be assigned in which any radiant is situated, if not the exact position -of the radiant itself. If, say, on Aug. 10, at midnight a swift, streaked meteor is seen shooting from the Pleiades towards Aldebaran, just risen, the radiant is either in Musca, Triangulum, or Andromeda. But if the meteor is slow, with a train, then we must go further back in the direction of its flight, and seek the radiant in the S. or S.W. sky. If the motion is very slow, the radiant may be as far away as Aquila. Second, the sensible position of the radiant. A low radiant yields long- pathed meteors, characterized by slowness of speed and a flaky appearance either of the streaks or trains. A radiant near the zenith gives short, darting meteors, with rather dense streaks or trains. These nearly vertical meteors have a less extensive range of atmosphere to penetrate than the hori- zontal meteors, which are sometimes abnormally long. In the case of brilliant meteors, however, the paths occasionally extend over considerable arcs though the radiant may be high. Third, the position of the radiant relatively to the path of a meteor. If a meteor is close to its radiant its track is usually slow, and appears greatly foreshortened by the effects of perspective. It is travelling (approaching) nearly in the line of sight, and 282 METEORS AND the streak or offcome of sparks is especially dense because it is seen through its entire depth ; and the nucleus in such a case has a brushy diffused appearance. Such meteors often traverse sinuous, or curved paths of 2, 3, or 4, and they are readily distinguishable from other meteors far from the radiants to which they belong. A good method of tabulating meteor-tracks is that adopted by Lieut.-Col. Tupman in his catalogue published by the British Association in 1874. I have adopted the same form, and herewith append a copy of my register of a few isolated bright meteors observed in the autumn of 1890 : Date 1890. G.M.T. Mag. Observed Path Length of Path. Dura- tion. Pro- Appearance, bable i Eadiant. From To E.A. Dec. E.A. Dec. Oct. 17 ... h rn 10 37 >! o o o o 219 +61 255 +65 16 sec. 3-5 V. slow, B. train. 204+56. 19 ... 10 35 1 6U+26 j 44+27^ 15 07 Swift, streak. Orion. 19 ... 12 O 326-8 319-10 7 0-5 Swift, streak. Orion. 25 ... 17 18 >2I 168 +34 180 +24 14* 0-8 Swift, streak. Lynx. 26 ... 7 33 # 329 +69 243 +51 42 4-0 Slow. 132 + 18. Nov. 1 7 1 >1 278 +49 244 +11 46 6-0 Very slow. J50+15. 1 9 17 >1 345 +11 307 + 1 39 4-0 Slow. '50+15. 5 10 40 >$ 28i-25 25^-29^ 5| 0-7 Swift, strk. 15 sec. Taurus. 16 11 15 * 274 +77 265|+ 6 7 10 1-5 Not very swift. Auriga. The duration of flight is a most important element to esti- mate correctly, as it affords data wherewith the real velocity may be computed, and enables the nature of the orbit in which the meteor is moving to be definitely assigned. This feature is, however, one of the most difficult of all to derive with satisfactory precision. In the case of very slow meteors lasting several seconds, it is easy by means of a stop-watch, or by other methods, to get the times of flight within narrow limits of error, but the swifter class of meteors complete their visible trajectories in the fraction of a second, and are gone before any effort can be made to gauge their durations, so that a value has to be attributed which is little better than a mere guess. METEORIC OBSERVATIONS. 283 Every adopted radiant-point should be based on at least five paths, unless the conditions are special, and these must show a very definite centre, and present family resemblances. It is often possible to detect a good centre from very few paths, when the radiant is low on the horizon, or when it occupies an isolated position. In recording meteors the details of their appearances should also be appended to the paths. Foreshortened and crooked courses, fluctuations of brightness, halting motion, spark- trains, phosphorescent streaks, broken streaks, and other features must be invariably noted when observed, as likely to assist in fully comprehending these bodies. A streak will sometimes brighten up perceptibly after the head has died out. One of the principal aims of future observers should be to ascertain the visible duration of meteor-showers, and the dis- placement or fixed position of the radiants during the period of their continuance. The Perseids seem to endure for forty- six nights (July 8-August 22) while the radiant moves from 3 + 49 to 76 + 57. The Lyrids also exhibit a shifting radiant, and it is highly probable some other showers are to be included in the same category. In investigating these, the observations of single nights should be kept separate, and the radiant determined from each set of paths. The positions when compared will then exhibit the rate and direc- tion of the displacement. As to radiants which are apparently stationary* during long intervals, these should be closely observed. Are the centres of radiation, as successively determined, identical, allowing for the slight errors of obser- vation ? Are they continuously in operation, or intermittent ? Meteors with motions in declination and near their radiants will be specially valuable in settling these questions, and if observed at more than one station will possess great signi- ficance. If it can be proved that a radiant is fixed and continuous during a few weeks, there can be no reason why it may not be stationary for a much more lengthy interval, unless the circumstances are exceptional. * A list of these was published in the ' Monthly Notices,' vol. 1. p. 466. See also ' Monthly Notices/ vol. xlv. pp. 93 et seq. 284 METEORS AND Though I have pointed out the urgency of noting the directions and durations of meteors, there are other features in such observations that must not be disregarded. If the paths are being recorded for the particular purpose of getting duplicate observations and calculating the heights, then it is desirable to note the beginning- and end-points of the flights as exactly as possible, for unless this is done the combined paths will show great discordances. Those who have acquired a familiar knowledge of the constellations will, however, experience little trouble in insuring accuracy in these records. Observers, particularly those residing in towns, must be constantly on their guard against mistakes in identifying meteors from terrestrial objects such as fire-balloons and the various forms of pyrotechnic display. That such caution is necessary will be admitted when we read the two following letters, which were published in the i Times y some years ago : " SIR, "A large meteor was seen to-night at 8.27, moving very slowly along the northern horizon, from west to east, at an altitude of about 8 deg. It was at least three times as brilliant as Venus, remaining visible for nearly five minutes, moving slower than any hitherto observed. I should be glad to receive observations made at more favourable stations. . . " I remain, Sir, your obedient Servant, " THOMAS CRUMPLEN. " Mr. Slater's Observatory, Euston Road, August 10th." " SIR, " The ' large meteor ' seen by Mr. Crumplen on Mon- day evening at 8.27, three times as brilliant as Venus, and moving from west to east, was a fire-balloon sent up shortly after 8 o'clock from the Eton and Middlesex Cricket Ground, Primrose Hill, as a finale to some athletic sports which had taken place during the afternoon. " I am, Sir, your obedient Servant, " B. C. C. " St. John's Wood, August 12th." METEORIC OBSERVATIONS. 285 In concluding this chapter I may briefly mention that an old idea concerning meteors was that they originated gales of wind, and that, in fact, they were the usual precursors of stormy weather. This belief is thus* expressed in Dryden's ' Virgil': " Oft shalt thou see, ere brooding storms arise, Star after star glide headlong down the skies, And, where they shot, long trails of lingering light, Sweep far behind, and gild the shades of night." THE STARS. CHAPTER XVI. THE STARS. Sidereal Work. Greek alphabet. Learning the Names of the Stars. The Constellation figures. Means of Measurement. Dividing power. Number of Stars. Magnitudes. The Milky Way. Scintillation of the Stars. Star-Disks. Distance of the Stars. Proper Motion of Stars. Double Stars and Binary Systems. Variable Stars. New or Temporary Stars. Star Colours. Groups of Stars. Further Observations. " Ten thousand suns appear Of elder beam ; which ask no leave to shine Of our terrestrial star, nor borrow light From the proud regent of our scanty day." BAHBAULD. THE planetary observer has to accept such opportunities as are given him ; he must use his telescope at the particular seasons when his objects are well presented. These are limited in number, and months may pass without one of them coming under favourable review. In stellar work no such irregularities can affect the progress of observations. The student of sidereal astronomy has a vast field to explore, and a diversity of objects of infinite extent. They are so various in their lustre, in their grouping, and in their colours, that the observer's interest is actively retained in his work, and we often find him pursuing it with unflagging diligence through many years. No doubt there would be many others employing their energies in this rich field of labour but for the unin- teresting character of star-disks, which are mere points of light, and therefore incapable of displaying any detail. Those who study the Sun, Moon, or planets have a large amount of surface-configuration to examine and delineate, and this is ever undergoing real or apparent changes. But this is wholly wanting in the telescopic images of stars, which exhibit a sameness and lack of detail that is not satisfying to the tastes of every observer. True there are some beautiful contrasts THE STARS. 287 of colour and many striking differences of magnitude in double stars; there are also the varying position and distance of binary systems, the curious and mysterious fluctuations in variable stars, and some other peculiarities of stellar pheno- mena which must, and ever will, attract all the attention that such important and pleasing features deserve. And these, it must be conceded, form adequate compensation for any other shortcomings. The observer who is led to study the stars by comparisons of colour and magnitude or measures of position, will not only find ample materials for a life-long research, but will meet with many objects affording him special enter- tainment. And his work, if rightly 'directed and accurately performed, will certainly add something to our knowledge of a branch in which he will certainly find much delectation. Greek Alphabet. The amateur must, at the outset of his career, thoroughly master the Greek alphabet. This will prevent many time-wasting references afterwards, and avoid the doubt and confusion that must otherwise result. The naked-eye stars in each constellation have Greek letters affixed to them on our celestial globes and star-maps. a Alpha Beta 7 Gamma 8 Delta e Epsilon Zeta 7? Eta 6 Theta i Iota K Kappa X Lambda a Mu v Nu f Xi o Omlcron TT Pi p Rho cr Sigma r Tau f Upsilon Phi X Chi T/r Psi co Omega. The letters are applied progressively to the stars (generally according to brightness) in each constellation. The Ist-mag. stars frequently have a duplicate name. Thus a Leonis is also known as Regulus, and a Canis Majoris as Sirius, the Dog-star. Learning the Names of the Stars. A knowledge of the 238 THE STARS. stars as they are presented in the nocturnal sky may be regarded as the entrance to the more advanced and difficult branches of the science, and forms the young observer's introductory lesson. When he has learnt a few of the prin- cipal constellations, and can point them out to his friends, he already begins to feel more at home with the subject, and regards it with a different eye to what he did before when the names and configurations of the stars were alike unknown to him. He no longer views the heavens as a mysterious assemblage of confusing objects, for here and there he espies certain well-known groups always preserving the same relative positions to each other. The unconscious gaze he formerly directed to the sky has given way to the intelligent look of recognition with which he now surveys the firmament. An acquaintance with the leading constellations, and with the names or the letters of the brighter stars in each, becomes very important in some departments of observation, and various methods have been suggested as likely to impress the positions and names on the memory. The beginner must first be content to get familiar with a few of the brighter stars, and make these the base for extending his knowledge. The objects are so numerous that it is impossible his primary attempts can be anything like complete. He must advance step by step in his survey, and feel his way cautiously, setting out from certain conspicuous stars with which he has already become conversant. A lantern and a series of star-maps are the only aids required, and with these he ought to make satis- factory progress. The stars as they are seen in the sky mav be compared with those figured in the maps, and their names and the constellations in which they lie may then be identi- fied. It is an excellent plan as conducing to fix the positions indelibly in the memory to construct maps from personal ob- servation, and to compare these afterwards with the published maps for identification of the constituent stars. This plan, if repeated several times, has the effect of impressing the positions of the leading stars forcibly upon the observer's mind. It is not intended to give, in this place, any details as to THE STARS. 289 the places or distribution of the stars. Without diagrams, such a description could not be made readily intelligible. To those, however, who are commencing their studies, 1 would recommend the northern sky as the most suitable region to aid their initiatory efforts. For " He who would scan the figured sky Its brightest gems to tell, Must first direct his mind's eye north And learn the Bear's stars well." The seven bright stars of Ursa Major are familiar to nearly everyone. Two of them, called the Pointers, serve to direct the eye to the Polar star, which, though not a brilliant one, stands out prominently in a region comparatively bare of large stars. It is important to know the Polar star, as it is situated near the centre of the apparent motion of the firmament. When the student has assured himself as to the northern stars Fig. 60. The constellation Orion. he will turn his attention southwards, and recognize the beautiful Orion and the curious groups in Taurus. He will also observe, much further east, the well-known sickle of Leo, and in time become acquainted with the many other constel- lations that make the winter sky so attractive. U 290 THE STARS. The Constellation Figures. The observer will soon realize that the creatures after which the constellations have been named bear no resemblance to the configuration of the stars they represent. If we look for a Bear amongst the stars of Ursa, for a Bull amid the stars of Taurus, or for a flying Swan in the stars of Cygnus we shall utterly fail to find it. The names appear to have been originally given, not because of individual likenesses between them and the star-groups to which they are applied, but simply on account of the neces- sity of dividing the sky into pasts, and giving each a distin- guishing appellation, so that it might be conveniently referred to. There were pressing needs for a system of stellar nomen- clature, and the plan of grouping the stars into imaginary figures was the one adopted to avoid the confusion of looking upon the sky as a whole. There are some who object to the method of the Chaldean shepherds because the series of grotesque figures on our star-maps and globes bear no natural analogies. But it would be unwise to attempt an innovation in what has been handed down to us from the myths of a remote antiquity, for " Time doth, consecrate, And what is grey with age, becomes religion." Means of Measurement. A micrometer becomes an indis- pensable instrument to those who make sidereal observations of an exact character. Without such means it will be impos- sible to determine either positions or distances except by mere estimation, and this is not sufficiently precise for double-star work. With a reliable micrometer * excellent results may be obtained, especially with regard to the varying angles of binary systems. Frequent remeasurement of these is de- sirable for comparison with the predicted places in cases where the orbits have been computed. In this department of astronomy the names of Herschel, South, Struve, Dawes, Dembowski, Burnham, and others are honourably associated, and it is notable that refracting-telescopes have accomplished * There are several forms of this instrument : for particulars of con- struction and use the reader is referred to Thornthwaite's 'Hints on Telescopes,' and Chambers's * Astronomy,' 4th ed. vol. ii. THE STARS. 291 nearly the whole of the work. But reflectors are little less capable, though their powers seem to have been but rarely employed in this field. Mr. Tarrant has lately secured a large number of accurate measures with a 10-inch reflector by Calver, and if care is taken to secure correct adjustment of the mirrors, there is no reason why this form of instrument Fig. 61. Diagram illustrating the Measurement of Angles of Position. (In measuring angles of position the larger star is always understood as central in the field. The north point is zero, and the angles are reckoned from this point towards the east. If a star has a faint component lying exactly east or following it, then the angle is 90 j if the smaller star is south, the angle is 180 ; and so on.) should not be nearly as effective as its rival. Mr. Tarrant advises those who use reflectors in observing double stars " to test the centering of the flat at intervals during the observa- tions, as the slightest shift of the large mirror in its cell will frequently occasion a spurious image which, if it by chance happens to fall where the companion is expected to be seen, will often lead to the conclusion that it has been observed. In addition to this, any wings or the slightest u 2 292 THE STARS. flare around a bright star will generally completely obliterate every trace of the companion, especially if close and of small magnitude, and such defects will in nine cases out of ten be found to be due to defective adjustment. Undoubtedly for very close unequal pairs the refractor possesses great advan- tages over a reflector of equal aperture ; in the case of close double stars the components of which are nearly equal there appears to be little, if any, difference between the two classes of instruments ; while for any detail connected with the colour of stars the reflector certainly comes to the fore from its being perfectly achromatic." These remarks from a practical man will go far to negative the disparaging statements some- times made with regard to reflectors and stellar work, and ought to encourage other amateurs possessing these instru- ments to take up this branch in a systematic way. Dividing Power. This mainly depends upon the aperture, and it was made the subject of careful investigation and experiment by Dawes, who found that the diameters of the star-disks varied inversely as the aperture of the telescope. Adopting an inch as the standard, he ascertained that this aperture divided stars of the sixth magnitude 4"'56 apart, and on this base he constructed the following table : Aperture iii inches. 1-0 . 1-6 . 2-0 . 2-25. 2-5 . 2-75 . 3-0 . 3-5 . 3-8 . 4-0 . 4-5 . 5-0 . 5-5 . 6-0 , Least separable distance. 4-56 2-85 2-28 2-03 1-82 1-66 1-52 1-30 1-20 1-14 1-0-1 0-91 0-83 0-76 Aperture in inches. 6-5 7-0 7-5 8-0 8-5 9-0 9-5 10-0 12-0 15-0 20-0 25*0 30-0 Least separable distance. . 0-70 . 0-65 . 0-61 . 0-57 . 0-536 . 0-507 . 0-480 . 0-456 , 0-380 , 0-304 , 0-228 . 0-182 0-152 THE STARS. 293 Dallmeyer, the optician, confirmed these values by re- marking : " In all the calculations I have made, I find that 4'33 divided by the aperture gives the separating power. Thus, 4'33 inches separates 1"." But a good deal depends upon the character of the seeing and upon other conditions. A large aperture will sometimes fail to reveal a difficult and close comes to a bright star when a smaller aperture will succeed. This is due to the position of the bright diffraction- ring, which in a large instrument may overlap the faint com- panion and obscure it, while in a small one the ring falls outside and the small star is visible*. Dawes concluded that u tests of separation of double stars are not tests of excellence of figure/' and he gave much valuable information with regard to micrometers and double-star observations generally in the ' Monthly Notices,' vol. xxvii. pp. 217-238. This paper will well repay attentive reading. Number of Stars. In the northern hemisphere there are about 5000 f stars perceptible to the naked eye. This is less than an observer would suppose from a casual glance at the firmament, but hasty ideas are often inaccurate. The scintil- lation of the stars and the fact that many small stars are momentarily glimpsed but cannot be held steadily have a tendency to occasion an exaggerated estimate of their numbers. Authorities differ as to the total of naked-eye stars. Sir R. S. Ball says " the number of stars which can be seen with the unaided eye in England may be estimated at about 3000." Gore gives 4000. Backhouse mentions 5600 as visible in the northern hemisphere. Argelander, who has charted 324,188 * Mr. George Knott, of Cuckfield, mentions that the radius of the first bright diffraction-ring of a stellar image, for a Tg-inch aperture, is 1"'01, and for one of 2 inches 3" '70 (' Observatory/ vol. vi. p. 19 ; see also vol. i. pp. 107 and 145). Mr. Dawes is quoted as giving 1 "'25 for a 7-inch, 1"*61 for a 5-inch, and 3"'57 for a 2-4-inch. These figures exceed the theoretical values, if the latter are adopted from Sir G. B. Airy's ' CJndu- latory Theory of Optics,' where, for mean rays, we have : Radius of object-glass in inches X radius of bright ring in seconds=3'70. t The number visible to different persons varies according to eyesight. Some observers see thirteen or fourteen stars in the Pleiades, while others caiinot discern more than six or seven. 294 THE STARS. stars between 2 S. of the equator and the N. pole, gives the following numbers of stars up to the 9th magnitude : 1st. 2nd. 3rd. 4th. 5th. 20 65 190 425 1100 6th. 7th. 8th. 9th. 3200 13,000 40,000 142,000 With every decrease in magnitude there is a great increase in numbers, and if this is extended to still smaller magnitudes down to the 15th or 16th we can readily understand that there exist vast multitudes of fainter stars. Paul Henry, of the Paris Observatory, says there are about 1,500,000 stars of the llth mag., and Dr. Schonfield, of Bonn, gives 3,250,000 as of the UTT mag. It is probable that by means of photography and the largest telescopes considerably more than 50 millions of stars may be charted, but this is a mere approximation. Roberts has photographed 16,206 stars within an area of four square degrees in a very rich region of the Galaxy near V Cyg n i? an d Gore computes that were the distribution equal to this over the whole firmament the number of stars would reach 167 millions. He also remarks that in the Paris pho- tographs of the Pleiades, 2326 stars are shown in a space covering about three square degrees, and this gives for the entire sky a total of 33 millions. It is, however, manifest that unusually plentiful spots in the heavens cannot be accepted as affording a criterion of the whole. Magnitudes. According to Argelander's figures, above quoted, each magnitude exhibits a rise of about 300 per cent. But authorities rarely agree as to scale, as the following com- parison between Sir J. Herschel and Struve will show : H. S. H. S. 4-0 3-6 8-0 7-3 4-5 4-1 8-5 7-7 5-0 4-6 9-0 8-1 5-5 5-05 9-5 8-5 6-0 5-5 10-0 8-8 6-5 5-95 10-5 9-1 7-0 6-4 ll'O 9-3 7-5 6-85 11-5 9*6 H. S. 12-0 9-8 12-5 10-0 13-0 10-18 13-5 10-36 14-0 10-54 14-5 10-71 THE STARS. 295 H. S. 15-0 10-87 16-0 11-13 17-0 11-38 18-0 11-61 19-0 11-82 20'0 12-00 Argelander's magnitudes come between those of Herschel and Struve. Such disagreements are perplexing to observers, and it is fortunate that in regard to the naked-eye stars we are now furnished with a more consistent and accurate series of magnitudes. Photometric determinations of the light of 4260 stars not fainter than the 6th mag., and between the N. pole and 30 S. declination, were made at Harvard College Observatory, and similar measures of 2784 stars between the N. pole and 10 S. declination were effected at the Oxford University Observatory, and the results published in 1885. The two catalogues are in very satisfactory agreement, the accordances within one tenth of a mag. being 31 per cent., within one quarter of a mag. 71 per cent., and within one third of a magnitude 95 per cent. The photometers used in the two independent researches were constructed on very different principles, and the substantial agreement in the results indicates that u a great step has been accomplished towards an accurate knowledge of the relative lustre of the stars " (' Monthly Notices/ vol. xlvi. p. 277). The Milky Way. On dark nights when the Moon is absent and the air clear, a broad zone of glimmering, filmy material is seen to stretch irregularly across the heavens. It may be likened to a milky river running very unevenly amongst the constellations, and showing many curves and branches along its course. On very favourable occasions the unaided eye glimpses many hundreds of glittering points on this light background. A field-glass reveals some thousands, and shows that it is entirely composed of stars the blended and confused lustre of which occasions that track of whiteness which is so evident to the eye. In a good telescope stars and star-dust exist in countless profusion, and great diversity is apparent in their numbers and manner of grouping. In certain regions the stars are 296 THE STARS. concentrated into swarms, and the sky is aglow with them ; while in others there are very few, and dark cavernous open- ings offer a striking contrast to the silvery sheen of surround- ing stars. There are many of these void spaces in Scorpio, and a circular one in Sagittarius K.A. 17 h 56 m , Dec. 27 51' has been particularly remarked. These inequalities of grouping may be easily recognized with the naked eye, espe- cially in Cygnus, where bright star-lit regions frequently alternate with dark void spaces. In the southern sky there is a noteworthy instance. Near the brilliant stars of Crux and Centaurus and closely surrounded by the Milky Way there is a large black vacancy very obvious at a glance, and so striking to ordinary observers that it is known as the " Coal- sack," a name applied to it by the early navigators of the southern seas. The course of the Milky Way may be described generally as flowing through Auriga, the club of Orion, feet of Gemini, western part of Monoceros, Argo Navis, Crux, feet of Cen- taurus, Circinus, Ara, where it separates into two branches, the western of which traverses the northern part of the tail of Scorpio, eastern side of Serpens, Taurus Poniatowski, A user, arid Cygnus. The eastern branch crosses the tail of Scorpio, the bow of Sagittarius, Antinous, Aquila, Yulpecula, and then enters Cygnus, where it reunites with the other branch. It thence passes through Cepheus, Cassiopeia, Perseus, and enters Auriga. In breadth it varies greatly, being in some places only 4 or 5, whereas in others it reaches 20. It is, of course, best visible when twilight is absent, but it is some- times very plain, even at midsummer, for at this season some of its more conspicuous sections are favourably placed for observation. It is supposed that fully nine tenths of the total number of stars in the firmament are included within the borders of the Milky Way. Some of the ancient philosophers, including Democritus, formed just conceptions as to the real nature of this appear- ance. Though they lacked instruments wherewith to observe the stars forming it, they yet saw them with the eye of reason. But very vague and incorrect notions prevailed in early times, when superstition was rife, as to many celestial THE STARS. 297 phenomena. Some of the ancient poets and learned men refer to the Galaxy as the path by which heroes ascended to heaven. Thus we read in Ovid : " A way there is in heaven's extended plain, Which when the skies are clear is seen below, And mortals, by the name of Milky, know ; The ground-work is of stars, through which the road Lies open to great Jupiter's abode." Scintillation of the Stars. The rapid variations of light known as the "twinkling " of the stars received notice from many ancient observers, including Aristotle, Ptolemy, and others, and they severally endeavoured to account for it, but not in a manner altogether satisfactory. At low altitudes bright stars exhibit this twinkling or scintillation in a striking degree, but it is much less perceptible in stars placed at con- siderable elevations. Sirius, the brightest star in the sky, is a noted twinkler. His excessive lustre and invariably low position are conditions eminently favourable to induce this effect. But the planets seldom exhibit scintillation in a very marked degree. The light of Jupiter and Saturn is steady, even when these planets are close to the horizon. Mercury, however, twinkles most obviously, and Venus and Mars, when low down, are often similarly affected, especially in stormy weather when the air is much disturbed. Hooke, in 1667, concluded that the scintillation was due "to irregular refractions of the light of the stars by differently heated layers of atmosphere/' M. Arago said it arose "from the peculiar properties possessed by the constituent rays of light, of moving with different velocities through the strata of the atmosphere, and of producing what are called interferences/' More recently, M. Montigney has conducted some interesting researches into this subject, and he believes " that riot only is twinkling caused, to a great extent, by the deviations of portions of a star's light altogether away from us by variable layers of atmosphere, but it is also affected, both in frequency and in the colours displayed, by the nature of the light emitted by the individual star." The planets are little subject to scin- tillation, as they present disks of sensible size, and thus are enabled to neutralize the effect of atmospheric interferences. 298 THE STARS. It is curious, however, that the steadiness of telescopic images does not appear to be much improved at high altitudes, and that the phenomenon of scintillation still operates powerfully as observed from mountainous stations. In February 1888, Dr. Pernter, of the Vienna Academy of Sciences, found " that the scintillation of Sirius was actually greater at the top of Sonnblick, 10,000 feet high, than it was at the base of the mountain, and he formed the opinion that scintillation has its origin in the upper strata of the atmosphere and not in the lower as usually assumed." It would appear from this that lofty situations do not possess all the advantages claimed for them in regard to the employment of large telescopes. Star-Disks. The stars as observed in telescopes are shorn of the false rays apparent to the naked eye, and they are seen with small spurious disks. That the disks are spurious is evident from the fact that the larger the telescope employed, the smaller the star-disks become. And moreover, when a star is occulted by the Moon, it disappears instantaneously. There is no gradual diminution of lustre ; the star vanishes with great suddenness. Bright stars, like Aldebaran or Regulus, have been watched up to the Moon's limb, and ob- servers have been sometimes startled at the abruptness with which they were blotted out. An appreciable disk could not be withdrawn in this instantaneous manner ; it would exhibit a perceptible decadence as the Moon increasingly overlapped it, but no such appearance is observed. On the occasion of the occultation of Jupiter on Aug. 7, 1889, the planet's dia- meter was 41"'4, and the disappearance occupied 85 seconds. Now had Aldebaran or Regulus a real disk of only 1" it would prevent their sudden extinctions, and their disappear- ances would be spread over perceptible though short intervals of time*. But there is every reason to conclude that the actual disks are to be represented by a small fraction of 1", so that the largest instrument and the highest powers fail to reveal it. In this connection, Mr. Gore, in his ; Scenery of the * About 2 seconds. Sir W. Herschel found the diameter of a Lyrse with a power of 6450 to be 0"'3553. Tycho Brahe, before the invention of telescopes, estimated the diameter of Sirius as 120". J. D. Cassini, with a telescope 35 feet long, found the diameter of the same star 5". THE STARS. 299 Heavens/ p. 152, says : " Let us take the case of a Centauri, which is, as far as is known at present, the nearest fixed star to the Earth. The distance of this star is about 25 billions of miles. From comparisons made between its light and the Moon, it has been found that its intrinsic brilliancy must be about four times that of the Sun. Supposing its greater lustre is due to its greater size a not improbable supposition it would subtend, if placed at the Sun's distance, an angle twice as great, or about 1, and hence we find that the angle sub- tended at its distance of 25 billions of miles would be about 7*gth of a second of arc, which the most powerful telescope yet constructed would be incapable of showing as a visible disk/' Distance of the Stars. The distances of the outer planets Uranus and Neptune, mentioned in an earlier chapter of this work, are sufficiently large to amaze us ; bat the distances of the stars may be said to be relatively infinite. For many years the problem of stellar distances defied all attempts to resolve it. At length, in 1838-39, Bessel^, Henderson, and Struve obtained results for three stars viz. 61 Cygni, a Cen- tauri, and as. Lyrse, which practically settled the question. More recent measures of stellar parallax, while correcting the earlier values, have virtually corroborated them ; though the figures adopted can only be regarded as approximations, owing to the difficult and delicate nature of the work. The binary star a. Centauri appears to be the nearest of all ; it has a parallax of 0"'75, and its distance from us is equal to 275,000 times the distance of the Sun. Light traversing space at the rate of 187,000 miles per second would occupy 4^ years in crossing this interval. In the Northern hemi- sphere 61 Cygni is the nearest star, with a parallax of //f 44 and a distance of about 470,000 times the Sun's distance. Light would take more than seven years in reaching us from this star. a. Lyrae has a parallax of 0"'15, equal to nearly 22 light- years. OL Crucis shows a very small parallax (0"'03), and its distance is excessively remote equal to about 108 light-years ! Proper Motion of Stars. A very slight motion affects the places of many of the so-called fixed stars. This must, after the lapse of long intervals of time, materially alter the con- 300 THE STARS. figuration of the constellations. But the change is a very gradual one, and must operate through many centuries before its effects will become appreciable in a general way. The greatest proper motion yet observed is that in regard to two small stars (one in Ursa Major and the other in Piscis Australis), which amounts to about 1" annually. Another motion has been recognized, viz. in the line of sight. Dr. Huggins made the initiatory efforts in this research by measuring the displacement of the F line in the spectrum of Sirius. The work has been actively pursued at the obser- vatories of Greenwich and Rugby, and with interesting results. While certain stars exhibit a motion of approach, others display a motion of recession. Thus Vega, Arcturus, and Pollux are approaching us at the rate of about 40 miles per second ; while Eigel is receding at the rate of 1.7 miles per second, Castor at the rate of 19, Regulus 14, Betelgeuse 25, and Aldebaran 31. Sirius, in the years from 1875 to 1878, was receding from us at the rate of 22 miles per second ; but this decreased in subsequent years, and in 1884-85 the star was approaching with a motion of about 22 miles per second. In 1886 and 1887 this rate was increased to about 30 miles per second, as observed both at Greenwich and Rugby. This confirms the idea that Sirius is moving in an elliptical orbit. Similar observations, in regard to the variable star Algol, have revealed that changes of velocity are connected with its changes of lustre. Before minimum the star recedes at the rate of 24^ miles per second, while after minimum the star approaches with a speed of 28 J miles per second (' Monthly Notices/ vol. 1. p. 241). Double Stars and Binary Systems. Telescopic power will often reveal two stars where but one is seen by the naked eye. Sometimes the juxtaposition of such stars is merely acci- dental : though they are placed nearly in the same line of sight the conjunction is an optical one only, and no con- nection apparently subsists between them. In other cases, however, pairs are found which have a physical relation, for one is revolving round the other ; and these are termed binary stars. Sir W. Herschel was the first to announce them, from definite observations, in 1802. Of double stars more than THE STARS. 301 10,000 are now known ; many of these are telescopic, but the list includes some fine examples of naked-eye stars. Double stars are excellent telescopic tests. A very close pair affords a good criterion as to the defining capacity of an instrument ; while a pair more widely separated and of greatly unequal magnitude, like that of a Lyrse, offers a test of the light-grasping power. But in these delicate observations, as, indeed, in all others, the character of the seeing exercises an important and variable influence. A double star that is well shown on one night becomes utterly obliterated on another, owing to the unsteadiness and flaring of the image. On such occasions as the latter one is reminded of the " twitching, twirling, wrinkling, and horrible moulding" of which Sir John Herschel complained, and which unfortunately forms a too common experience of the astronomical observer. A close double, of nearly equal magnitudes, requires a steady night, Fie. 62. Double Stars. /3 Orionis. y Leonis. a Ursse Minoris. y Virginia. d Cygni. y Arietis. y Androinedae. d Serpentis. such as is suitable for planetary details ; but a wide double consisting of a bright and a minute star rather needs a very clear sky than the perfection of definition. Certain doubles, such as 6 Aurigse, 8 Cygni, and f Herculis, are often more easily seen in twilight than on a dark sky ; and some expe- 802 THE STARS. CU Q 1*1 1 & 1 " H I .1 I I I 8.2 s -s S S t- t 00 00 00 GO 00 X O rH CO O <>1 IOGOOOOOCO*OT-HIOCO oifNcicpcp^f^^pipcp =6 666606666 OOOr^-i 05 CO 7-1 CD CD TH 00? CO 6 t- CO CD i i t CM t>- CO C* rH 00 irT CO l>- GO rHlOl HW CO O ^ CD 8 2 i i xo CD CO 5 O^ ^* ^1 ^>* QO O7) iO ^O T^ lO CO tO CO T^H C*^ CO ^r-t ,-HOf-i^O^O^CO -M *.S ,O C3 03 O> crj s-t T3 o W P ^ gtM HlflRl H S^ S* O fi d^-<^ . .9 =s *a "fe, b:3 * SD^ - eq ^ 4 a rt c '^w-ror-O < 1 <: J 5 .3 a> a 2 ^ 'S _S co > .S3 "d o3 -J2 PQ -1 C^ CO O CO CO CN ib TJH tO t>- > t^ CO CO 10 (^ !>. 1 P to CO b- S 1> CS N CO CO CO co >b I-H CO i a 55 S3 -I- 4- O <>J O'l 6 s $ CO r-t TJH W 5 O * f C3 8J w 5 3 I Jo M CC 1> CO* OS O i I i I i-l r-H CN 304 THE STARS. 2 .S Hil -o Its 111 ^ T^ 1 " Cu 5ts .Ss^a5 -^ irj ^ T^ 1 " Cu Hil 151 i s^a 1=6 g- i r-H 10 ooooiooo-^o T-H 10 t- ^ O CO 10 CC 10 O^rHTtl-HOO^CCi O CO CO ^H 1C 1O O O?O < OCNCOCq i 1 . 1 T-H >O CO TH 1 1 + I +4- !++ 1 i + -a s THE STARS. 305 i -S *** 05 -s n i a & 02. B 0! cc 02. 02, 02. 02. 02. E-H oi oi 'a 3 02. ds CM 1 os cb cb 1- 00 00 co co co fe 00 !>. OS g 1 OS ^ OS Os t- S5 fe OS t> !> iO 1 ds 00 g CO rH 8 OO CO g 8 CM ^^ O C<0 OS CM O CO CO 8 CO os rH i i CM ^ t- CO o OS rh HH Tjl IO OO i i i I rH "tf U4 * ^ "* CO l> rH CM oc ^ ip OS CO O CM CM iO O 6 i i CM 9 op o i OS l> CO OS t- rH rH CM CO OS rH CM OS GO CO O rH ii i < CM T-H P rt S 3 1 1Q ^ CM OS CO 00 ^ TH TH !> rH CO CO T 1 CM * CM CM r-l CM . co 1 - CO I 1 CM rH CM rH 10 O CM 1 8O CO CO CM i ( O CO 1 4- 1 + CM rH + + * 00 GO + CM O 7 + rH Tjl 1O CO CO CO 10 rH 00 CM CO rH cp i i lO 10 rH o^ 0i d 10 T-l CO iO CO rH 1 1 1 1 1 1 ip do r ( i 1 P rH CM CO S5 CO OS CO OS CO rH CO rH CO *- O CM CO CO 00 00 O CO r-> rH CO fr : : : i i : : : : 5 1 ft 1 b 3* ' 1 1 1 02. XP y a UrssB Min. . 1 CO 33Arietis .... 1 02. ]8 Geminorum a' Oapricorni. a Canis Min.. 02. e a Andromedas Jg 1 3 s i i CM CO lO xo iO 2 s 00 10 ^O S o cb c5 co S 306 THE STARS. rienced observers, conscious of this advantage, have secured excellent measures in daylight. Mr. Gledhill says : " Such stars as y Leonis and 7 Virginis are best measured before or very soon after sunset" (' Observatory/ vol. iii. p. 54). The determination of the angles of position and distance of double stars forms a very important and extensive branch of work in connection with sidereal astronomy. In cases where double stars preserve stationary places relatively to each other, there is clearly no need for frequent re-observation. But in those numerous instances where the two components form a binary system it is desirable to obtain as many measures as possible, so as either to verify the calculated orbit or to furnish the materials for an orbit if one has not been com- puted before. Dr. Doberck, whose name is well known in these researches, mentioned, in 1882, that ample data for purposes of computation had not been secured for more than thirty or forty binaries out of between five and six hundred such systems that were probably known to exist. Sir W. Herschel, in 1803, estimated the period of revolution of a Geminorum as 342 yrs. 2 mths. and of 7 Virginis as 1200 yrs. Orbits* do not appear, however, to have been computed until 1827, when Savery of Paris showed that the companion of Ursse Majoris was revolving in an ellipse with a period of 58 years. The accomplished Encke also turned his attention to this work, and adopted a more elaborate method ; and many others have pursued the subject with very interesting and valuable results. On pp. 302-305 is a selected list of some of the most noteworthy double and binary stars, arranged according to the distance between the components. In compiling the above list, I have used some of the latest measures available, as most of these doubles are binary systems, and therefore in motion, so that a few years effect a percep- tible difference in the angles of position and distance of the components. Some of the pairs are closing up, others are opening, and thus it happens that a binary star, divided with * Dr. Doberck gives some valuable information with reference to the computation of binary star-orbits in ' The Observatory/ vol. ii. pp. 110 and 140. THE STARS. 307 great difficulty to-day, may become an easy object some years hence, and vice versa. In fact, as telescopic tests they are constantly varying. Before leaving this part of the subject it may be interesting to refer individually to a few brilliant examples of double stars. Canis Majoris (Sirius). A red star according to ancient records, but it is now intensely white. In 1844 Bessel inferred from certain little irregularities in the proper motion of this star that it consisted of a binary system with a period of about half a century*. Peters confirmed this idea in 1851, and it was observationally verified eleven years afterwards. On Jan. 31, 1862, Alvan Clark, jun., while testing a new ]8|- inch refractor, discovered a very faint companion 10" distant. Measures in the few subsequent years proved that the position- angle was decreasing, while the distance showed a slight ex- tension. In 1872 it was about 11"*50, but since then the two stars have been approaching each other, and Mr. Burn- ham's measures in April 1890 gave the distance as only 4"*19. It is now, therefore, a very difficult object, and only visible in large instruments. In England it is never easy, owing to its southern position, and it has been little observed, but it is satis- factory to note that the large refractors at Washington, Princeton, and Chicago, U.S.A., have been often employed on this object in recent years. Mann gives a period of 51*22 years for this interesting binary, and places the time of peri- astron-passage as 1890'55. This differs from Gore's orbit, quoted in the table. ft Orionis (Rig el). A favourite test-object for small instru- ments. The companion has been seen with only 1^-inch aperture by experienced observers familiar with the object, * The star a Canis Minoris (Procyon) was also inferred to be a binary and to have a similar period. Several close companions appear to have been discovered (Ast. Nach. no. 2080). But Prof. Hall, using the 25-8- inch refractor at Washington, says : I have never been able to see any of these companions that would stand the test of sliding and changing the eyepiece, turning the micrometer, &c., and am therefore doubtful of their existence. This is an interesting star for the powerful telescopes of the future," It has been surmised that the companion is a non-luminous one, and therefore invisible. X2 308 THE STARS. and accustomed to its appearance in larger telescopes. The beginner may, however, esteem himself fortunate if he distin- guishes the smaller star with 3 inches of aperture. When he has done this he may afterwards succeed with 2^ inches only, and quite possibly with 2 inches. He can ascertain his ability in this direction by inserting cardboard diaphragms of the diameters referred to in the dew-cap of his telescope. This object is not a binary; the component stars are fixed relatively to each other, and merely form an optical double. The colours are pale yellow and sapphire blue. Burnham thought the smaller star was elongated, as though a very close double, but the 36-inch at Mount Hamilton has disproved the idea. a Lyres ( Vega] . Another well-known object, and one upon which amateurs are constantly testing their means. The companion star is extremely faint, and small instruments would have no chance with it but for its comparatively wide distance from Vega. Were it much nearer it would be obliterated in the glare. This is a more difficult pair than that of Rigel, though certain lynx-eyed observers have glimpsed the minute star with ridiculously small apertures. It is no mean feat, however, to discern the star with a 3-inch telescope. Webb saw it more easily with a power of 80 than with 144 on a 3 7 -inch. There are many other stars in the same field, though more distant than the companion alluded to. With power 60 on my 10-inch reflector, I counted eighteen stars in the field with'Vega on Oct. 9, 1889, though the full Moon was shining at the time. Several faint stars have been alleged to exist much closer to Yega than the well-known comes ; but these have resisted the great American refractors, and it may be safely assumed that they were ghosts produced by a faulty image. a Ursce Minoris (Polaris). This double, from its constant visibility in northern latitudes, from its unvarying brightness, and from the relatively stationary positions of the stars com- posing it, forms an excellent test for small instruments. But it is a comparatively easy object, and ought to be seen in a 2-inch telescope. With this aperture the primitive efforts of a young observer will probably be disappointing. If possible he should first look at the pair through a 3- or 4-inch, and THE STARS. 309 then he will know exactly what he may expect to see with inferior means. A difficult object is often readily glimpsed in a small telescope after the eye has become acquainted with it in a larger one. Experience of this kind is very requisite, and it is by thus educating the eye that observers are gra- dually enabled to reach objects which appeared hopelessly beyond them at their first attempts. The companion to Polaris, like that of Rigel and Vega, though situated in nearly the same line of sight is not physically related to the larger star, the contiguity of the objects being accidental. Some dubious observations have been made of comites nearer to Polaris than the one to which we have been adverting; but Burnham does not see these, and we are forced to conclude that they have no objective existence. a Scorpii (Antares). A fiery-red star, with a rather close, faint companion. This object being in 26 of S. declination is rarely seen with advantage in places with latitudes far north. Atmospheric disturbance usually affects the image in such degree that the smaller star is merged in the contor- tions of the larger. This pair is, however, interesting from the circumstance that it is frequently liable to occultation by the Moon. A night on which this double star can be dis- tinctly seen may be regarded as an exceptional one in point of definition. Jt appears to have been discovered nearly half a century ago bv Grant and Mitchel. Variable Stars. A proportion of the stars exhibit fluctua- tions in their visible brightness. In most cases, however, the variation is but slight, though there are instances in which the differences are considerable. The fluctuations are peri- odical in nature and capable of being exactly determined. But the character of the variation and the period are very dissimilar in different stars. Some are of short period, and their variations occupy a few days only ; others, however, are more gradual, and twelve months or more may represent the complete cycle of their changes. These alterations of bright- ness generally escape the notice of casual observers of the heavens. To them the stars appear as constant in their rela- tive magnitudes as they are in their relative positions. But a close observer of the firmament, who habitually watches and 310 THE STARS. records the comparative lustre of the stars, must soon discover numerous evidences of change. He will remark certain stars which alternately grow bright and faint, and, in fact, display a regular oscillation of brilliancy. In the case of a pair of stars he may possibly notice that the superior lustre is emitted first by one and then by the other. The observation of these variables dates from a period anterior to the invention of the telescope. Nearly three centuries ago Fabricius remarked that o Ceti (Mira) suffered a great diminution of light ; for while it was of the 3rd mag. in Aug. 1596, it became invisible in the following autumn. It was re-observed by Holwarda in 1639, and as he appears to have been the first to estimate its period, some authors, including Argelander, have credited him with the discovery. The star has a period of about 331'3 days. Its variations are somewhat erratic, for at max. it is sometimes only 4th mag., while at others it is as bright as 2nd mag., and its min. are equally inconsistent. /3 Persei (Algol) is another and perhaps the best known of all the variable stars. Its changes are very rapid, for it passes through its various gradations of brilliancy in less than three days. It was first noticed by Montanari in 1669, though it was left for Groodricke in 1782 to ascertain its period. The normal mag. of the star is 2'2, and it only shows distinct variation during the five hours which precede and follow a mini- mum, when it declines to 3- 7 mag. Its period is shortening for in 1782 it was 2 d 20 h 48 m 59 S 4, in 1842, 2 d 40 h 48 ra 55"% and at present Chandler finds it 2 d 20 h 48 m 51 s . As to the causes which contribute to these variations, they are invested in mystery. It has been conjectured that dark spots on the surfaces of the stars may, by the effects of rotation, introduce the observed alternations. Another surmise is that the tem- porary diminutions of lustre are to be ascribed to the inter- position of dark satellites, and this theory seems tenable in regard to stars of the Algol type. It is satisfactory to note that a large amount of systematic work is being done in this important and delicate branch of research. Such stars as are subject to variation have been classed as follows : 1. Temporary or new stars ; 2. Stars having long and pretty regular variation ; 3. Stars irregularly variable ; 4. Stars THE STARS. 311 Observer. oil! S^.^ & S i 00 ^ .-4- "1 1C 2 2 K 00 S *~ 2 ii t/r z^ 00 "~ ""' oo S2 ^ i- - r ~ l F-J Cj F 3oQ_ 1 _^_ (J - r -i r ^OOOO^^ ^ ^J" 'Si 'f S "O P- rg rQ _ jj" r~ ' *"* i-Q 1-3 * JS 6 "" ""' *^ ^ ~ " S S 2-| o | jl ,| jl * .5 H .c oi i r-l "^ 00 "^ CO ^ rH Iplfli .1 t^ !^ O2 O W PH O w o Period. s ro s s |a 1 & i ^ L O b^ O^ Ol r^ CO ^ tO ^ QO t< K^ t ~ 1 O g* "9* Q ^^gii (T3CO>%;J' 1 O l&CO '^O^r^rHOS^^^ 5* ? S B js js ^ x! ns A .a O C^ O i ... * i i r-l t C^ (M CO rH W 00 ?? ^ t- rH 00 -H 432 days ? o d 8 h 48 m rt< t-. i i 00 OS t iC t OS X 00 1 d C^ ^ C^J t vC d d OS t^* cq 01 cb co cb cb b cb os GO i^ ^ ^ oo co co ^ co '^ t>* CO rH 1C iC CO 17! t- 60 co cb cb cb t~ 4 cb cb S i o i COOC^O^O^CC(MO5OS(MOO + i + + 4- + + T~T+ + l ?1 I i i!;?l!? CO rH rH 1C I , iOXOCOCOC^CO(?^iCCOt>'001>.i-< c'ficbrH-^rciobi^O'* doi-ocbxc ClCi-H 1C^ lCi-lr-lTtlCOTtH(NO -QO(NCOCOiCCOCOt-^l>t^OOOCOTiH t^ CO ^^ ^^ CO O^ t^* CO G^ rH CO CO CO t^* lO t> 1^ CO 3$ O^ ^> rH i O iC T^ (M S j Name of Star ! ! I Mil i|! \\ I I : : o o .2 ^ g : p : 'S : * .rl a S *> -2 -c s o ^ .j g S .g ' a ^ g U U 1 8ll*54*aal ) i?g a.oCQ.X:P R-X/r4PnPo2 R-W*0 Hi s ill o o> & ^ S~ &i > W Or^ Q 4 ^ P P 1 Q2. X R-H *c *s -a ja 3. *o 312 THE STARS. varying in short periods ; 5. Stars of the type of Algol, which are liable to temporary diminutions of lustre. On the preceding page is a list of the most noteworthy variable stars. New or Temporary Stars. These stars (sometimes classed with variable stars) furnish us with rare instances of vast physical changes occurring among sidereal objects, usually so steadfast and endurable. The alternating lustre of certain variable stars represents phenomena of regular recurrence, and is probably to be explained by simple causes ; but the sudden outbursts and rapid decline of temporary stars are facts of a more startling character, and need a more exceptional explanation. The first of these objects re- corded in history appears to have been seen in Scorpio 134 years before the Christian era, and it suggested to Hippar- chus of Rhodes the idea of forming a catalogue of stars, so that in future ages observers might have the means of recog- nizing new stars or any other changes in the configuration of the heavens. Hipparchus completed his catalogue in 128 B.C. ; it contained 1025 stars, and forms one of the most valuable memorials we possess of the labours of the ancient astronomers. Another temporary star is said to have appeared in 130 A.D., but this and several other objects of presumably similar cha- racter noticed in later years may just possibly have been comets, and considerable doubt hangs over the descriptions. It will therefore be safest to confine our remarks to more modern and better attested instances of these phenomena* : 1572, November 11. The famous star of Tycho Brahe. He thus described his first view of it : " One evening when I was considering, as usual, the celestial vault, the aspect of which is so familiar to me, I perceived with indescribable astonishment a bright star of extraordinary magnitude near the zenith in the constellation of Cassiopeia." He adds : " The new star was destitute of a tail, or of any appearance of nebulosity; it resembled the other stars in all respects, only that it twinkled even more than stars of the first magnitude. In brightness it surpassed Sirius, a Lyrse, or Jupiter. It could * It is remarkable that nearly all the temporary stars have appeared in the region of the Milky Way. THE STARS. 313 be compared in this respect only to Venus when she is nearest the earth (when a fourth part of her illuminated surface is turned towards us). Persons who were gifted with good sight could distinguish the star in the daytime, even at noon, when the sky was clear." This brilliant NOVA began to fade early in Dec. 1572, and in April and May 1573 it resembled a star of the 2nd mag., in July and Aug. one of the 3rd mag., and in Oct. and Nov. one of the 4th mag. In March 1574 the star completely disappeared (to the naked eye), after a visibility of about 17 months. It exhibited some curious variations of colour. It was white when most brilliant ; it then became yellow, and afterwards red, so that its hue in the early part of 1573 was similar to that of Mars. But in May it again became white, and continued so until it ceased to be visible. The position of this star (for 1890) is K.A. O h 18 ffi 41 s , Dec. + 63 32''2. It was supposed to be a reapparition of the brilliant stars which shone between Cepheus and Cassio- peia in 945 and 1264, and to have possibly been associated with the " Star of Bethlehem ;" but there is no reliable evi- dence on which this view can be supported, as the alleged " stars " of 945 and 1264 were undoubtedly comets misde- scribed in old records. Cornelius Gemma is reputed to have seen the celebrated star of 1572 a few days before Tycho Brahe, viz., on November 8, 1572. 1604, October 10. Discovered by Brunowski, who an- nounced it to Kepler. It was brighter than a star of the 1st mag., also than Mars, Jupiter, or Saturn, which were not far distant at the time. It did not begin to fade immediately; for a month after its discovery it was still brighter than Jupiter, and of a white lustre. At the middle of November it surpassed Antares, but was inferior to Arcturus. In April 1605 it had fallen to the 3rd mag., and went on decreasing until in October it could scarcely be seen with the naked eye owing to the twilight resulting from its proximity to the Sun. In March 1606 it was invisible. The position of this object was about midway between f and 58 Ophiuchi, or at K.A. 17 h 24 m , Dec. -21 20 ; (1890). 1670, June 20. Discovered by the Carthusian Monk An- thelme in R.A. 19 h 43 m 3 s , Dec. + 27 3' (1890), a few degrees 314 THE STAKS. east of /9 Cygni. It was of the 3rd mag., and continued in view, with constantly fluctuating brightness, for nearly two years. At the end of March 1672 it was 6th mag., and has never reappeared. Hind found a small, hazy, and ill-defined star in the same place, but this is probably not the same as Anthelme's star of 1670. 1848, April 28. During the long interval of 178 years separating 1670 from 18-48 not a single new star appears to have revealed itself. Observers had multiplied, astronomical instruments had been much improved, star-catalogues were plentiful, and yet the sidereal heavens gave no intimation of a stellar outburst. No better proof than this could be afforded as to the great rarity of temporary stars. At length, in the spring of 1848, the spell was broken, and Mr. Hind announced that a new star of a reddish-yellow colour had appeared in Ophiuchus, R.A. 16 h 53 m 20 s , Dec. -12 43' (1890). He expressed himself as certain that no star brighter than the 9th mag. had been there previous to April 5. After rising to the 4th mag. it soon faded, and in 1851 could only be observed in large instruments. In 1875 it was still visible as a very minute star. 1860, May 21. M. Auwers, of Konigsberg, noticed a star of the 7th mag. near the centre of the bright resolvable nebula (M. 80), lying between a and @ Scorpii, R.A. 16 h 10 m 29 s , Dec. -22 42' (1890). On May 18 the star was not there, and it disappeared altogether in three weeks. It was independently seen by Pogson on May 28, and to him it seemed that " the nebula had been replaced by a star, so entirely were its dim rays overpowered by the concentrated blaze in their midst." 1866, May 12. Discovered by Birmingham at Tuam. It was of the 2nd mag., and situated in Corona, R.A. 15^ 54 m 54 s , Dec. +26 14' (1890). The outburst must have been very sudden, as Schmidt, at Athens, was observing this region three hours before the new star was detected, and is certain it was then fainter than the 4th mag. The star was found to be identical with one on Argelander's charts estimated as 9J mag. It faded from the 2nd to the 6th mag. by May 20, and was thereafter invisible to the naked eye. THE STARS. 315 1876, Nov. 24. A yellow star of the 3rd mag. was seen by the ever vigilant Schmidt at Athens near p Cygni, and where no such star existed on Nov. 20. The position of the object was R.A. 21 h 37 m 23 s , Dec. +42 20' (1890). It soon grew fainter, so that on Dec. 13 it was of the 6th mag. and devoid of colour. In the spectroscope it presented much the same lines as Birmingham's star of May 1866. In addition to the continuous spectrum it showed bright lines of hydrogen. 1885, August 31. Dr. Hartwig announced the appearance of a star-like nucleus in the great nebula (M. 31) of Andro- meda, R.A. O h 36 m 43 s , Dec. +40 40' (1890). Other ob- servers soon corroborated the discovery. The star appears to have been first seen on Aug. 19 ; it was not visible on the preceding night. On Sept. 1 its mag. was 6'5, on Sept. 2, 7-3, on Sept. 3, 7-2, Sept. 4, 8'0, Sept. 18, 9% &c. On Feb. 7, 1886, it had dwindled down to the 16th mag., according to an estimate made by Prof. Hall with the great Washington refractor. The spectrum was continuous, and Proctor and Gore considered " that the evidence of the spectroscope showed that the new star was situated in the nebula." The phenomena presented by the temporary stars alluded to are so different to those of ordinary variables that it is very questionable whether they ought to be classed together. Our knowledge of the former would no doubt progress more rapidly were they specially looked for and more instances discovered. Those who have acquired a familiar acquaintance with the naked-eye stars should examine them as often as possible with this end in view. Some of these objects lose light so quickly that unless they are caught near the maximum they are likely to escape altogether, and this shows the neces- sity of being constantly on the alert for their appearance. I have frequently, while watching for meteors, reviewed the different constellations in the hope of picking up a new object, but have never succeeded in doing so. Star Colours form another interesting department of side- real astronomy. It is obviously desirable to record the hues presented, not only by double stars and binary systems, but by isolated stars also, as changes of tint have been strongly suspected. Cicero, Seneca, Ptolemy, and others speak of 316 THE STARS. Sirius as a red star, whereas it is now an intense white; and if we rely on ancient descriptions similar changes appear to have affected some other prominent stars. But the old records cannot be implicitly trusted, owing to the errors of transcribers and translators ; and Mr. Lynn (' Observatory,' vol. ix. p. 104) quotes facts tending to disprove the idea that Sirius was formerly a red star. In the majority of cases double stars are of the] same colour, but there are many pairs in which the complementary colours are very decided. Chambers remarks that the brighter star is usually of a ruddy or orange hue, and the smaller one blue or green. " Single stars of a fiery red or deep orange are not uncommon, but isolated blue or green stars are very rare. Amongst conspicuous stars /3 Librae (green) appears to be the only instance." As an example of fiery-red stars Antares may be mentioned ; Alde- baran is deep reddish orange, and Betelgeuse reddish orange. Amongst the more prominent stars Capella, Rigel, and Pro- cyon may be mentioned as showing a bluish tinge, Altair and Vega are greenish, Arcturus is yellow, while Sirius, Deneb, Polaris, Fomalhaut, and Regulus are white. Mr. Birmingham published a catalogue of " The Red Stars " in the ( Transactions of the Royal Irish Academy, for August 1877, and Mr. Chambers has a working-catalogue of 7 1 9 such objects in the ' Monthly Notices,' vol. xlvii. pp. 348-387. The region of Cygnus appears to be especially prolific in red stars, and many of these objects are variable. In a paper read at a recent meeting of the Astronomical Society of the Pacific Mr. Pier- son stated that in binary systems where the stars are of equal magnitude the colours are invariably the same, while those differing in magnitude differ also in colour and the larger star is always nearer the red end of the spectrum than its secondary. In the estimation of star-colours reflecting-telescopes are very reliable owing to their perfect achromatism. Groups of Stars. Great dissimilarity is apparent in the clustering of stars. The heavens furnish us with all gra- dations from the loose, open groups like that in Coma Berenices, in the Pleiades, or in Prsesepe, to the compact globular clusters, in which some thousands of stars are so densely congregated that considerable optical power is re- THE STARS. 317 quired to disintegrate them. Some, it is true, yield more easily than others. The great cluster (Messier 13) in Hercules readily displays the swarms of stars of which it is composed ; but others are so difficult that it is only in the largest instruments they are resolved into star-dust. Further references to these wonderful objects will be made in the next chapter, and some of the principal examples described ; our purpose here is to allude to a few of the more scattered groups, and to some noteworthy instances of multiple stars. Coma Berenices. A naked-eye cluster, consisting of many stars, chiefly from the 5th to 6th mags. A telescope adds a number of smaller stars. Nebulae may be often swept up hereabouts, as it is not far north of the rich nebulous region of Yirgo. The Pleiades. Six stars are usually distinguished by the naked eye, and a seventh is occasionally remarked. Mostlin (the instructor of Kepler) counted fourteen, Miss Airy has drawn twelve, and Carrington, like Mostlin, saw fourteen. In 1877 I distinctly made out fourteen stars in this group. The telescope reveals a considerable number of small stars and Tempel's large nebula near Merope. Kepler saw thirty- two stars with a telescope, and Hooke seventy-eight ; but Wolf, at Paris, after three years of unremitting labour with a 4-foot reflector, catalogued 671 stars in the group. A photograph, however, with a 12-inch refractor showed 1421 stars; and a more recent negative includes no less than 2326. There is an interesting little triangle close to the brightest star, Alcyone ; and several of the leading stars are involved in nebulosity, discovered by means of photography. Prcesepe. A fine group of small stars, divisible by the unaided eye on a clear night. Chambers says the com- ponents are not visible without a telescope ; while Webb notes that the group is just resolvable by the naked eye. Thirty-six stars were glimpsed with Galilei's telescope ; but modern instruments show many more. Marth, using Lassell's 4-foot reflector at Malta, discovered several faint nebulae and nebulous stars in this cluster. X Persei. Perceptible to the eye as a patch of hazy material 318 THE STARS. lying between the constellations Cassiopeia and Persei. In a telescope it forms a double cluster, and is one of the richest and most beautiful objects that the sky affords. The tyro who first beholds it is astonished at the marvellous profusion of stars. It can be fairly well seen in a good field-glass, but its chief beauties only come out in a telescope, and the larger the aperture the more striking will they appear. It is on groups of this character that the advantage of large instruments is fully realized. The power should be very low, so that the whole of the two clusters may be seen in the field. An eyepiece of 40, field 65', on my 10-inch reflector, presents this object in its most imposing form. K Crucis. Sir J. Herschel's observations at the Cape have made this object familiar to northern observers. It is com- posed of more than 100 stars, from the 7th mag. downwards ; and some of the brighter ones are highly coloured, so that the general effect is greatly enhanced and fully justifies Herschel's statement that the group may be likened to " a superb piece of fancy jewelry." Ursce Majoris (Mizar) . This group is interesting both as a naked-eye and as a telescopic object. There is a 5th mag. star, named Alcor, about 11J' distant from Mizar, and the former was considered a good test-object for unaided vision by the Arabian astronomers. But the star has probably brightened ; for it can now be easily seen, and certainly offers no criterion of good vision. Mizar is a fine telescopic double, the companion being 4th mag. and distant 14J /X . Any small telescope will show it, and there is another 8th mag. star very near. a Orionis. This appears as a double-quadruple star, with several others in the same field. A 3-inch will reveal most of them, though some of the fainter stars in the group will be beyond its reach. 9 Orionis. In the midst of the great nebula of Orion there is a tolerably conspicuous quadruple star, the components of which form a trapezium. This is visible in a 2-inch re- fractor. In 1826 Struve discovered a fifth star, and in 1830 Sir J. Herschel found a sixth ; these were both situated a little outside the trapezium. All these stars have been seen THE STARS. 319 in a 3-inch telescope. The great 36-inch equatoreal at Mount Hamilton has added several others ; one was detected by Alvan G-. Clark (the maker of the object-glass) and another by Barnard. These were excessively minute, and placed within the trapezium. Barnard* has also glimpsed an extremely minute double star exterior to the trapezium, and forming a triangle with the stars A and C on the following diagram : Fig. 63. The Trapezium in Orion, as seen with the 36-inch refractor. Several observers, including Huggins, Salter, and others, had previously drawn faint stars in the interior of the tra- pezium ; but these could not be seen by Hall and Burnham in the large refractors at Washington and Chicago, and were thus proved to have no real existence. The new stars observed in the 3 6 -inch telescope are only just within the limits of its capacity, and therefore cannot be identical with * This expert comet-finder would appear to have more acute, sensitive vision on faint stars than Burnhain (see 'Monthly Notices,' vol. xlix. p. 354). 320 THE STARS. stars alleged to have been previously seen in small instru- ments. The fifth and sixth stars in the trapezium have been supposed to be variable, and not without reason ; possibly the others are equally liable to change, but this is only conjecture. Sir J. Herschel says that to perceive the fifth and sixth stars " is one of the severest tests that can be applied to a tele- scope " ('Outlines,' llth edit. p. 610); yet Burnham saw them both readily in a 6-inch a few minutes before sunrise on Mount Hamilton in September 1879. /5 and e Lyrse also form multiple groups, which will well repay observation either with large or small telescopes. Further Observations. Anyone who attempts to indicate with tolerable fulness the methods and requirements of obser- vation in the stellar department of astronomy will find a heavy task lies before him ; and it is one to which he will be unable to do justice in a small space, owing to the variety of matters to be referred to and the necessity of being particular in regard to each one. In what follows I shall merely make very brief allusions, as it is hoped the description already given of past work will be a sufficient guide for the future. Moreover, those who take up a special branch of inquiry will hardly rest content with the meagre information usually con- veyed in a general work on astronomy, but will consult those authorities who deal more exclusively with that branch. Double and binary stars may be said to form one department, variable and temporary stars another, the colours of stars a third, while many others may be signified such as the determination of star-magnitudes, positions, grouping, and distances ; also the proper motions and number of stars, besides photographic and spectroscopic work, each and all of which comprise a field of useful and extensive inquiry. The amateur will of course choose his own sphere of labour, consistently with his inclination and the character of his appliances. In connection with double stars, valuable work yet remains to be done, though the Herschels and the Struves gathered in the bulk of the harvest and Burnham has gleaned much that was left. With regard to bright stars, it may be assumed that very few, if any, close companions, visible in moderately small glasses, now await discovery, unless, indeed, THE STARS. 321 in cases where the star forms part of a binary system of long period, and the epoch of periastron has fallen in recent years. But with telescopic stars there must be many interesting doubles, some of them binaries, still unknown. These should be swept up and submitted to measurement. A great desideratum in this branch is a new general catalogue of double stars ; for such a work would greatly facilitate reference, and save the trouble of searching through different lists in order to identify an object. Burnham has given some practical hints on double-star work in the ' Sidereal Messenger/ and his remarks are reproduced in that excellent work * Astronomy for Amateurs/ As to variable stars, some of these permit of naked -eye estimation, others need a field-glass, and there are some which require to be followed in a good telescope. The observer who enters this department may either desire to find new objects or to obtain further data with regard to old ones. If the former, he cannot do better than watch some of the suspected variables in Gore's Catalogue of 736 objects, pub- lished by the Royal Irish Academy. Whether suspected or known variables are put under surveillance, the plan of com- parison will be the same. Several stars near the variable in position, and nearly equal in light, should be compared with it, and the differences in lustre, in tenths of a magnitude, recorded as frequently as possible. The extent and period of the variation will become manifest by a discussion of the results. The comparison-stars should of course be constant in light, and, if naked-eye stars, they may be selected from the Uranometria Nova Oxoniensis or i Harvard Photometry/ If telescopic stars are required, then recourse must be had to comprehensive charts such as Argelander's Durchmusterung, which includes stars up to 9J mag. Variable stars of the Algol type are especially likely to escape recognition, as they retain a normal brilliancy except during the few hours near the time of a minimum. As to star-colours, it must be admitted that our knowledge is in an unsatisfactory condition. The results of past obser- vation show discordances which are difficult to account for. When, however, all the circumstances are considered, we Y 322 THE STARS. need feel no surprise at this want of unanimity. In certain cases it is probable that actual and periodical changes occur in the colours of stars, though absolute proof is still required. Atmospheric variations unquestionably affect the tints of stars, and some alterations depend upon altitude, for a celestial object seen through the dense lower air-strata near the horizon will hardly preserve the same apparent hues when on the meridian. Telescopes are also liable to induce false impressions of colour, and especially by the employment of different eyepieces not equally achromatic. And the ob- server's judgment is sometimes at fault through physiological influences, or he may have a systematic preference for certain hues which little impress another observer. Those engaged in this branch feel the want of a reliable and ready means of comparison, and several have been tried ; but there are objections to their use, and it seems that the best objects are furnished by the stars themselves. Let the observer study the colours of well-known stars, and familiarize his eye with the distinctions in various cases (also with the differences due to meteorological effects &c.) ; he will then gradually acquire confidence, and may use these objects as standards. The difficulty will be that they cannot be directly compared, in the same field, with other stars ; but relative differences may be noted by turning the telescope from one object to the other. This will be better than forming estimates on the basis of an artificial method, which will sure to be troublesome to arrange, and probably erroneous in practice. In some stars the colour is so curious as to be attributed with difficulty, and with regard to faint stars colour-estimates are often un- reliable ; so that it is not desirable to go below the 9th mag. unless a very large instrument is employed. The necessity of being constantly on the look-out for temporary stars has been already mentioned. There is also the need for further observations of such of these objects as still exist. They are, however, very minute, and the observer will have to be careful as to their identity. Though no great revival in brilliancy is to be expected, these objects exhibited some singular fluctuations during their decline, and it is important to keep them under view as long as possible. THE STARS. 323 Many other departments of sidereal work are best left to the professional astronomer. The derivation of accurate star- places, proper motions, distances, &c. requires instruments of great refinement and trained hands to use them. Researches such as these do not come within the scope of ordinary amateurs. But a vast field is open to them in respect to double and variable stars ; and the physical relations of many of the former greatly intensify the interest in this branch, and make it necessary to secure frequent observations. 324 NEBULA AND CHAPTER XVIL NEBULA AND CLUSTERS OF STARS. Distinction. Large number of Nebulae and Clusters visible. Varieties of form and grouping. Distribution. Early Observations. Variable Nebulae. Nebulous Stars. The Magellanic Clouds. Double Nebulae. Real dimensions of Nebulas and Clusters. Round Nebulae and Clusters. Description of Objects. Further Observations required. Lists of selected Objects. Distinction. These objects, though classed together in cata- logues, offer some great distinctions which the observer will not be long in recognizing. It was thought at one period that all nebulae were resolvable into stars *", and that their nebulous aspect was merely due to the confused light of remote star-clusters. But modern telescopes, backed up by the unequivocal testimony of the spectroscope, have shown that purely nebulous matter really exists in space. The largest instruments cannot resolve it into stars, and it } ields a gaseous spectrum. The conjecture has been thrown out that it may be considered as the unformed material of which suns and planets are made. Large Number visible. D'Arrest once said that nebulse are so numerous as to be infinite, and his opinion is supported by the rapid increase in the number known. Let us make a comparison. Messier inserted in the Connaissanccs des Temps for 1783 and 1784 (published in 1781) a catalogue containing 103 nebulse and star-clusters. Of these 68 were new. In 1888 a new edition of Sir J. Herschel's catalogue * Sir W. Herschel at first entertained this view, finding that with every increase of telescopic power more nebulae were resolved. But in 1791 he said, "perhaps it has been too hastily surmised that all milky nebulosity is owing to starlight only." Lacaille had remarked in 1755 that " it is not certain the whiteness of parts of the Milky Way is caused by clusters of stars more closely packed together than in other parts of the heavens. ft CLUSTERS OF STARS. 325 of 1864 (revised and extended by Dreyer) was printed by the Eoyal Astronomical Society, and this includes 7840 objects 1 * The labours of the Herschels, of Lord Rosse, D' Arrest, Marth, Tempel, Stephan, and Swift have vastly augmented our knowledge in this branch since the time of Messier. Varieties of Form and Grouping. A telescope reveals all grades of condensation in stellar groups. Some consist of rather bright, scattered stars, and are easily resolved. Others contain more stars, but they are smaller, and greater power is required to show them. Others again are condensed into globular clusters needing high powers and good instruments to disconnect the mass of stars composing them. Some are faint, and the stars so minute that they are only to be distin- guished from nebula in the finest telescopes. As to the nebulae properly so called, they exist in all forms. They may be either round, elliptical, or in the form of a streak. Some are highly condensed in their centres, others present well- defined circular disks like planets, and a small proportion are in the form of ringsf. Many peculiarities of detail have been remarked, and a curious and complicated spiral structure has been discovered in certain prominent nebulae. One of these has been termed the " Whirlpool " Nebula from its singular convolution of form. Other objects have received distinctive appellations agreeably to their appearance. Thus, there is the " Dumb-bell" Nebula, the "Crab" Nebula, the "Horseshoe" Nebula, &c. Lord Eosse's 6-foot reflector is in a large degree responsible for the particular knowledge we possess of many of these objects. The large mirror commands a grasp of light which renders it very effective on forms of this character. An instrument of small diameter is quite inadequate to deal with them. They can be seen, it is true, and the general shape recognized in the most conspicuous examples, but their details of structure are reserved for the greater capacity of large apertures. * This is exclusive of 47 new nebulae discovered by Prof. Safford, which form the appendix to the catalogue. t Chambers says only four examples are known, but this is erroneous, as Lord Rosse's telescope has added five ring-nebulae to the four pre- viously catalogued. 326 NEBULA AND Distribution. With regard to distribution these objects exhibit the utmost irregularity, for in certain regions of the heavens they are found to be very plentiful, while in others they are singularly rare. Thus, in Virgo, Coma Berenices, Leo, and Ursa Major large numbers of nebulse abound, while in Hercules, Draco, Cepheus, Perseus, Taurus, Auriga, &c., very few are encountered. Taking the 7840 objects in the New General Catalogue of 1888 it will be found that their distribution in hours of Right Ascension is as follows : E.A. OH. I . II . III . IV . V . VI . VII . VIII . IX . X . XI Nebulae. K.A. 387 XII H. 428 XIII . 398 XIV . 300 XV . 276 XVI . 375 XVII . 171 XVIII . 196 XIX . 230 XX . 362 XXI . 404 XXII . 585 XXIII . Nebula?. 858 504 375 212 230 259 203 117 153 188 275 354 The maximum is therefore reached at XII hours, while the minimum is shown at XIX h. There is a secondary max. at I h., and a secondary min. at VI h. Early Observations. The nebula in Andromeda appears to have been the one first discovered, for the distinguished Persian astronomer Al-Sufi (who died in 986 A.D.) was undoubtedly acquainted with it. The nebula is figured upon a Dutch map of the stars nearly 400 years old. In 1612 Simon Marius redetected this object, and appropriately likened its appear- ance to that of a "candle shining through a piece of horn/' In 1618 the nebula in Orion was certainly known, for Cysatus of Lucerne compared it with the head of the fine comet visible in December of that year. Huygens alighted upon the same object in 1656, and appears to have been unconscious of its prior discovery. Only six "nebulse or lucid spots'" were known in 1716, and enumerated by Halley in the ' Phil. CLUSTERS OF STARS. 321 . Trans.' vol. xxix. These included those of Andromeda and Orion. A third was situated in the space between the bow and head of Sagittarius. This is M. 22, and consists of a bright globular cluster of stars. The fourth was the fine star- group involving o> Centauri, which Halley himself found in 1677. The fifth was another fine group in the right foot of Antinous. This is M. 11, and was discovered by Kirch in 1681. The sixth was the magnificent globular cluster (M. 13) in Hercules, discovered by Halley in 1714. In 1735 the Rev. W. Derham published a list of 16 of these objects, and in 1761 Lacaille summarized 42 nebulae and star-clusters which he had observed in the southern sky. This was followed by Messier's tables of 45 nebulas &c. in 1771, and of 103 in 1781 *. But these contributions, im- portant though they severally were, sunk into insignificance beside the splendid results obtained by Sir W. Herschel, who during his prolonged and systematic sweeps of the heavens picked up no less than 2500 new nebulae and clusters which he formed into three catalogues printed in the t Phil. Trans/ as follows : 1786, 1000 objects, 1789, 1000 ditto, 1802, 500 ditto. Variable Nebula?. It is in the highest degree probable that changes occur in the physical appearances of certain nebulae, though the opinion is not perhaps supported by a sufficient number of instances. Until Sir W. Herschel began his review of the heavens very few nebulae were known, and the information possessed about them was very incomplete. The early records, obtained with small and inferior telescopes, scarcely admit of comparison with recent observations, for in matters of detail little agreement will be found; and this proceeds certainly not so much from real changes in the objects as from differences due to the variety of instruments employed, to atmospheric vagaries, and to " personal equation." Bullialdus and Kirch in 1667 and 1676 and Le Gentil in 1759 supposed that remarkable changes were operating in the great elliptical nebula of Andromeda. But G. P. Bond fully * Some of tlie nebulae in Messier's list were discovered by Mechain at Paris, who, like Messier, earned celebrity by his cometary discoveries. He was born at Laon in 1744, and died at Valencia in 1805. 328 NEBULA AND investigated the evidence, and concluded that the variability of the object was by no means proved. Some observers have represented the nucleus as stellar, while others have drawn it as a gradual condensation, and Dr. Copeland has shown that different magnifying powers alter the aspect of the nucleus, " the lower powers making it more star-like, the higher ones more soft-looking and extensive/' Mairan and others entertained the view that the large irregular nebula in Orion was subject to change. This object received much attention from Sir W. Herschel, and he con- cluded that it underwent great alteration between 1774 and 1811. D'Arrest, from his own researches and a discussion of other results, expressed himself in 1872 that " the observed changes in this vast mass of gas seem exclusively to turn out O O / to be temporary fluctuations of brightness." Prof. Holden has arrived at a similar conclusion, and says : " The figure of the nebula has remained the same from 1758 till now (if we except a change in its apex about 1770, which seems quite possible) ; but in the brightness of its parts undoubted varia- tions have taken place, and such changes are still going on" (' Monograph of the Nebula in Orion,' p. 225). Hind discovered a faint nebula, with a diameter of about 1', on Oct. 11, 1852. It was situated in Taurus, the position being R.A. 4 h 15 m 33 s , Dec. +19 15'-6 (for 1890), or about 2 W. of the star e Tauri (mag. 3'7). D' Arrest, on Oct. 3, 1861, searched for this object, but found it had quite disappeared I A small round nebula was seen in 1868, about 4' preceding Hind's, but this resisted some later attempts at observation. In Oct. 1890, Burnham and Barnard, with the 36-inch refractor of the Lick Observatory, saw two nebulae here, one a very small, condensed nebula, with a stellar nucleus, and the other an exceedingly faint nebulosity about 45" in diameter (see ' Monthly Notices/ vol. li. pp. 94, 95). The nebula surrounding the star y Argus has been sus- pected of variation, particularly by Abbott, of Hobart Town, Tasmania. Yols. xxv., xxx., and xxxi. of the ( Monthly Notices ' contain many references to, and figures of, this * O. Struye had expressed views identical with these in 1857 (see 1 Monthly Notices/ vol. xvii. p. 230). CLUSTERS OF STARS. 329 interesting object. But the alleged changes have not been substantiated, and there seems no reason to doubt that they were purely imaginary. The trifid nebula in Sagittarius (M. 20) is supposed by Prof. Holden to have altered its position with reference to a triple star now situated in the S. following part of the nebula. Sir J. Herschel, more than half a century ago, had described this star as placed in the middle of the vacuity by which the nebula is divided. Dreyer, however, points out that the drawings of this object differ in many details, and that, though changes of brightness may have taken place, it is difficult to understand that the nebula should move so as to envelop the star in about 1835, " after which no sensible change occurred again, so far as published observations go/' The nebula (M. 17) just N. of the bow of Sagittarius was also inferred by Holden to have shifted its place rela- tively to the small stars figured by Lassell in this object; but Dreyer adduces facts which controvert this assumption. (See ' Monthly Notices,' vol. xlvii. pp. 412-420, where much valuable information will be found as to supposed variable nebula.) On Oct. 19, 1859, Tempel discovered a faint, large nebu- losity attached to the star Merope, one of the Pkiades, and at first mistook it for a diffused comet. Its position is E.A. 3 h 39 m '6, Dec. +23 26' (1890). An impression soon gained ground that this object was variable ; for while Schmidt, Ohacornac, Peters, and others saw it with small instruments, it could not be discerned by D'Arrest and Schjellernp with the large refractor at Copenhagen. Swift saw the nebula easily in 1874 with a 4J-inch refractor, and has observed it with the aperture contracted to 2 inches. Backhouse re-observed it in 1882 with a 4^-inch refractor. Yet in March 1881 Hough and Burnham sent a paper to the Koyal Astronomical Society with an endeavour to prove that the nebula did not exist ! They had frequently searched for it during the preceding winter, but not a vestige of the object could be seen in the 18J-inch refractor at Chicago, and they regarded the supposed nebula as due to the glow proceeding from Merope and neighbouring stars. But photography has 330 NEBULAE AND entirely refuted this negative evidence, and has shown, not only Tempel's nebula, but others involving the stars Maia, Alcyone, and Electra belonging to this cluster. As to the alleged variations in the Merope nebula, there is every reason to suppose these were not real. Proper motion has been suggested in regard to a very small, faint nebula (N.G.C. 3236) a few degrees following a Leonis. But Dreyer has disproved this by showing that there was no proper motion between 1865 and 1887, whence "it may be safely inferred that there has been none since 1 830,, unless we are to believe, in this and similar cases, that nebulae in the good old days moved about as they liked, but have been on their good behaviour since 1861." Nebulous Stars. This name was applied by Hipparchus and other ancient observers to the clusters of stars which, to the naked eye, appear as patches of nebulous light. Sir W. Herschel, in 1791, showed this designation to be incorrect, and used it in connection with stars actually involved in nebulosity. In sweeping the heavens he met with several instances of this kind. Thus, 3 E.S.E. of Persei he found a star of the 9th mag. surrounded by a nebula 3' in diameter. He picked up another close to the star 63 Geminorum. This is a remarkable object a star of the 9th mag. surrounded by two dark and two bright rings. On Feb. 3, 1864, Lord Rosse's telescope showed an opening in the outer bright ring, and the latter seemed connected with the inner bright ring ; so that the object presented the aspect of a spiral nebula with a star in the centre. The diameter of the whole nebulosity is 45". Key observed this object with an 18-inch reflector in 1868, and described it as symmetrical a central star, with intervening dark and bright rings. He found a power of 510 the best, for, " like the annular nebula in Lyra, it bears mag- nifying wonderfully well." Since Herschel's time many nebulous stars have been discovered. There is one of about 6th mag. in RA. 8 h 6 m 'l, Dec. -12 36'. The nebulosity round the star fades away gradually, and its extreme diameter is 157". There is a 7th mag. star at R.A. 21 h O m 14 s , Dec. + 67 44' involved in a very large, faint nebulosity. This is a striking object, and I have frequently picked it up while CLUSTERS OF STARS. 331 comet-seeking. The star has such a foggy, veiled appearance that on first remarking it the observer thinks his lenses are dewed, but on viewing neighbouring stars he sees them sharp and clear on the dark sky, and the contrast is very pro- nounced. The nebulous star is much isolated, though in a part of the sky where small stars abound. This is one of Herschel's discoveries and No. 7023 of the N. G. C. ; Dreyer says he has seen the nebulosity particularly distinct north and south of the star. In some cases a double star is involved in nebulosity, and there are instances in which two double stars are placed within an elliptical nebula. The Magellanic Clouds *. These are marked as Nubecula Major and Nubecula Minor on celestial globes and charts. They form two extensive aggregations of nebulae and star- clusters, and are readily visible to the naked eye in or near Hydrus, and not far from the south pole of the heavens, They may be likened to detached patches of the Milky Way. Sir J. Herschel says the Nubecula Major is situated between the meridians of 4 h 40 m and 6 h and the parallels of 66 and 72 of S. declination, and extends over a space of some 42 square degrees. The Nubecula Minor lies between O h 28 m and l h 15 m and 72 and 75 of S. decimation, and spreads over about 10 square degrees. The composition of these objects is very complex and diversified, and affords very rich ground for exploration with a large telescope. Nebulae exist in profusion and in every variety, and are intermingled with star-clusters varying in condensation from the compact globular form to groups more loosely scattered, and such as we often find in the Milky Way. Nearly three hundred nebulae and clusters are included in the major " cloud," while more than fifty others closely outlie its borders. In the minor about forty such objects have been discovered. It is very strange to find them collected together in this manner ; for in other regions of the firmament they are usually found separated, and certain classes appear to have their own special zones or localities of distribution. Sir J. Herschel pointed out that " globular clusters (except in one * Humboldt says this " name is evidently derived from the voyage of Magellan, although he was not the first who observed them." 332 NEBULA AND region of small extent) and nebulae 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 most remote possible from that circle, whereas in the Nubeculae they are indiscriminately mixed with the general starry ground and with irregular though small nebulae." Double Nebulae. Instances are not wanting of conspicuous double nebulae. M. 51 and 76, near 77 Ursse and 6 Andromedse, may be classed in this category. There is a very interesting, though a smaller object just W. of a and ft Geminoram, or in B.A. 7 h 18 m '6, Dec. +29 43'. Two bright, round nebula are separated by an interval of 28 // . These double nebulae are usually round, and are sometimes resolvable into stars. Whether they are physical or mere optical pairs has yet to be ascertained. So many examples exist that it seems highly probable they have a real connection, though no motion has yet been certainly detected to prove they are binary systems. Such motion may, however, be very slow, and require obser- vations extending over a much longer interval before it is revealed. Meal Dimensions of Nebulae and Clusters. It may be readily imagined that these objects are of immense size ; for though placed at distances of the utmost remoteness, they spread over perceptible and comparatively large areas. Gore remarks that, on the assumption that the globular cluster in Hercules (M. 13) is 5' in diameter, and its parallax one tenth of a second, its real diameter must be 3000 times the Sun's mean distance from the Earth, or nearly 280 billions of miles ! He further points out- that, though this group contains as many as 14,000 stars, according to Sir W. Herschel, yet each com- ponent may be separated many millions of miles from the others, owing to the vast dimensions of the group. Details like these are of course only approximate, as the distance of a nebula or star-cluster has not yet been definitely ascertained. The great nebulas of Orion and Andromeda must extend over prodigious regions in distance-space ; but to quote figures seems useless, in consequence of our inability to form just conceptions of such immensity. Round Nebulce and Clusters. Resolvable nebulae and CLUSTERS OF STARS. 833 clusters are frequently circular in outline. The central con- densation is an indication of their globular form, though not always so, for many of these objects become suddenly much brighter in the middle, and show an apparently stellar nucleus. The material or stars forming the object cannot therefore be equally distributed. Where it suddenly brightens there is a great condensation, and in some cases several of these are evident in the form of bright rings, intensifying as the nucleus is approached. This irregular aggregation denotes the opera- tion of "a force of condensation directed from all parts towards the centre of such systems/' In regard to planetary nebulas, they cannot be globular or they would exhibit a brightness increasing from the margin to the centre. Their even lumi- nosity throughout affords the evidence of a special structure. Sir J. Herschel thought the planetary nebula (M. 97) near /9 Ursaa Majoris must either be in the form of a hollow globe or a flat circular disk lying perpendicular to the line of vision. Description of Nebulae and Clusters of Stars. The latter objects are included in this chapter for several reasons. In a small telescope nearly all such clusters exhibit the aspect of nebulas, and they have been catalogued with them, though, as already explained, some great distinctions are to be drawn. To the naked eye the cluster Prsesepe, in Cancer, is usually visible as a patch of nebulosity, though on a very clear, dark night stars may be glimpsed sparkling about the spot, and a very small glass will suffice to show it as a nest of stars. This object, and some others of a more difficult character (their component stars being smaller and more compressed), are tabulated (I.) at the end of this chapter. A summary (II.) of globular clusters is also given, together with a list (III.) of nebulas, a few of which are resolvable into stars *. It must be under- stood that these selections, though comprising many notable objects, are by no means exhaustive, the intention being merely to indicate some typical examples of fine nebulas and clusters and of peculiarities of form or appearance, such as planetary, annular, elliptical, and centrally condensed nebulas and loose, compressed, and globular clusters. Some of these * I have selected the various objects in these lists from the New General Catalogue. 334 NEBULA AND objects deserve individual references, as they present interesting details to the telescopic observer and come within the reach of moderate appliances. Great Nebula in Andromeda (M. 31). This object has often been mistaken for a comet, for it is readily perceptible to the eye on a moonless night. It is very large 4 by 2 4, according to Bond, with a 15-inch refractor. He discovered a pair of dark streaks in the brightest region of the nebula, and these may be well seen in a 10-inch reflector. It is really triple ; for about 25' S. of the nucleus there is a very bright, round, resolvable nebula, discovered by Le Gentil, and a third, observed by Caroline Herschel, lies rather further to the N.W. Photographs by Roberts show dark rings dividing the bright interior parts of the nebula from the outer, and imparting to it a decided spiral tendency. This nebula has hitherto resisted attempts to resolve it into stars, though many hundreds have been seen in the foreground. But its spectrum is continuous, so that its stellar character is to be inferred. Great Nebula in Orion (M. 42). Visible to the naked eye just below a line connecting ft and Orionis, and involving Orionis. It exhibits an extremely complicated structure, and many of its irregular branches and condensations may be discerned in small instruments. Sir W. Herschel failed to resolve this object into stars with his 4-foot reflector; but Lord Rosse, in 1844, thought he had effected it with his 6-foot mirror, though the conclusion was premature. The spectroscopic researches of Huggins have shown this nebula to be composed of incandescent gases, so that the stars tele- scopically observed in it are probably in the foreground and entirely disconnected from the nebulous mass. Effective photographs have been taken of it by Draper, Common, and Roberts. It certainly forms one of the grandest objects in the heavens. 'The Planetary* Nebula (M. 97). Discovered by Mechain in 1781. In small telescopes it looks like a rather faint, round * These forms are more numerous than the annular nebulae. They often exhibit a blue colour, and the spectroscope shows them to consist of gas. CLUSTERS OF STARS. 335 mass of nebulosity, somewhat brighter in the middle than at the edges. In Lord Rosse's telescope it shows many details, including a spiral arrangement and two dark spots in the middle inclosing bright, eye-like condensations. The margin is fringed with protuberances, and from its peculiar aspect this object has been called the " Owl " Nebula. Diameter between 155" and 160". It may readily be picked up 2J S.E. of yS Ursae Majoris. It yields a gaseous spectrum. In Draco at R.A. 17 h 58 m 36 s , Dec. +66 38' there is a pretty small, but exceedingly bright planetary nebula. With a low power it looks like a star out of focus, but a high power expands it into a well-defined planetary disk. As observed in Lord Rosse's 3-ft. reflector on Sept. 17, 1873, this nebula exhibited " a round, well-defined disk of a full blue colour, light very equable, diameter 22"'4, surrounded by an extremely faint nebulosity. This is an excellent object of its class. Spiral Nebula (M. 51). Discovered by Messier on Oct. 13, 1773. It is situated in Canes Venatici, and 4 S.W. from 7) Ursae Majoris. An ordinary instrument will reveal it as a double nebula, and the two parts will be seen to differ greatly in size. Messier gave the distance separating them as 4 ; 35". Sir J. Herschel drew this object as a bright, centrally con- densed nebula, surrounded by a dark space and then by a luminous ring divided through nearly one half of its cir- cumference. Closely outlying this he placed a bright round nebula. Lord Rosse's 6-foot showed something very different. In April 1845 its spiral character was discovered ; coils of nebulosity were observed tending in a spiral form towards the centre, and the outlying nebula was seen to be connected with it. Some striking drawings have been published of this object. Those by Sir J. Herschel and Lord Rosse differ essentially, and would scarcely be supposed to represent the same nebula. ; but when we reflect that the instruments used were respectively of 18 inches and 72 inches aperture, the cause of the disparity becomes evident. Another fine example of a spiral nebula is M. 99, in the northern wing of Yirgo, and 8 E. of @ Leonis. This object was discovered by Mechain ; its spiral form of structure was detected by Lord Rosse in 1848. Diameter 2J'. Like 336 NEBULA! AND M. 51 it gives a continuous spectrum and is resolvable into stars. The Crab Nebula in Taurus (M. 1). Discovered by Bevis in 1731, and situated 1 N.E. of ? Tauri. Its diameter is 5J' by Fig. 64. 1. 1. Nebula with bright centre. 2. Planetary Nebula. 3. Ring-nebula in Lyra. 4. Star-cluster in Hercules. 3y. An early drawing with Lord Rosse's telescope shows it with numerous radiations; whence it was termed the Crab Nebula, from the supposed resemblance : but later obser- vations have given it quite another form. In 1877 there was sir CLUSTERS OF STARS. 337 no trace of the nebulous arras : it appeared as a well-defined, oval nebula with some irregularities of structure. This object is very plain in small telescopes, and may be readily picked up from its proximity to f Tauri ; but in such instruments it is void of detail, and merely presents a pale, oval nebu- losity. It has not been clearly resolved, though it has a mottled appearance, indicating a stellar composition, in large apertures. The Dumb-bell Nebula (M. 27). Discovered by Messier in 1779, and situated in Vulpecula a region very rich in small stars. Diameter about 7' or 8 ; . Its general form resembles a dumb-bell or hour-glass ; hence its name. Struve, Lord Rosse, and others have seen many stars in the nebulous mass, but the latter is not resolvable. I have seen seven stars in the nebula with a 10-inch reflector. Its peculiar shape is perceptible in a small instrument. This object frequently serves to illustrate books on Astronomy ; but the drawings by Sir J. Herschel, Lord Rosse, and others are curiously dis- cordant, and show how greatly differences in telescopic power may affect the observed appearance of an object. The Ring-Nebula in Lyra (M. 57). Discovered by Messier between the stars /3 and 7 Lyrse. Diameter 80" by 60". This object is bright, though rather small, and it will stand high powers. The dark centre may possibly be glimpsed in a 3-inch refractor ; I have seen it readily in a 4^-inch. It was at one period thought to be resolvable, but the spectro- scope has negatived the idea, and shown it probably consists of nitrogen gas. A small star near the centre was frequently seen in Lord Rosse's telescope; but the 36-inch refractor at Mount Hamilton reveals twelve stars projected on or within the ring, and several others have been suspected. There is a faint star exterior to the ring, and following it ; this is visible in small telescopes. The space within the ring is not quite dark, and the structure of the nebula is somewhat compli- cated as seen in large instruments. Another fine instance of an annular nebula may be found 3 preceding the 4th mag. star 41 Cygni, but it is not so large or conspicuous as that in Lyra. Its diameter is 47 /x by 41". Several stars were seen sparkling in it by Lord Rosse, who found the centre z 338 NEBULA AND was filled with faint light and the N. side of the ring broadest and brightest. Elliptical nebulce are well represented by the pair (M. 81 and 82) about 2 E. of d (22) Ursse Majoris. They are separated by about 38' of declination, so that they may be observed in the same field of a low-power eyepiece. The preceding one is very bright and large (8' by 2'). The following one is a ray or streak of nebulosity (l f by f). On May 21, 1871, the great Rosse telescope showed the latter as a most extraordinary object, at least 10' in length and crossed by several dark bands. Roberts photographed these nebulae on March 31, 1889. " The negative shows that the nucleus [of M. 81], which has not a well-defined boundary, is sur- rounded by rings of nebulous or meteoric matter, and that the outermost rings are discontinuous in the N.p. and S.f. directions." M. 82 is " probably a nebula seen edgeways, with several nuclei of a nebulous character involved, and the rifts and attenuated places in it are the divisions of the rings that would be visible as such if we could photograph the nebula from the direction perpendicular to its plane " (' Monthly Notices/ vol. xlix. p. 363). This fine pair may be easily picked up in a small instrument. Another grand object of this class (discovered by Caroline Herschel in 1783) lies in R.A. O h 42 m -2, Dec. -25 54', between the stars Ceti and a Sculptoris. Globular clusters furnish us with many examples of easily resolved and richly condensed balls of stars. M. 3 (dis- covered by Messier), M. 5 (discovered by G. Kirch), and M. 13 (discovered by Halley) may be selected as amongst the finest of these objects in the northern hemisphere. They are severally visible to the naked eye, and may be found in a telescope directed as follows : M. 3, between Arcturus and Cor. Caroli, and nearer the former ; M. 5, 7 S.W. of a Ser- pentis and close to the double star 5 Serpentis ; M. 13, one third the distance from rj to f Herculis. They are brilliant objects from 5' to 1' diameter. With power 60 on my 10-inch reflector they are spangled with stars, though not fully resolved. Smyth described M. 3 as consisting of about 1000 small stars, CLUSTERS OF STARS. 339 blazing splendidly towards the centre. Webb hardly resolved it with a 3/Q-inch refractor. Another fine object of this class (M. 80) will be encountered midway between a and /3 Scorpii. Sir W. Herschel described it as the richest and most compact group of stars in the sky, and it is noteworthy from the fact that a new star burst forth near its centre in 1860. There is a magnificent cluster, involving o> Centauri, which Sir J. Herschel considered as " beyond all comparison the richest and largest object of the kind in the heavens." It is visible to the naked eye as a 4th mag. star, but residents in northern latitudes are precluded from a view *of it. Pegasus also supplies us with some fine clusters ; Maraldi picked up two in 1746 (M. 2 and 15), and these will respectively be found 5 N. of /3 and 4 W.N. W. of e Pegasi. They are to be classed amongst the grandest objects of their kind. In Cygnus, at R.A. 20 h 41 m 7 s , Dec. +30 19', near K, and especially in the region immediately north-east, there exist irregular and extensive streams of faint nebulosity which may be said to form a telescopic milky way. Nebulae and stars are curiously grouped together, forming a remarkable arrange- ment which will well repay study. To see these objects satisfactorily, a moonless night, free from haze or fog, should be chosen, and the power should be moderately low, or some of the more feeble nebulous films will be lost. The observer may spend some agreeable hours in sweeping over this region, which is one of the best in a wonderfully rich constel- lation. Further Observations. The fact that Swift has discovered many hundreds of nebulae during the last few years affords indubitable proof that considerable numbers of these objects still await detection. No doubt they are generally small and faint, but it is necessary they should be observed and cata- logued, so that our knowledge in this department may be rendered as complete as possible. New nebulae are sometimes mistaken for expected comets, and occasionally give rise to misconceptions which would be altogether avoided were our data more exhaustive. Those who sweep for nebulae must have the means of deter- z2 340 NEBULAE AND mining positions, and a small telescope will be inadequate to the work involved. A reflector of at least 10 inches, or refractor of 8 inches, will be required ; and a still larger instrument is desirable, for to cope successfully with objects of this faint character needs considerable grasp of light. The power employed should be moderate; it must be high enough to reveal a very small nebula, but not so high as to obliterate a large, diffused, and faint nebula. In forming his first catalogue of 1000 nebulae, Sir W. Herschel used a New- tonian reflector of 18*7 inches aperture, power 157, field 15' 14"; Swift's recent discoveries were effected with a 16-inch refractor and a periscopic positive eyepiece, power 132, field 33'. With a low power a very extensive field will be obtained, and a large part of the sky may soon be examined, but it will be done ineffectively. It is better to use a mode- rately high power, and thoroughly sweep a small region. The work is somewhat different to comet-seeking ; it must proceed more slowly and requires greater caution, for every field has to be attentively and steadily scanned. If the telescope is kept in motion, a faint nebula will pass unseen. Some of these objects are so feeble that they are only to be glimpsed by averted vision. When the eye is directed, say, to the E. side, a faint momentary glow comes from the W. side of the field ; but the observer discerns nothing on looking directly for the object. On again diverting his gaze he receives another impression of faint nebulosity from the same point as before, and becomes conscious of its reality. Fre- quently, while comet-seeking, I meet with a small indefinite object, the character of which cannot be determined by direct scrutiny. On withdrawing the eye to another part of the field, however, the mystery is solved. If the object is a nebula, it flashes very distinctly on the retina ; but if a small cluster, the individual stars are seen sparkling in it. These indirect views are usually so effective that the trouble of applying higher powers is dispensed with. The glow from a faint nebula or comet often apparently fluctuates in a remarkable manner. Light-pulsations affect it, causing the nebulosity to be intermittently visible. It CLUSTERS OF STARS. 341 flashes out and enlarges, then becomes excessively feeble and indeterminate. The changes are not real, but due to the faint and delicate nature of the object, which is only fugitively glimpsed and presents itself differently with the slightest change in the manner of viewing it. Burnham has said there is no such thing as glimpsing an object ; but he is wrong. It is the intermediate step between steady visibility and absolute invisibility. The work of sweeping for nebulae is much delayed by the comparisons necessary for the identification of objects. The path may be smoothed by marking the known nebulae on a good chart, like Argelander's. The observer may then see, by reference, whether the objects he encounters have been picked up before. The labour of projecting all the nebulae contained in the New General Catalogue would of course be considerable, and the observer will probably find it expedient to select certain regions for examination, and map such nebulae as are included within their borders. The discovery of new nebulae offers an inviting field to amateurs. Vast numbers of these objects have escaped pre- vious observation, for though the sky has been swept again and again, its stores have not been nearly exhausted. Mr. Barnard recently stated that with the powers of the great 36- inch refractor the number of known nebulae (more than 8000) might readily be doubled ! As an example of their plentiful distribution in certain regions it may be mentioned that Mr. Burnham very recently discovered eighteen new nebulae in a small area of 1C' by 5''5 near the position in R.A. 13 h 38 m , Dec. 56 20' N. Near the pole of the northern heavens there exist many unrecorded nebulae, as this region does not appear to have been thoroughly examined with a large instrument. It is often the case that several nebulae are clustered near together. Whenever a new one is discovered the surrounding space should therefore be carefully surveyed in search of others. The region immediately outlying known objects may also be regarded as prolific ground for new discoveries. After several hours' employment in the work of searching for nebulae or comets the eye is enabled to discern faint objects which were invisible at first, as it is in a better condition to 342 NEBULA AND receive feeble impressions. While comet-seeking in 1889 and 1890 I discovered ten new nebulae, all near the N. pole, and their approximate positions are given below : Eef. No. Date of Discovery. Position 1890. Description. E.A. . Dec. + h m s o i 1. 1889, Aug. 26 4 29 59 75 25-2 F. S., b. M., * 12, n. p. 2. 1890, Nov. 7 4 40 19 78 7-9 R, S., E. 3. 1890, Oct. 19 4 46 38 68 9-8 R, S., E., b. M.N., F. double *s.f. 4. 1890, Nov. 16 5 50 7 80 31-0 v. R, S. 5. 1890, Nov. 9 6 11 45 83 1-9 R, S, E., in.b.M. 6. 1890, Oct. 17 6 59 26 85 45-0 v.F.,v.v.S., 12' s. s.f. N.G.O. 2300. 7. 1890, Nov. 7 7 8 52 80 7-4 v. R, p. S., 22' B. s. f. N.G.O. 2336. 8. 1890, Sept. 14 7 23 24 85 30-0 F., S., E., 46' s. f. N.G.C. 2300. 9. 1890, Sept. 8 8 21 37 86 7-4 p. F., S., m. b. M., * n. f. 10. 1890, Aug. 23 8 34 30 85 54-4 F., S., E., g. b. M., near pre- ceding. Abbreviations : F., faint ; S., small ; E., round ; M., middle ; N., nucleus, B., extended; v., very; b., brighter; n., north; s., south; f., following; p., pretty, preceding; m., much; g., gradually; *, star; N.G.C., New General Catalogue. No. 8 is placed centrally within a curious semicircle of stars, thus : Fig. 65. CLUSTERS OF STARS. 343 I. CLUSTERS OF STABS. No. N.G.C., 1888. No. M., 1781. Position, 1890. Description. E.A. Dec. 225. ... h m 37-1 +61 3 Stars 9th-10th mags. Between y and K 869. ... 2 11-3 +56 38 CassiopeisB. In Perseus. Stars 7th-14th mags. 1039. 34. 2 35-0 +42 18 A fine group, chiefly of 9th mag. stars. 1912. 38. 5 21-3 +35 44 Stars of various mags. In Auriga. 1960. 36. 5 29-0 +34 4 Stars of 9th-llth mags. Near 1912. 2099. 37. 5 45-1 +32 31 Stars and star-dust. 5 S. of 9 Aurigae. 2168. 35. 6 2-0 +24 21 Stars of 9th-16th mags, near i\ Gemi- norum. 2287. 14. 6 42-3 -20 38 Visible to naked eye. 4 S. of Sirius. 2437. 46. 7 36-8 -14 34 Nebula involved with cluster of 8th-13th mag. stars. 2477. ... 7 48-4 -38 16 Fine group of 12th mag. stars near Argus. 2516. ... 7 56-5 -60 34 Visible to naked eye. 200 stars of 7th- 13th mags. 2547. ... 8 7-4 -48 56 Vis.n.e. Stars 7th-l 6th mags. Diameter 20'. 2548. ... 8 8-3 - 5 28 Stars of 9th-13th mags. In Monoceros. 2632. 44. 8 34-0 +20 22 Prassepe. Group of bright stars vis. n. e. 2682. 67. 8 45-2 + 12 13 Large group of stars of 10th-15th mags. 4755. ... 12 47-1 -59 45 Very large group about K Crucis. 6121. 4. 16 16-9 -26 16 Close to Antares. Group and line of stars through it. 6603. 24. 18 12-0 -18 28 Stars of 15th mag. 3 N. of ft Sagittarii. 6611. 16. 18 12-7 -13 50 Group of at least 100 stars of various mags. 6705. 11. 18 451 - 6 24 Stars of llth mag. and fainter. Fine object. 6838. 71. 19 48-8 +18 29 Stars of llth-16th mags. In Sagitta. 7243. ... 22 10-9 +49 20 A clustering of many bright stars. 7654. 52. 23 19-4 +61 [rrregular group of 9th-13th mag. stars. 7789. ... 23 51-5 +56 6 Grand cluster of llth-18th mag. stars. 844 NEBULA? AND II. GLOBULAR CLUSTERS OF STARS. No. N.G.C., 1888. No. M., 1781. Position, 1890. Description. E.A. Dec. 104. ... h m 191 -72 42 P"ery large ; more than 15' diameter. 288. ... 47-3 -27 11 Slightly elliptical. Stars 12th-16th mags. 362. ... 58-5 -71 26 Stars 13th-14th mags. Diameter 4'. 1261. ... 3 9-3 -55 38 jarge. Stars and star- dust. 1851. ... 5 10-5 -40 10 Very bright and large. Fine object. 4147. ... 12 4-5 + 19 9 'retty large, round. Minute stars. 4590. 68. 12 33-7 -26 9 Vluch compressed group of 12th mag. stars. 5024. 53. 13 7-5 +18 45 ?ine object. Chiefly 12th inag. stars. 5139. ... 13 20-2 -46 44 Very large; diameter 20'. At w Cen- tauri. 5272. 3. 13 37-1 +28 56 Visible to naked eye. Diameter 7'. 5634. ... 14 23-8 - 5 29 Very bright, considerably large. Round. 5904. 5. 15 13-0 + 2 29 Visible naked eye. Stars 1 lth-15th mags. Diam. 5'. 5986. 15 38-8 -37 25 Stars of 13th-15th mags. In Lupus. 6093. 80. 16 10-5 -22 42 Stars of 14th mag. Between a and /3 Scorpii. 6205. 13. 16 377 +36 40 Visible naked eye. A grand object, in Hercules. 6218. 12. 16 41-5 - 1 45 Stars of 10th mag. and fainter. Diam. 4'. 6254. 10. 16 51-4 - 3 56 Stars of 10th-15th mags. Diameter 4'. 6266. 62. 16 54-2 -29 57 Stars of 14th-16th mags. In Scorpio. 6333. 9. 17 12-8 -18 24 Much compressed group of 14th mag. stars. Diam. 4'. 6341. 92. 17 13-8 +43 15 A mass of stars and star-dust. 7 N, TT Herculis. 6402. 14. 17 31-8 - 3 11 Chiefly stars 15th mag. Diameter 4'. 6656. 22. 18 29-7 -24 Stars of llth-15th mags. In Sagittarius. 6779. 56. 19 12-3 +30 Stars llth-1 4th mags. Between /3 Cygni and y LyraB. 6809. 55. 19 33-0 -31 14 Fine, large, round cluster of stars llth- 13th mags. 7078. 15. 21 24-7 + 11 41 Group of stars and star-dust. Diameter 5'. 7089. 2. 21 27-8 - 1 19 Exceedingly small stars. Diameter 5'. 7099. 30. 21 34-1 -23 41 Stars 12th -16th mags. Diameter 3'. CLUSTERS OF STARS. III. 345 No. N.G.C., 1888. No. M., 1781. Position, 1890. Description. R.A. Dec. 185. h m 32-9 +47 44 Very large ; pretty bright. Eesolvable into stars. 224. 31. 36-7 +40 40 Great nebula in Andromeda. 253. ... 42-2 -25 54 Very, very large and bright. 24' by 3 f . 598. 33. 1 27-6 +29 57'1 Exceedingly bright and large. Nucleus. 650. 76. 1 35-4 +51 1 Very bright double nebula. 1365. ... 3 29-4 -36 30 Very bright and large. Elliptical. 1501. ... 3 57-5 +60 37 Pretty bright planetary nebula. Diam. 1'. 1514. ... 4 2-4 +30 29 Star of 9th mag. in nebula 3' diameter. 1952. 1. 5 27-9 +21 56 Great Crab Nebula, near Tauri. Stars. 1976. 42. 5 29-9 - 5 28 Great nebula involving 9 Orionis. 1990. 5 30-6 - 1 16 Star (e Orionis) involved in nebulosity. 2070. ... 5 39-5 -69 9 Visible to naked eye. Great " looped " nebula. 2392. ... 7 22-7 +21 8 Nebulous star of 9th mag. 2403. 7 26-2 +65 50 Very large and bright. Elliptical. 2655. 8 41-2 +78 38 Very bright. Condensed in the middle. 2681. 2683. : 8 45-6 8 45-9 +51 44 +33 51 Very large and bright. Centre=star 10th mag. Very large and bright. Elliptical. 2841. 2903. * 9 14-4 9 25-9 +51 26 +22 Very large and bright. Centre = star 10th mag. Large, elliptical, double nebula. 3031. 81. 9 46-5 +69 35 Exceedingly bright and large. Ellip- tical. 3034. 82. 9 46-7 +70 13 A bright ray. In field with preceding. 3242. " 10 19-5 -18 5 Bright planetary nebula. Diameter 45". Blue. 3372. ... 10 40-8 -59 6 Great nebula surrounding t\ Argus. 3556. 11 5-4 +56 16 Large, rather bright. Elliptical. 3587. 97. 11 8-4 +55 37 Fine planetary nebula. Diameter 3'. Near /3 Ursa Majoris. 346 NEBULAS. III. NEBULA (continued] No. N.a.a, 1888. No. M., 1781. Position, 1890. Description. E.A. Dec. 3623. 65. h m 11 13-2 +13 42 Large, bright, elliptical. Near following one. 3627. 66. 11 14-5 + 13 36 Large elliptical nebula. Near /3 Leonis. 4254. 99. 12 13-3 + 15 2 Very fine 3-branched spiral nebula. 4321. 100. 12 17-4 + 16 26 Very large 2-branched spiral nebula. 4382. 85. 12 19-9 +18 48 Very bright ; pretty large. Bound. 4472. 49. 12 24-2 + 837 Bright ; round. Besolvable into stars. 4486. 87. 12 25-3 +13 Large; round. Bright centre. Third of three. 4565. ... 12 30-9 +26 36 A ray of bright nebulosity E. of Coma. 4736. 94. 12 45-7 +41 43 Large and bright. Nucleus. Besolvabte. 5128. ... 13 19-0 -42 27 Very large and bright. Elliptical. Bifid. 5194. 51. 13 25-2 +47 46 Great spiral nebula near t\ Ursae Maj. 5236. 83. 13 30-8 +29 18 Fine object. 3-branched spiral. 5367. ... 13 51-1 -39 27 Very large and bright. Condensed in the middle. 5907. ... 15 13-0 +56 44 Large, elliptical. Another very close to it. 6369. ... 17 22-6 -23 40 Pretty bright, small ring-nebula. 6514. 20. 17 55-7 -23 1 Bright; large. Trifid. Double star involved. 6523. 8. 17 56-9 -24 23 Bright, with loose cluster of stars. 6618. 17. 18 14-4 -16 13 Bright and extremely large. 2-hooked. 6720. 57. 18 49-5 +32 54 Bing-nebula between (3 and y Lyras. 6826. ... 19 41-8 +50 16 Pretty large and bright planetary nebula. 6853. 27. 19 54-9 +22 25 The " Dumb-bell " Nebula. Tine object. 6960. ... 20 41-1 +30 19 Large and bright. K Cygni involved. 7009. 7662. ... 20 58-2 23 20-6 -11 48 +41 56 Very bright, small, planetary nebula. Elliptical. Very bright, pretty small, planetary or ring-nebula. * NOTES AND ADDITIONS. 347 NOTES AND ADDITIONS. LARGE AND SMALL TELESCOPES. P. 19. With reference to mountainous sites for large instruments, a remark in Sir Isaac Newton's l Opticks ' (1730) may be quoted: " Telescopes . . . cannot be formed so as to take away that confusion of rays which arises from the tremors of the atmosphere. The only remedy is a most serene and quiet air, such as may perhaps be found on the tops of the highest mountains above the grosser clouds." P. 27. Lieut. Winterhalter, of the United States Navy, recently visited a large number of European observatories, and in describing that of Nice says : " M. Perrotin declares that two hours' work with a large instrument is as fatiguing as eight with a small one, the labour involved increasing in proportion to the cube of the aperture, the chances of seeing decreasing in the same ratio, while it can hardly be said that the advantages increase in like proportion." The Nice Observatory, and its splendid instruments (including a 30-inch refractor), are due to the munificence of M. Bischoffsheim, who has expended about five million francs upon them. P. 36. The large refractor to be erected on Wilson's Peak of the Sierra Madre range of mountains, in Southern Cali- fornia, is to be 40 inches in diameter. The rough unground disks of glass are already in the hands of the Clarkes, of Cambridgeport, Mass. It is estimated that the complete object-glass and cell will cost something like $65,000, and the focal length of the instrument will be about 58 feet. THE SUN. P. 100. The last minimum of sun-spot frequency appears to have occurred at the middle of 1889. Conspicuous spots 348 NOTES AND ADDITIONS. were very rare in the first half of 1890, but some fine groups were presented in the last half of the year. On Aug. 31 I saw a group extending over 113,000 miles in length, and on Nov. 27 there was another, which measured 123,700 miles. P. 111. Thompson's cardboard disks have been favourably spoken of as enabling observers to determine the positions of spots at any season of the year. MERCURY. P. 137. At the meeting of the British Astronomical Association on Nov. 26, 1890, Mr. G. F. Chambers expressed his firm belief in the existence of an intra-Mercurial planet. The President (Capt. W. Noble) in his inaugural address pointed out the desirability of effecting further observations, both of Mercury and Venus, with a view to redetermine their rotation-periods. He justly remarked that moderately small instruments might be fittingly employed in the work, and that Schiaparellr's deductions (mentioned on pp. 142 and 149) ought to be accepted with extreme reserve pending their verification. MARS. P. 160. Prof. W. H. Pickering observed some of the canals on Mars in 1890 with a 12-inch refractor, but was not able to double any of them. He says that, in examining these objects, the power employed should not " exceed one or two hundred." This is quite contrary to the advice of others, who recommend high magnifiers ; and perhaps it accounts for Prof. Pickering's failure in recognizing the duple canals. With the great 36-inch refractor Mr. Keeler saw, on July 5 and 6, 1890, some curious white spots on the edges of the gibbous limb of Mars, something similar to those visible on the unilluminated part of the lunar disk. The canals were observed as feeble diffused bands. The two satellites were seen by a lady visitor, though previously unaware of their existence. P. 161. The method of deriving the rotation-period of NOTES AND ADDITIONS. 349 Mars is exemplified by Mr. Proctor in the ' Monthly Notices/ vol. xxviii. p. 38. An interesting paper, " On the Deter- mination of the Rotation-Period of Jupiter in 1835," will be found in the ' Memoirs,' vol. ix. PLANETOIDS. P. 167. The 308th planetoid was discovered by Charlois on March 5, 1891. JUPITER. P. 170. Dupret, in Algiers, saw Jupiter with the naked eye on Sept. 26, 1890, and following days, twenty minutes before sunset. P. 191. M. Gruillaume, during a recent transit of the shadow of Jupiter's second satellite, observed a duplicate shadow, fainter than the ordinary one, which partly covered its southern side. COMETS. P. 250. On Nov. 16, 1890, Dr. Spitaler, while looking for Zona's Comet with the 27-inch refractor of the Vienna Observatory, discovered a new and very faint comet only 23' distant from the object of his search. That two of these bodies should be found almost simultaneously and so near together must be regarded as a very singular coincidence. METEORS. P. 261. Mr. Proctor held the view that certain meteorites may have originally been ejected from the Sun. A recent writer thus summarizes our knowledge of them : " That they are independent bodies, moving in orbits of their own in space ; that these dark bodies are abundant in the inter- planetary spaces ; that those within the near range of solar or planetary attraction move with great velocity ; that many swarms of them follow well-known orbits ; and that, in general, their origin is undoubtedly the same as that of other celestial bodies " (' Sidereal Messenger,' June 1890, p. 284). 350 NOTES AND ADDITIONS. P. 267. On May 2, 1890, a brilliant fireball, leaving a long train of fire and smoke, and exploding with a noise like thunder, was seen at many places in Northern Iowa, Minne- sota, U.S.A. Some fragments of the meteor fell on a farm a few miles from the south line of Minnesota. The largest piece was sold by auction for $100, but it soon transpired that the person who sold it was only the lessee and not the owner of the ground on which the meteor fell. The aerial visitor and its purchase-money were therefore peremptorily seized by legal authorities, pending the decision of a Court of Justice as to the rightful ownership. P. 267. On December 14, 1890, at 9 h 42 m a large fireball of dazzling lustre, and giving a report like thunder, was widely observed in the southern parts of England. At the end-point the fireball appears to have been only 8 miles in height, and over a point near Brentwood, in Essex. THE STARS. P. 309. Prof. Chandler, of Cambridge, Mass., estimates that the total number of variable stars visible with a common field-glass is about 2000, but with a large telescope there are probably hundreds of thousands within reach. He further states that quite five sixths of the variable stars are reddish in colour, and that the redness is usually a function of the length of the period of variation. The redder the star the longer its period. P. 312. In a recent communication to the Academy of Sciences, M. Lescarbault (the alleged discoverer of Vulcan in 1859) announced that on the night of January 11, 1891, he discovered a bright body in Leo which he could not identify in any star-map, and hence concluded it to be a new star, or one suddenly increased in brilliancy. The " new star,'* however, subsequently turned out to be the planet Saturn ! This ridiculous mistake (so easily avoidable with a little care) will naturally divest the supposed discovery of Vulcan of the importance attached to it by some writers, for M. Lescar- bault obviously lacks the experience and caution necessary to command credit. NOTES AND ADDITIONS. 351 AND CLUSTERS OF STARS. P. 327. Mr. Roberts, from a comparison of his photo- graphs, has found distinct evidence of variability in the nucleus of the great nebula in Andromeda. In some of the photographs the nucleus is shown to be stellar, while in others there is no trace of this. Mr. Roberts remarks : " We may reasonably infer that the nucleus of the nebula is variable, and that it will be practicable to study the character of the variability without the necessity of giving long exposures of the plates." The period of the variation has now to be deter- mined, and it is advisable that telescopic observations of the nucleus should be made with the view of confirming the pho- tographic results. It would be premature to regard the changes as demonstrated before they have been submitted to thorough investigation. P. 327. In the Comptes Rendus for March 2, 1891, M. Bigourdan has a paper on the variability of the nebula N.G.C. 1186, situated near Algol. This nebula was dis- covered by Sir W. Herschel in 1785, and though Sir J. Herschel re-observed it in 1831, Lord Rosse looked for it without success in 1854 and 1864. On Nov. 8, 1863, D'Arrest failed to detect the nebula, though he searched for it with assiduity at a time when the sky was very favourable. He was led to conclude that the object did not exist. M. Bigourdan finds that the nebula is again visible in the position indicated by the two Herschels, viz. R.A. 2 h 54 m 20 s , Dec. +42 10 7 , he having observed it on Jan. 31 and Feb. 26, 1891. It is difficult to believe that this object could have escaped the scrutiny of Lord Rosse and D'Arrest in 1854, 1863, and 1864 ; hence the variation is probably real. The nebula may be easily found, as it is very near the binary B.D. + 42 (1123 G.C.), the position of which for 1891 is R.A. 2 h 58 m 6 s , Pec. +42 29' (< Nature,' March 12, 1891). P. 329. While examining the Pleiades on the night of November 14, 1890, Mr. Barnard discovered a new and considerably bright, round, cometary nebula 36" S. and 9" following Merope. The reason why this nebula has not been detected by photography is because it is so near Merope that 352 NOTES AND ADDITIONS. the over-exposed light from the star obliterates it. But it is certainly very strange that the object alluded to has never been telescopically discovered before ; for the Pleiades have been scrutinized repeatedly with all sorts of telescopes, and particularly since Tempel announced his discovery of a large faint nebula involving Merope in 1859. Mr. Barnard says the new nebula is 30'' in diameter, and that it is visible in a 12-inch refractor when Merope is hidden with a wire. INDEX. Action in Sun-spots, Cyclonic, 108. Active volcanoes on the Moon, 120. Adams theoretically discovers Neptune, 222. Advantage of Equatoreals, 54. Aerolites, 264. Air and water on the Moon, Absence of, 115. Algol, 310. Alleged satellite of Venus, 152. Almanacks, 83. Alphabet, Greek, 287. Alpine Valley, 127. Altitudes of markings on Jupiter, 185. Amateur's first view of Mercury, 139. Ancient ideas concerning meteors, 260. Andromeda, Great Nebula in, 334. Andromedes, 276. Angles of Position, Measurement of, 291, 306. Announcement of a new comet, 244. Annual rate of cometary discoveries. 255. Antares, 309. Anthelme, Discoverer of a new star in 1670, 313. Apennines, 132. Aperture and Power required for Comet- seeking, 252. Apparitions, Meteoric, 261. Appearance of Comets, 228. of Mars, 155. Aquarids, 275. Archimedes, 127. Argelander's magnitudes of stars, 294. Aristarchus, 120. Ascertaining positions of Comets, 257. Aspect of the rings of Saturn, 204. Atmosphere of Jupiter, 177. of Mars, 161. of Mercury, 139. of Venus, 151. Atmospheric undulations, 29. Attractions of Telescopic work, 85. Auwers, Discoverer of a new star in 1860, 314. Bacon, Eoger, Early hints on refracted rays, 3. Barnard, His cometary discoveries, 255. observes Brooks's multiple Comet, 239. observes new stars in the Trape- zium, 319. observes a new nebula in the Pleiades, 351. Beauty and brilliancy of Venus, 145. Belts of Sun-spots, 104. on Jupiter, 172. on Saturn, 198. on Uranus, 218. Berth on's dynamometer, 50. Biela's Comet, 238. Bigourdan observes a variable Nebula, 351. Binary Stars, 300. Birmingham discovers a new star in 1866, 314. Bond, G. P., discovers Crape-ring of Saturn, 202. Brahe's, Tycho, new star of 1572, 312. Bright objects near the Sun, 107. Brightness and position of Jupiter, 170. Brooks on Comet-seeking, 253. on Occultation of Jupiter, 187. on Shower of telescopic Meteors, 272, 274. Brooks's double Comet of 1889, 239. Brorsen's Comet, 239. Browning and reflectiug-telescopes, 60. Brunowski discovers a new star, 31 3. Burnham, Discoverer of double Stars, 31, 320. discovers a group of 18 new ne- bulge, 341. , Measures of the companion to Sirius, 307. on the inutility of " stops," 58. Calver compares light of reflectors and refractors, 37. , Maker of glass specula, 16, 17. Canal-shaped markings on Mars, 159. Canis Majoris a, 307. 2 A 354 INDEX. Cassegrain's reflecting-lelescope, 10. Cassini, Diameter of his object-glasses, 9. discovers four satellites and the divided ring of Saturn, 9. , Observations of Jupiter, 172. , of Saturn, 198. , of Venus, 147. Celestial Globe, 63. Centauri a, Diameter and distance, 299. Ceres, 168. Chambers on Coloured Stars, 316. on the intra-Mercurial Planet, 348. Chandler on Variable Stars, 350. Changes, Lunar, 120. on Jupiter, 182. on Mars, 163. on Saturn, 206. Charts of Mars, 158. Cheapness of Telescopes, 57. Choice of Telescopes, 38. Clark, Alvan, & Sons, make large object- glasses, 18. , discovers the companion to Sirius, Cleaning lenses, 59. Clusters of Stars, 317. Coggia's Comet of 1874, 233. Colour of Jupiter, 171. of Mars, 155. of Saturn, 195. of Uranus, 217. Colouring of the eclipsed Moon, 119. Colours of Stars, 315. Coma Berenices, 317. COMETS AND COMET-SEEKING, 227. Ideas concerning Comets, 227. , Appearance of Comets, 228. Large number visible, 228. Nature of apparition, 229. Tenuity, 229. Differences of orbit, 230. Discoveries of Comets, 230. Large Comets, 231. Periodical Comets, 234. Halley's Comet, 236. Encke's Comet, 236. Biela's Comet, 238. Brooks's double Comet, 239. Brorseii's Comet, 239. Faye's Comet, 240. D'Arrest's Comet, 240. Pons-Wirmecke's Comet, 240. Tuttle's Comet, 241. Grouping of Periodical Comets, 241. Further Observations required, 243. Nomenclature of Comets, 246. Curiosities of Comets, 248. Naked-eye Comets, 248. Comet-seeking, 249. English weather and Comet-seek- ing, 251. COMETS AND COMET-SEEKING (cant.). Aperture and Power required, 252. Annual rate of discovery, 255. Telescopic Comets, 256. Ascertaining positions, 257. Dr. Doberck's hints, 258. Prizes for Discoveries, 258. Common, His large Reflectors, 15 ; Their performance, 28. Computation of a Meteor's real path, 278. Conjunctions, Planetary, 225. Constellation figures, The, 290. Cooke & Sons mount a 24'8-inch re- fractor, 18 ; Its barren record, 25. Copernicus, 127. Course of the Milky Way, 296. " Crab " Nebula, in Taurus (M. 1 ), 336. Crape-ring of Saturn, 202. Crucis K:, Cluster at, 318. Curiosities of Comets, 248. Cyclonic action in Sun-spots, 108. Cygnus, Nebulous streams in, 339. Dallmeyer on Dividing power, 293. D'Arrest's Comet, 240. Dawes's observations of Jupiter, 173. observations of Saturn's Crape- ring, 202. , On Dividing power, 292. Solar Eyepiece, 92. Definition in towns, 81. Deimos, Outer Satellite of Mars, 165. Democritus explains the Milky Way, 2. Dennett announces the Eed Spot on Jupiter, 173. Denning's Cornet, 243. Denza on the Meteors of Nov. 27, 272. Derham, his list of Nebulas, 327. Description of Nebulas and Clusters of Stars, 333. Determination of the Sun's rotation- period, 104. Detonating Fireballs, 267. Dewing of Mirrors, 62. Diffraction-rings, 293. Dimensions of Nebulae and Scar-clusters, 332. of Sun-spots, 94. Disappearance of Saturn's ring, 205. Discordant observations of Saturn, 204. Discoveries of Comets, 230. of Nebulas, 341. Discovery of Neptune, 221. of Planetoids, 167. of Uranus, 215. Distance of the stars, 299. Distinction between Nebulas and Star- clusters, 324. Distribution of Nebulas in R.A., 326. Disturbances, Recurrent solar, 110. Dividing power, 292. INDEX. 355 Divisions in outer ring of Saturn, 201. Doberck, Dr., On the Invention of the Telescope, 5. , On Comet-seeking, 258. Dollond patents his Achromatic Tele- scope, 12. , His object-glasses, 16. Donati's Comet of 1858, 233. Dorfel mountains, 131. Double Comets: Biela's, 238. Brooks's, 239. Nebulae, 332. Stars, 300, 302. Draco, planetary nebula in, 335. Drawing, 73. Drawings of Jupiter, 185. Dumb-bell Nebula (M. 27), 337. Duration of meteor-flights, 282. Duration of Silver-on-glass films, 60. Dynamometer, Berthon's, 50. Early observations of Jupiter, 172. of nebulae and star-clusters, 326. of Neptune, 222. of Saturn, 197. of the Sun, 88. of Uranus, 216. of Venus, 147. Earthshine on the Moon, 116. Eccentric position of Saturn's rings, 204. Eclipses of Jupiter's satellites, 189. of the Moon, 118. of the Sun, 97. Elger's lunar observations, 127, 131. Drawings of lunar objects, 129, 130, 132. Ellipse, 230. on Jupiter, Gledhill's, 173. Elliptical nebulae, 338. Elongations of Mercury, 138. of Venus, 145. Encke's Comet, 236. division in Saturn's ring, 202, 208. English weather and Comet-seeking, 251. Equatoreal spots on Jupiter, Bright, 175, 181. , Dark, 181. Equatoreals, Advantage of, 54. Exceptional position of Sun-spots, 111. Eyepiece, Field of, 50. Eyepieces, 46. , Single-lens, 47. Fabricius observes Sun-spots, 89. Faculae, Sudden outburst of, 108. Faint objects, Observation of, 72. Faintness of the markings on Venus, 150. Falls of stone and iron, 266. Faye's Comet, 240. Field of eyepiece, Diameter of, 50. Figures, The Constellation, 290. Fireball of Nov. 23, 1877, 267. Fireballs, 267. , Heights of, 268. First view of Mercury, Amateur's, 139. Formations, Lunar, 123. Foucault parabolizes and silvers glass Speculse, 15. Fracastor, His remarks on lenses in 1538, 4. Friendly Indulgences, 74. Future, Past and, 84. Future eclipses of the Moon, 118. of the Sun, 98. Galaxy, or Milky Way, The, 295. Galilei and the invention of the Tele- scope, 2, 4, 5. , Discovery of Jupiter's satellites, 187. , His first instrument and disco- veries, 6, 7. Galle observes Saturn's crape-ring, 202. observes Neptune, 222. Geminids, 276. Glass, Opera, 61. Gledhill's ellipse on Jupiter, 173. Globe, Celestial, 63. Globular clusters, 338. , List of, 344. Gore, Diameter of a Centauri, 299. , Dimensions of a Star-cluster, 332. , Stellar distribution, 294. Greek alphabet, 287. Gregory invents a reflecting-Telescope, 10. Grimaldi, 129. Grouping of periodical Comets, 241. Groups of Stars, 316. Grubb, Maker of a 4- foot Cassegrainian reflector, 14 ; Performance of, 25. , Maker of a 27 -inch refractor, 18 ; Performance of, 27. Hall, Chester More, invents achromatic Object-glass, 11. Hall, Prof., discovers a white spot on Saturn, 199. , Observations of Saturn's satellites, 213. on the great Washington refractor, 26. , Eemarks on large and small tele- scopes, 31. Halley's Comet, 236. list of Nebulae in 1716, 326. Harriott, Early observer of Sun-spots, 89,90. Hartwig, Discovers a new Star in An- dromeda, 315. 3o6 INDEX. Heights of FirebaUs, 268 ; of Meteors, 277. Heis, His labours in Meteoric astro- nomy, 262. Hencke, Discoverer of Planetoids, 167. Henry, Bros., make a 30-inch refractor, 18 ; Performance of, 27. observe the belts on Uranus, 21 8. Herschel, Prof. A. S., observes meteors, 262. Herschel, Sir J., Comet of 1861, 233. , Description of K Crucis, 318. , Disappearance of Saturn's ring, 205. , Eediscovers Uranus, 219. , Satellites of Uranus, 220. , Texture of Comets, 229. , Thickness of Saturn's ring, 205. , Trapezium of Orion, 318, 320. Herschel, Sir W., and Cometary dis- covery, 231 . , His discovery of Nebulas, 327. , His discovery of nebulous Stars, 330. , His discovery of Uranus, 215 ; of Satellites, 220. , His method of observing Sun- spots, 91. , His observations of Jupiter, 1 82. r, His observations of Mercury, 142. , His observations of Saturn, 199. -, His observations of Venus, 149. , Nucleus of Comet of 1811, 232. observes Binary Stars, 300, 306. , Performance of 4-ft. reflector, 21. , Remarks on eyepieces, 47. , Rotation of Jupiter's Satellites, 189. , Singular figure of Saturn, 196. Hevschel's, Sir W., Telescopes, 12, 13, 39. Hevelius, diameter of his object-glasses, 9. Hind, Discoverer of a new Star in 1848, 314. , Discoverer of Planetoids, 167. , Discoverer of a variable Nebula, 328. Hipparchus forms a Star-catalogue, 312. Hoek on the origin of Comets, 243. Hooke's observations of Jupiter, 172. Hough's observations of Jupiter, 174, 182. Hewlett's observations of Sun-spots, 101, 102. Huygens on the invention of the Tele- scope, 2. , Discoveries on Saturn, 8, 198. , Length of his instruments, 8. Huygens's Negative eyepiece, 8, 46. Hyginus, The rill or cleft of, 130. Hyperbola, 230. Identity of Meteors and Comets, 262. Increasing number of Telescopes, 57. Intra-Mercurial Planet, 137. Jansen, Zachariah, Inventor of the Tele- scope, 4. Johnson's projections of Solar Eclipses, yy Juno, 168. JUPITER, 170. An interesting object, 170. Brightness and position, 170. Period &c., 171. Belts and spots, 172. Observations of Hooke, Cassini, and others, 172. The " Ellipse " of 1869-70," 173. The red spot, 173. Rotation of red spot, 175. Rotation of bright equatoreal spots, 175. Rotation of dark spots in N. hemi- sphere, 175. Rotation-period, 176. Nature of the red spot, 177. Bright equatoreal spots, 181. Dark equatoreal spots, 181. New belts, 182. Changes on the planet, 182. Further observations required, 183. Occultations by the Moon, 185. The four satellites, 187. Their eclipses, occultations, and transits, 189. The planet without visible satellites, 192. Spots on the Satellites, 193. Occultation of a Star by Jupiter, 193. Keeler, White spots and canals on Mars, 348. Kitchiner, The inutility of large Tele- scopes, 35. , Singular form of Saturn, 196. Klein's supposed new crater near Hygi- nus, 122. Large and small telescopes compared, 20, Comets, 231. number of Comets, 228. refractor intended for California, 36, 347. Lassell, His large reflecting-telescopes, 14 ; Their performance, 24. discovers the satellite of Neptune, 224. glimpses a belt on Uranus, 217. Leander McCormick refractor, 26. Learning the names of the Stars, 287. Leibnitz mountains, 131. INDEX. 357 Le Mairean or Herschelian telescope, 13. Lenses, Cleaning, 59. out of centre, 55. Leonids, 276. Lescarbault rediscovers Saturn, 350. Le Verrier, Theoretical discoverer of Neptune, 222. Lick, James, Founder of the Lick Ob- servatory, 18. Lick refractor, Performance of the, 27. Light of Comets, Fluctuating, 245. Limited means no obstacle, 51. Lippersheim, Hans, Inventor of the telescope, 4, 5. Lunar changes, 120. formations, 123. Lyrae a, 308. Lyrids, 275. Madler's observations of Lunar objects, 127, 131. of Mars, 158. of Yenus, 149. Magellanic clouds, 331. Magnitudes of Stars, 294. Marius, Simon, observes Jupiter's satel- lites, 6. observes Nebula in Andromeda, 326. Markings on Mercury, Surface-, 142. on Venus, 147. , Faintness of, 150. MARS, Appearance of, 155. Period &c., 155. Phase, 156. Surface-configuration, 156. Charts and nomenclature, 158. Discovery of satellites and canal- shaped markings, 159. Summary of observations, 160. Eotation, 161. Further observations required, 162. Changes on the Planet, 163. Satellites, 164. Occupations by the Moon, 166. Martin's 4-foot reflector at Paris, 15 ; Its performance, 25. 29-inch refractor at Paris, 18. Maunder on Sun-spots, 93. Maxima and minima of Sun-spots, 100. Means of measurement, 290. MERCURY, 137. Supposed planet Vulcan, 137. Visibility, 138. Period &c., 138. Elongations, 138. Amateur's first view, 139. Phases, 139. Atmosphere, 139. Telescopic observations, 140. MERCURY (cont.'). Schiaparelli's results, 141. Observations of Schroter and W. Herschel, 142. Surface-markings, 142. Transits across the Sun, 143. Occultations, 144. Messier, The Comet-hunter, 249. Messier's lists of Nebulae, 327. large Comet of 1769, 232. METEORS AND METEORIC OBSERVATIONS, 260. Ancient ideas, 260. Meteoric apparitions, 261. Eadiation of Meteors, 262. Identity of Meteors and Comets, 262. Aerolites, 264. Fireballs, 267. Heights of Fireballs, 268. Meteorite from Biela's Comet, 270. Differences of motion, 271. Nomenclature of Meteor-systems, 271. Meteor- storms, 271. Telescopic Meteors, 272. Meteor-showers, 274. Varieties of Meteors, 276. Meteor of Dec. 28, 1888, 277. Average heights of Meteors, 277. Computation of Meteor-heights, 278. Meteoric observations, 279. Meteors and terrestrial objects, 284. and gales of wind, 285. Method, 78. Milky Way or Galaxy, 2, 295. Minimum of Sunspots, 347. Mirrors, Dewing of, 62. MOON, Attractive aspect of the, 113. Diameter and distance, 114. Crateriform aspect, 114. Absence of air and water, 115. Only one hemisphere visible, 115. Earthshine, 116. Telescopic observations, 116. Eclipses, 118. Physical changes, 120. Active volcanoes, 120. Crater Aristarchus, 120. Linne, 121. near Hyginus, 122. General description of formations, 123. Description of special objects, 125- 132. Objects near terminator, 133. Occupation of Stars, 135. Visibility of new and old Moon, 136. Moonlight and planetary definition, 187. Motion of light, 190. 358 INDEX. Motion of Stars in the line of sight, 300. of Sun-spots, Proper, 106. Mounting of Telescopes, 45. Naked-eye views of Comets, 248. of Jupiter in daylight, 170. of Jupiter's satellites, 188. of Mercury, 139. of Sun-spots, 89. of Uranus, 217. of Venus in transit, 105. of Vesta, 168. Names of the Stars, Learning the, 287. Nasmyth and Carpenter describe Plato, 126. Nasmvth's Telescopes, 16. " Willow-leaves," 101. Nature of Cometary apparitions, 229. of the red spot on Jupiter, 177. NEBULAE AND CLUSTERS OP STARS, 324. Distinction, 324. Large number visible, 324. Varieties of form and grouping, 325. Distribution in E.A., 326. Early observations, 326. Variable Nebulas, 327. Nebulous Stars, 330. The Magellanic Clouds, 331. Double Nebula, 332. Beal dimensions of Nebulas and Clusters, 332. Bound Nebulas and Clusters, 332. Description of Nebulas and Clusters, 333. Great Nebula in Andromeda, 334. in Orion, 334. Planetary Nebulas, 334. Spiral Nebula, 335. Crab Nebula in Taurus, 336. Dumb-bell Nebula, 337. King Nebula in Lyra, 337. Elliptical Nebulas, 338. Globular Clusters, 338. Further observations, 339. Discovery of new Nebulas, 341. New Nebulas discovered at Bristol, 342. List of Clusters of Stars, 343. of globular Clusters, 344. of Nebulas, 345. Nebulous Stars, 330. Neison, Lunar observations, 128, 129. NEPTUNE, Discovery of, 221. Observations in 1795, 222. Period &c., 223. Observations, 223. Supposed ring, 223. The satellite, 223. New or temporary Stars, 312. Newton, 128. Newton, Sir Isaac, Experiments on Colours, 9. , His reflecting-telescope, 11. , On mountainous sites for scopes, 347. Noble, Occultation of Jupiter, 186. , Occultation of Saturn, 210. tele- on observations of Mercury and Venus, 348. Nomenclature of Comets, 246. of Lunar formations, 123. of Mars, 158. of Meteor-systems, 271. Number of Comets visible, Large, 228. of Nebulas and Star-clusters, 324. of Planetoids, 167. of Stars, 293. Observations of Neptune, 223. required of the Sun, 97. of the Moon, 116. of Mercury, 143. of Venus, 152. of Mars, 162. of Jupiter. 183. of Saturn, 205. of Uranus, 219. of Comets, 243. of Meteors, 279. of Stars, 320. of Nebulas, 339. , Solar, 88. Observatories, 64. Observer's aims, 42. Observing, Open-air, 75. Observing-seats, 53. Occultations of Jupiter, 185. of Jupiter's satellites, 189, 190. of Mars, 166. of Mercury, 144. of Begulus by Venus, 154. of Saturn, 209. of Star by Jupiter, 193. of Venus, 153. of Vesta, 169. Olbers discovers Pallas and Vesta, 167. , His Comet of 1815, 235, 241. , Observer of Comets, 250. Open-air observing, 75. Opera-glass, 61. Orbits of Comets, Differences in, 230. Orion, Great Nebula in, 334. , The constellation, 289. Orionids, 275. Orionis (3, 307. 9, 318. a, 318. Outbursts of Faculas, 108. Palisa, Discoverer of Planetoids, 167. Palitzch, Discoverer of Halley's Comet, 236. INDEX. 359 Pallas, 168. Parabola, 230. Past and future work, 84. Periodical Comets, 234. , Grouping of, 241. Periodicity of Jupiter's markings, 184. of Sunspots, 100. Perrotin observes the belts on Uranus, 218. the canals on Mars, 27, 160. on work with a 30-inch refractor, 347. Perry on drawing Sun-spots, 93. observes veiled Sun-spots, 110. Persei /3 (Algol), 310. - X, 317. Perseids, 275. . Their shifting radiant-point, 283. Perseverance, 79. Petavius, 128. Peters, Discoverer of Planetoids, 167. Phasej Epochs of similar, 117. of Jupiter, 172. - of Mars, 156. Phases of Mercury, 139. of Venus, 147. Phobos, Inner Satellite of Mars, 165. Photography, 82. Photometric measures of Starlight, 295. Physical aspects of Comets, 244. changes on the Moon, 120. Pickering on the canals of Mars, 348. Planetary bodies on the Sun, 105. conjunctions, 225. Nebula, 334. Planetoid, The 308th, 349. PLANETOIDS, Number of. 167. History of their discovery, 167. Occupation of Vesta, 167. Dimensions and brightness, 168. Plato, 125. Polaris, 308. Pons, Discoverer of many Comets, 250. Pons's Comet of 1812, 241, 242, 245. Pons-Winnecke's Comet, 240. Powers, Method of determining, 49. , Overstating, 49. , Eequisite magnifying, 48. Praesepe, 317. Preparation of the observer, 66. Princeton refractor, Performance of, 26. Prizes for Cometary discoveries, 258. Proctor on Amateur observers, 163. on Sun-ejected Meteors, 349. Projection of satellites of Jupiter, 190. of Stars on the Moon, 135. Prominences, Solar, 111. Proper motion of spots on Jupiter, 173. of Stars, 299. of Sun-spots, 106. Publications, Astronomical, 83. Pulkowa, The 30-inch refractor at, 27. Quadrantids, 274. Eadiation of Meteors, 262. Ramsden's positive eyepiece, 47. Ranyard, Absorption of light by object- glasses, 37. Recording Meteor-tracks, 280. Records, 72. Recurrent disturbances on the Sun, 110. forms on the Sun, 111. Red spot on Jupiter, Appearance of, 173; Rotation of, 175; Nature of, 177. Refracting-lenses or burning-glasses, 3. Refracting-telescope, 12. Refractors and Reflectors, 39. Rheita, Valley near, 131. Rigel.. 307. Ring nebula in Lyra, 337. of Neptune, Supposed, 223. of Saturn, Division in the outer, 201. The Crape, 202. Rings of Saturn, 201. , Aspect of the, 204. , Eccentric position of the, 204. , Thickness, 205. Roberts's photographs of the Nebula in Andromeda, 334,351. of Nebulas in Ursa Major, 338. Rosse, Lord, Large reflecting-telescopes, 14 ; Their performance, 21. Rotation of Comets, Visible evidences of, 246. of Jupiter, 176, 348. of Mars, 161, 348. of Mercury, 142. of Saturn, 199. of the Sun, 103. of Uranus, 217. of Venus, 149. Round Nebulae and Clusters, 332. Safarik on Telescopic Meteors, 273. Saros, The, 99. Satellite of Neptune, 223. of Venus, Alleged, 152. Satellites of Jupiter, 187. of Mars, 164. of Saturn, 211. of Uranus, 220. SATURN, 195. Apparent lustre, 195. Period &c., 196. " Square-shouldered " aspect, 196. Early observations, 197. His belts and spots, 199. Rotation-period, 199. The Rings, 201. Divisions in outer ring, 201. 360 INDEX. SATURN (cont.\ Crape-ring, 202. Discordant observations, 204. Eccentric position of rings, 204. Aspect of the rings, 204. Further observations, 205. Occupations of Saturn, 209. The Satellites, 211. Transits of shadow of Titan, 213. Occultations of Stars by Saturn, 214. Scheiner's early observations of Sun- spots, 89. Schiaparelli, Observations of Mars, 159. , Observations of Mercury, 141. associates Comets and Meteors, 264. Schmidt announces change in a lunar crater, 121. , Discoverer of a new Star, 315. Schroter's observations of Mercury, 142. of Saturn, 200. . of Venus, 148. Scintillation of Stars, 297. Scorpii a, 309. Shadows cast by Faculae, 109. Short's reflectors, 12. Showers of Meteors, 274. Sidereal work, 286. Silver-on-glass films, Duration of, 60. Sirius, 300, 307. Small telescopes, 31. and Mars, 160. and Solar work, 90. Solar Eclipse of Aug. 19, 1887, 98. Eclipses visible in England, 98. observations, 88. Prominences, 111. Southern Comets, Large, 233, 234. Spiral Nebula, 335. Spitaler's Comet of 1890, 349. " Square-shouldered " aspect of Saturn, 196. Star-disks, 298. Stars, Nebulous, 330. , Occupation of, 135. visible through Comets, 246. STARS, THE, 286. Sidereal work, 286. Greek Alphabet, 287. Learning the names of the Stars, 287. The constellation Orion, 289. The constellation Figures, 290. Means of Measurement, 290. Dividing power, 292. Number of Stars, 293. Magnitudes, 294. The Milky Way, 295. Scintillation of" Stars, 297. Star-disks, 298. Distance of the Stars, 299. STARS, THE (cont.\ Proper motions of Stars, 299. Double Stars and binary systems, 300. List of Double Stars, 302-5. a Canis Majoris, 307. /3 Orionis, 307. a Lyrse, 308. a Ursse Minoris, 308. a Scorpii, 309. Variable Stars, 309. o Ceti and /3 Persei, 310. List of Variable Stars, 311. New or temporary Stars, 312. Description of temporary Stars, 312. Star-colours, 315. Groups of Stars, 316. Coma Berenices, 317. The Pleiades, 317. Praesepe, 317. X Persei, 317. K Crucis, 318. Ursae Majoris, 318. ff Orionis, 318. Orionis, 318. Further Observations, 320. " Stops," Utility of, 58. Storms, Meteor, 271. Straight Wall, 130, 131. Streak seen at Jask, Meteor-, 278. Structure of Sun-spots, Crateriform, 101. SUN, THE : Diameter and Distance, 87. Solar observations, 88. Spots on the Sun, 88. Early observations, 88. Small telescopes and solar work, 90. Tinted glass, 91. Solar diagonal, 92. Drawing Sun-spots, 93. Ascertaining dimensions, 94. Sun-spot of June 19, 1889, 95. Eclipses of the Sun, 97. Periodicity of Spots, 100. Crateriforra Structure, 101. " Willow Leaves," 101. Rotation of the Sun, 103. Determining the Period, 104. Planetary bodies in transit, 105. Proper motion of Sun-spots, 106. Rise and decay of spots, 106. Black nuclei in the Umbra 1 , 106. Bright objects near Sun, 107. Cyclonic Action, 108. Sudden outbursts of Faculrc, 108. Shadows cast by Faculaj, 109. Veiled Spots, 110. Recurrent disturbances, 110. Recurrent forms, 111. Exceptional position of Spots, 111. The Solar prominences, 111. INDEX: 361 Sun-spots, 88, 347. Superstitious ideas on Comets, 227. Surface configuration of Mars, 156. markings on Mercury, 142. on Venus, 147. Sweeping for Comets, 249. for Nebulas, 340. Swift, Discoverer of Comets, 252. , of Nebula, 339. Tails of Comets, 244. Tarrant on Double Stars, 291. Telescope, Invention and Development, Telescopes, Cheapness and increasing number of, 57. , Choice of, 38. , Large and small, 20. , Mounting of, 45. , Testing, 43. Telescopic Comets, 256. Meteors, 272. - Work, Attractions of, 85. Tempers Comets, 241. Nebula in the Pleiades, 329. observation of Aristarchus, 120. Temporary Stars, 312. Tenuity of Comets, 229. Terby's White Spot on Saturn's rings, 203. Terminator, Moon's age and Objects near, 133. Testing Telescopes, 43. Test-objects, 55. Theophilus, 128. Thornthwaite, Method of Solar observa- tion, 92. Tinted glass for Solar observation, 91. Titan, Transit of, 213. Total Eclipses of the Moon, 118. - of the Sun, 99. Transits of Intra-Mercurial Planets, 105, 137. of Jupiter's satellites and their shadows, 189, 191. of Mercury, 143. of Venus, 153. Trans-Neptunian planet, 224. Tupman, Method of tabulating Meteors, 82. -, Eemarks on a Fireball, 267. Tuttle's Comet, 241. Twilight on Venus, 151. Twinkling of the Stars, 297. Tycho, 127. URANUS, Discovery, 215. Mistaken for a Comet, 215. True character revealed, 216. Period &c., 217. Observations, 217. URANUS (cont.) His belts, 218. Further observations, 219. The satellites, 220. Ursas Majoris , 318. Ursa3 Minoris a, 308. Utility of " stops," 58. Variable Nebulae, 327, 351. Stars, 309. , List of, 311. , Observations of, 321. Variations in the light of Comets, 245. Varieties of form and grouping in Nebulas and Star-clusters, 325. of Meteors, 276. Vega, 301, 308. Veiled Sunspots, 110. VENUS, Beauty and brilliancy of, 145. Period &c., 146. As a telescopic object, 146. Surface-markings, 147. Rotation-period, 149. Faintness of the markings, 150. Twilight, 151. Alleged satellite, 152. Further observations, 152. Transits, 153. Occultations, 153. Vesta, 168 ; Occultation of, 169. Visibility of Mercury, 138. Vision, 70. Vulcan, Supposed planet, 137, 348. Wargentin describes a Lunar Eclipse, 119. Warner's Comet-Prizes, 259. Washington Kefractor, The great, 25, 36. Weather and Comet-seeking, English, 251. Webb, Lunar observations, 127, 130, 131. , Markings on Mercury, 143. Williams, Observations of canals of Mars, 160. , of Jupiter, 175, 177. , of Jupiter's satellites, 191. of Plato, 126. " Willow-Leaves," The Solar, 101. Wind, Its influence on definition, 69. With of Hereford, Maker of glass spe- cula, 15. Wolf on large and small Telescopes, 25, 35. Working-lists, 68. Young, Performance of 23-inch re- fractor, 26. observes belts on Uranus, 217. on the successes of small instru- ments, 34. PRINTED BY TAYLOR AND FRANCIS, RED LION COURT, FLEET STREET. * t * # UNIVERSITY OF CALIFORNIA LIBRARY BERKELEY Return to desk from which borrowed. This book is DUE on the last date stamped below. ASTRONOMY LIBRARY OCT 311952 JAN 4 1954 MAY 2 7 1969 LD 21-100m-ll,'49(B7146sl6)476