STARGAZING: PAST AND PRESENT. L I 15 I? A ii - UN I V KK.s i'i-'i' op ;4 *" R.S. NEWALLS TELESCOPE STAEGAZING PAST AND PRESENT. BY J. NOEMAN LOCKYEE, E.E.S., *\ CORRESPONDENT OF THE INSTITUTE OF FRANCE. EXPANDED FROM SHORTHAND NOTES OF A COURSE OF ROYAL INSTITUTION LECTURES, WITH THE ASSISTANCE OF G. M. SEABROKE, F.R.A.S. LIBRA K i MACMILLAN AND CO. 1878. [The Rigid of Translation and Reproduction is Reserved,] f, ON DON : H. Of. AY, SONS, AND TAYfOR, BREAD STREET HILf., K.C. PREFACE. IN the year 1870 I gave a course of eight Lectures on Instrumental Astronomy at the Royal Institution. The Lectures were taken down by a shorthand writer, my intention being to publish them imme- diately. In this, however, I was prevented by other calls upon my time. In 1875 my friend Mr. Seabroke generously offered to take the burden of preparing the notes for the press off my shoulders ; I avail myself of this opportunity of expressing my very great obligations to him for his valuable services in this particular as well as for important help in the final revision of the proofs. On looking over the so completed MSS., however, I saw that the eight hours at my disposal had not permitted me to touch upon many points of interest which could hardly be omitted from the book. Besides this, the progress made in the various instrumental methods in the interval, and the results obtained by them, had been very remarkable. I felt, therefore, that the object I had in view, namely, to further the cause of physical astronomy, vi PREFACE. by creating and fostering, so far as in me lay, a general interest in it, and by showing how all de- partments of physical inquiry were gradually being utilized by the astronomer, would only be half attained unless the account were more complete. I have, therefore, endeavoured to fill up the gaps, and have referred briefly to the new instruments and methods. The autotype of the twenty-five inch refractor is the gift of my friend Mr. Newall, and I take this opportunity of expressing my obligation to him, as also to Messrs. Cooke, Grubb and Browning for several of the woodcuts with which the chapters on the Equatorial are illustrated; and to Mr. H. Dent- Gardner for some of those illustrating Clock and Chronometer Escapements, and for revising my account of them. Nor can I omit to thank Mr. Cooper for the pains he has taken with the woodcuts, especially those copied from Tycho Brahe's description of his Observatory, and Messrs. Clay for the careful manner in which they have printed the book. J. NORMAN LOCKYEE. November \Qth, 1877. CONTENTS. BOOK I. THE PRE-TELESCOPIC AGE. CHAP. PAGE I. THE DAWN OF STARGAZING .......... 1 II. THE FIRST INSTRUMENTS ........... 16 III. HlPPARCHUS AND PTOLEMY ........... 25 IV. TYCHO BRAKE ...... . ........ 37 BOOK IT. THE TELESCOPE. V. THE REFRACTION OF LIGHT .......... 55 VI. THE REFRACTOR .............. 73 VII. THE REFLECTION OF LIGHT .......... 90 VIII. THE REFLECTOR ............... 1 IX. EYEPIECES ................. 109 X. PRODUCTION OF LENSES AND SPECULA ....... 117 XI. THE " OPTICK TUBE " ............. 139 XII. THE MODERN TELESCOPE ... ...... 152 viii CONTENTS. BOOK III. TIME AND SPACE MEASURERS. CHAP. PAGE XIII. THE CLOCK AND CHRONOMETER 175 XIV. CIRCLE READING 211 XV.-THE MICROMETER . . 218 BOOK IV. MODERN MERIDIONAL OBSERVATIONS. XVI. THE TRANSIT CIRCLE 233 XVII. THE TRANSIT CLOCK AND CHRONOGRAPH 253 XVIII. " GREENWICH TIME," AND THE USE MADE OF IT . . . 271 XIX. OTHER INSTRUMENTS USED IN ASTRONOMY OF PRECISION 284 BOOK V. THE EQUATORIAL. XX. VARIOUS METHODS OF MOUNTING LARGE TELESCOPES . 293 XXL THE ADJUSTMENTS OF THE EQUATORIAL ...... 328 XXII. THE EQUATORIAL OBSERVATORY 337 XXIII. THE SIDEROSTAT 343 XXIV. THE ORDINARY WORK or THE EQUATORIAL 349 CONTENTS. i x BOOK VI. ASTRONOMICAL PHYSICS. CHAP. PACK XXV. THE GENERAL FIELD OF PHYSICAL INQUIRY .... 371 XXVI. DETERMINATION OF THE LIGHT AND HEAT OF THE STARS 377 XXVII. THE CHEMISTRY OF THE STARS : CONSTRUCTION OF THE SPECTROSCOPE 386 XXVIII. THE CHEMISTRY OF THE STARS (CONTINUED) : PRINCIPLES OF SPECTRUM ANALYSIS 401 XXIX. THE CHEMISTRY OF THE STARS (CONTINUED) : THE TELESPECTROSCOPE 422 XXX. THE TELEPOLARISCOPE 441 XXXI. CELESTIAL PHOTOGRAPHY. THE WAYS AND MEANS . . 454 XXXII, CELESTIAL PHOTOGRAPHY (CONTINUED) : SOME EESULTS 463 XXXIII. CELESTIAL PHOTOGRAPHY (CONTINUED) : KFCENT KESULTS 469 LIST OF ILLUSTRATIONS. F1O. PAGE 1. The heavens according to Ptolemy 3 2. The zodiac of Denderah ... 7 3. Illustration of Euclid's state- ments 10 4. The plane of the ecliptic ... 13 5. The plane of the ecliptic, show- ing the inclination of the earth's axis 14 6. The first meridian circle ... 20 7. The first instrument graduated into 360 (west side) . . . _ . 21 8. Astrolabe (armillse sequatorise of Tycho Brahe) similar to the one contrived by Hippar- chus 26 9. Ecliptic astrolabe (the armillse zodiacales of Tycho Brahe), similar to the one used by Hipparchus ..'.... 28 10. Diagram illustrating the preces- sion of the equinoxes . . .31 11. Eevolution of the pole of the equator round the pole of the ecliptic caused by the preces- sion of the equinoxes ... 32 12. The vernal equinox among the constellations, B.C. 2170 . . 34 13. Showing how the vernal equinox has now passed from Taurus and Aries . 34 14. Instrument for measuring alti- tudes 35 15. Portrait of Tycho Brahe (from original painting in the posses- sion of Dr. Crompton, of Man- chester) 39 16. Tycho Brahe's observatory on the island of Huen 43 17. Tycho Brahe's system .... 46 18. The quadrans maximus repro- duced from Tycho's plate . . 48 19. Tycho's sextant 50 20. Yiew and section of a prism . . 56 FIG. PAGK 21. Deviation of light in passing at various incidences through prisms of various angles . . 57 22. Convergence of light by two prisms base to base .... 59 23. Formation of a lens from sections of prisms 60 24. Front view and section of a double convex lens .... 61 25. Double concave, plane concave, and concavo-convex lenses . 61 26. Double convex, plane convex, and concavo-convex lenses . 62 27. Convergence of rays by convex lens to principal focus ... 62 28. Conjugate foci of convex lens . 63 29. Conjugate images 64 30. Diagram explaining Fig. 29 . . 64 31. Dispersion of rays by a double concave lens 65 Horizontal section of the eye- ball 66 Action of eye in formation of images 68 34. Action of a long-sighted eye . . 69 35. Diagram showing path of rays when viewing an object at an easy distance 70 36. Action of short-sighted eye . . 71 37. Galilean telescope 73 38. Telescope .75 39. Diagram explaining the magni- fying power of object-glass . 76 40. Schemer's telescope 78 41. Dispersion of light by prism . . 80 42. Diagram showing the amount of colour produced by a lens . . 81 43. Decomposition and recomposi- tion of light by two prisms . 83 44. Diagram explaining the forma- tion of an achromatic lens . 84 45. Combination of flint- and crown- glass lenses in an achromatic lens .... 86 Xll LIST OF ILLUSTRATIONS. FIG. PAGE 46. Diagram illustrating the irration- ality of the spectrum ... 87 47. Diagram illustrating the action of a reflecting surface . . .91 48. Experimental proof that the angle of incidence = angle of reflection 92 49. Convergence of light by concave mirror 94 50. Conjugate foci of convex mirror 94 51. Formation of image of candle by reflection 95 52. Diagram explaining Fig. 51 . . 96 53. Eeflection of rays by convex mirror 98 54. Eeflecting telescope (Gregorian) 101 55. Newton's telescope J 02 56. Eeflecting telescope (Cassegrain) 103 57. Front view telescope (Herschel) 103 58. Diagram illustrating spherical aberration 105 59. Diagram showing the proper form of reflector to be an ellipse 106 60. Huyghens' eyepiece .... 110 61. Diagram explaining the achro- maticity of the Huyghenian eyepiece Ill 62. Eamsden's eyepiece 112 63. Erecting or day eyepiece . . .113 64. Images of planet produced by short and long focus lenses, &c 123 65. Showing in an exaggerated form how the edge of the speculum is worn down by polishing . 128 65*. Section of Lord Eosse's polish- ing machine 131 66. Mr. Lassell's polishing machine . 132 67. Simple telescope tube, showing arrangement of object-glass and eyepiece 140 68. Appearance of diffraction rings round a star when the object- glass is properly adjusted . . 141 69. Appearance of same object when object-glass is out of adjust- ment 141 70. Optical part of a Newtonian reflector of ten inches aper- ture 143 71. Optical part of a Melbourne re- flector 143 72. Mr. Browning's method of sup- porting small specula . . .144 73. Support of the mirror when ver- tical : . 146 74. Division of the speculum into equal areas 147 75. Primary, secondary, and tertiary systems of levers shown separately 148 76. Complete system consolidated into three screws . 148 FIG. PACK 77. Support of diagonal plane mirror (Front view) . . . .150 78. Support of diagonal plane mirror (Side view) .... 150 79. A portion of the constellation Gemini seen with the naked eye 154 80. The same region, as seen through a large telescope . . 155 81. Orion and the neighbouring constellations 156 82. Nebula of Orion 157 83. Saturn and his moons .... 160 84. Details of the ring of Saturn . 161 85. Ancient clock escapement . .177 86. The crown wheel 178 87. The clock train 1 80 88. Winding arrangements . . .181 89. The cycloidal pendulum . . .185 90. Graham's, Harrison's, and Greenwich pendulums . . . 188 91. Greenwich clock : arrangement for compensation for baro- metric pressure 194 92. The anchor escapement . . .197 93. Graham's dead beat . . . .199 94. Gravity escapement (Mudge) . 200 95. Gravity escapement (Bloxam) . 202 96. Greenwich clock escapement . 204 97. Compensating balance . . . 207 98. Detached lever escapement . . 208 99. Chronometer escapement . . 209 100. The fusee 209 101. Diggs' diagonal scale . . . .213 102. The vernier 214 103. System of wires in a transit eyepiece 220 104. Wire micrometer 221 105. Images of Jupiter 224 106. Object-glass cut into two parts 225 107. The parts separated, and giving two images of any object . . 225 108. Double images seen through Iceland spar 227 109. Diagram showing the ordinary and extraordinary rays in a crystal of Iceland spar . . . 227 110. Crystals of Iceland spar . . .228 111. Double image micrometer . . 229 112. Tycho Brahe's mural quadrant 235 113. Transit instrument (Transit of Venus Expedition) .... 236 114. Transit instrument in a fixed observatory . , 237 115. Diagram explaining third ad- justment 239 116. The mural circle 241 117. Transit circle, showing the addi- tion of circles to the transit instrument 242 118. Perspective view of Greenwich transit circle 243 119. Plan of the Greenwich transit circle 245 LIST OF ILLUSTRATIONS. MO. PAGE 120. Cambridge (U.S.) meridian circle . . 248 121. Diagram illustrating how the pole is found 249 122. Diagram illustrating the differ- ent lengths of solar and sidereal day . 255 123. System of wires in transit eye- piece 257 124. The Greenwich chronograph. (General view) 261 125. Details of the travelling car- riage which carries the mag- nets and prickers. (Side view and view from above) . . . 262 126. Showing how on the passage of a current round the soft iron the pricker is made to make a mark on the spiral line on the cylinder 263 127. Side view of the carriage carry- ing the magnets and the pointer that draws the spiral 263 128. Wheel of the sidereal clock, and arrangement for making contact at each second . . . 266 129. Arrangement for correcting mean solar time clock at Greenwich 268 130. The chronopher 276 131. Keflex zenith tube 286 132. Theodolite 288 133. Portable alt-azimuth .... 289 134. The 40-feet at Slough . . .294 135. Lord Rosse's 6-feet 295 136. Eefractor mounted on alt-azi- muth tripod for ordinary star- gazing 296 137. Simple equatorial mounting . 298 138. Cooke's form for refractors . . 300 139. Mr. Grubb's form applied to a Cassegrain reflector .... 301 140. Grubb's form for Newtonians . 303 141. Browning's mounting for New- tonians 304 142. The Washington great equato- rial 309 143. General view of the Melbourne reflector . 312 144. The mounting of the Melbourne telescope 313 145. Great silver-on-glass reflector at the Paris observatory . . .316 1 46. Clock governor 319 147. Bond's spring governor . . . 320 148. Foucault's governor .... 323 149. Illuminating lamp for equa- torial 150. Cooke's illuminating lamp . . 151. Dome 152. Drum ......... 153. New Cincinnati observatory (Font elevation) 154. Cambridge (U.S.) equatorial . *IG. 155. 156. 157. 158. 159. 160. 161. 162. 163. 164. 165. 166. 167. 168. 170. 171. 172. 173. 174. 175. 176, 177, 178, 179, 180, 181. 182. 183. 184. 185. 186. 187. 325 326 OOQ ooo 338 188. 189. 338 190. 339 | 191. Section of main building United States naval observa- tory 341 Foucault's siderostat . . . 344 The siderostat at Lord Lindsay's observatory 348 Position circle 353 How the length of a shadow thrown by a lunar hill is mea- sured 354 The determination of the angle of position of the axis of Saturn's ring 358 Measurement of the angle of position of the axis of a figure of a comet 359 Double star measurement . . 360 Ring micrometer 368 Thermopile and galvanometer . 374 Rumford's photometer . . . 378 Bouguer's photometer . . . 379 Kepler's diagram 387 Newton's experiment, showing the different refrangibilities of colours 388 First observation of the lines in the solar spectrum . . . 391 Solar spectrum 392 Student's spectroscope . . . 393 Section of spectroscope . . 394 Spectroscope with four prisms . 396 Automatic spectroscope (Grubb's form) 397 Automatic spectroscope (Brown- ing's form) 397 Last prism of train for return- ing the rays 398 Spectroscope with returning beam 399 Direct-vision prism .... 399 Electric lamp 404 Electric lamp arranged for throwing a spectrum on a screen 405 Comparison of the line spectra of iron, calcium, and alumi- nium, with common impurities 406 Coloured flame of salts in the flame of a Bunsen's burner . 408 Spectroscope arranged for show- ing absorption 409 Geissler's tube 413 Spectrum of sun-spot . . . .415 Diagram explaining long and short lines 416 Comparison of the absorption spectrum of the sun with the radiation spectra of iron and calcium, with common im- purities 418 Comparison prism 423 Comparison prism 423 Foucault's heliostat .... 424 Object-glass prism 426 XIV LIST OF ILLUSTRATIONS. FIG. PAGE 192. The eyepiece end of the Newall refractor 427 193. Solar telespectroscope (Brown- ing's form) 428 194. Solar telespectroscope (Grubb's form) 428 195. Side view of spectroscope . . 429 196. Plan of spectroscope .... 429 197. Cambridge star spectroscope elevation 430 198. Cambridge spectroscope plan . 430 199. Direct-vision star spectroscope (Secchi) 431 200. Types of stellar spectra . . . 433 201. Part of solar spectrum near F . 436 202. Distortions of F line on sun 438 203. Displacement of F line on edge of sun 439 204. Diagram showing the path of the ordinary and extraordi- nary ray in crystals of Ice- land spar 445 205. Appearance of the spots of light on the screen shown in the preceding figure, allowing the ordinary ray to pass and rotating the second crystal . 446 206. Appearance of spots of light on screen on rotating the FIG. PAGE second crystal, when the ex- traordinary ray is allowed to pass through the first screen. 447 207. Instrument for showing polari- zation by reflection .... 448 208. Section of plate-holder . . . 456 209. Enlarging camera 458 210. Instantaneous shutter . . . 460 211. Photoheliograph as erected in a temporary observatory for photographing the transit of Venus in 1874 461 212. Copy of photograph taken during the eclipse of 1869. 474 213. Part of Beer and Madler's map of the moon 476 214. The same region copied from a photograph by De La Eue . 477 215. Comparison between Kirchhoff's map and Kutherfurd's photo- graph 480 216. Arrangement for photographi- cally determining the coinci- dence of solar and metallic lines 481 217. Telespectroscope with camera for obtaining photographs of the solar prominences . . . 482 BOOK I. THE PRE-TELESCOPIC AGE. LI B II A \i i UNIVKUSiTY OF STARGAZING: PAST AND PRESENT. CHAPTER I. THE DAWN OF STARGAZING. SOME sciences are of yesterday ; others stretch far back into the youth of time. Among these there is one of the beginnings of which we have lost all trace, so coeval o o * was it with the commencement of man's history ; and that science is the one of which we have to trace the instrumental developments. Although our chief task is to enlarge upon the modern, it will not be well, indeed it is impossible, to neglect the old, because, if for no other reason, the welding of old and new has been so perfect, the conquest of the unknown so gradual. The best course therefore will be to distribute the different fields of thought and work into something like marked divisions, and to commence by dividing the whole time during which man has been observing the heavens into two periods, which we will call the Pre- telescopic and the Telescopic Ages. The work of the Pre-telescopic age of course includes all the early observations made by the unaided eye, while that of 2 STARGAZING : PAST AND PRESENT. [BOOK i. the Telescopic age includes those of vastly different kinds, which that instrument had rendered possible ; so that it divides itself naturally into some three or four sub-ages of extreme importance. It is unnecessary to say one word here on the im- portance of the invention of the telescope ; it is well for the present purpose, however, to emphasize the further distinctions we obtain when we consider the various additions made from time to time to the telescope. The Telescope, in fact, was comparatively little used until astronomy annexed that important branch of physics to its aid which gave us a Clock a means of dividing time in the most accurate manner. In quite recent times the addition of the Camera to the Telescope marks an important advance ; indeed the importance of photography is not yet recognised in the way it should be. Then, again, there is the addition of the Spectroscope, which, though it is only now beginning to yield us rich fruit, really dates from the beginning of the present cen- tury. This is an ally to tjie telescope of such power that he would be a bold man who would venture to set bounds to the conquests their combined forces will make. Now not only is it essential for the proper under- standing of the instruments used nowadays in every observatory, by every stargazer, to go back to the origin of the science of observation, but in no other way can one fully see in what way the new instrumental methods have added themselves to the old ones. Further, it is of importance to go back to the actual old field of work in which the geometric conceptions which grew up in the minds of the men of ancient time conceptions which we are now utilizing and extend- CHAP. 1.] THE DAWN OF STARGAZING. ing were gradually elaborated. To do this, there is no better way than to dwell very briefly on the work actually done by the old astronomers. This programme, then, being agreed to, the first point is to trace the progress of astronomical instruments down to the time of Copernicus and Galileo. During all this period the most generally received doctrine was, Fiu. 1. The Heavens according to Ptolemy. that the earth was the centre of the visible heavens ; and although there were many variations of this, still the arrangement of Ptolemy, Fig. 1, is a good type of the ideas of the ancients. B 2 4 STARGAZING : PAST AND PRESENT. [BOOK I. We begin with man's first feeble efforts, the work which man was enabled to do by his unaided eye ; and we end with the tremendous addition which he got to his observing powers by the invention of the telescope. The first instrument used for astronomical observa- tions was none of man's making. In the old time the only instrument was the horizon ; and, truth to tell, in a land of extended plains and isolated hills, it was not a bad one. Hence it was, doubtless, that ob- servations in the first instance were limited to certain occurrences such as the risings and settings of the stars and the relative apparent distances of the heavenly bodies from each other. So far as we are able to learn from ancient authors, the observations next added were those of the con- junctions of the planets and of eclipses. The Egyptians are stated to have recorded 373 solar, and 832 lunar eclipses ; and this statement is probably correct, as the proportions are exact, and there should be the above number of each in from 1,200 to 1,300 years. The Chinese also record an observation, made between the years 2514 and 2436 B.C., of five planets being in conjunction. The Chaldeans appear to have observed the motions of the moon, and an observation in 2227 B.C. is recorded ; but these old dates are probably fictitious. It is impossible to regard without surprise the general attention given to astronomical investigation in those early days compared with what we find now. Yet if we attempt to build up for ourselves any idea as to the problems of which the ancients attempted the solution, it is difficult if not impossible to do it ; we cannot realize the blank which the heavens presented CHAP, i.] THE DAWN OF STARGAZING. 5 to them, so many great men have lived between their time and our own, by whose labours we, even if un- consciously, have profited. The first idea seems to have been to observe which stars were rising or setting at seed or harvest time, to divide the heavens into Moon Stations, and then to mark astronomically their monthly and yearly festivals. If one looks into the old records we find that all the labours of man which had to be performed in the country or elsewhere were determined by the rising or setting of the stars. All the exertions of the navigator and the agriculturist were thus regulated. Of the planets in those early times we hear little, except from the Chinese annals which record conjunctions. This was before man began to use the sun as a stand- point, and hence it is that there are so many references in the ancient writers to the rising and setting of the most striking star cluster the Pleiades, and the most striking constellation Orion. It is known that the year, in later times at all events, began in Egypt when the brightest star in the heavens, Sirius, the dog-star, rose with the sun, this day being called the 1st of the month Thotli, 1 which was the commencement of the Sothiac period of 1461 years. It would appear that observations of culminations, that is, of the highest points reached by the stars, were not made till long after horizon observations were in full vigour ; and here it is a question whether pyramids and the like were not the first astronomical instruments constructed by man, because for great nicety in such observations a nicety, let us say, sufficient to determine astronomically by means of culminations the time for i Corresponding to 20th July, 139 B c. 6 STARGAZING : PAST AND PRESENT. [BOOK i. holding a festival a fixed instrument of some kind was essential. The rich mine recently opened up by Mr. Haliburton and Mr. Ernest de Bunsen concerning the survival in all nations in our own one takes the name the Feast of All Souls' of ancient festivals governed by the midnight culmination of the Pleiades will doubtless ere long call general attention to this earliest form of accurate astronomical observation, and the determination by Professor Piazzi Smyth of the fact that in 2170 B.C., when the Pleiades culminated at midnight at the vernal equinox, the passages in the north and south faces of the pyramid of Gizeh were directed, the southern one to this culmination, and the northern one to the then pole star, a Draconis, at its transit, about 4 from the pole. Hence one may regard the pyramid as the next astro- nomical instrument to the horizon. While then it is possible that such culmination observations soon replaced in some measure that class of observations which here- tofore had been made on the horizon, another teaching of horizon observations became apparent. By and by travellers observed that as they travelled northwards the stars that were just visible on the southern horizon, when culminating, gradually disappeared below it. These ob- servations were at once seized on, and Anaximander accounted for them by supposing that the earth was a cylinder. 1 The idea of a sphere did not come till later ; when it did come then came the circle as an astrono- mical instrument. For let us consider that a person on the earth stands, say, at the equator ; then he will just be able to see along his north and south horizon the stars pointed to by the axis of the globe : if now he is transported northwards, his horizon will chancre o 1 Anaximander flourished about 547 B.C. CHAP. I.J THE DAWN OF STARGAZING. with him ; he will no longer be able to see the southern stars, but the northern ones will gradually rise above his horizon till he gets to the north pole, when the north pole star, instead of being on his horizon, as was the case when he was at the equator, will be over his head. So by moving from the equator to the pole (or a quarter of FIG. 2. The Zodiac of Denderah. the distance round the earth) the stars have moved from the horizon to the point overhead, or the zenith, that is also a quarter of a circle. So it appears that if an observer moves to such a distance that the stars appear to move over a certain division of a circle with reference to the horizon, he must have moved over an equal 8 STARGAZING : PAST AND PRESENT. [BOOK i. division on the earth's surface. Then, as now, the circle in the Western world was divided into 360, so that the observer in moving 1 by the stars would have moved over -5T7F of the distance round the earth, on the assump- tion that the earth is a globe ; and if the distance over which the observer has moved be multiplied by 360, the result will be the distance round the earth. Now let us see how Posidonius a long time afterwards (he was born about. 135 years B.C.) applied this con- ception. He observed that at Ehodes the star Canopus grazed the horizon at culmination, while at Alexandria it rose above it 7J. Now 7J is T V of the whole circle ; so he found that from the latitude of Ehodes to that of Alexandria was T V of the circumference of the earth. He then estimated the distance, getting 5,000 stadia as the result ; and this multiplied by 48 gave him 240,000 stadia, his measure of the circumference of the earth. When the sun's yearly course in the heavens had been determined, it was found that it was restricted to that band of stars called the Zodiac, Fig. 2 ; the sun's position in the zodiac at any one time of the year being found by the midnight culmination of the stars opposite the sun ; this and the apparent and heliacal risings and settings were alone the subjects of observation. It is obvious, then, that when observations of this nature had gone on for some time, men would be anxious to map the stars, to make a chart of the field of heaven ; and such a work was produced by Autolycus three and a half centuries before Christ. We also owe to Auto- lycus and Euclid, who flourished about the same time (300 B.C.), the first geometrical conceptions connected with the apparent motions of the stars. CHAP, i.] THE DAWN OF STAKGAZING. 9 In the theorems of Autolycus there is a particular reference to the twelve parts of the zodiac, as denoted by constellations. The following are the most important propositions which he lays down : 1. " The zodiacal sign occupied by the sun neither rises nor sets, but is either concealed by the earth or lost in the sun's rays. The opposite sign neither rises nor sets, i.e., visibly, i.e., after sundown, but it is visible during the whole night. 2. " Of the twelve signs of the zodiac, that which precedes the sign occupied by the sun rises visibly in the morning ; that which succeeds the same sign sets visibly in the evening. 3. " Eleven signs of the zodiac are seen every night. Six signs are visible, and the five others, not occupied by the sun, afterwards rise. 4. "Every star has an interval of five months between its morning and its evening rising, during which time it is visible. It has an interval of at least thirty days between its evening setting, and its morning rising during which time it is invisible." (That is, the space passed over by the sun in its annual path is such that a star which you see on one side of the sun, when the sun rises at one time, would be seen a month afterwards on the other side of the sun.) Autolycus makes no mention of the planets. Their irregular movements rendered them unsuited to the practical object which he had in view. He is, however, stated by Simplicius, as quoted by Sir G. C. Lewis to have proposed some hypothesis for explaining their anomalous motions, and to have failed in his attempt. Euclid carries the results arrived at in this early pre-telescopic age much further ; in a little-known treatise, the Phenomena, 1 he thus sums up the know- ledge then acquired : 1 Quoted by Sir G. C. Lewis in his Astronomy of the Ancients, p. 187. 10 STARGAZING : PAST AND PRESENT. [BOOK i. " The fixed stars rise at the same point, and set at the same point ; the same stars always rise together, and set together, and in their course from the east to the west they always preserve the same distance from one another. Now, as these appearances are only consistent with a circular movement, when the eye of the observer is equally distant from the circumference of the circle in every direction (as has been demonstrated in the treatise on Optics), it follows that the stars move in a circle and are attached to a single body, and that the vision is equally distant from the circumference. FIG. 3. Illustration of Euclid's statements. D& the region of the always visible. which rise and set. P the star between the Bears. C BA the regions of the stars " A star is visible between the Bears, not changing its place, but always revolving upon itself. Since this star appears to be equally distant from every part of the circumference of each circle described by the other stars, it must be assumed that all the circles are parallel, so that all the fixed stars move along parallel circles, having this star as their common pole. " Some of these neither rise nor set, on account of their moving in elevated circles, which are called the 'always visible ' They are the stars which extend from the visible pole to the Arctic circle. Those which are nearest the pole describe the smallest CHAP, i.] THE DAWN OF STARGAZING. 11 circle, and those upon the Arctic circle the largest. The latter appears to graze the horizon. "The stars to the south of this circle all rise and set, on account of their circles being partly above and partly below the earth. The segments above the earth are large and the segments below the earth are small in proportion as they approach the Arctic circle, because the motion of the stars nearest the circle above the earth is made in the longest time, and of those below the earth in the shortest. In proportion as the stars recede from this circle, their motion above the earth is made in less time, and that below the earth in greater. Those that are nearest the south are the least time above the earth, and the longest below it. The stars which are upon the middle circle make their times above and below the earth equal ; whence this circle is called the Equinoctial. Those which are upon circles equally distant from the equinoctial make the alternate segments in equal times. For example, those above the earth to the north correspond with those below the earth to the south ; and those above the earth to the south correspond with those below the earth to the north. The joint times of all the circles above and below the earth are equal. The circle of the milky way and the zodiacal circle being oblique to the parallel circles, and cutting each other, always have a semicircle above the earth. " Hence it follows that the heaven is spherical. For if it were cylindrical or conical, the stars upon the oblique circles, which cut the equator, would not in the revolution of the heaven always appear to be divided into semicircles; but the visible segment would sometimes be greater and sometimes less than a semicircle. For if a cone or a cylinder were cut by a plane not parallel to the base, the section is that of an acute-angled cone, which resembles a shield (an ellipse). It is, therefore, evident that if a figure of this description is cut in the middle both in length and breath, its segments will be unequal. But the appearances of the heaven agree with none of these results. Therefore the heaven must be supposed to be spherical, and to revolve equally round an axis of which one pole above the earth is visible and the other below the earth is invisible. 12 STARGAZING : PAST AND PRESENT. [BOOK i. " The Horizon is the plane reaching from our station to the heaven, and bounding the hemisphere visible above the earth. It is a circle ; for if a sphere be cut by a plane the section is a circle. " The Meridian is a circle passing through the poles of the sphere, and at right angles to the horizon. " The Tropics are circles which touch the zodiacal circle, and have the same poles as the sphere. The zodiacal and the equinoctial are both great circles, for they bisect one another. For the beginning of Aries and the beginning of the Claws (or Scorpio) are upon the same diameter ; and when they are both upon the equinoctial, they rise and set in conjunction, having between their beginnings six of the twelve signs and two semicircles of the equinoctial; inasmuch as each beginning, being upon the equinoctial, performs its movement above and below the earth in equal times. If a sphere revolves equally round its axis, all the points on its surface pass through similar axes of the parallel circles in equal times. Therefore these signs pass through equal axes of the equinoctial, one above and the other below the earth; consequently the axes are equal, and each is a semicircle ; for the circuit from east to east and from west to west is an entire circle. Consequently -the zodiacal and equinoctial circles bisect one another; each will be a great circle. Therefore the zodiacal and equinoctial are great circles. The horizon is likewise a great circle ; for it bisects the zodiacal and equinoctial, both great circles. For it always has six of the twelve signs above the earth, as well as a semicircle of the equator. The stars above the horizon which rise and set together reappear in equal times, some moving from east to west, and some from west to east." We have given this long extract in justice to the men of old, containing as it does many of those geometrical principles which all our modern instruments must and actually do fulfil. It is true that the present idea of the earth's place in the system is different. Euclid imagined the earth to be CHAP, i.] THE DAWN OF STARGAZING. 13 at the centre of the universe. It is now known that the earth is one of various planets which revolve round the sun, and further, that the journey of the earth round the sun is so even and beautifully regulated that its motion is confined to a single plane. Year after year the earth goes on revolving round the sun, never departing, except to a very small extent, from this plane, which is one of the fundamental planes of the astronomer and called the Plane of the Ecliptic. Suppose this plane to be a tangible thing, like the surface of an infinite ocean, the sun will occupy a FIG. 4. The Plane of the Ecliptic. certain position in this infinite ocean, and the earth will travel round it every year. If the axis of the earth were upright, one would repre- sent the position of things by holding a globe with its axis upright, so that the equator of the earth is in every part of its revolution on a level with this ecliptic sea. But it is known that the earth, instead of floating, as it were, upright, as in Fig. 4, has its axis inclined to the plane of the ecliptic, as in Fig. 5. It is also known that by turning a globe round, dis- tant objects would appear to move round an observer on the globe in an opposite direction to his own motion,. 14 STARGAZING : PAST AND PRESENT. [BOOK r. and these distant objects would describe circles round a line joining the places pointed to by the poles of the earth, i.e., round the earth's axis. It is now easy to explain the observations referred to by Euclid by supposing the surface of the water in the tub to represent the plane of the ecliptic, that is, the plane of the path which the sun apparently takes in going round the earth ; and examining the relative positions of the sun and earth represented by two floating balls, the latter having a wire through it in- clined to the upright position ; it will be seen at once by FIG. 5. The Plane of the Ecliptic, showing the Inclination of the Earth's Axis. turning the ball on the wire as an axis to represent the diurnal motion of our earth, how Euclid finds the Bear which never sets, being the place in the heavens pointed to by the earth's pole ; and how all the stars in different portions of the heavens appear to move in complete circles round the pole-star when they do not set, and in parts of circles when they pass below the horizon. By moving the ball representing the earth round the sun and so examining their relative positions, during the course of a year it will be seen how the sun appears to travel through all the signs of the zodiac in succession CHAP, i.] THE DAWN OF STAEGAZING. 15 in his yearly course, remaining a longer or shorter time above the horizon at different times of the year. For it will be seen that if the spectator on the globe, when in the position that its inclined axis, as shown in Fig. 5, points towards the sun, were looking at the sun from a place where one can imagine England to be at midday, the sun would appear to be very high up above the horizon ; and if he looked at it from the earth in the opposite part of its orbit it would be very low and near the horizon. When the earth, therefore, occupied the intermediate positions, the sun would be half way between the extreme upper position and the extreme lower position as the earth moves round the sun, and in this way the solstices, equinoxes, and the seasonal changes on the surface of our planet, are easily explained. CHAPTER II. THE FIRST INSTRUMENTS. . THE ancients called the places occupied by the sun when highest and lowest the Solstices, and the inter- mediate positions the Equinoxes. The first instrument made was for the determination of the sun's altitude in order to fix the solstices. This instrument was called the Gnomon. It consisted of an upright rod, sharp at the end and raised perpendicularly on a horizontal plane, and its shadow could be measured in the plane of the meridian by a north and south line on the ground. Whenever the shadow was longest the sun was naturally lowest down at the winter solstice, and vice versd for the summer solstice. Here then we leave observations on the horizon arid come to those made on the meridian. The Gnomon is said to have been known to the Chinese in the time of the Emperor Yao's reign (2300 B.C.), but it was not used by the Greeks ' till the time of 1 This instrument is also reported to have been used by the Chaldeans in 850 B.C. ; the invention of it being attributed to Anaxi- mander. This philosopher, says Diogenes Laertes, observed the revo- lution of the sun, that is to say, the solstices, with a gnomon ; and probably he measured the obliquity of the ecliptic to the equator, which his master had already discovered. CHAP, ii.] THE FIRST INSTRUMENTS. 17 Thales, about 585 B.C., who fixed the dates of the solstices and equinoxes, and the length of the tropical ' year that is, the time taken by the sun to travel from the vernal equinoctial point round to the same point again. The next problem was to discover the inclination of the ecliptic, or, what is the same thing, the amount that the earth's equator is inclined to the ecliptic plane (represented by the surface of the water in our tub). Now in order to ascertain this, the angular distance between the positions occupied by the sun when at the solstices must be measured ; or, since one solstice is just as much below the equinoctial line as the other is above it, we might take half the angle between the solstices as being the obliquity required. The first method of measuring the angle was to measure the length of the sun's shadow at each sol- stice, and so, by comparison of the length of the shadow with the height of the gnomon, calculate the difference in altitude, the half of which was the angle sought. And this was probably the method of the Chinese, who obtained a result of 23 38' 11" in the time of Yao ; and also of Anaximander in his early days, who obtained a result of 24. But before trigo- nometrical tables, the first of which seem to have been constructed by Hipparchus and Ptolemy, were known, in order to find this angle it was constructed geome- trically, and then what aliquot part of the circumference it was, or how much of the circumference it contained was determined ; for the division of the circle into 360 is subsequent to the first beginning of astronomy and hence it was that Eratosthenes said that the distance c 18 STARGAZING : PAST AND PRESENT. [BOOK i. from the tropics was J of the circumference, and not that it was 47 46' 26". The gnomon is, without exception, of all instruments the one with which the ancients were able to make the best observations of the sun's altitude. But they did not give sufficient attention to it to enable it to be used with accuracy. The shadow projected by a point when the sun is shining is not well defined, so that they could not be quite certain of its extremity, and it would seem that the ancient observations of the height of the sun made in this manner ought to be corrected by about half the apparent diameter of the sun ; for it is probable that the ancients took the strong shadow for the true shadow ; and so they had only the height of the upper part of the sun and not that of the centre. There is no proof that they did not make this correction, at least in the later observations. In order to obviate this inconvenience, they subse- quently terminated the gnomon by a bowl or disc, the centre of which answered to the summit ; so that, taking the centre of the shadow of this bowl, they had the height of the centre of the sun. Such was the form of the one that Manlius the mathematician erected at Eome under the auspices of Augustus. But in comparatively modern times astronomers have remedied this defect in a still more happy manner, by using a vertical or horizontal plate pierced with a circular hole which allows the rays of the sun to enter into a dark place, and in fact to form a true image of the sun on a floor or other convenient receptacle, as we find is the case in many continental churches. Of course at this early period the reference of any GHAP. ii.] THE FIRST INSTRUMENTS. 10 particular phenomenon to true time was out of the ques- tion. The ancients at the period we are considering used twelve hours to represent a day, irrespective of the time of the year the day always being reckoned as the time between sunrise and sunset. So that in summer the hours were long and in winter they were short. The idea of equal hours did not occur to them till later ; but no observations are closer than an hour, and the smallest division of space of which they took notice was something like equal to a quarter or half of the moon's diameter. When we come down, however, to three centuries before Christ, we find that a different state of things is coming about. The magnificent museum at Alexandria was beginning to be built, and astronomical observations were among the most important things to be done in that vast establishment. The first astronomical workers there seem to have been Timocharis and Aristillus, who began about 295 B.C., and worked for twenty-six years. We are told that they made a catalogue of stars, giving their positions with reference to the sun's path or ecliptic. It was soon after this that the gnomon gave way to the invention of the Scarphie. It is reaJly a little gnomon on the summit of which is a spherical segment. An arc of a circle passing out of the foot of the style was divided into parts, and we thus had the angle which the solar ray formed with the vertical. Nevertheless the scarphie was subject to the same inconveniences, and it required the same corrections, as the gnomon ; in short, it was less accurate than it. That did not, however, hinder Eratosthenes from making use of it to measure the size of the earth and the inclination of the ecliptic c 2 20 STARGAZING : PAST AND PRESENT. [BOOK i. to the equator. The method Eratosthenes followed in ascertaining the size of the earth was to measure the arc between Syene and Alexandria by observing the altitude of the sun at each place. He found it to be A of the circumference and 5,000 stadia, so that if TCF of the circumference of the earth is 5,000 stadia, the whole circumference must be 50 times 5,000, or 250,000 stadia l FKJ. 6. The First Meridian Circle. A*nd now still another instrument is introduced, and we begin to find the horizon altogether disregarded in favour of observations made on the meridian. The instrument in question was probably the inven- tion of Eratosthenes. It consisted of two circles of nearly the same size crossing each other at right angles, (Fig. 6) ; one circle represented the equator and the other the meridian, and it was employed as follows : The circle A was fixed perfectly upright in the 1 28/279 miles. CHAP. II.] THE FIRST INSTRUMENTS. 21 meridian, so that the greatest altitude of the sun each day could be observed; the circle B was then placed exactly in the plane of the earth's equator by adjusting the line joining c and D to the part of the heavens between the Bears, about which the stars appear to revolve. This done, the occurrence of the equinox was waited for, at which time the shadow of the part of the FIG. 7. The First Instrument Graduated into 360 (West Side). circle E must fall upon the part marked F, so as exactly to cover it. We now come to the time when the circle began to be divided into 360 divisions or degrees about the time of Hipparchus (160 B.C.). There are two instruments described by Ptolemy for measuring the altitude of the sun in degrees instead of in fractions of a 'circle. They, like the gnomon, were used for determining the altitude of the sun. The first, Fig. 7, consisted of two circles of copper, one, c D, larger than the other, 22 STARGAZING : PAST AND PRESENT. [BOOK i. having the smaller one, B, so fitted inside it as to turn round while the larger remained fixed. The larger was divided into 360, and the smaller one carried two pointers. This instrument was placed perfectly up- right and in the plane of the meridian, and with a fixed point, c, always at the top by means of a plumb-line hanging from c over a mark, D. On this small circle are two square knobs projecting on the side, E and F. When the sun was on the meridian the small circle was turned so as to bring the shadow of the knob E over the knob F, and then the degree to which the pointer pointed was read off on the larger circle. And of course, as the position of the knobs had to be changed as the sun moved in altitude, the angle through which the sun moved was measured, and the circle being fixed, the sun's altitude could always be obtained. The other instrument consisted of a block of wood or stone, one side of which was placed in the plane of the meridian ; and on the top corner of this side was fixed a stud ; and round it as a centre a quarter of a circle was described, divided into 90. Below this stud was another, and by means of a plumb-line one stud could always be brought over the other ; so that the instru- ment could always be placed in a true position. At midday then, when the sun was shining, the shadow of the upper stud would fall across the scale of degrees, and at once give the altitude of the sun. Ptolemy, who used this instrument, found that the arc included between the tropics was 47f. The result of all these accurate determinations of the solstices and equinoxes was the fixing of the length of the year. CHAP. ii. J THE FIRST INSTRUMENTS. 23 We have so far dealt with the methods of observation which depend upon the use of the horizon and of the meridian; we will now turn our attention to extra- meridional observations, or those made in any part of the sky. Before we discuss them, let us consider the principles on which we depend for fixing the position of a place on a globe. On a terrestrial globe there are lines drawn from pole to pole, called meridians of longitude ; and if a place, is on any one meridian it is said to be in so many degrees of longitude, east or west of a certain fixed meridian, as there are degrees intercepted between this meridian and the one on which the place is situated. There are also circles at right angles to the above and parallel to the equator ; these are circles of latitude, and a place is said to have so many degrees N. or S. latitude as the circle which passes through it intercepts on a meridian between itself and the equator, so that the latitude of a place is its angular distance from the equator, and the longitude is its angular distance E. or W. of a fixed meridian that of Greenwich being the one used for English calculation ; and each large country takes the meridian of its central observatory for its starting-point. The distance round the equator is some- times expressed in hours instead of degrees ; for as the earth turns round in twenty-four hours, so the equator can be divided into hours, minutes, and seconds. So that if a star be just over the meridian of Greenwich, which is 0' 0", or O h O m s longitude at a certain time, in an hour after it will be over a meridian 15 or one hour west of Greenwich, and so on, till at the end of twenty-four hours it would be over Greenwich again. Now let us turn to the celestial globe. 24 STARGAZING : PAST AND PRESENT. [BOOK I. What we call latitude and longitude on a terrestrial globe is called declination and right ascension on the celestial globe, because in the heavens there is a latitude and longitude which does not correspond to our lati- tude and longitude on the earth. If we imagine the lines of latitude and longitude on the earth to be pro- jected, say as shadows thrown on the heavens by a light in the centre of the earth, the lines of right ascension (generally written R.A.) and declination (written Dec. or D.) will be perfectly depicted. But there is another method of co-ordinating the stars, in which we have the words latitude and longitude used also, as we have said, for the heavens ; meaning the distance of a star from the ecliptic instead of the equator, and its distance east or west measured by meridians at right angles to the ecliptic. This premised, we are in a position to see the enormous advance rendered possible by the methods of observation introduced by Hipparchus and Ptolemy. CHAPTEE III. HIPPARCHUS AND PTOLEMY. AMONG the astronomers of antiquity there are two figures who stand out in full relief Hipparchus and Ptolemy. The former, "the father of astronomy," is especially the father of instrumental astronomy. As he was the first to place observation on a sure basis, and left behind him the germs of many of our modern instru- ments and methods, it is desirable to refer somewhat at length to his work and that of his successor, Ptolemy. Hipparchus introduced extra-meridional observa- tions. He followed Meton, Anaximander, and others in observing on the meridian instead of on the horizon, and then it struck him that it was not necessary to keep to the meridian, and he conceived an instrument, called an Astrolabe, fixed on an axis so that the axis would point to the pole-star, like the one represented in Fig. 8. This engraving is of one of Tycho Brahe's instruments, which is similar to but more elaborate than that of Hipparchus no drawing of which is extant, c, D, is the axis of the instrument pointed to the pole of the heavens ; E, B, c, the circle placed North and South representing the meridian ; R, Q, N, the circle placed at right angles 26 STARGAZING : PAST AND PRESENT. [BOOK i. to the polar axis, representing the equator, but in the instrument of Hipparchus it was fixed to the circle E, iT, c, and not movable in its own plane as this one is. M, L, K, is a circle at right angles to the equator, and moving round the poles, being a sort of movable meri- FIG. 8. Astrolabe (Armilke Aequatoriae of Tycho Brahe) similar to the one contrived by Hipparchus. dian. Thus, then, if the altitude of a star from the equator (or its declination) was required to be observed, the circle was turned round on the axis, and the sights, Q, M, moved on the circle till they, together with the sight A, pointed to the star ; the number of degrees between one of the sights and the equator, was then CHAP, in.] HIPPARCHUS AND PTOLEMY. 27 read off, giving the decimation required. The number of degrees, or hours and minutes, of Right Ascension, from K to E could be then read off along the circle R, Q, N, giving the distance of the object from the meridian. As the stars have an apparent motion, the difference in right ascension between two stars only could be obtained by observing them directly after each other, and allowing for the motion during the interval between the two observations. In this manner, then, Hipparchus could point to any part of the heavens and observe, on either side of the meridian, the sun, moon, planets or any of the stars, and obtain their distance from the equatorial plane ; but another fixed plane was required ; and Hipparchus, no longer content with being limited to measuring distances from the equator, thought it might be possible to get another starting-point for distances along the equator. It was the determination of this plane, or starting-point from which to reckon right ascension, that was one of the difficulties Hipparchus had to encounter. This point he decided should be the place in the heavens where the sun crosses the equator at the spring equinox. But the stars could not be seen when the sun was shining ; how, then, was he to fix that point so that he could measure from it at night ? He found it at first a tremendous problem, and at last hit upon this happy way of solving it. He reasoned in this way : "As an eclipse of the moon is caused by the earth's shadow being thrown by the sun on the moon, if this happen near the equinox, the sun and moon must then be very near the equator, and very near the ecliptic in fact, near the intersection of the two fundamental planes which are supposed to cross 28 STARGAZING: 'PAST AND PRESENT. [BOOK i. each other. If I can observe the distance, measured along the equator, between the moon and a star, I shall have obtained the star's actual place, because, of course, if the moon is exactly opposite the sun, the sun will be 180 degrees of right ascension from the moon, and the right ascension of the sun being known it will give FIG. 9. Ecliptic Astrolabe (the Annillse Zodiacales of Tycho Brahe), similar to the one used by Hipparchus. me the position of the star." This method of observa- tion was an extremely good one for the time, but it could only have been used during an eclipse of the moon, and when the sun was so near the equator that its distance from the equinoctial point along the ecliptic, as calculated by the time elapsed since the equinox, CHAP, in.] HIPPARCHUS AND PTOLEMY. 29 differed little from tlie same distance measured along the equator, or its right ascension, so that the right ascen- sion of the sun was very nearly correct. Hipparchus hit upon a very happy alteration of the same instrument to enable him to measure latitude and longitude instead of declination and right ascension in fact, to measure along the ecliptic instead of the equator. Instead of having the axis of the inner rings parallel to the axis of the earth, as in Fig. 9, he so arranged matters that the axis of this system was separated from the earth's axis to the extent of the obliquity of the ecliptic, the circle R, Q, N, therefore instead of being in the plane of the equator, was in that of the ecliptic. Then it was plain to Hipparchus that he would, instead of being limited to observe during eclipses of the moon, be able to reckon from the sun at all times ; because the sun moves always along the ecliptic and the latitude of the sun is nothing. We will now describe the details of the instrument. There is first a large circle, E, B, c, Fig. 9 (which is taken from a drawing of this kind of instrument as con- structed subsequently by'Tycho Brahe), fixed in the plane of the meridian, having its poles, D, c, pointing to the poles of the heavens ; inside this there is another circle, F, i, H, turning on the pivots D, c, and carrying fixed to it the circle, o, P, arranged in a plane at right angles to the points I, K, which are placed at a distance from c and D equal to the obliquity of the ecliptic; so that I and K represent the poles of the ecliptic, and the circle, o, P, the ecliptic itself. There is then another circle, R, M, turning on the pivots I and K, repre- senting a meridian of latitude, and along which it is measured. Then, as the sun is on that part of the ecliptic SO STARGAZING : PAST AND PRESENT. [BOOK i. nearest the north pole, in summer, its position is repre- sented by the point F on the ecliptic, and by N at the winter solstice ; so, knowing the time of the year, the sight Q can be placed the same number of de- grees from F as the sun is from the solstice, or in a similar position on the circle o P as the sun occupies on the ecliptic. The circle can then be turned round the axis c, D, till the sight Q, and the sight opposite to it, Q', a?e in line with the sun. The circle, o, R, will then be in the plane of the ecliptic, or of the path of the earth round the sun. The circle, R, M, is then turned on its axis, I, K, and the sights, R, R, moved until they point to the moon. The distance Q, L, measured along o, P, will then be the difference in longitude of the moon and sun, and its latitude, L, R, measured along the circle R, M. But why should he use the moon \ His object was to determine the longitude of the stars, but his only method was to refer to the motion of the sun, which could be laid down in tables, so that its longitude or distance from the vernal equinox was always known. But we do not see the stars and the sun at the same time ; therefore in the day time, while the moon was above the horizon, he determined the difference of longitude between the sun and the moon, the longitude of the sun or its distance from the vernal equinox being known by the time of the year ; and after the sun had set he determined the difference of longitude between the moon and any particular star ; and so he got a fair representation of the longitude of the stars, and succeeded in tabulating the position of 1,022 of them. CHAP. III.] HIPPARCHUS AND PTOLEMY. 31 It is to the use of this instrument that we owe the discovery of the precession of the equinoxes. After Hipparchus had fixed, the position of a num- ber of stars, he found that on comparing the place amongst them of the sun at the equinoxes in his day with its place in the time of Aristillus that the positions differed that the sun got to the equinox, or point where it crossed the equator, a short time before it FIG. 10. Diagram Illustrating the Precession of the Equinoxes. got to the place amongst the stars where it crossed the time of Aristillus ; in fact, he found that the in equinoctial points retrograded along the equator, and Ptolemy (B.C. 135) appears to have established the fact that the whole heavens had a slow motion of one degree in a century which accounted for the motion of the equinoxes. Let us see what we have learned from the observation of this motion, for motion there is, and the ancients must be looked on with reverence for their skill in 32 STARGAZING : PAST AND PRESENT. [BOOK i. determining it with their comparatively rude instru- ments. In Fig. 10, A represents the earth at the vernal equinox, and at this time the sun appears near a certain star, s, which was fixed by Aristillus ; but in the time of Hipparchus the equinox happened when the sun was near a star, s', and before it got to s. Now we know FIG. 11. Revolution of the Pole of the Equator rouud tiie J^ole of the Ecliptic caused by the Precession of the Equinoxes. that the sun has no motion round the earth, and that the equinox simply depends on the position of the earth's equator in reference to the ecliptic ; so that in order to produce the equinox when the earth is at E and before it get to A, its usual place, all we have to do is to turn the pole of the earth through a small arc of the dotted CHAP, in.] HIPPARCHUS AND PTOLEMY. 33 circle, and so alter its position to that shown at F, when its equator and poles will have the same posi- tion as regards the sun as they have at A, so the equinox will happen when the earth is at E, and before it reaches A. This may be practically represented by taking an orange and putting a knitting-needle through it, and drawing a line representing the equator round it, and half immersing it in a tub of water, the surface of which represents the ecliptic. We are then able to examine these motions by moving the orange round the tub to represent the earth's annual motion, and at the same time making the orange slowly whobble like a spinning- top just before it falls, by moving the top of the knitting-needle through a small arc of a circle in the same direction as the hands of a clock at ever^ revolution of the orange round the centre of the tub. The points where the equator is cut by the surface of the water (or ecliptic) will then change, as the orange whobbles, and the line joining them, will rotate, and as the equinox happens when this line passes through the sun, it will be seen that this will take place earlier at each revolution of the orange round the tub. The equinox will therefore appear to happen earlier each year, so that the tropical year, or the time from equinox to equinox, is a little shorter than the sidereal year, or the time that the earth takes to travel from a certain place in its orbit to the same again ; for if the earth start from an equinoctial point, the equinox will happen before it gets to the same place where the equinoctial point was at starting. This is called the precession of the equinoxes. This discovery must be regarded as the greatest triumph obtained by the early stargazers, and there is D 34 STARGAZING : PAST AND PRESENT. [BOOK i. much evidence to show that when the zodiac was first marked out among the central zone of stars, the Bull and not the Ram was the first of the train. Even the Ram, owing to precession, is no longer the leader, for the sign Aries is now in the constellation Pisces. FKJ. 12. The Vernal Equinox among the Constellations, B.C. 2170. FIG. 13. Showing how the Vernal Equinox has now passed Jrom Taurus and Ai ies. The two accompanying drawings by Professor Piazzi Smyth of the position of the vernal equinox among the stars in the years 2170 B.C. and 1883 A.D. will show how precession has brought about celestial changes which have CHAP. III.] HIPPAECHUS AND PTOLEMY. 35 not been unaccompanied by cliauges of religious ideas and observances in origin connected with the stars. We now come to Ptolemy. There was another instru- ment used by Ptolemy, and described by him, which we may mention here ; it was called the Parallactic Kules, so named perhaps because that ancient astronomer used it first for the observation of the parallax of the moor?. It consists of three rods, D E, D F, E F, Fig. 14, two of which formed equal sides of an isosceles triangle ; and FIG. 14. Instrument for Measuring Altitudes. the third, which had divisions on it, made the one at the base, or was the chord of the angle at the summit. One of the equal sides, D F, was furnished with pointers, over which a person observed the star, whilst the other, D E, was placed vertically, so that they read off the divisions on E F, and then, by means of a table of chords, the angle was found ; this angle was the distance of the star from the zenith. Ptolemy, wishing to observe with great accuracy the position of the moon, made himself an D 2 36 STARGAZING : PAST AND PRESENT. [BOOK i. instrument of this kind of a considerable size ; for the equal rulers were four cubits long, so that its divisions might be more obvious. He rectified its position by means of a plumb-line, Purbach, Regiomontanus, and Walther, astronomers of the fifteenth century, employed this manner of observing, which, considering the youth of astronomy, was by no means to be despised. This instrument, constructed with great care, would have sufficiently been useful as far as concerns certain measure- ments and would have furnished results sufficiently exact ; but the part of ancient astronomy that failed was the way of measuring time with any precision. There were astronomers who proposed clepsydras for this purpose ; but Ptolemy rejected them as very likely to introduce errors ; and indeed this method is subject to much inconvenience and to irregularities difficult to prevent. However, as the measurement of time is the soul of astronomy, Ptolemy had recourse to an- other expedient which was very ingenious. It consisted in observing the height of the sun if it were day, or of a fixed star if it were night, at the instant of a phenomenon of which he wished to know the time of occurrence, for the place of the sun or star being known to some minutes of declination and right ascension as also was the latitude of the place, he was able to calcu- late the hour ; thus when they observed, for example, an eclipse of the moon, it was only necessary to take care to get the height of some remarkable star at each phase of the eclipse, say at the commencement and at the end, in order to be able to conclude the true time at which it took place. This was the method adopted by astronomers until the introduction of the pendulum. CHAPTEK IV. TYCHO BRAKE. LEAVING behind us the results of the researches of Ptolemy, who succeeded Hipparchus and whose methods have been described, and passing over the astronomy of the Arabs and Persians as being little in advance of Hipparchus and Ptolemy, we come down to the sixteenth century of our era. Here we find ourselves in presence of the improvements in instruments effected by a man whose name is con- spicuous Tycho Brahe a Danish nobleman who, in the year 1576, established a magnificent observatory at Huen, which may be looked upon as the next building of importance after that great edifice at Alexandria which has already been referred to. What Hipparchus was to the astronomy of the Ancients such was Tycho to the astronomy of the Middle Ages. As such his life merits a brief notice before we proceed to his work. He was born at Knuds- thorp, near Helsingborg, in Sweden, in 1546, and went to the University of Copenhagen to prepare to study law ; while there he was so struck with the prediction of an eclipse of the sun by the astrological almanacks that he gave all his spare time to the study of astronomy. In 38 STARGAZING : PAST AND PRESENT. [BOOK i. 1565 his imcle died and Tycho Brahe fell into possession of one of his uncle's estates ; and as astronomy, or astrology as it was then called, was thought degrading to a man in his position by his friends, who took offence at his pursuits and made themselves very objectionable, he left for a short stay at Wittenberg, then he went to Eostock and afterwards to Augsburg, where he constructed his large quadrant. He returned to his old country in 1571 ; while there, Frederick II., King of Denmark, requested him to deliver a course of lectures on astronomy and astrology and became his most liberal patron. The King granted to Tycho Brahe for life the island of Huen, lying between Denmark and Sweden, and built there a magnificent observatory and apartments for Tycho, his assistants and servants. The main building was sixty feet square, with observing towers on the north and south, and a library and museum. Tycho called this Uraniberg the city of the heavens ; and he after- wards built a smaller observatory near called by him Sternberg city of the stars, the former being insuffi- ciently large to contain all his instruments. The following is a list of these instruments as given in Sir David Brewster's excellent memoir of Brahe, in Martyrs of Science : In the South and greater Observatory. 1. A semicircle of solid iron, covered with brass, four cubits radius. 2. A sextant of the same materials and size. 3. A quadrant of one and a half cubits radius, and an azimuth circle of three cubits. 4. Ptolemy's parallactic rules, covered with brass, four cubits in the side. 5. Another sextant, 6. Another quadrant, like No. 3. CHAP, TYCHO BRAKE. 3 ( J FIG. 15. Portrait of Tycho Brahe (from original painting in the possession of Dr. Crompton, of Manchester). JL i 13 U ,\ ;* i UNI V Kits 1TV OF CHAP, iv.] TYCHO BRAKE. 41 7. Zodiacal armillaries of melted brass, and turned out of the solid, of three cubits in diameter. Near this observatory there was a large clock with one wheel two cubits in diameter, and two smaller ones which, like it, indicated hours , minutes, and seconds. In the South and lesser Observatory. 8. An armillary sphere of brass, with a steel meridian, whose diameter was about four cubits. In the North Observatory. 9. Brass parallactic rules, which revolved in azimuth above a brass horizon, twelve feet in diameter. 10. A half sextant, of four cubits radius. 11. A steel sextant. 1 2. Another half sextant with steel limb, four cubits radius. 13. The parallactic rules of Copernicus. 14. Equatorial armillaries. 15. A quadrant of a solid plate of brass, five cubits in radius, showing every ten seconds. 16. In the museum was the large globe made at Augsburg. In the Sterriberg Observatory. 1 7. In the central part, a large semicircle, with a brass limb, and three clocks, showing hours, minutes, and seconds. 18. Equatorial armillaries of seven cubits, with semi-armillaries of nine cubits. 19. A sextant of four cubits radius. 20. A geometrical square of iron, with an intercepted quadrant of five cubits, and divided into fifteen seconds. 21. A quadrant of four cubits radius, showing ten seconds, with an azimuth circle. 22. Zodiacal armillaries of brass, with steel meridians, three cubits in diameter. 23. A sextant of brass, kept together by screws, and capable of being taken to pieces for travelling with. Its radius was four cubits. 24. A movable armillary sphere, three cubits in diameter. 25. A quadrant of solid brass, one cubit radius, and divided into minutes by Nbnian circles. 26. An astronomical radius of solid brass, three cubits long. 27. An astronomical ring of brass, a cubit in diameter. 28. A small brass astrolabe. 42 STARGAZING : PAST AND PRESENT. [BOOK i. Tycho Bralie carried on his work at Uraniberg for twenty-one years, and appears to have been visited by many of the princes of the period and by students anxious to learn from so great a man. In Frederick's treatment of Tycho Bralie we have an early and muni- ficent and, in its results, most successful instance of the endowment of research. On the death of Frederick II., in 1588, Christian IV. came to the throne. The successor cared little for astronomy, and his courtiers, who were jealous of Tycho' s position, so acted upon him that the pension, estate and canonry with which Tycho had been endowed were taken away. Unable to put up with these insults and loss of his money, he left for Waiidesburg in 1597, where he was entertained by Count Henry Rantzau. It was now that he wrote and published the Astronomies instauratce Mechanica, a copy of which, together with his catalogue of 1000 stars, was sent to the Emperor Rudolph II., who invited him to go to Prague. This he accepted, and he and his family went to the castle of Benach in 1599, and a pension of 3000 crowns was given to him. Ten years afterwards he removed with his instruments into Prague to a house purchased and presented to him by the Emperor ; here he died in the same year. The wonderful assistance which Tycho Brahe was able to bring to astronomy shows that then, as now, any considerable advance in physical investigation was more or less a matter of money, and whether that money be found by individuals or corporations, now or then, we cannot expect any considerable advance without such a necessary adjunct. The principal instruments used at first by Tycho Brahe resembled the Greek ones, except that they were much CHAP. TV.] TYCHO BRAIIB. 43 44 STARGAZING : PAST AND PRESENT. [BOOK i. larger. Hipparchus was enabled to establish the position of a heavenly body within something less than one degree of space some say within ten minutes ; but there was an immense improvement made in this direction in the instruments used by Tycho. One of the instruments which he used was in every way similar to the equatorial astrolabe designed by Hipparchus, and was called by Tycho, the 'armillae equatorise ' (Fig. 8). With that instrument in connec- tion with others Tycho was enabled to make an immense advance upon the work done by Hipparchus. Tycho, like Hipparchus, having no clock, in the modern sense, was not able to determine the difference of time between the transit of the sun or a particular star over the meridian, so that he was compelled to refer everything to the sun at the instant of observation, and he did that by means of the moon. Hipparchus, as we have seen, determined the difference of longitude, or right ascension, between the sun and the moon and between the moon and the stars, in the manner already described, and so used the moon as a means of determining differences between the longitude or right ascension of the sun and the stars. Now Tycho, using an instrument similar to that of Hipparchus, saw that he would make an improvement if instead of using the Moon he used Venus ; for the measure of the surface of the moon was considerable, and could not be easily reckoned, and its apparent position in the heavens was dependent on the position of a person on the earth, because it is so near the earth that it has a sensible parallax, that is, a person at the equator of the earth might see the moon in the direction of a certain star ; but, on going to the pole, the moon would appear below CHAP, iv.] TYCHO BRAHE. 45 the line of the star. If one were looking at a kite in the air to the south and then walked towards the south, the kite would gradually get over head, and on proceeding further it would get north. To persons at different stations the kite wtould appear in different positions, and the nearer the kite was to the observer the less distance he would have to go to make it change its place. So also with the moon ; it is so near to us that a change of place on the earth makes a consider- able difference in the direction in which it is seen. Instead, therefore, of using the Moon, Tycho used Venus, and so mapped 1,500 stars after determining their absolute right ascensions, in this manner without the use of clocks. Fig. 8 shows the instrument called the "armillse equatorise," which he constructed, and which was based upon the principle of that which Hipparchus had used. Here the axis of motion, c, D, of these circles is so arranged that it is absolutely parallel to the axis of the earth ; but instead of the circle R, Q, N, representing the equator, being fixed, it revolved in its own plane while held by the circle G, H, I, making its use probably more easy, but leaving the principles unaltered. Tycho Brahe also used another similar instrument of much larger size for the same purposes as the one we have just considered. It consisted of a large circle, which was seven cubits in diameter, corresponding to the circle K, L, M, Fig. 8 ; and carrying the sights in the same manner, it was placed in a circular pit in the ground, with its diameter pointing towards the pole. This was used for measuring declinations. The circle B, Q, N, Fig. 9, was represented by a fixed circle carried on pillars surrounding the pit, and along which the right ascension of the star was measured. This iDstrument, 46 STARGAZING : PAST AND PHESENT. [BOOK i. therefore, was more simple than the smaller one, and probably much more accurate. Tycho was not one of those who was aware of the true system of the universe ; he thought the earth fixed, as Ptolemy and* others did; but whether we suppose the earth to be movable in the middle of the vault of stars or stationary, in either case that position is absolutely immaterial in ascertaining the right ascension of stars. If one takes the terrestrial globe, and looks upon the meridians, it is at once clear that the distance from FIG. 17. Tyclio Brake's System. meridian to meridian remains unaltered, whether the globe is still or turning round : so the stars maintain their relative positions to each other, whether we con- sider the earth in motion or the sphere in which the stars are placed to revolve round it. The introduction of clocks gave Tycho the invention of the next instrument, which was the transit circle. At this time the pendulum had not been invented ; but it struck him and others that there was no necessity for having two or more circles rotating about an axis parallel CHAP, iv.] TYCHO BRAHE. 47 to the earth's axis, as in the astrolabes or armilloe, but only to have one circle in the plane of the meridian of the place. So that, by the diurnal movement of the earth round its own axis, all the stars in the heavens would gradually and seriatim be brought to be visible along the arc of the circle, so he arranged matters in the following way. The stars were observed through a hole in a wall and through an eyehole, sliding on a fixed arc. The number of degrees marked at the eyehole on the arc at once gave the altitude of the heavenly bodies as seen through that hole. If a star was very high, it would be necessary for an observer to place his eye low down to be able to see it. If it were near the horizon, he would have to travel up to the top of this circle to determine its altitude, and having done that, and knowing the latitude of the place of observation, the observer will be able to determine the position of the star with reference to the celestial equa- tor. The actual moment at which the star was seen was noted by the clock, and the time that the sun had passed the hole being also previously noted, the length of time between the transits was known ; and as the stars appear to transit or pass the meridian every twenty-four hours, it was at once known what part of the heavens was inter- cepted between the sun and the star in degrees, or, as is usually the case, the right ascension of the star was left expressed in hours and minutes instead of degrees ; thus he had a means of determining the two co-ordinates of any celestial body. The places of the comet of 1677, which Tycho dis- covered, and of many stars, were determined with abso- lute certainty ; but astronomers began to be ambitious. It was necessary in using this instrument to wait till a 48 STARGAZING : PAST AND PRESENT. [BOOK i. celestial body got to the meridian. If it was missed, then they had to wait till the next day ; and further, they had no opportunity whatever of observing bodies which set in the evening. QVADRANS MAXIMYS CHALI BEUS Q.UADRATO INCLUSUS, ET Horizoi\ti Azimuthali chalybeo infiftens. FIG. 18. The Quadrans Maximus reproduced from Tycho's plate. Seeing, therefore, that clocks were improving, it was suggested by one of Tycho's compeers, the Landgrave of Hesse-Cassel, that by an instrument arranged some- thing like Fig. 18, it would be possible to determine the exact position of any body in the heavens when CHAP, iv.] TYCHO BRAHE. 49 examined out of the meridian, and so they got again to extra-meridional observations. The instrument used by Tycho Brahe for the purpose, called the Qaadrans Maximus, is represented in Fig. 18. In this there is the quadrant B,D, one pointer being placed, as shown at the bottom, near H, and the other at the top, c. These pointers or sights were directed at the star by moving the arm c, H, on the pivot A, and turning the whole arm and divided arc round on the axis N, R. The altitude of the star is then read off on the quadrant B, D, and the azimuth, or number of degrees east or west of the north and south line, is then read off on the circle Q, R, s. The screws Y, Y, served to elevate the horizontal circle, and level it exactly with the horizon, and the plum- mets w and v, hanging from G, were to show when the circle was level or not ; for the part A, G, being at right angles to the circle should be upright when the circle is level, so that if A, G, is upright in all positions when moved round the circle in azimuth, the circle is horizontal. Here, then, is an instrument very different in principle from what we had before. In this case the heavens are viewed from the most general standpoint we can obtain the horizon ; but observations such as these refer to the position of the place of observation absolutely, without any reference to the position of the body with respect to the equator or the ecliptic ; but knowing the latitude of the place of observation and the time, it was possible for a mathematical astronomer to reduce the co-ordinates to right ascension and declination, and so actually to look at the position of these bodies with reference to the celestial sphere. Tycho also had various other instruments of the same kind, differing only in the position of the quadrant D, B, E 50 STARGAZING : PAST AND PRESENT. [BOOK i. and of the circle on which the azimuth was measured. These instruments are the same in principle as our modern alt-azimuth, which will be described hereafter, S'EXTANS ASTRONOMICVS TRIGONICVS PRO DISTANTIIS rimandis. FIG. 19, Tycho's Sextant. one form having a telescope and the other being without it. Fig. 19 is yet another very important instrument in- vented by Tycho Brahe ; it is the prototype of our modern much used sextant. It was used by Tycho Brahe for CHAP, iv.] TYCHO BRAKE. 51 determining the distance from one body to another in a direct line ; a star or the moon, say, was observed by the pointers c, A, while another was observed by the pointers N, A, by another observer. The number of degrees then between JNT and c gave the angular distance of the two bodies observed. This instrument was mounted at E, so that it could be turned into any position. Not only then had this instrument its repre- sentative in our present sextant, but it was used in the same way, not requiring to be fixed in any one position. We also find represented in Tycho Brahe's book another form of the same instrument, the sight A being next the observer, instead of away from him, so that he could observe the two stars through the sights N and c with- out moving the eye. In this form only one observer was required instead of two as in the last. There was also another instrument, Fig. 6, used by this great astronomer, very similar to Ptolemy's parallactic rules, used for measuring zenith distances, or the distances of stars from the part exactly overhead. The star or moon was observed by the sights H, I, and the angle from the upright standard D, K, given by divisions on the rod E, F, D, E being placed exactly upright by a plummet, and being also able to turn on hinges at B and c, any part of the sky could be reached. There is one more of his instruments that needs notice he had so many of all kinds that space will not allow reference to more than a very few. This one was for measuring the altitudes of the stars as they passed the meridian ; it is a more con- venient form of the mural quadrant, and instead of a hole in the wall, there are sights on a movable arm, working over a divided quadrant fixed in the plane of the meridian, just like the quadrant outside the E 2 52 STARGAZING : PAST AND PRESENT. [BOOK r. horizontal circle, so the observer had no reason to move up or down according as the star was high or low. Here then ends the pre-telescopic age. Tycho was one of the very last of the distinguished astronomers who used instruments without the telescope. We began with the horizon, and we have now ended with the meridian. We also end with a power of determining the position of a heavenly body to ten seconds of space, the instrument of the Greeks reading to 10' and those of Tycho to 10". We began with the immovable earth fixed in the midst of the vault of the sky, and on this assumption Tycho Brahe made all his observations, which ended in enabling Kepler to give us the true system of the world, which was the requisite basis for the crowning triumph of Newton. BOOK II. THE TELESCOPE. CHAPTER V. THE REFRACTION OF LIGHT. IT is difficult to give the credit of the invention of the telescope to any one particular person, for, as in the case of most instruments, its history has been a history of improvements ; and whether we should give the laurel to Jansen, Baptista Porta, Galileo or to others whose names are unknown, is an invidious task to decide ; we will therefore not enter in any way into the question, interesting though it be, as to who was the inventor of the " optick tube/' as the tele- scope was called by its first users. The telescope is not a thing in the ordinary sense it is a combination of things, the things being certain kinds of lenses, concave and convex, known and used as spectacles long before they were combined to form the telescope. The first telescopes depended on the refraction of light; others, to which attention will be called in a future chapter, depended on reflection. In order to understand the action of a lens, it is necessary to understand the action of a prism. By the aid of Fig. 20 the action of the lenses of which telescopes are constructed will be understood. A prism is a piece of glass, or other transparent substance, the sides 56 STARGAZING : PAST AND PRESENT. [BOOK n. of which are so inclined to each other that its section is a triangle, and its action on light passing through it is to change the direction of the course of the beam. If we examine Fig. 21 we shall understand the action clearly. It is a known law, that when a beam of light falls obliquely on the surface of a medium more dense than that through which it has been passing, its direction is changed to a new one, nearer the line drawn at right angles to that surface, called the normal. For instance, FIG. 20. View and Section of a Prism. the ray s, i, falling on the prism at I, is bent into the course T, E, which is in a direction nearer to that of N, i, produced inside the prism. On emerging, the reverse takes place, and the ray is bent away from the normal E, N X , and takes the course E, R. In the second diagram, Fig. 21, the ray s, I, called the incident ray, coincides with the normal to the surface, so it is not refracted until it reaches the second surface, when it has its path changed to E, R, instead of taking its direct course shown by the dotted line. This bending of the ray / J - CHAP. v.J THE REFRACTION OF LIGHT. 57 is very plainly shown with an electric lamp and screen. If a trough with parallel sides be placed so as to inter- cept part of the light coming from the electric lamp, FIG. 21. Deviation of Light in Passing at Various Incidences through Prisms of Various Angles. so that part shall pass through it and part above, we have the image of the hole in the diaphragm of the lantern on the screen unchanged. Now, if the trough be filled with water, no difference whatever is made 58 STARGAZING : PAST AND PRESENT. [BOOK n. in the position of the light on the screen, because the water, which is denser than the air, is contained in a trough with parallel sides ; but by opening the sides like opening a book, or by interposing another trough with inclined sides, shaped like a V> that parallelism is destroyed, and then the light passing through it will be deflected upwards from its original course, and will fall higher on the screen ; by opening the sides more and more, one is able to alter the direction of the light passing through the prism, which has been con- structed by destroying the parallelism of the two sides. The refraction of light then depends upon the density of the substance through which it passes, on the angle of incidence of the ray, on the angle of the prism, arid also on the colour of the light, about which we shall have something to say presently. Let us now pass from the prism to the lens ; for having once grasped the idea of refraction there will be no difficulty in seeing what a lens really is. With the prism just considered, placed so that a ver- tical section is represented by a V> a ra y is thrown upwards ; if another similar prism be placed with its base in contact with the base of the other, and its apex upwards, so that its section will be represented by a V reversed, A, it is clear this will turn tjie rays down- wards, so that the rays, on emerging from both prisms will tend to meet each other, as shown in Fig. 22, where one ray is turned down to the same extent that the other is turned up ; so that by the combination of two prisms the two rays are brought to a point, which is called a focus. Now, if instead of putting the prisms base to base, they are put apex to apex, a contrary action takes place, and by this means one is able to CHAP, v.] THE KEFRACTION OF LIGHT. 59 cause two rays of light to diverge instead of converging, so that the prisms, placed apex to apex, cause the rays to diverge, and when placed base to base they cause the light to converge. If instead of having two prisms merely, there be taken a system having different angles at their apices, and from each prism there be cut a section, beginning by cutting off the apex of the most powerful prism, a slice from below the apex of the next, and a slice below the corresponding part of the next, and so on ; and then if these slices be laid on each other so as to FIG. 22. Convergence of Light by Two Prisms Base to Base. form a compound prism, and another similar prism be placed with its base to this one, we get what is represented in Fig. 23. These different slices of prisms become more and more prismatic, that is, they form parts of prisms of greater angle, as they approach the ends. We can imagine a section of such a system as thin as we please. Suppose we had such a section and put it in a lathe, rotating it on the axis A B, we should describe a solid figure, and if we suppose all the angles rounded off, so that it is made thinner and thinner as we recede from the centre, the prism system is turned 60 STARGAZING : PAST AND PRESENT. [BOOK n. into a lens having the form represented in Fig. 24. In a similar manner, lenses thinner in the middle than at the edges, called concave lenses, can be constructed, some forms of which are represented in section in Fig. 25. It is also obvious that convex lenses of all curves and FIG. 23. Formation of a Lens from Sections of Prisms. combinations of curves can be made, some of which appear in Fig. 26. The action of such lenses upon the light proceed- ing from any source may now be considered. If there is a parallel beam proceeding from a lamp, or from the sun, and it falls on the form of lens, called a convex lens, which bulges out in the middle, we learn from Fig. CHAP, v.] THE REFRACTION OF LIGHT. 61 27, that the upper part acts like the upper prism just considered and turns the light down, and the lower acts FIG. 24. Front View and Section of a Double Convex Lens. in the reverse manner and turns the light up, and the sides act in a similar manner ; and as the inclination FIG. 25. Double Concave, Plane Concave, and Concavo-Convex Lenses. of the surfaces of the lens increases as we approach the edge, the rays falling on the parts near the edge are 62 STARGAZING : PAST AND PRESENT. [BOOK IT. turned out of their course more than those falling near the centre, so that we have the rays converged to a point F, called the focus of the lens ; and as the rays from an FIG. 26. Double Convex, Plane Convex, and Concavo-Convex Lenses. electric lamp are generally rendered parallel by means of the lenses in the lantern, called the condensers, the rays from such a lamp falling on a convex lens will come to a focus at just the same distance from the lens, called FIG. 27. Convergence of Rays by Convex Lens to Principal Focus. its principal focal length as they would do if they came from the sun or stars. So far we have brought rays to a focus, and on holding CHAP, v.] THE REFRACTION OF LIGHT. 63 a piece of paper at the focus of the convex lens, as just mentioned, there appears on it a spot of light ; and every one knows that if this experiment be performed with the sun, one brings all the rays falling on the lens almost to a point, and the longer waves of light will set fire to the paper ; and on this principle burning-glasses are constructed. If, however, the rays are not parallel when falling on the lens, but diverging, they are not brought to a focus so near the lens, and the nearer the luminous source or object is, the further off will the light be brought to a focus on the other side. FIG. 28. Conjugate Foci of Convex Lens. If matters are reversed, and the luminous source be placed in the focus, the rays of light, when they leave the lens, will converge to the position of the original source ; so that there are two points, one on either side of the lens, which are the foci of each other s, s', Fig. 28, called conjugate foci; as one approaches the lens the other recedes, and vice versd, and it is obvious that when the one approaches the lens so as to coincide with the principal focus, the other recedes to an infinite distance, and the emergent rays are parallel. Now let us consider how images are formed. If we 64 STARGAZING : PAST AND PRESENT. [BOOK rr. take a candle, Fig. 29, and hold the lens a little distance away from, it, then, on placing a screen of FIG. 29. Conjugate Images. paper just on the other side of the lens, there will be a small flame depicted on it, an exact representation of FIG. 30. Diagram explaining Fig. 29. the real flame : and it is formed in this way : Consider the rays proceeding from the top of the flame, which are CHAP, v.] THE REFRACTION OF LIGHT. 65 represented separately in Fig. 30, where A represents the top. One of these rays, A a, passing through the centre of the lens 0, will be unaffected because the surfaces through which it passes are parallel to each other ; and we know from the property of the lens that all the other rays from A will, on passing through it, be brought to a focus somewhere on A a, depending on the curvature of the lens, and in the case of our lens it is at a. In like manner also all the rays from B are brought to a focus at 6, and so on with all other parts of A, B, which in this case represents the flame, each will have Fro. 31. Dispersion of Rays by a Double Concave Lens. its corresponding focus ; there being cones of rays from every point of the object and to every point of the image, having for their bases the convex lens, and we get an image or exact representation of our candle flame. Tt will further be noticed that the image a b is smaller than A B, in proportion as the distance a b is less than A B ; so that if we increase the focal length of the lens till a b is twice the distance away from the lens, it will become double its present size. If now the flame be brought nearer the lens, its image a b becomes indistinct ; and we must move the screen further away in order to render the image again clear ; p 66 STARGAZING : PAST AND PRESENT. [BOOK n. hence the place of the focus depends on the distance of the object, and the candle and its image must correspond to two conjugate foci. If now rays be passed from the lantern or sun through a concave lens, Fig. 31, they are not brought to a focus, but are dispersed and travel onwards, as if they came from a point, F, which is called its virtual focus ; and if rays are first converged by a convex lens, and then, before CA FIG. 32, Horizontal Section of the Eyeball. Scl, the sclerotic coat ; On, the cornea ; R, the attachments of the tendons of the recti muscles ; Ch, the choroid ; Gp, the ciliary processes ; Cm, the ciliary muscle ; Ir, the iris ; Aq, the aqueous humour ; Cry, the crystalline lens ; Vi, the vitreous humour ; Rt, the retina ; Op, the optic nerve ; Ml, the yellow spot. they reach the focus are allowed to fall on a concave one, we can, by placing the lenses a certain distance apart, render the converging rays again parallel ; or we can make them slightly divergent, as if they came, not from an infinite distance, but from a point a foot or two off. The application of this arrangement will appear hereafter. CHAP, v.] THE REFKACTION OF LIGHT. 67 What has now been said on the action of the convex lens will enable us to consider the optical action of the eye, without which we do little in astronomy. As to the way that the brain receives impressions from the eye we need say nothing, for that belongs to the domain of physiology, except indeed this, that an image is formed on the retina by a chemical decomposition, brought about by the dissociating action of certain rays of light in exactly the same way as on a photographic plate. Optically considered, the eye consists of nothing more than a convex lens, Cry, Fig. 32, and a surface, Rt, extending over the back of the eyeball, called the retina, on which the objects are focussed, but the rays of light falling on the cornea Cn, are refracted somewhat, so that it is not quite true to say that the crystalline lens does all the work, but for our present purpose it is sufficiently correct, and we shall consider their combined action as that of a single lens. The outer coat of the eyeball, shown in section in Fig. 32, is called the sclerotic, with the exception of that more convex part in front of the eye, called the cornea ; behind this comes the aqueous humour and then the iris, that membrane of which the colour varies in different people and races. In the centre of this is a circular aperture called the pupil, which contracts or expands according to the brightness of the objects looked at, so that the amount of light passing into the eye is kept as far as possible constant. Close behind the iris comes the crystalline lens, the thickness of which can be altered slightly by the ciliary muscle. In the space between the lens and the back of the eye is a transparent jelly-like substaoce called the vitreous humour. Finally comes the retina, a most delicate surface chiefly com- F 2 68 STARGAZING : PAST AND PRESENT. [BOOK n. posed of nerve fibres. It is on this surface, that the image is formed by the curved surfaces of the anterior mem- branes, and through the back of the eyeball is inserted the mass of filaments of the optic nerve making communica- tion with the brain ; these filaments on reaching the inside of the eye spread out to receive the impressions of light. Here then, we hare a complete photographic camera ; the crystalline lens and cornea, separated by the aqueous humour, representing the compound-glass camera lens, and the retina standing in the place of the sensitive plate. V FJG. 33. Action of Eye in Formation of Images. The path of the light forming an image on the retina is shown in Fig. 33, where A B is the object, and a b its image, formed in exactly the same way as the image of the candle-flame which we have just considered ; in fact, the eye is exactly represented by a photographic camera, the iris acting in the same manner as the stops in the lens, limiting its available area, and by con- tracting, decreasing the amount of light from bright objects, and at the same time increasing the sharp- ness of definition, for in the case of the eye, the luminous rays obey the known laws of propagation of light in media of variable form and density, and we have CHAP, v.] THE REFRACTION OF LIGHT. 69 only simple refraction to deal with. The next matter to be considered is that the nearer the object A B is to the eye, the larger is the angle A, 0, B, and also a, o, 6, and therefore the image on the retina is larger ; but there is a limit to the nearness to which the object can be brought, for, as we found with the candle, the distance between the lens and the image must be increased as the object approaches, or the curvature of the lens itself must be altered, for if not the ray forming the rays from each point of the object will be too divergent for the lens to be able to bring them to a focus. Now in the eye Fib. 34. Action of a Long-sighted Eye. there is an adjustment of this sort; but it is limited so that objects begin to get indistinct when brought nearer the eye than perhaps six inches, because the rays become too divergent for the lens to bring them to a focus on the retina, and they tend to cdme to a focus behind the retina; as in Fig. 34 ; but we may assist the eye lens by using a glass convex lens in front of it, between it and the object. It is for this reason that spectacle glasses are used to enable long-sighted persons to see clearly. We may also use a inuch stronger lens, and so get the object very near the lens and eye, as in Fig. 35, where 70 STARGAZING : PAST AND PRESENT. [BOOK n. a b is the object so near the eye that, if it were not for the lens L, its image would not come to a focus on the retina at all. The effect of the lens is to make the rays proceeding in a cone from a and b less divergent, so that after passing through it, they proceed to the eye- lens as if they were coming from the points A and B, a foot or so away from the eye, and so the object a b appears to be a much larger object at a greater distance from the eye. FIG. 35. 1. Diagram showing path of rays when viewing an object at an easy distance. 2. Object brought close to eye when the lens L is required to assist the eye-lens to observe the image when it is magnified. A convex lens then has the power of magnifying objects when brought near the eye, and its action is clearly seen in Fig. 35, where the upper figure shows the arrow at as short a distance from the eye as it can be seen distinctly with an ordinary eye, and the lower figure shows the same arrow brought close to the eye, and rendered dis- tinctly visible by the lens when a magnified image is thrown on the retina, as if there was a real larger arrow somewhere between the dotted lines at the ordinary CHAP, v.] THE REFRACTION OF LIGHT. 71 distance of distinct vision. It is also obvious that the nearer the object can be brought to the eye-lens the more magnified it is, just as an object appears larger the nearer it is brought to the unaided eye. We have been hitherto dealing with the effect of a convex lens on the rays passing to the eye. We will now deal with a concave one. We found that the power of adjustment of the normal eye was sufficient to bring parallel rays, or those proceeding from a very distant object, and also slightly FIG. 36. Action of Short-sighted Eye. diverging rays, to a focus on the retina. Parallel or slightly divergent rays are most easily dealt with, and slightly convergent rays can also be focussed on the retina ; but if the eye-lens is too convex, as is the case with short-sighted people, Fig. 36, a concave lens of slight curvature is used to correct the eye-lens and bring the image to a focus on the retina instead of in front of it. If the rays are very convergent, as those proceeding from a convex lens and coming to a focus, the lens of a normal eye will bring them to a focus far in front of the retina, as if the person were very short-sighted. But by interposing a sufficiently powerful concave lens the rays are made less convergent or parallel, and the eye- 72 STARGAZING : PAST AND PRESENT. [BOOK n. lens brings them to a focus on the retina, as if they came from a near object, so the use of convex and concave lenses placed close to the eye is to render divergent or convergent rays nearly parallel, so that the eye-lens can easily focus them, and therefore one of the conditions of the telescope is that the rays which come into our eye shall be parallel or nearly so. lj 1 ii ii A a i UN1VKKS1TV OF CALlFOIiXlA. CHAPTEK VI. THE REFRACTOR. IN the telescope as first constructed by Galileo there are two lenses, so arranged that the first, a convex one, A B Fig. 37, converges the rays, while the second, c D, FIG. 37. Galilean Telescope. A B, convex lens converging rays; CD, concave lens sending thein parallel again and fit for reception by the eye. a concave one, diverges them, and renders them parallel, ready for the eye ; the rays then, after passing through c D, go to the eye as if they were proceeding along the dotted lines from an object M M, closer to the eye instead of from a distant object, and so, by means of the telescope, the object appears large and close. It is this that constitutes the telescope. But now-a- days we have other forms, as we are not content with 74 STARGAZING : PAST AND PRESENT. [BOOK n. the convex combined with the concave lens, and modern astronomy requires the eyepiece to be of more elaborate construction than those adopted by Galileo and the first users of telescopes, although this form is still used for opera-glasses and in cases where small power only is required. Having the power of converging the light and forming an image by the first convex lens or object glass, as we saw with the candle flame (Fig. 29), and an opportunity of enlarging this image by means of a mag- nifying or convex eyepiece, we can bring an image of the moon, or any other object, close to the eye, and examine it by means of a convex lens, or a combination of such lenses. So we get the most simple form of refracting telescopes represented in Fig. 38, in which the rays from all points of the object let us take for instance an arrow are brought to a focus by the object-glass A, forming there an exact representation of the real arrow. In the figure two cones of rays only are delineated, namely, those forming the point and feather of the arrow, but every other point in the arrow is built up by an infinite number of cones in the same way, each cone having the object-glass for its base, By means of the lens c we are able to examine the image of the arrow B, since the rays from it are thus rendered parallel, or nearly so, and to the eye they appear to come from a much larger arrow at a short distance away. We can draw their apparent direction, and the apparent arrow (as is done in Fig. 37 by the dotted lines), and so the object appears as magnified, or, what comes to the same thing, as if it were nearer. The difference between this form and that contrived by Galileo is this : in the latter the rays are received by the eyepiece while converging, and rendered parallel CHAP, vi.] THE REFRACTOR. 75 by a concave lens, while in the former case the rays are received by the eyepiece on the other side of the focus, where they have crossed each other and are diverging, and are rendered parallel by a convex lens. We may now sum up the use of the eye-lens. The image is brought to a focus on the retina, because the object is some distance off, and the rays from every point, (as from A and B, Fig. 35), on reaching the eye, are nearly parallel ; but it is not necessary that they should be absolutely parallel, as the eye is capable of a small ad- justment, but if one wishes to see an object much nearer FIG. 38. Telescope. A, object-glass, giving an image at B ; c, lens for magnify- ing image B. (as in the lower figure), it is impossible to do it unless some optical aid is obtained, for the rays are too divergent, and cannot be brought to a focus on the retina. What does that optical aid effect ? It enables us to place the object in the focus of another lens which shall make the rays parallel, and fit for the lens of the eye to focus on the retina, and since the object can by this means be brought close to the lens and eye, it forms a larger image on the -retina. Dependent on this is the power of the telescope. We shall refer later on to the mechanical construction of the telescope. Here it may be merely stated that 76 STARGAZING : PAST AND PRESENT. [BOOK n. the smaller ones consist of a brass tube, the object- glass held in a brass ring screwed in at one end of the tube and a smaller tube carrying the eyepiece sliding in and out of the large tube and sometimes moved by a rack and pinion motion, at the other. The larger ones as mounted for special uses will also be fully described farther on. The power of the telescope depends on the object- glass as well as on the eyepiece ; if we wish to magnify the moon, for instance, we must have a large image of the FIG. 39. Diagram Explaining the Magnifying Power of Object-glass. moon to look at, and a powerful lens to see that image. By studying Fig. 39 the fundamental condition of producing a large image by a lens will be seen. Suppose we wish to look at an object in the heavens, the diameter of which is one degree ; if the lens throws an image of that body on to the circumference of a circle of 360 inches, then, as there are 360 degrees in a circle, that image will cover one inch ; let the circle be 360 yards, and the image of a body of one degree will cover one yard ; and to take an extreme case and suppose the circumference of the circle to be 360 miles, then the image will be one mile in diameter. CHAP. vi.J THE REFRACTOR. 77 This is one of the principal conditions of the action of the object-glass in enabling us to obtain images which can be magnified by a lens, and by such magnification made to appear nearer to us than they are. Galileo used telescopes which magnified four or five times, and it was only with great trouble and expense that he produced one which magnified twenty-three times. Now, after what has been said of focal length, one will not be surprised to hear of those long telescopes produced in the very early days, a few of which are still extant ; these show as well as anything the enormous difficulty which the early employers of telescopes had to deal with in the material they employed. One can scarcely tell one end of the telescope from the Qther ; all the work was done in some cases by an object-glass not more than half an inch in effective diameter. It might be supposed that those who studied the changes of places and the positions of the heavenly bodies would have been the first to gain by the invention of the telescope, and that telescopes would have been added to the instri^ments already described, replacing the pointers. For sucl} a use as this a telescope of half an inch aperture would have been a great assistance. But things did not happen so, because the invention of the telescope gave such an impetus to physical astronomy that the whole heavens appeared novel to mankind. Groups of stars appeared which had never been seen before; Jupiter and Saturn were found to be attended by satellites ; the sun, the immaculate sun, was deter- mined after all to have spots, and the moon was at once set upon and observed with diligence and care ; so that there was a very good reason why people should not limit the powers of the telescope to employing it to determine 78 STARGAZING: PAST AND PRESENT. [BOOKI positions only. The number of telescopes was small, and they could not be better employed than in taking a survey of all the marvellous things which they revealed. It was at this time that the modern equatorial was fore- shadowed. Galileo, and his contemporaries Scheiner and others, were observing sun-spots, and the telescope, Fig. 40, which Scheiner arranged, a very rough instru- FIG. 40. Schemer's Telescope. ment, with its axis parallel to the earth's axis, and allowed to turn so that Schemer might follow the sun for many hours a day, was one of the first. This instru- ment is here reproduced, because it was one of the most important telescopes of the time, and gathered in to the harvest many of the earliest obtained facts. Since by means of little instruments like these, so much of beauty and of marvel could be discovered in the skies, it is no wonder that every one who had CHAP, vi.] THE REFKACTOE. 79 anything to do with telescopes strained his nerves to make them of greater power, by which more marvels could be revealed. It was not long before those little instruments of Scheiner expanded into the long telescopes to which reference has been made. But there was a difficulty introduced by the length of the instrument. The length of the focus necessary for magnification spread the light over a large area, and therefore it was necessary to get an equivalent of light by increasing the aperture of the object-glasses in order that the object might be sufficiently bright to bear considerable magnification by the eyepiece, and now arose a tremendous difficulty. One part of refraction, namely, deviation, enables us to obtain, but the other half, dispersion, prevents our obtaining, except under certain conditions, an image we can make use of. By dispersion is meant the pro- perty of splitting up ordinary light into its component colours, of which we shall say more in dealing with spectrum analysis. If we wish to get more light by increasing the aperture of the telescope, the deviation of the light passing through the edge of the object- glass is increased, and with it the dispersion, the result of this increase of deviation. If the light of the sun be allowed to fall through a hole into a darkened chamber, and then through a prism, Fig. 41, it is re- fracted, and instead of having an exact reproduction of the bright circle we have a coloured band or spectrum. The white light when refracted is not only driven out of its original course deviated but it is also broken up dispersed into many colours. We have a con- piderable amount of colour ; and this the early observers found when they increased the size of their telescopes, 80 STARGAZING : PAST AND PRESENT. [BOOK IT. for it must be remembered that a lens is only a very complex prism. First, they increased the size by enlarging the object-glasses, and not the focal length ; but when they had done that they had that extremely objection- able colour which prevented them seeing anything FIG. 41. Dispersion of Light by Prism. well. The colour and indistinctness came from an overlapping of a number of images, as each colour had its own focus, owing to varying refrangibilities. They found, therefore, that the only effective way of in- creasing the power of the telescope was by increasing its focal length so as to reduce the dispersing action as much as possible, a,nd so enlarging the size of the CHAP, vi.] THE REFRACTOR. 81 actual image to be viewed, without at the same time increasing the angular deviation of the rays transmitted through the edges of the lens. The size of the image corresponding to a given angular diameter of the object is in the direct proportion of the focal length, while the flexure of the rays which converge to form any point of it is in the same proportion inversely. To take an example. In the case of an object-glass of crown-glass, the space over which the rays are dispersed is one-fiftieth of the distance through which FIG. 42. Diagram Showing the Amount of Colour Produced by a Lens. they are deviated, and it will be seen by reference to Fig. 42, that if the red rays are at R, and the blue at B, the distance A B is fifty times R B, and as these distances depend on the diameter of the lens only, we can increase the focal length, and so increase the size of the image without altering the dispersion R B, and so throw the work of magnifying on the object-glass instead of on the eyepiece, which would magnify R B equally with the image itself. So that in that time, and in the time of Huyghens, telescopes of 100, 200, and 300 feet focal Tr 82 STARGAZING : PAST AND PRESENT. [BOOK n. length were not only suggested but made, and one enthusiastic stargazer finished an object-glass, the focal length of which was 600 feet. Telescopes of 100 and 150 feet focal length were more commonly used. The eyepiece was at the end of a string, and the object- glass was placed free to move on a tall pole, so that an observer on the ground, by pulling the string, might get the two glasses in a line with the object which he wished to observe. So it went on till the time of Sir Isaac Newton, who considered the problem very carefully but not in an absolutely complete way. He came to the con- clusion, as he states in his Optics, that the improvement of the refracting telescope was " desperate ; " and he gave his attention to reflecting telescopes, which are next to be noticed. Let us examine the basis of Sir Isaac Newton's state- ment, that the improvement of the refracting telescope was desperate. He came to the conclusion that in re- fraction through different substances there is always an unchanged relation between the amount of dispersion and the amount of deviation, so that if we attempt to correct the action of one prism by another acting in an opposite direction in order to get white light, we shall destroy all deviation. But Sir Isaac Newton happened to be wrong, since there are substances which, for equiva- lent deviations, disperse the light more or less. So by means of a lens of a certain substance of low dispersive power we can form an image slightly coloured, and we can add another lens of a substance having a high dis- persive power and less curvature and just reverse the dispersion of the first lens without reversing all its deviating power. CHAP, vi.] THE REFRACTOR. 83 The following experiments will show clearly the appli- cation of this principle. We first take two similar prisms arranged as in Fig. 43. The last through which the light passes corrects the deviation and dispersion of the first. We then take two prisms, one of crown glass and the other of flint glass, and since the dispersion of the flint is greater than that of the crown, we imagine with justice that the flint-glass prism may be of a less angle than the other and still have the same dispersive power, and at the same time, seeing that the angles FIG. 43. Decomposition and Recomposition of Light by Two Prisms. of the prisms are different, we may expect to find that we shall get a larger amount of deviation from the crown-glass prism than from the other. If then a ray of light be passed through the crown- glass prism, we get the dispersion and deviation due to the prism A Fig. 44, giving a spectrum at D. And now we take away the crown glass and pla,ce in its stead a prism of flint glass inverted ; the ray in this instance is deviated less, but there is an equal amount of colour- ing at D'. If now we use both prisms, acting in opposite G 2 84 STARGAZING : PAST AND PRESENT. [BOOK ii. directions, we shall be able to get rid of the colours, but not entirely compensate the deviation. We now place the original crown -glass prism in front of the lantern and then interpose the flint-glass prism, so that the light FIG. 44. Diagram Explaining the Formation of an Achromatic Lens. A, crown- glass prism ; B, flint-glass prism of less angle, but giving the same amount of colour ; c, the two prisms combined, giving a colourless yet deviated band of light at D". shall pass through both. The addition of this prism of flint, of greater dispersive power, combines, or as it were shuts off, the colour, leaving the deviation uncom- pensated, so that we get an uncoloured image of the hole- CHAP. vi.'J THE REFRACTOR. 85 in front of the lantern at D". ' This is the foundation of the modern achromatic telescope. Another method of showing the same thing is to bring a V-shaped water-trough into the path of the rays from the lantern ; then, while no water is in it, the beam of light passing through it is absolutely uncoloured and undeviated. In this case we have no water inclosed by these surfaces, and it is not acting as a prism at all. If, however, a prism of flint glass, a substance of high dispersive power, is introduced into it, with its refracting edge upwards, it destroys the condition we had before, and we have a coloured band on the screen, because the glass tha,t the prism is made of has the faculty of strong dispersion in addition to its deviation. We can get rid of that dispersion by throwing dispersion in a contrary direction by filling up the trough with water, and so making, as it were, a water prism on either side of the glass one, water being a substance of low dis- persive power. We have a colourless beam thrown on the screen, which is deviated from the original level, because the water prisms are together of a greater angle than the glass one. The experiments of Hall and Dolland have resulted in our being able to combine lenses in the same way that we have here combined prisms, bearing in mind what has been said in reference to the action of lenses being like that of so many prisms ; and we may consider two lenses, one of crown and the other of flint glass, Fig 45. The crown glass being of a certain curvature will give a certain dispersion ; the flint glass, in consequence of its great dispersive power, will require less curvature to correct the crown glass. What will happen will be this : assuming the second lens to be away, the rays will 86 STARGAZING : PAST AND PRESENT. [BOOK n. emerge from the first (convex) lens and form a coloured image at A. But if the second flint-glass concave lens be interposed it will, by means of its action in a contrary direction, undo all the dispersion due to this first lens and a certain amount of deviation, so that we shall get the combination giving an almost colourless image at B. It will not be absolutely colourless, for the reasons which will be now explained. If light be passed through different substances placed in hollow prisms, or through prisms of flint and crown glass, and the spectra thus produced be observed, we find there are FIG. 45. Combination of Flint- and Crown-glass Lenses in an Achromatic Lens. important differences. "When we expand the spectra considerably, we see that the action of these different substances is not absolutely uniform, some colours ex- tending over the spectrum further than others. In the case of one kind of glass the red end of the spectrum is Crushed up, while in the other we have the red end expanded. This is called the irrationality of the spectrum pro- duced by prisms of different substances. The crown and the flint-glass lenses and for telescopes we must use such glass give irrational spectra, so that the achromatic telescope is not absolutely achromatic, in consequence CHAP, vi.] THE REFRACTOR. 87 of this peculiarity ; for if R, G, B, Fig. 46, are the centres of the red, green, and violet in the spectrum given by a prism composed of the glass of which one lens is made, and R', G', B', are those of the other, if the lenses are placed so as to counteract each other, and are of such curves that the reds and violets are combined, the greens will remain slightly outstanding. Suppose, as in the drawing, the second prism disperses the violet as much as the first one does, then, when these are reversed they will exactly compensate red and violet. But the second one acts more strongly on the green than the first, which will be over-compensated ; and if we weaken the second FIG. 46. Diagram Illustrating the Irrationality of the Spectrum, prism so that the green and red are correct, then the violet will be slightly outstanding, which in practice is not much noticed, except with a very bright object when there is always outstanding colour. This is, however, not a matter of any very great im- portance for ordinary work, since the visual rays all lie in the neighbourhood of the yellow, so that opticians take care to correct their lenses for the rays in this part of the spectrum, and at the same time, as a matter of necessity, over-correct for the violet rays, that is, reverse the dispersion of the exterior lens, so that the violet rays have a longer instead of a shorter focus than the 88 STARGAZING : PAST AND PRESENT. [BOOK n. red, and, therefore, in looking at a bright object, such as a first magnitude star, it appears surrounded by a violet halo ; with fainter objects the blue light is not of sufficient intensity to be visible. It is, therefore, always preferable to correct for the most visible rays and leave the outstanding violet to take care of itself; but never- theless various proposals have been made to get rid of it. Object-glasses containing fluids of different kinds have been tried, but they have never become of any practical value, and it does not seem probable that they ever will. In order to get rid of the outstanding violet colour when the remainder of the spectrum was corrected, Dr. Blair constructed object-glasses the space between the lenses of which were filled with certain liquids, generally a solution of a salt of mercury or antimony, with the addition of hydrochloric acid ; for in the spectrum given by the metallic solution the green is proportionally nearer the red than is the case with the spectrum produced by hydrochloric acid, so that by the adjustment of the different solutions he exactly destroyed the outstanding colour of the ordinary combination. In this way Sir John Herschel tells us he was able to construct lenses of three inches aperture and only nine inches focal length, free from chromatic and spherical aberration. It was proposed by Mr. Barlow to correct a convex crown-glass lens for chromatic aberration by a hollow concave lens containing bisulphide of carbon, a highly dispersive fluid, having double the power of flint glass. This lens was placed in the cone of rays between the object-glass and the eyepiece. Its surfaces were con- cavo-convex, calculated to destroy spherical aberration, and its distance from the object-glass was varied until CHAP, vi.] THE REFRACTOR. 89 exact achromatism was obtained. A telescope on this principle of eight inches aperture was made by Mr. Barlow, which proved highly satisfactory. In the early part of the last century it was proposed by Wolfius to interpose between the object-glass and eyepiece a concave lens in order to give greater magnification of the image, with a slight increase of focal length ; if an ordinary lens be used the achromatism of the images given by the object- glass will be destroyed* Messrs. Dolland and Barlow, however, proposed to make the concave lens achromatic, so that the image is as much without colour when the lens is used as without it. Mr. Dawes found such a lens to work extremely well. These lenses, usually called " Barlow lenses," are generally made about one inch in diameter, and by varying their distance from the eye- piece the image is altered in size at pleasure. In the reflecting telescope, with which we will now proceed to deal, there is an absence of colour ; but the reflector is not without its drawbacks, for there are imperfections in it as great as those we have been considering in the case of the 'refractor. it AHY , r: R s i T v , p j CALIFORNIA. J CHAPTER VII. THE REFLECTION OF LIGHT. WE have now dealt with the refraction of light in general, includiDg deviation and dispersion, in order to see how it can assist us in the formation of the telescope ; and we have shown how the chromatic effect of a single lens can be got rid of by employing a compound system composed of different materials, and so we have got a general idea of the refracting telescope. We have now to deal with another property of light, called reflec- tion ; and our object is to see how reflection can help us in telescopes. In the case of reflection we get the original direction of the ray changed as in the case of refraction, but the deviation is due to a different cause. Take a bright light, a candle will do, and a mirror fixed so that the light falls on its surface and is thrown back to the eye, Fig. 47, we see the image of the candle apparently behind the mirror ; the rays of light falling on the mirror are reflected from it at exactly the same angle at which they reach it. This brings us in the presence of the first and most important law of reflection ; and it is this, at whatever angle the light falls on a mirror, at that angle will it be reflected. As it is usually expressed, the angle CHAP. VIL] THE REFLECTION OF LIGHT. 91 of incidence, which is the angle made by the incident ray with an imaginary line drawn at right angles to the mirror, called the normal, is equal to the angle of reflection, that is, the angle contained by the reflected ray, and the normal to the surface. In order, therefore, to find in what direction a ray of light will travel after striking a flat polished surface, we must draw a line at right angles to the surface at the point where the ray impinges on it, then the reflected ray will make an angle with the normal equal to that which the incident ray FIG. 47. Diagram Illustrating the Action of a Reflecting Surface. makes, or the angles of incidence and reflection will be equal. Very simple experiments, which every one can make will show us the laws which govern the phenomena of reflection. Let us employ a bath of mercury for a reflecting surface, and for a luminous object a star, the rays of which, coming from a distance which is prac- tically infinite, to the surface of the earth, may be considered exactly parallel. The direction of the beams of light coming from the star, and falling on the 92 STARGAZING : PAST AND PRESENT. [BOOK ii. mirror formed by the mercury, is easily determined by means of a theodolite, Fig. 48. If we look directly at the star, the line i' s' of the telescope indicates the direction of the incident luminous rays, and the angle FIG. 48. Experimental Proof that the Angle of Incidence = Angle of Reflection. s' T' N', equal to the angle ?, I, N, is the angle of inci- dence, that is to say, that formed by the luminous ray with the normal to the surface at the point of incidence. In order to find the direction of the reflected lumi- nous rays, we must turn the telescope on its axis, until CHAP, vii.] THE REFLECTION OF LIGHT. 93 the rays reflected by the surface of the mercury bath enter it and produce an image of the star. When the image is brought to the centre of the telescope, it is found that the angle R' i' N' is equal to the angle of reflection N, I, R. Thus, in reading the measure on the graduated circle of the theodolite the angle of reflection can be compared with the angle of incidence. Now, whatever may be the star observed, and what- ever its height above the horizon, it is always found that there is perfect equality between these angles. Moreover, the position of the circle of the theodolite which enables the star and its image to be seen evi- dently proves that the ray which arrives directly from the luminous point and that which is reflected at the surface of ths mercury are both in the same vertical plane. Now this demonstrates one of the most important laws of reflection. The laws of refraction do not deal directly with the angles themselves, but with the sines of the angles ; in reflection the angles are equal ; in refraction the sines have a constant relation to each other. So far we have dealt with plane surfaces, but in the case of telescopes we do not use plane surfaces, but curved ones, so we will proceed at once to discuss these. In Fig. 49, A represents a curved surface, such as that of a concave mirror, the centre of curvature being c. Now we can consider that this curved surface is made up of an infinite number of small plane surfaces, and since all lines drawn from the centre, c, to the mirror, will be at right angles to the surface at the points where they meet it, we find, from our experiment with the plane mirror, that rays falling on the mirror at these 94 STARGAZING: PAST AND PRESENT. [BOOK ir. points will be reflected so that the angles on either side of each of these lines shall be equal ; so, for instance, in Fig. 49, we wish to find to what point the upper ray will be reflected, and we draw a line from the centre, c, to the FIG. 49. Convergence of Light by Concave Mirror. point where it falls on the mirror, and then draw another line from that point making the angle of reflection equal to that made by the incident ray, and we can consider the small surface concerned in reflection flat, so that the ray will in this case be reflected to F. If now we take FIG. 50. Conjugate Foci of Convex Mirror. any other ray, and perform the same operation we shall find that it is also reflected nearly to F, and so on with all other parallel rays falling on the mirror ; and this point, F, is therefore said to be the focus of the mirror. CHAP. VII.] THE REFLECTION OF LIGHT. 95 If now the rays, instead of falling parallel on the mirror, as if they came from the sun or a very distant object, are divergent, as if they came from a point s, Fig. 50, near the mirror, the rays approach nearer to the lines drawn from the centre to the mirror, one of which is FIG. 51. Formation of Image of Candle by Reflection. represented by the dotted line ; or, in other words, the angles of incidence become reduced, and so the angles of reflection will also be reduced, and the focus of the rays from s will approach the centre of the mirror, and be at s ; just so it will be seen that if an illuminated point 96 STARGAZING : PAST AND PRESENT. [BOOK n. be at s, its focus will be at s, and these two points are therefore called conjugate foci. If a candle is held at a short distance in front of a concave mirror, as represented in Fig. 51, its image appears on the paper between the candle and the mirror, so that the rays from every point of the flame are brought to a focus, and produce an image just as the im- age is produced by a convex lens. If we study Fig. 52 the formation of this image will be clearly understood. First we must note that the rays A, c, a, and B, c, b, which FIG. 52. Diagram explaining Fig. 51. pass through the centre of curvature of the mirror c, will fall perpendicularly on the surface, and be reflected back on themselves, so that the focus of the part A of the arrow will be somewhere on A a, and that of B on B b, and by drawing another ray we shall find it reflected to , which will be the focus of the point A, and so also by drawing another line from B, we shall find it is reflected to 6, which is the focus of the part B ; and we might re- peat this process for every part of the arrow, and for every ray from those parts. We now see that since the CHAP. VIL] THE REFLECTION OF LIGHT. 97 rays A a and B b cross each other at c, the distance from a to b bears the same proportion to the distance from A to B as their respective distances from the point c ; or, in other words, the image is smaller than the object in the same proportion as the distance from the image to c is smaller than the distance from the object to c. Now, in dealing with the stars, which are at a practically in- finite distance, the rays are parallel, and will be brought to. a focus half-way between the mirror and its centre of curvature. In this case, therefore, the distance from the image to the mirror is equal to that from the image to the centre, so that we can express the size of the image by saying that it is smaller than the object, in proportion as its distance from the mirror is smaller than the distance of the object from c ; and as it makes little difference whether we measure the distance of the stars from c or from the mirror, and as c is not always known, we can take the relation of the distances of the object and image from the mirror as representing the proportionate sizes of the two. We will now consider the case of rays falling on a mirror curved the other way, that is, a convex mirror. Let us consider the ray impinging at D, Fig. 5 3, which would go on to c, the centre of the mirror. Now, as CD is drawn from the centre, it is at right angles to the mirror at D, and the ray L D, being in the same straight line on the opposite side, will also be at right angles, and will be reflected back on itself. Now take the ray I A, draw c E through A, then E A will be perpendicular to the surface at A, and i A E will be the angle of incidence, and E A G the angle of reflection, so that this ray A G will be reflected away from L D, and so will all the other rays falling on the mirror as K B : and if we continue H 98 STARGAZING : PAST AND PRESENT. [BOOK n. the lines G A and H B backwards, they will meet at M, and therefore the rays diverge from the mirror as if they came from a point at M, and this point is called the virtual focus. So much for parallel rays. Next let us consider another case which happens in the telescope, namely, where converging rays fall on a convex mirror, as in Fig. 53, where we consider the light proceeding to the mirror from a converging lens along the lines H B and G A, these FIG. 53. Reflection o,t Rays, "by Convex Mirror. will be made parallel, at B K and A F, after reflection, and it is manifest that by making the mirror sufficiently con- vex, these rays, tending to come to a focus at M, could be rendered divergent ; and if the curvature is decreased by making the centre of curvature at a certain distance beyond c, it will be seen at once by the diagram that these rays will after reflection, converge towards L and will come to a focus in front of the mirror at a point further in front than c is behind it, so that they have CHAP, vii.] THE REFLECTION OF LIGHT. 99 been rendered less convergent only by the mirror in this supposed case. It will be seen from what has been stated here and in Chapter V., that we get nearly the same results from reflection as we did from refraction when we were con- sidering the functions of glasses instead of mirrors ; that a concave mirror acts exactly as a convex lens, and vice versa, so that they can be substituted the one for the other. If we take a mirror, and allow the light to fall on it from a lamp, no one will have any difficulty in seeing that the mirror grasps the beam, and forms an image which is seen distinctly in front of the mirror, just as one gets an image from a convex lens behind it. H 2 CHAPTER VIII. THE KEFLECTOK. THE point we have next to determine is how we can utilise the properties of reflection for the purposes of astronomical observation. Many admirable plans have been suggested. The first that was put on paper was made by Gregory, who pointed out that if we had a concave mirror, we should get from this mirror an image of the object viewed at the focus in front of it, as in Fig. 51. Of course we cannot at once utilise this focal image by using an eyepiece in the same way as we do in a refractor, because the observer's head would stop the light, and the mirror would be useless, and all the suggestions which have been made, have reference to obtaining the image in such a position that we are able to view it conveniently. Gregory, the Scottish astronomer above referred to, in 1663 suggested a method, and it has turned out to be a good one, of utilizing reflection by placing a small mirror D c, Fig. 54, on the other side of the focus A of the large one, at such a distance that the image at A is again focussed at B by reflection from the small mirror ; and at B we get of course an enlarged image of A. The rays of light proceeding to B would, CHAP. VIIL] THE REFLECTOR. 101 however, be intercepted by the large mirror, unless an aperture were made in the large mirror of the size of the small one through which the rays could pass and be rendered parallel by means of an eyepiece placed just behind the large mirror. So that towards the object is the small mirror c, and there is an eyepiece E, which enables the image of the object to be viewed after two reflections, first from the large mirror and then from the small one. Mr. Short (who made the best tele- scopes of this construction, and did much for the optical FIG. 54 Reflecting Telescope (Gregorian). science of the last century) altered the position of the small mirror with reference (:o the focus of the large one, by sliding it along the tube by a screw arrangement, F, and so was enabled to focus both near and distant objects without altering the eyepiece. But before this was put into practice, Sir Isaac New- ton (in 1666) made telescopes on a totally different plan. The eyepiece of the Newtonian telescope is at the side of the tube, and not at the end, as in Gregory's. We have next to inquire how this arrangement is carried out, and, like most things, it is perfectly simple when one 102 STARGAZING : PAST AND PRESENT. [BOOK ii. knows how it is done. There is a large mirror at the bot- tom of the tube as in the Gregorian, but not perforated, and the focus of the mirror would be somewhere just in front of the end of the tube. Now in this case we do not allow the beam to get to the focus at all in the tube or in front of it ; but before it comes to the focus it is re- ceived on a small diagonal plane surface ra, and thus it is at once thrown outwards at right angles through the side of the tube, and comes to a focus in front of an eyepiece, placed at the side, ready to be viewed the same as an image from a refractor (Fig. 55). FIG. 55. Newton's Telescope. The next arrangement is one which Mr. Grubb has recently rescued from obscurity, and it is called the Cassegrainian form. It will be seen on referring to that, Fig. 56, if the small mirror, c, were removed, the rays from the mirror A B would come to a focus at F. , In the Gregorian construction a concave reflector was used outside that focus (at c, Fig. 54), but Casse- grain suggested that if, instead of using a concave reflector outside the focus, a reflector with a convex surface were placed inside it, we should arrive at very nearly the same result, provided we retain the hole in the large mirror. The converging rays from A B will fall on the CHAP. VIII.] THE REFLECTOR. 103 convex surface of the mirror c, which is of such a curvature and at such a distance from F, the focus of the large mirror, that the rays are rendered less con- verging, and do not come to a focus until they reach D, FIG. 56. Reflecting Telescope (Cassegrain). where an image is formed ready to be viewed by the eyepiece E. It appears from this, that the convex mirror is in this case acting somewhat in the same manner as the concave lens does in the Galilean telescope. FIG. 57. Front View Telescope (Herschel). Then, lastly, we have the suggestion which Sir William Herschel soon turned into more than a suggestion. The mirror M in this arrangement is placed at the bottom of the tube as in the other forms, but, instead of being placed flat on the bottom it is slightly tipped, so that 104 STARGAZING : PAST AND PRESENT. [BOOK n. if the eyepiece is placed at the edge of the extremity of the tube all parallel rays falling on the mirror are reflected to the side of the tube at the top where the eyepiece is, instead of being reflected to a convex or other mirror in the middle. This is called the front view telescope, and it enabled Sir William Herschel to make his discoveries with the forty-feet reflector. With small telescopes this form could not be adopted, as the observer's head would cover some part of the tube and obstruct the light, but with large telescopes the amount of light stopped by the head is small in proportion to what would be lost by using a small mirror. These are in the main the four methods of arranging reflecting telescopes the Gregorian, the Cassegrainian, the Newtonian, and the Herschelian. In order to make large reflectors perfect large tele- scopes of short focus, because that is one of the require- ments of the modern astronomer we have to battle against spherical aberration. We have already seen that the power of substances to refract light differs for different colours, and we have seen the varied refraction of different parts of the spectrum, and the necessity of making lenses achromatic. Now there is one enormous advantage in favour of the reflector. We do not take our light to bits and put it together again as with an achromatic lens. But curiously enough, there is a something else which quite lowers the position of the reflector with regard to the refractor. Although, in the main all the light falling in parallel lines on a concave surface is reflected to a focus, this is only true in a general sense, because, if we con- sider it, we find an error which increases very rapidly CHAP, viii.] THE REFLECTOR. 105 as the diameter of the mirror increases or as the focal length diminishes. For instance, D i, Fig. 58, is the segment of a circle, or the section of a sphere if we deal with a solid figure. D c, E G and H i, are three lines representing parallel rays falling on different parts of it. According to that law which we have considered, we can find w^here the ray E G will fall. We draw a line L, G, from the centre to the point of reflection, and make FIG. 58. Diagram Illustrating Spherical Aberration. the angle F G L, equal to the angle of incidence E G L ; then F will be the focus, so far as this part of the mirror is concerned. Now let us repeat the process for the ray H I, and we shall find that it will be reflected to K, a point nearer the mirror than F, and it will be seen that the further the rays are from the axis D c, the further from the point F is the light reflected ; so that if we consider rays falling from all parts of the reflecting surface, a not very large but a distinctly visible surface is covered with 106 STARGAZING : PAST AND PRESENT. [BOOK n. light, so that a spherical surface will not bring all the rays exactly to a point, and with a spherical mirror we shall get a blurred image. We can compare this imperfection of the reflector, called spherical aberration, with the chromatic aberration of the object-glass. Newton early calculated the ratio of imperfection depending upon these properties of light, first of dis- persion and then of spherical aberration, and he found that in the refracting telescope the chromatic aberration was more difficult to correct and get rid of than the spherical aberration of the reflector, so that in Newton's FIG. 59. Diagram Showing the Proper Fbrm of Reflector to be an Ellipse. time, before achroiiiatic lenses were constructed, the reflector with its aberration had the advantage. It must now be explained how this difficulty is got over. What is required to produce a mirror capable of being used for astronomical purposes, is to throw back the edges of the mirror to the dotted line ACI, Fig. 58, which will make the margin of the mirror a part of a less concave mirror, and so its focus will be thrown further from itself to F, instead of to K. Now let us consider what curve this is, that will throw all the rays to one point. It is an ellipse, as will be seen by reference to Fig. 59, in which, instead of having a spherical surface the CHAP, vni.] THE REFLECTOR. 107 section of which is a circle, we deal with a surface whose section is an ellipse. It will be seen in a moment, that by the construc- tion of an ellipse any light coming in any direction from the point A, which represents one of the foci of the curve, must necessarily be reflected back to the other focus, B, of the curve, for it is a well-known property of this curve that the angles made with a tangent c D, by lines from the foci are equal ; and the same holds good for the angles made at all other tangents ; and it will be seen at once that this is better than a circular curve, because by making the distance between the foci almost infinite we shall have the star or object viewed at one focus and its image at the other ; if we use any portion of the reflecting surface we shall still get the rays reflected to one point only. It must also be noticed, that unless we have an ellipse so large that one focus shall represent the sun or a particular star we want to look at, this curve will not help us in bringing the light to one point, but if we use the curve called the parabola, which is practically an ellipse with one focus at an infinite distance, we do get the means of bringing all the rays from a distant object to a point. Hence the reflector, especially when of large diameter, is of no use for astronomical purposes without the parabolic curve. That it is extremely difficult to give this figure may be gathered from Sir John Herschel's statement, that in the case of a reflecting telescope, the mirror of which is forty-eight inches in diameter and the focal distance of which is forty feet, the distance between the parabolic and the spherical surface, at the edges of the mirror, will be represented by something less than a twenty-one thousandth part of an inch, or, more 108 STARGAZING : PAST AND PRESENT. [BOOK n. accurately, ramr inch. In Fig. 58 the point A represents the extreme edge of the curve of the parabolic mirror, and D that of the circular surface before altered into a parabola. At the time of Sir William Herschel the practical difficulties in constructing large achromatic lenses led to the adoption by him of reflectors beginning with small apertures of six inches to a foot, and increasing till he obtained one of four feet in diameter and forty-six feet focal length. This has been surpassed by Lord Eosse, whose well-known telescope is six feet diameter, and fifty-three feet focal length. Mr. Lassell, Mr. De La Kue, M. Foucault and Mr. Grubb, have also more recently succeeded in bringing reflectors to great perfection. How the work has been done will be fully stated in the sequel. LI HI? CHAPTER IX. EYEPIECES. WE have considered the telescope as a combination of an object-glass and eyepiece in the one case, and of a speculum and eyepiece in the other ; that is to say, we have discussed the optical principles which are applied in the construction of refracting and reflecting telescopes, the telescope being taken as consisting of an object-glass or speculum and an eyepiece of the most simple form, viz., a simple double convex lens. We must now go into detail somewhat on the subject of eyepieces, and explain the different kinds. It will be recollected that when we spoke of the object-glass, its aberration, both chromatic and spherical, was mentioned. Now every ordinary lens has these errors, and eyepieces must be corrected for them, but this is not done in exactly the same way as with object-glasses. In the case of eyepieces the error is corrected by using two lenses of such focal lengths or at such a distance apart that each counteracts the defects of the other; not by using two kinds of glass as in the case of the object-glass, but by so arranging the lenses that the coloured rays produced by the first lens shall fall at 110 STARGAZING: PAST AND PRESENT. [BOOK n. different angles of incidence on the second and become recombined. Let us take the case of a well-known eyepiece, called the Huyghenian eyepiece, after its inventor. It con- sists of two plano-convex lenses, A and B Fig. 60, with their convexities turned towards the object-glass, arid having their focal lengths in the proportion of three to one. The strongest lens, A, being next the eye, the lens B is placed inside the focus of the object-glass, so that it assists in bringing the image, say of a double star, to FIG. 60. Huygliens' Eyepiece. a focus at F, half way between the lenses, and nearer to the object-glass than it would have been without the lens. This image is then viewed by the eye-lens, A, and a magnified image of it seen apparently at F', as has been before explained. Now let us see how the field - lens renders this combination achromatic. Let us consider the path of a ray falling on the lens near B, shown in section in Fig. 61 : it is there refracted, but, the blue rays being refracted more than the red, there will be two rays produced, r and v, giving of course a coloured edge to the image ; but when this image is CHAP, ix.] EYEPIECES. HI viewed by the eye-glass, A, it no longer appears coloured, for the ray v, falling nearer the axis of A, is less bent than r, and they are rendered nearly parallel and appear to proceed from the point F' where the whole image appears without colour. In order to get the best result with this form of eyepiece the focal length of the field- lens should be three times that of the eye-lens and they should be placed at a distance of half their joint focal lengths apart. FIG. 61. Diagram Explaining the Achromaticity of the Huyghenian Eyepiece. The next eyepiece which comes under consideration is that called Kamsden's, Fig. 62. It consists of two plano-convex lenses of the same focus, A and B, placed at a distance of two-thirds of the focal length of either apart ; they are both on the eye side of the focus of the telescope, and act together, to render the rays parallel and give a magnified virtual image of F'F. This eyepiece is not strictly achromatic, but it suffers 112 STARGAZING : PAST AND PRESENT. [BOOK n. least of all lenses from spherical aberration ; it also has the advantage of being placed behind the focus of the object-glass, which makes it superior to others in instruments of precision, as we shall presently see. It must be remembered that these eyepieces give an inverted image or rather the object glass gives an in- verted image, and the eyepiece does not right it again ; but there are eyepieces that will erect the image, and Kheita's is one of this kind. It is, as will be seen from Fig. 63, merely a second application of the same means that first inverts the object, namely, a second small FIG. 62. Bamsden's Eyepiece. telescope. A is the object-glass, a b the image inverted in the usual way ; B is an ordinary convex lens sending the rays from a and b parallel. Now, instead of placing the eye at c, as in the ordinary manner, another small lens, acting as an object-glass, is placed in the path of the rays, bringing them to a focus at a, 6', and forming there an erect image which is viewed by the eye-lens D. This is the erecting eyepiece or " day eyepiece/' of the common " terrestrial telescope." Dollond substituted an Huygheuian eyepiece for the eye-lens D, and so made what is called his four-glass eyepiece. CHAP, ix.] EYEPIECES. 113 Dr. Kitchener devised and Mr. GL Dollond made an alteration in this eyepiece in order to vary its power at pleasure. It is done in this way : The size of the image a! V depends upon the relation of the distances a B and E cr', which can be varied by altering the distance of the combination of the lenses B and E, from the image a 6, and so making a' V larger and at a focus further from E; the tube carrying D slides in and out, so that it can be focussed on a' V at whatever distance from E it may be. This arrangement is called Dollond's Pancratic eyepiece. On the sliding tube carrying the lens D, or rather the FIG. 63. Erecting or day eyepiece. Huyghenian eyepiece in place of the single lens, are marked divisions, showing the power of the eyepiece when drawn out to certain lengths, so that if we want the eyepiece to magnify say 100 times, the tube carrying the eye-lens is drawn out to the point marked 100, and the whole eyepiece moved in or out of the telescope tube by the focussing screw, until the image of the object viewed is focussed in the field of the eyepiece D. To increase the power, we have only to draw out the eyepiece D, and move the whole combination nearer to the object- glass so as to throw the image a! V further from the lens E. This eyepiece, though* so convenient for chang- ing powers, is little used, owing perhaps chiefly to i 114 STARGAZING : PAST AND PRESENT. [BOOK ij. four lenses being required instead of two, hence a loss of light, so a stock of eyepieces of various powers is generally found in observatories. When very high powers are required, a single plano-convex lens is some- times used, but although there is less loss of light in this case, the field of view is so contracted in comparison with that given with other eyepieces that the single lens is seldom used. This form is, however, adopted in Dawes' solar eyepiece, to be hereafter mentioned, and a number of lenses are in this case fixed in holes near the circumference of a disc of metal which turns on its* centre, so that by rotating the disc the lenses come in succession in front of the focUs of the object-glass, and the power can be changed almost instantaneously. In order that objects near the zenith may be observed with ease, a diagonal reflector is sometimes used, so that the eye looks sidewise into the telescope tube instead of directly upwards. This reflector may take the form of two short pieces of tube joined together at right angles, and having a piece of silvered glass or a right-angled prism at the angle, so that when one tube is screwed into the telescope, the rays of light falling on the reflector are sent up the other, in which the ordinary eyepiece is placed. The eyepieces just described are suitable, without further addition, for observing all ordinary objects, but when the sun has to be examined a difficulty presents itself. The heat rays are brought to a focus along with those of light, and with an object-glass of more than one or two inches aperture there is great danger of the heat cracking the lenses, but with such telescopes the interposition and neglect of this may cost an eye pf smoked or strongly-coloured glass in front of the CHAP, ix.] EYEPIECES. 115 eye is generally sufficient to protect it from the intense glare. With larger telescopes, however, dark glasses are apt to split suddenly and allow the full blaze of sunlight to enter the eye and do infinite mischief, and some other method of reducing the heat and light is required. Perhaps the most simple method of effect- ing this object is to allow the light to fall on a diagonal plane glass reflector at an angle of 45, which lets the greater part of the light and heat pass through, reflect- ing only a small portion onwards to the eyepiece and thence to the eye ; a coloured glass is, however, required as well, and the glass reflector must form part of a prism of small angle, otherwise there will be two images, one produced by each surface. Another arrangement is to reflect the rays from the surfaces of two plates of glass inclined to them at the polarizing angle, so that by turning the second plate, or a Nicols' prism, in its place round the ray as an axis, the amount of light allowed to pass to the eye can be varied at pleasure. The late Mr. Dawes constructed a very convenient solar eyepiece, depending on the principle of view- ing a very small pbrtion of the sun's image at one time, and thereby diminishing the total quantity of heat passing through the eye-lens. The details of the eyepiece are as follows : very minute holes of varying diameters are made in a brass disc near its circum- ference, and as this is turned each successive hole is brought into the centre of the field of view and the common focus of the eye -lens and object-glass. Small areas on the sun of different sizes can thus be examined at pleasure. A number of eye-lenses of different powers arranged in a disc of metal can be successively brought I 2 116 STARGAZING : PAST AND PRESENT. [BOOK n to bear, giving a means of quickly varying the power, while coloured glasses of different shades can be passed in front of the eye in the same manner. The surface of the disc of brass containing the holes is covered on one side that on which the sun's image falls with plaster of Paris, which, being a bad conductor, prevents the heat from affecting the whole apparatus. The true magnifying power of the eyepiece is found by dividing the focal length of the object-glass by that of the eyepiece ; in practice it is found approximately by comparing the diameter of the object-glass with that of its image formed by the eyepiece when the telescope is in its usual adjustment ; the former divided by the latter giving the power required. The diameter of the image can be measured by a small compound microscope carry- ing a transparent scale in its focus, when the image of the object-glass is brought to a focus and enlarged on the scale and then viewed y together with the divisions, by the microscope ; or the image can be measured with tolerable accuracy by Mr. Berthon's dynameter, consist- ing of a plate of metal traversed longitudinally by a wedge-shaped opening. This is placed close to the eye-lens in the case of the Huyghenian eyepiece, or at the point where the image of the object-glass is focussed with other forms of eyepieces, and the plate moved until the sides of the wedge-shaped opening are exactly tan- gential to the image ; the point of the opening at which this occurs is read off on a scale, which gives the width of opening at this point and therefore the diameter of the image. CHAPTER X. PRODUCTION OF LENSES AND SPECULA. BEFORE we go on to the use and various mountings of telescopes, the optical principles of which have been now considered, a few words may be said about the materials used and the method of obtaining the necessary and proper curves. Object-glasses, of course, have always been made of glass, and till a few years ago specula were always made of metal ; but so soon as Liebig discovered a method of coating glass with a thin film of metallic silver, Steinheil, and after him the illustrious Foucault, so well known for his delicate experiments on the velocity of light and his invention of the gyroscope, suggested the construction of glass mirrors coated by Liebig's process with an exceedingly thin f}lm of silver, chemi- cally deposited. This arrangement much reduced the price of reflectors and rendered their polishing extremely easy, and at the present time diseg of glass up to four feet in diameter are being thus produced and formed into mirrors, though in the opinion of competent judges this size is likely to be the limit for some time. But there is this important difference, that although glass is now used both for reflectors and refractors, almost -any glass, 118- STARGAZING: PAST AND PRESENT. [BOOK ir. even common glass, will do, if we wish to use it for a speculum ; but if we wish to grind it into lenses it is impossible to overrate the difficulty of manufacture and the skill and labour required in order to prepare it for use, first in the simple material, and then in the finished form in which it is used by the astronomer. In a former chapter we considered some chefs-d'ceuvre of the early opticians, some specimens of a quarter or half-an-inch in diameter, with extremely long focus ; and as we went on we found object-glasses gradually increasing in diameter, but they were limited to the same material, namely, crown glass. Dollond, whose name we have already mentioned in connection with that of Hall, gave us the foundation of the manufacture of the precious flint glass, the con- nection of which with crown glass he had insisted upon as of critical importance. The existence of a piece of flint glass two inches in diameter was then a thing to be devoutly desired, that is to say, flint glass of sufficient purity for the purpose ; it could not be made of a size larger than that, and not only was the material wanted, but the material in its pure state. In the year 1820 we hear of a piece of flint glass six inches in diameter, and in 1859 Mr. Simms reported that a piece of flint glass of seven and three-quarter inches was produced, six inches of which were good for astrono- mical purposes. But even at this time they did these things better in Germany and Switzerland, where M. Guinand made large discs at the beginning of the present century. He was engaged by Fraunhofer and Utzschneider at their establishment in Bavaria in 1805, and by his process achromatics of from six to nine inches in diameter were constructed. Afterwards Merz, the CHAP, x.] PRODUCTION OF LENSES AND SPECULA. 119 successor of Fraunhofer, succeeded in obtaining flint glass of the then unprecedented diameter of fifteen inches. Now we have in part turned the tables, and Mr. Chance, of Birmingham, owing to the introduction of foreign talent, has since constructed discs of glass of a workable diameter of twenty-five inches for Mr. NewalTs telescope, and for the American Government he has com- pleted the large discs used in constructing the refractor of 26 inches' diameter for the observatory at Washington (the Americans are never content till they go. an inch beyond their rivals), while M. Feil of Paris, a descendant of the celebrated Guinand, has also made one of nearly 28 inches' diameter for the Austrian Government. Messrs. Chance and Feil, however, have the monopoly , of this manufacture, and the production of these discs is a. secret process. What we know is that the glass is prepared in pots in large quantities, it is then allowed to cool, and is broken up in order that it may be determined which portions of the glass are worth using for optical purposes. These are gathered together and fused at a red heat into a disc, and it is this disc which, after being annealed with the utmost care, forms the basis of the optician's work. For the glass used for reflectors, purity is of little moment, as we only require a surface to take a polish, since we look on to it, and not through it ; but in the case of the glass that has to be shaped into a lens the purity is of the utmost importance. The practical and scientific optician, on his commencement to make an object-glass, will grind the two surfaces of both flint and crown as nearly parallel as possible, and polish them. In this state he can the better examine them as to veins, striae, and other defects, which would be fatal to any- 120 STARGAZING : PAST AND PRESENT. [BOOK n. thing made out of it. He has next to see that the annealing is perfectly done by examining the discs with polarized light, to see by the absence of the " black cross " that there is no unequal tension. It is so difficult to run the gauntlet through all these difficulties when the aperture is considerable that refractors of forty inches' aperture may be perhaps despaired of for years to come, though the glassmaker is willing to try his part. Next, as to metallic specula. As we are dealing with the instruments that are now used, we will be content with considering the compounds that have been made successfully, and omit the variations which have never been brought into practice. To put it roughly, the metal used for Lord Bosse's reflector consisted of two parts of copper and one part of tin ; but here we have an idea of the Scylla and the Chary bdis which are always present in these inquiries. If we use too much tin, which tends to give a surface of brilliancy to the speculum, a few drops of hot water poured on it will be enough to shiver it to atoms. This brittleness is objec- tionable, and what we have to do is to reduce the quantity of tin. But then comes the Charybdis. If we do this, the colour is no longer white, but it is yellow, and in addition we have introduced a surface that quickly tarnishes instead of a surface which remains bright. The proportions which seem to answer best are copper sixty- four parts and tin twenty-nine. Lord Eosse, we believe, uses 31 '79 per cent, of tin ; or very nearly the above proportions. Mr. Grubb in the Melbourne mirrors used copper and tin in the proportion of 32 to 14' 77. Having the metal, we have roughly to cast it in the shape of a speculum, but if an ordinary casting is made in a sand mould the speculum metal is so spongy that we CHAP, x.] PRODUCTION OF LENSES AND SPECULA. 121 can do nothing with it. If it is put in a close mould it will probably be cast very well, but it will shiver to atoms with a very slight change of temperature. The difficulty was got over by Lord Eosse, using an open mould called a " bed of hoops ; " the bottom of the mould being composed of strips of iron set edgeways, held together by an iron ring and turned to the proper convexity ; sand is then placed round the iron to form the edges, the metal is then poured in, and the bubbles and vapours run down through the small aper- tures at the bottom of the mould, so that the speculum is fairly cast. Mr. Lassell proposed a different method, which was introduced by Mr. Grubb in his arrangements for the Melbourne telescope. Instead of having the bottom of the bed of hoops perfectly horizontal it is slightly inclined ; the crucible, which contains the metal of which the speculum is to be cast, is then brought up to it the amount of metal being something under two tons in the case of the Melbourne telescope and the bed of the mould is kept tipped up as the metal is poured into it, and so arranged as to keep the melted metal in contact with one side ; and as it gets full it is brought into a perfectly horizontal position. Having cast the speculum, the next thing is to put it in an annealing oven, raised to a temperature of 1,000, where it is allowed to cool slowly for weeks till it has acquired nearly the ordinary temperature. On being removed from the oven the speculum is placed on several thicknesses of cloth and rough ground on front, back, and edge. Having got the material roughly into form we now pass on to see what is done next. In the case of the reflector, whether of metal or glass, 122 STAKGAZING : PAST AND PRESENT. [BOOK u. the optician next attempts to get a perfectly spherical surface of the proper curvature for the required focus. In the case of the refractor matters are somewhat more complicated ; we have there four spherical surfaces to deal with, and the optician has work to do of quite a different kind before he even commences to grind. ' Presuming the refractive and dispersive properties of the glass not known, it will be necessary to have a small bit of glass of the same kind to experiment with. That the optician inay make no mistake in this im- portant matter, some glass manufacturers make the discs with projecting pieces to be cut off; these the object- glass maker works into prisms to determine the exact refraction and dispersion, including the position in the spectrum of the Fraunhofer lines C and G-, for both the crown and flint glass. With these numbers and the desired focal length he has all the necessary data for the mathematical operation of calculating the powers to be given to the two lenses flint and crown, and the radii of curvature of the four surfaces in order that the object- glass may be aplanatic or free frorn aberration both spherical and chromatic. The problem is what mathe- maticians call an indeterminate one, as an infinite num- ber of different curvatures is possible. Assume, however, the radius of curvature of one surface, and all the rest are limited. In assuming the radius of curvature on one of the crown-glass surfaces, it is well to avoid deep ones. It is better to divide the refraction of the four surfaces as equally as the nature of the problem will admit, as any little deviation from a true spherical figure in the polishing will produce less effect in injuring the per- formance of the object-glass from surfaces so arranged than if the curves were deep. CIIAP. x.] PRODUCTION OF LENSES AND SPECULA. 123 But whatever curves he chooses he goes to work so that the spherical aberration of the compound lens shall be eliminated as far as possible, and the chromatism in one lens shall be corrected by the other, or in other words, that what is called the secondary spectrum shall be as small as possible ; and it is to be feared that this will never be abolished. 1 FIG. 64. Images of planet produced by short and long focus lenses of the same" aperture giving images of different si^e, but with the same amount of colour round the edges. This matter requires a somewhat detailed treatment in order thaj; it may be seen how the four surfaces to which reference has been made are determined. The chromatic dispersion, in the case of the object-glass, may be roughly stated to be measured by about one fiftieth of the aperture. Suppose for instance the discs, Fig. 64, to represent the image of any object, say the 1 Professor Stokes and Mr. Vernon Harcourt some time ago made experiments with phosphatic glass, and some of this material was worked into a lens by Mr. Grubb, who states that " the result was successful so far as the obtaining of specimens of phosphatic glass with rational spectra ; but phosphatic glass is almost unworkable, and when the experiment was tried on a siliceous glass it failed. Some alleviation of this secondary spectrum can be got by using a triple objective, but with, of course, a corresponding loss of light." 124 STARGAZING : PAST AND PRESENT. [BOOK n. planet Jupiter. Then round that planet we should have a coloured fringe, and the dimensions of that coloured fringe, that is, the joint thickness of colour at A and D, will be found by dividing the diameter of the object- glass used by fifty. Now this is absolutely independent of the focal length of the telescope ; therefore one way of getting rid of it is to increase the focal length of telescopes ; and as the size of the image depends on focal length, and has nothing whatever to do with aperture, we may imagine that with the same sized object-glass, instead of having a little Jupiter as on the left of Fig. 64, we may have a very large Jupiter, due to the increased focal length of the tele- scope. Then, it may be asked, how about the chromatic aberration ? It will not be disturbed. The aperture of the object-glass remains unaltered, and there is no more chromatic aberration here than in the first case ; so that the relation between the visible planet Jupiter and the colour round it is changed by altering the focal length. But as we have seen, we are able by means of a combination of flint and crown glass to counteract this dispersion to a very great extent. How then about spherical aberration ? Up to the present we have assumed that all rays falling on a convex lens are brought to a point or focus, but this is not strictly true, for the edges of a lens turn the rays rather too much out of their course, so that they will not come to a point ; just as the rays re- flected from a spherical mirror do not form a single focus. The marginal rays will be spread over a certain circular surface, just as the colour due to chromatic aberration covered a surface surrounding the focus. It was ex- plained that for the same diameter of lens the circle of CHAP, x.] PRODUCTION OF LENSES AND SPECULA. 125 colour remained the same, irrespective of focal length, but in the case of spherical aberration this is not so ; it diminishes as the square of the focal length increases ; that is to say, if we double the focal length we shall not only halve, but half-halve, or quarter the aberration. Newton calculated the size of the circle of aberration in comparison with that due to colour, and he found that in the case of a lens of four inches diameter and ten feet focus, the sperical aberration was eighty-one and a half times less than that of colour. It is found that by altering the relative curvatures of the surfaces of the lens, this aberration can be corrected without altering the focal length ; for any number of lenses can be made of different curvatures on each side but of the same thickness in the middle, so that they have all the same focal length, but the one, having one surface about three times more convex than the other, will have least aberration, so that it is the adaptation of the surfaces of lenses to each other that exercises the art of the optician. So far we have got rid of this aberration in a single lens ; it can also be done in the ca,se of achromatic lenses. The foci of the two lenses in an achromatic combination must bear a certain relation to each other, and the cur- vatures of the surfaces must also have a certain relation for spherical aberration. In the achromatic lens there are four surfaces, two of which can be altered for one aberration and two for the other. For instance, in the case of the lens, Fig. 45, where the interior surfaces of the lenses are cemented together, although shown separate for clearness, we find that if the exterior sur- face of the crown double convex lens be of a curvature struck by a radius 672 units in length, and the exterior surface of the flint glass lens to a curvature due to a 126 STARGAZING : PAST AND PRESENT. [BOOK n. radius of 1,420 units, the lens will be corrected for spherical aberration, and these conditions leave the in- terior surfaces to be altered so that the relation between the powers of the lenses is such as to give achromatism. The flint is as useful in correcting the spherical aberration as the chromatic aberration ; for although the relative thicknesses of the flint and crown are fixed in order to get achromatism, still we have by altering both the curvatures of each lens equally, and keeping the same foci, the power of altering the extent of spherical aberration; and it is in the applications of these two conditions that much of the higher art of our opticians is exercised. We have now therefore practically got rid of both aberrations in the modern object-glass, and hence it is that lenses of the large diameter of twenty-five and twenty-six inches are possible. The nearest approach to achromatism is known to be made when looking at a star of the first or second mag- nitude, the eyepiece being pushed out of focus towards the object-glass, the expanded disc has its margin of a claret colour. When the eyepiece is pushed beyond the focus outwards the margin of the expanded disc is of a light green colour. If the object-glass is well corrected for spherical aber- ration, the expanded discs both within and without the focus will be constituted of a series of rings equally dense with regard to light throughout, with the exception of the marginal ring, which will be a little stronger than the rest. Having determined the radius of curvature of surface, both he who grinds the speculum, whether of speculum metal or glass, and he who grinds the object-glass, starts CHAP, x.] PRODUCTION OF LENSES AND SPECULA. 127 fair ; only one has four times the work to do that the other has. The grinding is managed in a simple way, and the process of grinding or polishing an object-glass or speculum, either of glass or of metal, is the same. Supposing we wish to make a reflecting telescope of six feet focus, or a surface of an object-glass of twelve feet radius, all we have to do is to get a long rod, a little more than twelve feet long, and pin it to a wall at its upper end so that it can swing, pendulum fashion ; then at a distance of twelve feet below the point of suspension a pin is stuck through the rod and its point made to scratch a line on a sheet of metal laid against the wall ; then this line will be part of a circle struck with a radius of twelve feet. If then the plate be cut along this line we get a convex and a concave surface of the desired radius, and then we can take a block of iron or brass and turn its surface, convex or concave, to fit the sheet of metal or template. For a reflector we should make a convex tool, and for a refractor a concave one. Generally this grinding tool is divided into squares or furrows all over it, in order that the emery which is used in rough grinding may flow freely about with the water. A disc of glass is then laid on the tool, or the tool on the glass, the two being pressed together by a weight or spring ; emery powder, with water, is strewn between them, and one is rubbed over the other by a machine similar to those used for polishing, which we shall explain presently. This operation is continued until the glass is ground all over, and in this process of rough grinding the rough emery is used between the tool and the glass, so that whatever irregularities the glass or tool may have they are got rid of, and it is easy to obtain a spherical surface, and indeed, it is the only 128 STARGAZING : PAST AND PRESENT. [BOOK IT. surface that can be obtained. Then finer and finer emery is used, till it ceases to be a sufficiently fine sub- stance to use, and a surface of iron or lead is also too hard a surface. Now the polishing begins, and the optician and amateur avail themselves of a suggestion due to Sir Isaac Newton, who always saw much further through things than other people. Even when he first began to make the first reflector, he used pitch, a substance not too hard, nor yet too soft, and one that can be regulated by temperature ; therefore for polishing, instead of having a tool made of metal, pitch laid on glass or wood and supplied with rouge and water is used. This polisher of pitch is divided into squares by channels to allow free flow of rouge and FIG. 65. Showing in an exaggerated forhi how the edge of the speculum is worn down by polishing. water, and is laid on the mirror or object-glass, or vice versd, and moved about over it. When the maximum of polish is attained the work is done, and the object-glass finished, as here we have to do with a spherical surface. In the grinding of the two discs for Mr. NewalTs telescope 1,560 hours were con- sumed, the thickness of the crown disc having been reduced one inch in the process. In the case of specula, however, there is more to be done ; and it is in this polishing of specula that the curve is altered from a circle to a parabola by using a certain length of stroke, size of polisher, consistency of pitch, and numbers of other smaller matters, the CHAP, x.] PRODUCTION OF LENSES AND SPECULA. 129 proper proportionment of which constitutes the practical skill of the optician, and it is in accomplishing this that the finest niceties of manipulation come into play, and the utmost patience is required. 1,170 hours were occupied in the grinding and polishing of the four-feet Melbourne speculum. This was equivalent to 2,050,000 strokes of the machine at 33 per minute for rough and 24 for fine grinding. Dr. Eobinson, in his description of the grinding operations, states that at the edge of one of the four-feet specula the distance of its parabola from the circle was only 0*000106". In the early times of specula the polishing was in- variably done by hand, a handle being cemented by pitch to the back of the speculum to work it with. Mudge tells us that at first, when the mirror was rough from the emery grinding, it was worked round and round on the pitch, which was supplied with rouge and water and cut by channels into small squares, carrying the edge but little over the polisher, an oc- casional cross stroke being made. The effect of this was to press the pitch towards the centre where the polish always commenced, and gradually spread to the circumference. As soon as the polishing was complete the speculum was worked by short straight strokes across the centre, tending to bring it back to a sphere ; then the circular strokes were recommenced to restore the paraboloid form : these were continued for a short time only, otherwise it would pass the proper curve and require reworking with straight strokes again. By this method some small mirrors of first-class definition were constructed. When Sir W. Herschel began his labours he con- structed a machine for working the speculum over the K 130 STARGAZING : PAST AND PRESENT. [BOOK n. polisher ; the polisher was a little larger than the mirror, the proportion given by him from a number of trials being 1-06 to 1. The speculum was held in a circular frame, which was free to turn round in another ring or frame ; this frame was moved backwards and forwards by a vibrating lever to which it was attached by rods, carrying the speculum over the polisher. This motion he designates the stroke. Besides this there was the side motion produced by a rod attached to the side of the frame opposite to that to which the rods giving it the stroke were attached and at right angles to the direction of stroke : this rod was in connection, by means of intermediate levers, with a pin on a rachet wheel, which was turned a tooth at a time by a rod in connection with the lever giving the stroke motion, so that the rod giving the side motion was pushed and pulled back by the pin on the rachet wheel every time it turned round, which it did every twenty or thirty strokes. There were also teeth on the ring fastened round the edge at the back of the speculum, into which claws worked which were attached by rods to a point on the lever a little distance from the attachment of the rod giving the stroke, so that the claws had a less motion than the speculum and its ring, and consequently pulled the ring, and with it the speculum, round a tooth or more at each stroke. The polisher was also turned round in the same manner in a contrary direction to the motion of the speculum. The speculum had therefore three motions, a revolving one on its centre, a stroke, and a side motion, making its centre describe a number of parallel lines over the polisher on each side of its centre. Sir W. Herschel gives as a good working length of stroke, 0'29, and 0'19 side motion measured from CHAP, x.] PKODUCTION OF LENSES AND SPECULA. 131 side to side, the diameter of the speculum being 1. To produce a seven-inch mirror with this instrument he would work continuously for sixteen hours, his sister " putting the victuals by bits into his mouth." Lord Eosse adopted a similar arrangement; the polisher, KL, Fig. 65* was worked over the speculum in straight strokes with side motion, the requisite straight motion being given by a crank-pin and rod and the side motion by the continuation of this latter rod on the other side of the polisher working in a guide on another crank-pin, which threw it from side to side as the wheel carrying the pin revolved. The trough E F carrying the speculum also revolved slowly, and the requisite motions FIG. 65.* Section of Lord Rosse's polishing machine. were given by pulleys and straps of various sizes under the table on which the machine rested ; the weight of the polisher was in a great measure counterpoised by strings from its upper surface to a weighted lever M above. The polisher was free to turn in its ring, which it did once in about twenty strokes, and for the six feet specu- lum the velocity of working was about eight strokes a minute, the length of stroke being one-third of the diameter of the speculum, and that of the side motion one-fifth. The speculum was polished on the same system of 2 132 STARGAZING : PAST AND PRESENT. [BOOK IT. levers that were afterwards to support it, in order that no change of form might be produced in moving it to a different mounting. The consistency of the pitch is a matter of importance, Mr. Lassell's test of the requisite hardness being the number of impressions left by a FIG. 66. Mr. Lassell's polishing machine. sovereign standing on edge on it ; this should leave three complete impressions of the milled edge in one minute at the ordinary temperature of the atmosphere. Fig. 66 represents the machine contrived by Mr. Lassell for his method of polishing, and shows what CHAP, x.] PRODUCTION OF LENSES AND SPECULA. 133 a complicated arrangement is essential in order to arrive at any good result in these matters. The speculum is placed on a bed, and above it is a train of wheels terminating in a crank-pin that gives motion to the polisher, which is made to take a very devious path by the motion of the wheels above. The pin giving motion to the polisher a at its centre can be set at a variable distance from the axis of the lowest pinion F to which it is attached, by moving it in its slide, so that when the pinion is turned, the pin and centre of the polisher describe a circle. The pinion in question is carried on a slide c above it, attached to the main vertical driving shaft A, so that as the shaft revolves the centre of the pinion describes a circle of a diameter variable at pleasure by moving it in the slide c, the result of the two motions being that the centre of the polisher describes circles about a moving centre, and consequently in constantly varying positions on the specu- lum. Motion is given to the vertical shaft by the cog- wheel and endless screw above> worked by some prime mover, and as the cog-wheels on the shaft E parallel to the main shaft are carried round the latter by the arm D holding them, they are caused to revolve by gearing into the fixed wheel B 3 through the centre of which the main shaft passes, and they in their turn impart motion to the pinion carrying the pin giving motion to the polisher. The speculum is also maintained in slow rotation by the wheel and endless screw below it. The speculum and its supports are surrounded by water con- tained in a circular trough not shown in the engraving, so that the consistency of the pitch shall be constant. This arrangement, pure and simple, was found to bring on the polish in rings over the speculum, and as an 134 STARGAZING : PAST AND PRESENT. [BOOK n. improvement, the speculum, or rather the system of levers supporting it, was carried on a plate which had the power of sliding backwards and forwards on the wheel turning it round ; the edges of this plate pressed against a fixed roller, and it was made of such a shape that as it revolved it was forced to take a side motion as its edges passed by the fixed roller, so that the specu- lum had a side motion in addition to the rotatory one. Mr. De La Kue improved on this by giving the speculum a rotatory motion irrespective of that of the sliding plate, so that the side motion should not always be along the same diameter of the speculum. This was done by allowing the speculum to turn freely on a pivot on the sliding plate, and giving it a rotatory motion by means of a cord going round the plate carrying the speculum supports. As a further improvement Mr. De La Rue controls the motion of the polisher on the central pin, giving it motion by a crank carrying a system of wheels in place of the lowest crank, so that the pin gets a rotatory motion in addition to these. Mr. Grubb's arrangement for polishing is different The speculum is made to rotate, the polisher is made to execute curves variable at pleasure by altering the throw of the cranks which move rods attached to the centre of the polisher, giving it a motion similar to that of Mr. LasselFs machine. The polisher moves a little off the edge, so that the edge is worn down more than the centre, thus giving the parabolic form. M. Foucault, of whom we have already spoken, pro- ceeds in a different manner in parabolising his glass mirrors. He first obtains a spherical surface, fairly reflec- tive, by grinding. He then alters the surface to a para- boloid form by handwork, only testing the surface from CHAP, x.] PRODUCTION OF LENSES AND SPECULA. 135 time to time to ascertain the parts requiring reduction by the polishing pad. The method of testing is as beau- tiful as it is simple. The approximate estimate of the curvature of the speculum is made by placing a small and well-defined object, such as the point of a pin, close to the centre of curvature and examining its image formed close by its side with a lens. As a nicer test, he places an object having parallel sides, say a flat ruler, near the centre of curvature, and views its image with the naked eye at the distance of distinct vision, then each point of the edge is seen by rays converging only from a small portion of the surface of the mirror, the remainder of the diverging cone from each point of the edge passes on beside the eye, and by moving the eye about, any point of the edge can be seen formed by rays proceeding from any particular part of the mirror, viz., that part in line with the eye and point of the edge examined ; if the curvature be not uniform the edge will appear distorted, and points on it will appear in different positions, as rays from different parts of the mirror are received by the eye as it is moved, making the edge appear to move in waves. Finally, he allows light from a very small hole in a metal plate near the centre of curvature to fall on the mirror, and places the eye just on the side opposite to the point where the image is formed, so as to receive the rays as they diverge after having come to a focus. The whole of the light thus passes into the eye, and the mirror is seen illuminated in every part. A sharp edge of metal is then gradually brought into the focus, when the illumination of the mirror decreases, and just before the light disappears the irregularities will plainly appear, showing themselves by patches of light, which prove that those parts still bright are so inclined as to reflect the rays 136 STARGAZING : PAST AND PRESENT. [BOOK ir. by the side of the true focus. By moving the metallic edge so as to advance upon the focus from all sides, a very good idea of the irregularities may be obtained. If, however, the surface be truly spherical, the light will disappear regularly over the whole surface. M. Foucault commences by making the surface truly spherical, and then by polishing off in concentric circles, increasing the polishing from the centre, an elliptic and at last a parabolic curve is attained. The ellipse is tested from time to time by removing the perforated plate further and further away from the mirror until the ellipse becomes practically a parabola. The great advantage of this method is, that the effect of the polishing can be examined as it proceeds, and the work can always he applied wherever necessary, and the test is entirely inde- pendent of hot-air currents which are seen to fluctuate over the mirror as waves of light, leaving the irregularities of form permanently marked. It further appears that the method may be varied to form a first-rate test of a finished mirror already mounted ; for one has nothing to do but bring a star into the field of view, and remove the eyepiece, and bring the eye into such a position as to receive the diverging rays from the focus of the star. A knife is then gradually moved across in front of the eye, say from the right ; then if the mirror commences to get darkened on the right side distinctly before the left the knife is on the mirror side of the focus ; if, however, the left side of the mirror becomes darkened first it is on the eye side of the focus. After a few trials it can be got to cut across the focus and darken the mirror at all points at once, and show up all irregularities. We have now, then, by one system or another, got our mirror, either of speculum metal or of glass, and if of the CHAP, x.] PRODUCTION OF LENSES AND SPECULA. 137 latter substance we have to silver it ; processes have been published by Mr. Browning, and M. Martin, 1 by which, on the plan proposed in the first instance by Liebig, an extremely thin coating of silver is deposited on the glass. This film is susceptible of taking a high polish, which, in the case of small mirrors, can be renewed as often as is wished without repolishing the mirror ; the resilvering of one of large aperture however is a most 1 Mr. Browning's method of silvering glass specula is as follows : Prepare three standard solutions : r , T , . ( Crystals of nitrate of silver 90 grains ) T^- i Solution A < _ . J . . V Dissolve. ( Distilled water 4 ounces J T ,. T> ( Potassa, pure by alcohol .. 1 ounce | T >>. , Solution B <> x<1 ' F J nK } Dissolve. | Distilled water 25 ounces J Solution C /Milk-sugar (in powder)... J ounce ) Digsolm ( Distilled water 5 ounces j Solutions A and B will keep, in stoppered bottles, for any length of time ; Solution C must be fresh. To prepare sufficient for silvering an 8 in. speculum, pour two ounces of Solution A into a glass vessel capable of holding thirty-five fluid ounces. Add, drop by drop, stirring all the time (with a glass rod), as much liquid ammonia as is just necessary to obtain a clear solution of the grey precipitate first thrown down. Add four ounces of Solution B. The brown-black precipitate formed must be just re-dissolved by the addition of more ammonia, as before. Add distilled water until the bulk reaches fifteen ounces, and add, drop by drop, some of Solution A, until a grey precipitate, which does not re-dissolve after stirring for three minutes, is obtained ; ^then add fifteen ounces more of distilled water. Set this solution aside to settle ; do not filter. When all is ready for immersing the mirror, add to the silvering solution two ounces of Solution C, and stir gently and thoroughly. Solution C may be filtered. The mirror should be suspended face downwards about J-inch deep in the liquid, by strings attached to pieces of wood fastened to the back of the mirror with pitch, and before being immersed should be cleaned with nitric acid and washed with distilled water. The silver- ing is completed in about an hour, and when finished the surface 138 STARGAZING : PAST AND PRESENT. [BOOK n. formidable affair. To those who wish to silver their own mirrors, let us say that it should be done in summer, or in a room kept by a stove at an equable summer heat, and the silvering solution should be kept for a day or more to settle, and for probably some chemical change to take place before the reducing solution is added. It will be found easy enough to silver the small planes for Newtonian reflectors, but large mirrors require much greater care and trouble. should be washed in distilled water and dried, and then polished with soft leather, finishing with a little rouge. The following method is used by M. Martin : Make solutions : 1. Nitrate of silver 4 per cent. 2. Nitrate of ammonia 6 ) perfectly free from 3. Caustic potash 10 ,, j carbonates. 4. Dissolve twenty-five grammes of sugar in 250 grammes of water j add three grammes of tartaric acid ; heat it to ebullition during ten minutes to complete the conversion of sugar ; cool down, and add fifty cubic centimetres of alcohol in summer to prevent fermentation, add water to make the volume to \ litre in winter and more in summer. Clean well the surface of the glass. Take equal quantities of the four solutions : mix 1 and 2 together, and 3 and 4 also together : mix the two, pouring it at once into the vessel where the silvering is to be done. The mirror is suspended face downwards in the liquid, and the deposit begins after about three minutes, and is finished after twenty minutes. Take out the mirror, clean well with water, dry it in the air, and rub it then gently with a very fine leather. CHAPTER XL THE " OPTICK TUBE/' HAVING now obtained the lenses and specula we come, in order to complete our consideration of the purely optical portion of the subject, to the question of mounting these lenses and specula in tubes and thus connecting them with the eyepieces so as to become of practical utility. We will first consider the adjustment of lenses in a tube, the combination forming a simple telescope that can be supported, in any manner desirable, by mountings we shall presently consider, according to the purpose for which it is required. The adjustment of specula will be considered as we advance further. The smaller telescopes consist of a brass tube, the object- glass, held in a brass ring, being screwed in at one end of the tube : a smaller tube sliding in and out of the other end of the large tube, generally moved by a rack and pinion motion, carries the eyepiece. In larger telescopes the mounting is similar, only somewhat more elaborate, the object-glass being carried in a brass cell, or a steel one if the dimensions are very large. This screws into the ring at the end of the tube, and this ring can be slightly tipped on either side by set screws, so that the object-glass can be brought exactly at right angles to the axis of the tube. 140 STARGAZING : PAST AND PRESENT. [BOOK n. It is important, in order that an object-glass shall per- form its best, that the lenses forming it shall be properly centred : this is generally done by the maker once and for ever. Wollaston pointed out an ingenious method of centring them- it is as follows : The eyepiece is removed, and a lighted candle put in its place : the object-glass is then examined from the opposite side, when, if all the lenses are correctly placed, the images of the candle produced by the successive reflections of the candle from the surfaces of the lenses will be concentric, and in a straight line from the candle through the centre of the system of lenses, a fact easily judged of, by moving the eye slightly from side to side, and if they FIG. 67. Simple telescope tube, showing arrangement of object-glass and eye- piece. are not, they are easily corrected by tipping the lens in fault slightly in the cell. In case the lenses are cemented together, this method of course is applicable in setting the object-glass at right angles to the axis of the tube. , The adjustment of an object-glass can also be judged of by examining a star as it is thrown in and out of focus by the focusing screw ; the disc of the star should be perfectly round in and out of focus, and the rings produced by interference should also be circular when in focus, and the disc of lightj when out of focus, must be circular. Any elongation of the disc or rings, or a " flare " appearing, shows a want of a slight altera- tion of the setting screw, on the same side of the object-glass as the " flare " or elongation appears. CHAP, xi.] THE " OPTICK TUBE. 141 In some object-glasses the curves of the two interior surfaces are such that three pieces of tin foil are placed at equal distances round the edge to prevent the central portions from coming in contact. The flexure of small object-glasses by their own weight is of little importance, because every surface is affected alike ; but when the aperture is large special precautions have to be taken. The late Mr. Cooke when he had completed the 25-inch object-glass for Mr. Ne wall's telescope, introduced a system of counterpoise levers just within the edge which helped to support the Fig. 68. Appearance of diffraction FIG. 69. Appearance of same object rings round a star when the object- when, object-glass is out of adjust- glass is properly adjusted. ment. object-glass in all positions. Mr. Grubb states that with an aperture of 15 inches, supported on three points, there is decided evidence of flexure, and he pro- poses, in the 2 7 -inch Vienna refractor, not only to introduce six intermediate supports, thereby following in the footsteps of Mr. Cooke, but with larger apertures to introduce boldly a central support, or to hermetically seal the tube and fill it with compressed air. He has calculated that in the case of an object-glass 40 inches aperture, weighing 600 Ibs., two-thirds of its weight could be supported by an air pressure of one-third of a pound to the square inch. 142 STARGAZING : PAST AND PRESENT. [BOOK IT. The tube of the telescope when of large size is usually made of iron or wood, and a tube of the latter substance may be made very light and yet. sufficiently strong, by wrapping layers of veneer round a central core and fasten- ing the layers firmly with glue. There are generally two or more tubes sliding inside each other at the eye end of the telescope, to carry the eyepiece so as to give plenty of power of adjustment of the length of the tube to suit the different eyepieces, or other instruments used in their place. The tube then is ready to be adapted to any of the mountings to be hereafter considered. We now come to the mounting of specula, and when we recollect the enormous weights of some of the specimens to which we have referred, it will be obvious that some additional precautions, which are not at all necessary in the case of a refractor, must be taken to insure success. In reflecting telescopes, the speculum is carried at the bottom of a tube in a sort of tray or cell, which can be adjusted by screws at the back, so as to set the mirror at right angles to the tube, and the conditions of support should be such that the mirror should be as free from strain as if it were floating in mercury. A system of lateral supports in all positions is also necessary. The action of the telescope depends greatly on the backing of the speculum, and numerous methods of carrying specula on soft backing and systems of levers have been suggested, all aiming at carrying them so that they are free from all possible strain and flexure oc- casioned by their own weight. For smaller mirrors a soft back of flannel or cloth can be used, and a leather strap placed round the mirror and its back, so as to form CHAP. XI.] THE "OPTIOK TUBE. 143 144 STARGAZING : PAST AND PRESENT. [BOOK ii. the side of a sort of circular tray, will give it sufficient support when inclined to the horizontal. Mr. Browning adopts the plan of making the back of the mirror and its support perfectly flat, so as not to require levers or soft backing ; this arrangement would probably fail for mirrors larger than one foot in diameter, although answering admirably for those of less size. % We will now consider the methods of mounting specula of larger size, and will take as an instance the mounting of some of the largest specula in existence which must act so as to prevent flexure in any position of the speculum. The speculum is, in the case of the FIG. 72. Mr. Browning's method of supporting small specula. The bottom of the speculum A is a carefully prepared plane surface, and the outer rim of the inner iron cell B, on which it rests, is also a plane. The speculum is kept in this cell by the ring G G, and it may be removed from, and replaced in, the telescope, without altering its adjustment. Melbourne telescope, of the weight of something like two tons. When it is inclined at any considerable angle to the horizon, it is apt to bend over at the top, and thus destroy its proper curvature ; and when horizontal, if not equally supported, it will also bend, and unless some measures are taken to prevent this flexure it will so entirely alter its figure by its own weight as to render minute observations of any delicate stars absolutely impossible. Mr. Lassell was the first to suggest an arrangement for preventing this flexure. Through the back of the speculum case the case which holds and supports the HAP. xi.] THE "OPTICK TUBE." 145 speculum, which :we shall have to speak about presently he inserts a large number of very small levers, the centres of which are fixed to the exterior part of this case, the forward part of each resting against a small aperture made in the back of the speculum. The ends of the levers furthest from the speculum are crowned with small weights, the weights varying on different parts of the speculum. Now so long as the speculum is perfectly horizontal, i.e. so long as the zenith is being observed, these levers will have no action whatever ; but the moment the reflector is brought into any other position, as, for instance, when we wish to observe a star near the horizon, the more the mirror is inclined to the horizon the greater will be the power of these small levers, and at length their total effect comes into action when a star close to the horizon is being observed. Then the whole weight of the mirror is carried by these levers acting at points all over its back. In the Melbourne reflector, which has recently been finished, Mr. Grubb manages this somewhat differently, as will be seen by Figs. 73 76. In Fig 73 the speculum is in a vertical position. It is supported in a frame, B B, all round it, which consists of a slightly flexible hoop of metal a little larger than the speculum. This. in its turn is supported by a large fixed hoop, A A, having a hook-shaped section. This hoop is attached to the tube of the telescope c c. The hoop, B B, is rather larger than the part of A on which it hangs, so that it can adjust itself to the form of the mirror ; and not only is the mirror supported in the hoop B B, like as in a strap in the position shown, but in every other position of the tube the speculum still hangs evenly supported. 146 STARGAZING : PAST AND PRESENT. [BOOK jr. As we have already seen, there is another point to consider. Not only must we be able to support the mirror when inclined to the horizon, but we must support it bodily at the end of the tube when it is horizontal. We will next examine an arrangement adopted by Mr. Grubb, similar to that adopted by others, for supporting the Melbourne speculum, and we cannot do better than quote Mr. Grubb's own explanation of it. He says : FIG. 73. Support of the mirror when vertical. " To understand it, suppose the speculum to be divided into forty-eight portions, as in Fig. 74, each of them being exactly equal in area, and consequently in weight. Now, if the centre of gravity of each of these pieces rested on points which would bear up with a force = the weight of each segmental piece, it is evident that there would be no strain in the mass from segment to segment. " This is exactly what is accomplished by this system ; in fact, if when the speculum is resting on these supports CHAP. xi.J THE " OPTICK TUBE." 147 it could be divided up into segments corresponding to those lines, they would have no inclination to leave their places, showing a perfect absence of strain across those lines. Suppose now the points representing the centres of gravity of these segments were supported on levers and triangles, so as to couple them together, as at A, Fig. 75, and each of these couplings to be supported from a point a, representing the centre of gravity of the sum of the segments supported by that particular couple, and it Fio. 74. Division of the speculum into equal areas. is evident that there can be no strain between the com- ponents of these couples. Again, let these points, a, be coupled together by the system shown at B, Fig. 75, and their centres of gravity, fr, coupled as at c, and it is evident that the whole weight of the speculum ultimately condensed by this system into these points is supported on forty- eight points of equal support being the centres of gravity of the forty-eight segments at Fig. 75. In Fig. 76 is seen the whole system complete. It consists of three screws passing through the back of the speculum L 2 148 STARGAZING : PAST AND PRESENT. [BOOK ii. box (which serve for levelling the mirror), the points of which carry levers (primary system) supporting triangles on their extremities (secondary system), from the ver- tices of which are hung two triangles and one lever (tertiary system). All the joints of this apparatus are capable of a small rocking motion, to enable them to take their positions when the speculum is laid upon them. " In the system of levers made by Lord Eosse for his six-feet speculum, the primary, secondary, and tertiary systems were piled up one over the other, so that the FIG 75. Primary, secondary, and tertiary systems of levers shown separately. FIG. 76. Complete system consolidated into three screws. distance from the support of the primary to the back of the speculum was about fifteen inches. This, as will be readily seen on consideration, introduced a new strain when the telescope was turned off the zenith, and had to be counterpoised by another very complicated system of levers. But in the Melbourne telescope, by the substitution of cast-steel for cast-iron, and by hanging the tertiary system from the secondary, and allowing it (the tertiary) to act in some places through the secondary, the whole system is reduced to three and a half inches CHAP, xi.] THE " OPTICK TUBE." 149 in height, and the distance from the support of the pri- mary lever to the back of the speculum is only one and three-quarter inch, by which means this cumbersome apparatus is entirely done away with. " The ultimate points of the tertiary system are gun- metal cups, which hold truly ground cast-iron balls with a little play, and when the speculum is laid on these it can be moved about a little by a person's finger with such ease as to seem to be floating in some liquid." It may perhaps be thought that it would be better to support these great specula on a flat surface, and it might be, if we could do so without extreme difficulty ; but Lord Eosse has stated that if we attempt to support a large speculum on a surface extremely flat, a thread placed across that surface, or even a piece of dust, is quite enough to bend the mirror and render it absolutely useless. That will show the extreme importance of the support of the speculum. Let us then assume that we have the speculum and the tube perfectly adjusted. The next thing, in all con- structions except the Herschelian, is to apply the second small reflector, concave in the case of the Gregorian, convex in the case of the Cassegrainian, and plane in the case of the Newtonian. This small mirror is generally supported by a thin strip of metal firmly fastened to the side of the tube, with power of movement parallel to the axis of the telescope, in the case of the Gregorian and Cassegrainian, for the purpose of focussing. In the Newtonian, the reflecting diagonal prism or plane mirror, inclined at an angle of 45 to the axis, is preferably supported in the manner suggested by Mr. Browning. See Figs. 77 and 78. In these B B B represent strips of strong chronometer 150 STARGAZING : PAST AND PRESENT. [BOOK ii. spring steel, placed edgewise towards the speculum ; by these the prism or small mirror D is suspended. The mirror thus mounted, does not produce such coarse rays on bright stars as when it is fixed to a single stout arm ; it is also less liable to vibration, which is very injurious to distinct vision, or to flexure, which interferes with the accuracy of the adjustments. The most usual form of reflector is the Newtonian, large numbers of which kind are now made ; and just as the object-glasses of refractors require adjusting, so do not only the large mirror, but also the "flat" or diagonal FIG. 77. Support of diagonal plane mirror (Front view). FIG. 78. Support of diagonal plane mirror (Side view). mirror of this form. In the Newtonian the flat must be adjusted first ; to do this, first place the large mirror in its cell in the tube, and secure it by turning it in the bayonet joint, with the cover on the mirror. Then remove the glasses from one of the eyepieces, insert it into the eyetube, and fix the diagonal mirror loosely in its position. Then, looking through the eyetube, move the diagonal mirror, by means of the motions which are provided, until the reflected image of the cover of the speculum is seen in the centre of it. CHAP, xi ] THE OPTICK TUBE." 151 This is accomplished by first loosening the milled- headed screw behind the mirror, and turning the mirror until the image of the speculum cover appears central in one direction. The screw at the back of the mirror enables the reflected image to be brought central in the other direction. Next comes the turn of the large mirror. Take off the cover by screwing off the side opening and place the eye at the eyetube after having removed the eyepiece ; the reflection of the diagonal mirror will be seen in the reflected image of the speculum. The adjusting screws, at the back of the speculum, must then be moved until the diagonal mirror is seen in the centre of the speculum. The adjustment should then be complete. This may be judged of by bringing a star to the centre of the field, and sliding the focussing-tube in or out, when the circle of light should expand equally, and its centre should remain central in the field. As another test a bright star should be viewed with a high power, and the image examined ; if it is round and the circles of light round it are concentric without rays in any one direction, then all is correct; but if a flare is seen, it is evidence that the part of the diagonal mirror towards which the flare extends must be moved from the eye by the setting- screws at the back. r, i u a A i< i- TJNIVKRSITY CALIFORNIA. CHAPTER XII. THE MODERN TELESCOPE. THE gain to astronomy from the discovery of the tele- scope has been twofold. We have first, the gain to physical astronomy from the magnification of objects, and secondly, the gain to astronomy of position from the magnification, so to speak, of space, which enables minute portions of it to be most accurately quantified. Looking back, nothing is more curious in the history of astronomy than the rooted objection which Hevel and others showed to apply the telescope to the pointers and pinnules of the instruments used in their day ; but doubtless we must look for the explanation of this not only in the accuracy to which observers had attained by the old method, but in the rude nature of the telescope itself in the early times, before the introduction of the micrometer. We shall show in a future chapter how the modern accuracy has step by step been arrived at ; in the present one we have to see what the telescope does for us in the domain of that grand physical astronomy which deals with the number and appearances of the various bodies which people space. Let us, to begin with, try to see how the telescope helps us in the matter of observations of the sun. The CHAP, xii.] THE MODERN TELESCOPE. 153 sun is about ninety millions of miles away ; suppose, therefore, by means of a telescope reflecting or refracting, whichever we like, we use an eyepiece which will magnify say 900 times, we obviously bring the sun within 100,000 miles of us ; that is to say, by means of this telescope we can observe the sun with the naked eye as if it were within 100,000 miles of us. One may say, this is something, but not much ; it is only about half as far as the moon is from us. But when we recollect the enormous size of the sun, and that if the centre of the sun occupied the centre of our earth the circumference of the sun would extend considerably beyond the orbit of the moon, then one must acknowledge we have done something to bring the sun within half the distance of the moon. Suppose for looking at the moon we use on a telescope a power of 1,000, that is a power which magnifies a thousand times, we shall bring the moon within 240 miles of us, and we shall be able to see the moon with a telescope of that magnifying power pretty much as if the moon were situated somewhere in Lancashire Lancaster being about 240 miles from London. It might appear at first sight possible in the case of all bodies to magnify the image formed by the object-glass to an unlimited extent by using a suffi- ciently powerful eyepiece. This, however, is not the case, for as an object is magnified it is spread over a larger portion of the retina than before ; the brightness, therefore, becomes diminished as the area increases, and this takes place at a rate equal to the square of the increase in diameter. If, therefore, we require an object to be largely magnified we must pro- duce an image sufficiently bright to bear such magnifica- tion ; this means that we must use an object-glass or 154 STARGAZING : PAST AND PRESENT. [BOOK n speculum of large diameter. Again, in observing a very faint object, such as a nebula or comet, we cannot, by decreasing the power of the eyepiece, increase the brightness to an unlimited extent, for as the power decreases, the focal length of the eyepiece also in- creases, and the eyepiece has to be larger, the emergent pencil is then larger than the pupil of the eye, and consequently a portion of the rays of the cone from each point of the object is wasted. We get an immense gain to physical astronomy by the revelations of the fainter objects which, without the telescope, would have remained invisible to us ; but, as we know, as each large telescope has exceeded preceding ones in illuminating power, the former bounds of the visible creation have been gradually extended, though even now we cannot be Fl on 9 oftife con- sa i for tiiere are others beyond the the naked eye. region which the most powerful telescope reveals to us ; though we have got only into the surface we have increased the 3, 000 or 6,000 stars visible to the naked eye to something like twenty millions. This space-penetrating power of the tele- scope, as it is called, depends on the principle that whenever the image formed on the retina is less than sufficient to appear of an appreciable size the light is apparently spread out by a purely physiological action until the image, say of a star, appears of an appreciable diameter, and the effect on the retina of such small points of light is simply proportionate to the amount of light received, whether the eye be assisted by the telescope or not ; the stars always, except when suffi- CHAP, xii.] THE MODERN TELESCOPE. 155 ciently bright to form diffraction rings, appearing of the same size. It, therefore, happens that as the apertures of telescopes increase, and with them the amount of light, (the eyepieces being sufficiently powerful to cause FIG. 80. The same region, as seen through a large telescope. all the light to enter the eye,) smaller and smaller stars become visible, while the larger stars appear to get brighter and brighter without increasing in size, the image of the brightest star with the highest power, if we neglect rays and diffraction rings, being really much 156 STARGAZING : PAST AND PRESENT. [BOOK ii. smaller than the apparent size due to physiological effects, and of this latter size every star must appear. The accompanying woodcuts of a region in the con- stellation of Gemini as seen with the naked eye and with a powerful telescope will give a better idea than mere language can do of the effect of this so-called space- penetrating power. With nebulae and comets matters are different, for these, FIG. 81. Orion and the neighbouring constellations. even with small telescopes and low powers, often occupy an appreciable space on the retina. On increasing the aperture we must also increase the power of the eyepiece, in order that the more divergent cones of light from each point of the image shall enter the pupil, and therefore increase the area on the retina, over which the increased amount of light, due to greater aperture, is spread; the brightness therefore is not increased, unless indeed we CHAP, xii.] THE MODERN TELESCOPE. 157 were at the first using an unnecessary high power. On the other hand, if we lengthen the focus of the object-glass, and increase its aperture, the divergence of the cones of light is not increased and the eyepiece need not be altered, but the image at the focus of the object-glass is increased in size by the increase of focal FIG. 82. Nebula of Orion. length, and the image on the retina also increases as in the last case. We may, therefore conclude that no comet or nebula of appreciable diameter, as seen through a telescope having an eyepiece of just such a focal length as to admit all the rays to the eye, can be made brighter by any increase of power, although it may easily be made to appear larger. 158 STARGAZING : PAST AND PRESENT. [BOOK n. Very beautiful drawings of the nebula of Orion and of other nebulae, as seen by Lord Eosse in his six- foot reflector, and by the American astronomers with their twenty- six inch refractor, have been given to the world. The magnificent nebula of Orion is scarcely visible to the naked eye ; one can just see it glimmering on a fine night ; but when a powerful telescope is used, it is by far the most glorious object of its class in the Northern hemisphere, and surpassed only by that surrounding the variable star rj Argus in the Southern. And although, of course, the beauty and vastness of this stupendous and remote object increase with the increased power of the instrument brought to bear upon it, a large aperture is not needed to render it a most impressive and awe-inspiring object to the beholder. In an ordinary 5-foot achromatic, many of its details are to be seen under favourable atmospheric conditions. Those who are desirous of studying its appearance, as seen in the most powerful telescopes, are referred to the plate in Sir John Herschel's "Results of Astronomical Observations at the Cape of Good Hope," in which all its features are admirably delineated, and the positions of 150 stars which surround 6 in the area occupied by the Nebula, laid down. In Fig. 82 it is represented in great detail, as seen with the included small stars, all of which have been mapped with reference to their positions and brightness. This then comes from that power of the telescope which simply makes it a sort of large eye. We may measure the illuminating power of the telescope by a reference to the size of our own eye. If one takes the pupil of an ordinary eye to be something like the fifth of an inch in diameter, which in some cases is an CHAP, xii.] THE MODERN TELESCOPE. 159 extreme estimate, we shall find that its area would be roughly about one-thirtieth part of an inch. If we take Lord Kosse's speculum of six feet in diameter the area will be something like 4,000 inches : and if we multiply the two together we shall find, if we lose no light, we should get 120,000 times more light from Lord Kosse's telescope than we do from our unaided eye, everything supposed perfect. Let us consider for a moment what this means ; let us take a case in point. Suppose that owing to imperfec- tions in reflection and other matters two-thirds of the light is lost so that the eye receives 40,000 times the amount given by the unaided vision, then a sixth magni- tude star a star just visible to the naked eye would have 40,000 times more light, and it might be removed to a distance 200 times as great as it at present is and still be visible in the field of the telescope, just as it at present is to the unaided eye. Can we judge how far off the stars are that are only just visible with Lord Kosse's instrument ? Light travels at the rate of 185,000 miles a second, and from the nearest star it takes some 3^ years for light to reach us, and we shall be within bounds when we say that it will take light 300 years to reach us from many a sixth magnitude star. But we may remove this star 200 times further away and yet see it with the telescope, so that we can probably see stars so far off that light takes 60,000 years to reach us, and when we gaze at the heavens at night we are viewing the stars not as they are at that moment, but as they were years or even hundreds of years ago, and when we call to our assistance the telescope the years become thousands and tens of thousands expressed in 160 STARGAZING: PAST AND PRESENT. [BOOK n. miles these distances become too great for the imagination to grasp ; yet we actually look into this vast abyss of space and see the laws of gravitation holding good there, and calculate the orbit of one star about another. Whether the telescope be of the first or last order of excellence, its light-grasping powers will be practically the same ; there is therefore a great distinction to be drawn between the illuminating and defining power. The former, as we have seen, depends upon size (and subsidiarily upon polish), the latter depends upon the accuracy of the curvature of the surface. FIG. 83. Saturn and his moons (general view with a S^-iucli object-glass.) If the defining power be not good, even if the air be perfect, each increase of the magnifying power so brings out the defects of the image, that at last no details at all are visible, all outlines are blurred, or stellar character is lost. The testing of a glass therefore refers to two different qualities which it should possess. Its quality as to material and the fineness of its polish should be such that the maximum of light shall be transmitted. Its quality, as to the curves, should be such that the rays passing through every part of its area shall converge absolutely to the same point, with a chromatic aberration CHAP. XII.] THE MODERN TELESCOPE. 101 a o f M 162 STAKGAZING : PAST AND PRESENT. [BOOK n. not absolutely nil, but sufficient to surround objects with a faint violet light. In close double stars therefore, or in the more minute markings of the sun, moon, or planets, we have tests of its defining power ; and if this is equally good in the instruments examined, the revelations of telescopes as they increase in power are of the most amazing kind. A 3f -inch suffices to show Saturn with all the detail shown in Fig. 83, while Fig. 84 shows us the further minute structure of the rings which comes out when the planet is observed with an aperture of 26 inches. In the matter of double stars, a telescope of 2 inches aperture, with powers varying from 60 to 100, should show the following stars double : Polaris. y Arietis. a Geminorum. a Piscium. p Herculis. y Leonis. ju Draconis. Ursse Majoris. Cassiopeae. A 4-inch aperture, powers 80-120, reveals the duplicity of ft Orionis. a Lyrse. S Geminorum. Hydrae. Ursse Majoris. 2 Andromeda. 19 Draconis. /* 2 Bo6tis. CHAP, xii.] THE MODERN TELESCOPE. 163 The " spurious disk," which a fixed star presents, as seen in the telescope, is an effect which results from the passage of the light through the object-glass ; and it is this appearance which necessitates the use of the largest apertures in the observation of close double stars, as th& size of the star's disk varies, roughly speaking, in the inverse ratio of the aperture of the object-glass. In our climate, which is not so bad as some would make it, a 6- to an 8 -inch glass is doubtless the size which will be found the most constantly useful ; a larger aperture being frequently not only useless, but hurtful. Still, 4 or 3f inches are apertures by all means to be encouraged ; and by object-glasses of these sizes, made of course by the best makers, views of the sun, moon, planets, and double stars may be obtained, sufficiently striking to set many seriously to work as amateur observers, and with a prospect of securing good, useful results. Observations should always be commenced with the lowest power, gradually increasing it until the limit of the aperture, or of the atmospheric condition at the time, is reached. The former may be taken as equal to the number of hundredths of inches which the diameter of the object-glass contains. Thus, a 3f-inch object-glass, if really good, should bear a power of 375 on double stars where light is no object ; the planets, the Moon, &c., will be best observed with a much lower power. (See chapter on eyepieces.) Care should be taken that the object-glass is properly adjusted. And we may here repeat that this may be done by observing the image of a large star out of focus. If the light be not equally distributed over the image, or the diffraction rings are not circular, the screws of the M ( 2 164 STARGAZING : PAST AND PRESENT. [BOOK n. cell should be carefully loosened, and that part of the cell towards which the rings are thrown very gently tapped with wood, to force it towards the eyepiece, or the same purpose may be effected by means of the set- screws always present on large telescopes, until perfectly equal illumination is arrived at. This, however, should only be done in extreme cases ; it is here especially desirable that we should let well alone. The convenient altitude at which Orion culminates in these latitudes renders it particularly eligible for obser- vation ; and during the first months of the year our readers who would test their telescopes will do well not to lose the opportunity of trying the progressively difficult tests, both of illuminating and separating power, afforded by its various double and multiple systems, which are collected together in such a circumscribed region of the heavens that no extensive movement of their instruments an important point in extreme cases will be necessary. Beginning with S, the upper of the three stars which form the belt, the two components will be visible in almost any instrument which may be used for seeing them, being of the second and seventh magnitudes, and well separated. The companion to ft though of the same magnitude as that to 8, is much more difficult to observe, in^ consequence of its proximity to its bright primary, a first-magnitude star. Quaint old Kitchener, in his work on telescopes, mentions that the companion to Eigel has been seen with an object-glass of 2f-inch aperture ; it should be seen, at all events, with a 3 -inch, f, the bottom star in the belt, is a capital test both of the dividing and space-penetrating power, as the two bright stars of the second and sixth magnitudes, of CHAP, xii.] THE MODERN TELESCOPE. 165 which the close double is composed, are exactly 2j" apart, while there is a companion to one of these components of the twelfth magnitude about f" distant. The small star below, which the late Admiral Smyth, in his charming book, " The Celestial Cycle," mentions as a test for his object-glass of 5*9 inches in diameter, is now plainly to be seen in a 3f . The colours of this pair have been variously stated ; Struve dubbing the sixth mag- nitude which, by the way, was missed altogether by Sir John Herschel " olivaceasubrubicunda." That either our modern opticians contrive to admit more light by means of a superior polish imparted to the surfaces of the object-glass, or that the stars themselves are becoming brighter, is again evidenced by the point of light preceding one of the brightest stars in the system composing *j CHAPTER XX. VARIOUS METHODS OP MOUNTING LARGE TELESCOPES. WE have already gone somewhat in detail into the construction of the transit circle, which is almost the most important of modern astronomical instruments. We then referred to the alt-azimuth, in which, instead of dealing with those meridional measurements which we had touched upon in the case of the transit circle, we left, as it were, the meridian for other parts of the sphere and worked with other great circles, passing not through the pole of the heavens, but through the zenith. AVe now pass to the "optick tube," as used in the physical branch of astronomy, and we have first to trace the passage from the alt-azimuth to the Equatorial, as the most convenient mounting is called. This equatorial gives the observer the power of finding any object at once, even in the day-time, if it be above the horizon ; and, when the object is found, of keeping it stationary in the field of view. But although this form is the most convenient, it is not the one universally adopted, because it is expensive, and because, again, till within the last few years our opticians were not able to grapple with all its difficulties. Hence it is that some of the instruments which have 294 STARGAZING : PAST AND PRESENT. [BOOK v. been most nobly occupied in investigations in physical astronomy have been mounted in a most simple manner, some of them being on an alt-azimuth mounting. Of these the most noteworthy example is supplied by the forty-feet instrument erected by Sir William Herschel at Slough. FIG. 134. The 40-feet at Slough. Lord Rosse's six-feet reflector again is mounted in a different manner. It is not equatorially mounted ; the tube, supported at the bottom on a pivot, is moved by manual power as desired between two high side walls, carrying the staging for observers, and so allowing the CHAP, xx.] METHODS OF MOUNTING LARGE TELESCOPES. 295 293 STARGAZING : PAST AND PEESENT. [BOOK v. telescope a small motion in right ascension of about two hours. Our amateurs then may be forgiven for still FIG. 116. liclractoi mounted 011 Alt-azimuth Tripod for ordinary Stargazing. adhering to the alt-azimuth mounting for mere star- gazing purposes. We must recollect that, with the alt-azimuth, we are CHAP, xx.] METHODS OF MOUNTING LARGE TELESCOPES. 297 able to measure the position of an object with reference to the horizon and meridian ; but suppose we tip up the whole instrument from the base, so that, instead of having the axis of the instrument vertical, we incline it so as to make the axis, round which the instrument turns in azimuth, absolutely parallel to the earth's axis. Of course, if we were using it at the north pole or the south pole, the axis would be absolutely vertical, as when it is used as an alt-azimuth, or otherwise it would not be absolutely parallel to the axis of the earth. On the other hand, if we were using it at the equator, it would be essential that the axis should be horizontal, since to an observer at the equator the earth's axis is perfectly horizontal ; but, for a middle latitude like our own, we have to tip this axis about 51^ from the horizontal, so as to be in proper relationship with, i.e. parallel to, the earth's axis. Having done this, we can, by turning the instrument round this axis, called the polar axis, keep a star visible in the field of view for any length of time we choose by exactly counteracting the rotation of the earth, without moving the telescope about its upper, or what was its horizontal, axis. The lower circle of the instru- ment will then be in the plane of the celestial equator, and the upper one, at right angles to it, will enable us to measure the distance from that plane, or the declina- tion of an object, while the lower circle will tell us the distance of the object from the meridian in hours or degrees. With the aid of good circles and good clocks, we can thus determine a star's position. Fig. 137 shows an Equatorial Stand, one of the first kind of equatorials used by astronomers. We see at once the general arrange- 298 STARGAZING : PAST AND PRESENT. [BOOK v. ments of the instrument. In the first place, we have a horizontal base, D, and on it, and inclined to it, is a disc of metal, c ; again on this disc lies another disc, A, B, which can revolve round on c, being held to it by a central stud, so that when A B is in the plane of the earth's equator its axis points to the pole and is parallel to the axis of the earth. On the upper disc there are two supports for the axis of the telescope, E, which is at right angles to the polar axis and is called the declination axis of the telescope ; round it the tele- scope has a motion in a direction from the pole to the equator. FIG. 137. Sim] ile Equatorial Mounting. In the equatorial mounting, clockwork is introduced, and after the instrument has been pointed to any par- ticular star or celestial body, the clock is clamped to the circle moving round the polar axis, and so made to drive it round in exactly the time the earth takes to make a rotation. By a clock is meant an instru- ment for giving motion, not with reference to time, but so arranged that, if it were possible to use it continu- ously, the motion would exactly bring the telescope round once in the twenty-four sidereal hours which CHAP, xx.] METHODS OF MOUNTING LARGE TELESCOPES. 299 are necessary for the successive transits of stars over the meridian. There is an objection to the form of instrument given above, the telescope cannot be pointed to any position near the pole, since the stand comes in the way. This is obviated in the various methods of mounting, which we shall now pass under review. The German Mounting. This is the form now almost universally adopted for refractors and reflectors under 20 inches aperture. The polar axis has attached to it at right angles a socket through which the declination axis passes, and this axis carries the telescope at one end and a counterweight at the other. The polar axis lies wholly below the de- clination axis, and both are supported by a central pillar entirely of iron, or partly of stone and partly of iron. By the courtesy of Messrs. Cooke and Sons, Mr. Howard Grubb, and Mr. Browning, we are enabled to give examples of the various forms of this mounting now in use in this country for instruments of less than 20 inches aperture. In Fig. 138, we have the type form of Equatorial Refractor introduced some 30 years ago by the late Mr. Thomas Cooke. The telescope is represented parallel to the polar axis, which is inclosed in the casing supported by the central pillar, and carries one large right ascension circle above and another smaller one below, the former being read by microscopes attached to the casing. The socket or tube carrying the declination axis is connected with the top of the polar axis. To this the 300 STARGAZING : PAST AND PBESENT. [BOOK v. declination circle is fixed, while an inner axis fixed to the telescope carries the verniers. FIG. 138. Cooke's form for Refractors. The clock is seen to the north of the pillar. While this is driving the telescope, rods coming down to the CHAP, xx.] METHODS OF MOUNTING LARGE TELESCOPES. 301 eyepiece enable the observer to make any small altera- Fio. 139. Mr. Grubb's form applied to a Cassegrain Reflector. tions in right ascension or declination ; indeed in all modern instruments everything except winding the clock 302 STARGAZING : PAST AND PRESENT. [HOOK v. is done at the eyepiece, so that the observer when fairly at work is not disturbed. The lamp to illuminate the micrometer wires is shown near the finder. The friction rollers, which take nearly all the weight off the surfaces of the polar axis, are connected with the compound levers shown above the casing of the polar axis. In Fig. 139 we have Mr. Grubb's revision of the German form. The pillar is composite, and the support of the upper part of the polar axis is not so direct as in the mounting which has just been referred to. There are, however, several interesting modifications to which attention may be drawn. The lamp is placed at the end of the hollow polar axis, and supplies light not only for the micrometer wires, but for reading the circles ; the central cavity of the lower support is utilised for the clock, which works on part of a circle, instead of a complete one, as in the instrument already described. In the case of Newtonian reflectors the observer requires to do his work at the upper end of the tube ; this therefore should be as near the ground as possible. This is accomplished by reducing the support to a mini- mum. Figs. 140 and 141 show two forms of this mount- ing, designed by Mr. Grubb and Mr. Browning. The two largest and most perfectly mounted refractors on the German form at present in existence are those at Gateshead and Washington, U.S. The former belongs to Mr. Newall, a gentleman who, connected with those who were among the first to recognise the genius of our great English optician, Cooke, did not hesitate to risk thousands of pounds in one great experiment, the success of which will have a most important bearing upon the astronomy of the future. CHAP. xx.J METHODS OF MOUNTING LARGE TELESCOPES. 303 In the year 1860 the largest refractors which had been turned out of the^Optical Institute at Munich under the FIG. 140. Grubb's form for Newtonians. control, first, of the great Fraunhofer, and afterwards of Merz, were those of 177 square inches area at Poulkowa 304 STARGAZING : PAST AND PRESENT. [BOOK v. FIG. 141. Browning's mounting for Newtonians. CHAP, xx.] METHODS OF MOUNTING LARGE TELESCOPES. 305 and Cambridge (U.S.). Our own Cooke, who was rapidly bringing back some of the old prestige of Dollond and Tulley's time to England a prestige which was lost to us by the unwise meddling of our excise laws and the duty on glass, 1 which prevented experiments in glass- making had completed a 9 J inch for Mr. Fletcher and a 10 inch for Mr. Barclay; while in America Alvan Clarke had gone from strength to strength till he had completed a refractor of 1 8j inches for Chicago. The areas of these objectives are 67, 78 '5, and 268 inches respectively. Those who saw the great Exhibition of 1862 may have observed near the Armstrong Gun trophy two circular blocks of glass some 26 inches in diameter and about two inches thick standing on their edges. These were two of the much-prized " discs " of optical glass manufactured by Messrs. Chance of Birmingham. At the close of the Exhibition they were purchased by Mr. Newall, and transferred to the workshops of Messrs. Cooke and Sons at York. The glass was examined and found perfect. In time the object-glass was polished and tested, and the world was in possession of an astronomical instrument of nearly twice the power of the 18^ inch Chicago instrument 485 inches area to 268. Such an achievement marks an epoch in telescopic astronomy, and the skill of Mr. Cooke and the muni- ficence of Mr. Newall will long be remembered. The general design and appearance of this monster 1 It is not too much to say that the duty on glass entirely stifled, if indeed it did not kill, the optical art in England. We were so depen- dent for many years upon France and Germany for our telescopes, that the largest object-glasses at Greenwich, Oxford, and Cambridge are all of foreign make. X 306 STARGAZING : PAST AND PRESENT. [BOOK v. among telescopes will be gathered from the general view given in the frontispiece, for which we are indebted to Mr. Newall. It is the same as that of the well-known Cooke equatorials ; but the extraordinary size of all the parts has necessitated the special arrangement of most of them. The length of the tube, including dew-cap and eye- end, is 32 feet, and it is of a cigar shape, the diameter at the object-end being 29 inches, at the centre of the tube 34 inches, and at the eye-end 22 inches. The cast-iron pillar supporting the whole is 19 feet in height from the ground to the centre of the declination axis, when horizontal ; and the base of it is 5 feet 9 inches in diameter. The trough for the polar axis alone weighs 14 cwt., the weight of the whole instrument being nearly 6 tons. The tube is constructed of steel plates riveted together, and is made in five lengths screwed together with bolts. The flanges were turned in a lathe, so as to be parallel to each other. It weighs only 13 cwt., and is remark- ably rigid. Inside the outer tube are five other tubes of zinc, increasing in diameter from the eye to the object-end ; the wide end of each zinc tube overlapping the narrow end of the following tube, and leaving an annular space of about an inch in width round the end of each for the purpose of ventilating the tube, and preventing, as much as possible, all interference by currents of warm air with the cone of rays. The zinc tubes are also made to act as diaphragms. The two glasses forming the object-glass weigh 144 lb., and the brass cell weighs 80 lb. The object-glass has an aperture of nearly 25 inches, or 485 inches area, and in CHAP, xx.] METHODS OF MOUNTING LARGE TELESCOPES. 307 order as much as possible to avoid flexure from unequal pressure on the cell, it is made to rest upon three fixed points in its cell, and between each of these are arranged three levers and counterpoises round a counter-cell, which act through the cell direct on to the glass, so that its weight in all positions is equally distributed among the twelve points of support, with a slight excess upon the three fixed ones. The focal length of the lens is 29 feet. Attached to the eye-end of the tube are two finders, each of 12*5 inches area ; they are fixed above and below the eye-end of the main tube, so that one may be readily accessible in all positions of the instrument. It is also supplied with a telescope having an object-glass of 33 inches area. This is fixed between the two finders, and is for the purpose of assisting in the observations of comets and other objects for which the large instrument is not so suitable. This assistant telescope is provided with a rough position circle and micrometer eyepieces. Two reading microscopes for the declination circle are brought down to the eye- end of the main tube ; the circle 38 inches in diameter is divided on its face and edge, and read by means of the microscopes and prisms. The slow motions in declination and R. A. are given by means of tangent screws, carrying grooved pulleys, over which pass endless cords brought to the eye-end. The declination clamping handle is also at the eye- end. The clock for driving this monster telescope is fixed to the lower part of the pillar, and is of comparatively small proportions, the instrument being so nicely counterpoised that a very slight power is required to be exerted by the clock, through the tangent screw, on the driving-wheel x 2 308 STARGAZING : PAST AND PRESENT. [BOOK v. (seven feet in diameter), in order to give the necessary equatorial motion. The declination axis is of peculiar construction, necessi- tated by the weight of the tubes and their fittings, and corresponding counterpoises on the other end, tending to cause flexure of the axis. This difficulty is entirely overcome by making the axis hollow, and passing a strong iron lever through it having its fulcrum imme- diately over the bearing of the axis near the main tube, and acting upon a strong iron plate rigidly fixed as near the centre of the tube as possible, clear of the cone of rays. This lever, taking nearly the whole weight of the tubes, &c., off the axis, frees it from all liability to bend. The weight of the polar axis on its upper bearing is relieved by anti-friction rollers and weighted levers ; the lower end of the axis is conical, and there is a corre- sponding conical surface on the lower end of the trough ; between these two surfaces are three conical rollers carried by a loose or " live " ring, which adjust them- selves to equalize the pressure. The hour-circle on the bottom of the polar axis is 26 inches in diameter, and is divided on the edge, and read roughly from the floor by means of a small diagonal telescope attached to the pillar ; a rough motion in K. A. by hand is also arranged for, by a system of cogwheels, moved by a grooved wheel and endless cord at the lower end of the polar axis, so as to enable the observer to set the instrument roughly in K. A. by the aid of the dia- gonal telescope. It is also divided on its face, and read by means of microscopes. The declination and hour- circle will probably be illuminated by means of Geissler tubes, and the dark and bright field illuminations for the micrometers will be effected by the same means. CHAP, xx.] METHODS OF MOUNTING LARGE TELESCOPES. 309 So soon as the success of the Newall experiment was put beyond all question by Cooke, Commodore B. F. Sands, the superintendent of the U.S. Naval Observatory, sent a deputation, consisting of Professors S. Newcomb, FIG. 142. The Washington Great Equatorial. Asaph Hall, and Mr. Harkness, accompanied by Mr. Alvan Clarke, to examine and report upon the Newall telescope, and the result was that they commissioned Alvan Clarke to construct a large telescope for that country. 310 STARGAZING : PAST AND PRESENT. [BOOK v In the Washington telescope the aperture of the object-glass is 26 inches that is, one inch larger than the English type-instrument. The general arrangements are shown in the accompanying woodcut. It will be seen that the mounting is much lighter than in the English instrument, and a composite pillar gives place for the clock in the central cavity. The English Mounting. In the English mounting the telescope, like a transit instrument, has on each side a pivot, and these pivots rest on a frame somewhat larger than the telescope, pointing to the pole and supported by two pivots, one at the bottom resting on bearings near the ground, and the other carried by a higher pillar clear of the observer's chair. The motions of the telescope are similar to those given by the German mounting in all essentials ; the Greenwich equatorial is mounted in this manner. It is carried in a large cylindrical frame, supported at both ends by two pillars above by a strong iron pillar, while the other end rests on a firm stone pillar, going right to the earth, independently of the flooring. This mounting, though preferred for the large instrument at Greenwich, has been discarded generally, as the long polar axis is necessarily a serious element of weakness ; the telescope is supported on its weakest part, and it is liable to great changes from contraction and expansion of the frame. The Forked Mounting. It is now getting more usual to mount Newtonians of large dimensions equatorially, in spite of the immense weight to be carried. One of the first methods was to CHAP, xx.] METHODS OF MOUNTING LARGE TELESCOPES. 311 use a polar axis in the same manner as for a refractor, only that it bifurcated at the top, forming there a fork, and between this fork the telescope is swung, after the same manner as a transit. This method of mounting was adopted by M. Foucault in the case of his first large silvered - glass reflector. The height of the bifurcation is dependent on the distance between the centre of gravity of the tube and the speculum, and if we use an extremely light tube, or if, as it is the fashion to abolish them now altogether for reflectors, - we use a skeleton tube of iron lattice work, this bifurca- tion of the polar axis need not be of any great length. The polar axis being entirely below the telescope and being driven by the clock, we have a perfect method of mounting a speculum of any weight we please. This arrangement was first suggested and carried into effect by Mr. Lassell for his four-foot Newtonian, which was mounted at Malta. The polar axis was a heavy cone- shaped casting resting on its point below, and moving on its largest diameter just below the base of the fork. Lord Ro?:r\i \. CHAPTER XXIII. THE SIDEROSTAT, AT one of the very earliest meetings of the Royal Society, the difficulties of mounting the long focus lenses of Huyghens being under discussion, Hooke pointed out that all difficulties would be done away with if instead of giving movement to the huge telescope itself, a plane mirror were made to move in front of it. This idea has taken two centuries to bear fruit, and now all acknow- ledge its excellence. One of the most recent additions to astronomical tools is the Siderostat, the name given to the instrument suggested by Hooke. By its means we can make the sun or stars remain virtually fixed in a horizontal tele- scope fixed in the plane of the meridian to the south of the instrument, instead of requiring the usual ponderous mounting for keeping a star in the field of view. It consists of a mirror driven by clockwork so as to continually reflect the beam of light coming from a star, or other celestial object, in the same direction ; the principle consisting in so moving the mirror that its normal shall always bisect the angle subtended at the mirror by the object and the telescope or other apparatus on which the object is reflected. It was Foucault who, towards the end of his life, 344 STARGAZING : PAST AND PRESENT. [BOOK v. thought of the immense use of an instrument of this kind as a substitute for the motion of equatorials ; he, however, unfortunately did not live to see his ideas realized, but the Commission for the purpose of carrying out the publication of the works of Foucault directed : ::: ' ; ' "" " ; ' Hi !;''' " - FIG. 156. Foucault 's Siderostat. Mr. Eichens to construct a siderostat, and this one was presented to the Academy of Science on December 13th, 1869, and is now at the Paris Observatory. Since that date others have been produced, and they have every chance of coming largely into use, especially in physical astronomy. Fig. 156 shows the elevation of the instru- CHAP, xxni.] THE SIDEROSTAT. 345 ment, the mirror of which, in the case of the instrument at Paris, is thirty centimetres in diameter, and is supported by a horizontal axis upon two uprights, which are capable of revolving freely upon their base. The back of the mounting of the mirror has an extension in the form of a rod at right angles to it, by which it is connected with the clock, which moves the mirror through the medium of a fork jointed at the bottom of the polar axis. The length of the fork is exactly equal to the distance from the horizontal axis of the mirror to the axis of the joint of the fork to the polar axis, and the direction of the line joining these two points is the direction in which the reflected ray is required to proceed. The fork is moved on its joint to such a position that its axis points to the object to be viewed, and, being carried by a polar axis, it remains pointing to that object as long as the clock drives it, in the same manner as a telescope would do on the same mounting. Then, since the distance from the axis of the mirror to the joint of the fork is equal to the distance from the latter point to the axis of its joint to the sliding tube on the directing rod, an isosceles triangle is formed having the directing rod at its base ; the angles at the base are therefore equal to each other. Further, if we imagine a line drawn in continuation of the axis of the fork towards the object, then the angle made by this line and that from the axis of the mirror to the elbow joint of the fork (the direction of the reflected ray) will be equal to the two angles at the base of the isosceles triangle ; and, since they are equal to each other, the angle made by the directing rod and the axis of the fork (or the incident ray) from the object, is equal to half the angle made by the latter ray and the direction of the reflected ray ; and if lines are drawn through the surface 346 STARGAZING : PAST AND PRESENT. [BOOK v. of the mirror in continuation of the directing rod and the line from the elbow joint to the axis of the mirror ; and a line to the point of intersection be drawn from the object, this last line will be parallel to the axis of the fork, and the angle it makes with the continuation of the directing rod, or normal to the surface of the mirror, will be half the angle made by it and the line representing the reflected ray. Therefore the angle made by the inci- dent ray and the required direction of the reflected ray is always bisected by the normal, so that the reflected ray is constant in the required direction. The clock is driven in the usual manner by a weight. A rod carries the motion up to the system of wheels by which the polar axis is rotated. As this axis rotates it carries with it the fork, which transmits the required motion to the mirror. And as the fork alters its direc- tion the tube slides upon the directing rod, thus altering the inclination of the mirror. In order to vary the position of the mirror without stopping the instrument there are slow motion rods or cords proceeding from the instrument which may be carried to any distance desirable. The polar axis is set in the meridian similarly to an equatorial telescope, the whole apparatus being firmly mounted upon a massive stone pillar which is set several feet in the ground, and rests upon a bed of concrete, if the soil is light. A house upon wheels, running upon a tramway, is used to protect the instrument from the weather, and when in use this hut is run back to the north, leaving the siderostat exposed. In the north wall of the observatory is a window, and the telescope is mounted horizontally opposite to it : so the observer can seat himself comfortably at his work, and by his guide CHAP. XXIII.] THE SIDEROSTAT. 347 348 STARGAZING : PAST AND PRESENT. [BOOK v. rods direct the mirror of the siderostat to almost any part of the sky, viewing any object in the eyepiece of his telescope without altering his position. In spec- troscopy and celestial photography its use is of immense importance, for in these researches the image of an object is required to be kept steadily on the slit of the spectroscope or on the photographic plate, and for this purpose a very strongly-made and accurate clock is required to drive the telescope and mounting, which are necessarily made heavy and massive to prevent flexure and vibration. The siderostat, on the other hand, is extremely light, without tube or accessories, and a light, delicate clock is able to drive it with accuracy, while the heavy telescope and its adjuncts are at rest in one position. The sun and stars can, there- fore, as it were, be "laid on" to he observer's study to be viewed without the shifting of the observatory roof and equatorial, or of the observing chair, which brings its occupant sometimes into most uneasy positions. We figure to ourselves the fu ture of the physical obser- vatory in the shape of an ordinary room with siderostat outside throwing sunlight or rays from whatever object we wish into any fixed instrument at the pleasure of the observer. There are, however, inconveniences attending its use in some cases ; for instance, in measur- ing the position of double stars, the diurnal motion gradually changes their position in the field of the telescope, so that a new zero must be constantly taken or else the time of observation noted and the necessary corrections made. CHAPTER XXIV. THE ORDINARY WORK OF THE EQUATORIAL. THE equatorial enables us to make not only physical observations, but differential observations of the most absolute accuracy. First we may touch upon the physical observations made with the eyepiece alone star-gazing, in fact. The Sun first claims our attention : our dependence on him for the light of day, for heat, and for in fact almost everything we enjoy, urges us to inquire into the physics of this magnificent object. Precautions must however be taken ; more than one observer has already been blinded by the intense light and heat, and some solar eyepiece must be used. For small telescopes up to two inches, a dark glass placed between the eye and the eyepiece is sufficiently safe ; for larger apertures, the diagonal reflector, or D awes' solar eyepiece, already described, comes into requisition. Another method of viewing the sun is to focus the sun's image with the ordinary eyepiece on a sheet of paper or card, or, better still, on a surface of plaster of Paris carefully smoothed. The bright ridges or streaks, usually seen in spotted regions near the edge, called the faculse, and the mottled surface, appearing, according to Nasmyth, like a number 350 STARGAZING : PAST AND PRESENT. [BOOK v. of interlacing willow-leaves the minute " granules " of Dawes, are best seen with a blue glass ; but for observing the delicately-tinted veils in the umbrse of the spots a glass of neutral tint should be used. The Moon is a fine object even in small telescopes. The best observing time is near the quarters, as near full moon the sun shines on the surface so nearly in the same direction as that in which we look, that there is no light and shade to throw objects into view. Hours may be spent in examining the craters, rilles, and valleys on the surface, accompanied with a good descriptive map or such a book as that which Mr. Neison has recently published. The planets also come in for their share of examina- tion. Mercury is so near the sun as seldom to be seen. Venus in small telescopes is only interesting with reference to her changes, like the moon, but in larger ones with great care the spots are visible. Mars is interesting as being so near a counterpart of our own planet. On it we see the polar snows, continents and seas, partially obscured by clouds, and these appearances are brought under our view in succession by the rotation of the planet. With a good six-inch glass and a power of 200 when the air is pure and the opposition is favourable, there is no difficulty in making out the coast-lines, and the various tones of shade on the water surface may be observed, showing that here the sea is tranquil, and there it is driven by storms. Up to very lately it was the only planet of considerable size further off the sun than Venus that was supposed to have no satellite ; two of these bodies have however been lately discovered by Hall with the large Washington refractor of twenty- six inches diameter, and they appear to be the tiniest CHAP, xxiv.] ORDINARY WORK OF THE EQUATORIAL. 351 celestial bodies known, one of them in all probability not exceeding 10 miles in diameter. Jupiter and Saturn are very conspicuous objects, and the eclipses, transits, and occultations of the moons, and the belts of the former and rings of the latter, are among the most interest- ing phenomena revealed to us by our telescopes, while the delicate markings on the third satellite of Jupiter furnish us with one of the most difficult tests of defini- tion. Uranus and Neptune are only just seen in small telescopes, and even in spite of the use of larger ones, we are in ignorance of much relating to these planets. The amateur will do well to attack all these with that charming book, the Rev. T. W. Webb's Celestial Objects for Common Telescopes, in his hand. To observe the fainter satellites of the brighter planets, or, indeed, faint objects generally, near very bright ones, the bright object may be screened by a metallic bar, or red or blue glass placed in the common focus. So much with regard to our own system. When we leave it we are confounded with the wealth of nebulae, star- clusters, and single or multiple systems of stars, which await our scrutiny. With the stars, not much can be done without further assistance than the eyepiece alone. The colours of stars may however be observed, and for this purpose a chromatic scale has been proposed, and a memoir thereon written, by Admiral Smyth, for comparison with the stars. The colour of a star must not be confused with the colours often very vivid - produced by scintillations, these rapid changes of bright- ness and colour depending on atmospheric causes. Of the large stars, Sirius, Vega and Eegulus are white, while Aldebaran and Betelgueux are red. In many double and multiple stars however the contrast of colours shows 352 STARGAZING : PAST AND PRESENT. [BOOK v. up beautifully ; in Cygni for instance we have a yellow and blue star, in 7 Leonis, a yellow and a green star ; and of such there are numerous examples. Interesting as all these observations are, a new life and utility are thrown into them when instead of using a simple eyepiece the wire micrometer is introduced. This, as we have before stated, generally consists of one wire, or two parallel wires, fixed, and one or two other wires at right angles to these, movable across the field. This micrometer is used in connection with a part of the eyepiece end of the telescope, which has now to be described. This is a circle, the fineness of the gradua- tion of which increases with the size of the .telescope, read by two or four verniers. The circle is fixed to the telescope, while the verniers are attached to the eyepiece, carrying the micrometer, which is rotated by a rack and pinion. The whole system of position circle (as it is called) and wire micrometer, is in adjustment when (l) the single or double fixed wires and the movable ones cross in the centre of the field, and (2) when with a star travelling along the single fixed or between the two fixed wires, the upper vernier reads 180 and the lower one reads zero. This motion across the field gives the direction of a parallel of declination ; that is to say, it gives a line parallel to the celestial equator, and, knowing that, one will be able at once, by allowing the object to pass through the field of view, to get this datum line. For instance, supposing the whole instrument is turned round on the end of the telescope, so that one of the two wires x and y, Fig. 104, at right angles to the thin wires for measuring distance, shall lie on a star during all its motion across the field of view ; then those two wires. CHAP, xxiv.] ORDINARY WORK OF THE EQUATORIAL. 353 being parallel to the star's motion, will represent two parallels of declination ; and we use the direction of the parallels of declination to determine the datum point at right angles to them, that is, the north point of the field. We have then & position micrometer, that is, one in which the field of view is divided into four quad- rants, called north preceding, north following, south preceding, and south following, because if there be an FIG. 158. Position Circle. object at the central point it will be preceded and fol- lowed by those in the various quadrants. The movable wires lie on meridians and the fixed ones on parallels when adjusted as above. The position circle is often attached to, and forms part of, the micrometer instead of being fixed to the telescope, and in screwing it on from time to time, the adjustment of the zero changes, and the index error A A 354 STAEGAZING : PAST AND PRESENT. [BOOK v. must be found each time the micrometer is put on the telescope. In practice it is usual to take the north and south line as the datum line, and positions are always expressed in degrees from the north round by east 90, south 180, and west 270, to north again in the direction contrary to that of the hands of a clock. The angle from the east and west line being found by the micrometer, 90 is either added or subtracted, to give the angular measurement from north. But to make these measurements we want a clock ; a clock which, when we have got one of these objects in the middle of the field of view, shall keep it there, and enable the tele- scope to keep any object that we may wish to observe fixed absolutely in the field of view. But in the case of faint objects this is not enough. We want not only to see the object, but also the wires we have referred to. Now then the illuminating-lamp and bright wires, if necessary, come into use. The following, Fig. 159, will show how we proceed if we merely wish to measure a distance, the value of the divisions of the micrometer screw having been previously determined by allowing an equatorial star to transit. It represents the position of the central and the movable wire when the shadow thrown by the central hill of the the lunar crater Copernicus is being measured to deter- mine the height of the hill above the floor of the crater. It has been necessary to let the fixed wire lie along the shadow ; this has been done by turning the micrometer ; but there is no occasion to read the vernier. Except on the finest of nights the stars shake in the field of view or appear woolly, and even on good nights the readings made by a practised eye often differ, inter CHAP, xxiv.] ORDINARY WORK OF THE EQUATORIAL. 355 56, more than would be thought possible. In measuring distances we have supposed for simplicity that we find the distance that one wire has to be moved from coincidence with the fixed wire from one point to another, and theoretically speaking the pointer should point to o on the screw head when the wires are over each other, and then when the wires are on the points, the reading of the FIG. 159. How the Length of a Shadow thrown by a Lunar Hill is measured. screw head divided by the number of divisions corre- sponding to I" will give the distance of the points in seconds of arc. But in practice it is unnecessary to adjust the head to o when the wires coincide, and the unequal expansion of the metals of the instrument, due to changes of temperature, would soon disarrange it. It is also some- what difficult to say when the wires exactly coincide, A A 2 35G STARGAZING : PAST AND PRESENT. [BOOK v. and an error in this will affect the distance between the points. It is therefore found best to only roughly adjust the screw head to o, and then open out the wires until they are on the points and take a reading, say twenty-two ; the screw is then turned in the opposite direc- tion and the movable wire passed over to the other side of the fixed one, and another reading taken, say eighty-two; now the screw has to be moved in the direction which decreases the readings on its head from one hundred downwards, as the distance of the wires increases, so that we must subtract the reading eighty-two from a hundred to give the number of divisions from the o through which the screw is turned, and the reading in this direction we will call the indirect reading, in contradistinction to the direct reading taken at first. So far we have got a read- ing of twenty-two direct and eighteen indirect, which means that we have moved the screw from twenty-two on one side of o to eighteen on the other side, or through forty divisions, and in doing so the movable wire has been moved from the distance of the two points on one side of the fixed wire to the same distance on the other, or through double the distance required. Therefore forty divisions is the measure of twice the distance, and the half of forty, or twenty divisions, is the measure of the distance itself between the two points to which our attention has been directed, whether stars, craters in the moon, spots on the sun, and the like. Let us consider what is gained by this method over a measure taken by coincidence of the wires as a starting- point, and opening out the wires until they cut the points. In the method we have just described there are two chances of error in taking the measurements the direct and indirect ; but the result obtained is divided CHAP, xxiv.] ORDINARY WORK OF THE EQUATORIAL. 357 by two, so that the error is also halved in the final result. Now by taking the coincidence of the wires as the zero, or starting-point, the measure is open to two errors, as in the last case the error of measurement of the points, plus the error of coincidence of wires, an error often of considerable amount, especially as the warmth of the face and breath causes considerable alteration in the parts of the instrument, making a new reading of co- incidence necessary at each reading of distance. As the result is not divided by two, as in the first case, the two errors remain undivided, so we may say that there is the half of two errors in one case and two whole errors in the other. Here then we use the micrometer to measure dis- tances ; but from a very short acquaintance with the work of an equatorial it will at once be seen that one wants to do something else besides measure distances. For instance, if we take the case of the planet Saturn, it would be an object of interest to us to determine how many turns, or parts of a turn, of the screw will give the exact diameter of the different rings ; but we might want to know the exact angle made by the axis with the direction of the planet's motion, across the field, or with the north and south line. If we have first got the reading when the wires are in a parallel of declination, and then bring Saturn back again to the middle of the field and alter the di- rection of the wires until they are parallel to the major axis of the ring, we can read off the position on the circle, and on subtracting the first reading from this, we get the angle through which we have moved the wires, made by the direction of the ring with the parallel of declination, which is the angle required. We are thus 358 STARGAZING : PAST AND PRESENT. [BOOK v. not only able to determine the various measurements of the diameter of the outer ring by one edge of the ring falling on one of the fine wires, and the other edge on the other wire, but, by the position circle outside the micrometer we can determine exactly how far we have moved that system, and thus the angle formed by the axis of the ring of the planet at that particular time. The uses of the position micrometer as it is called are very various. In examination of the sun it is used to Fiu. 160. The Determination of the Angle of Position of the axis of Saturn's Ring. ascertain the position of spots on the surface, and the rate of their motion and change. The lunar craters require mapping, and their distances and bearing from certain fixed points measuring, for this then the position micrometer comes into use. The varying diameters and the inclinations of the axes of the planets and the periods of revolution of the satellites are determined, and the position of their orbits fixed, in like manner. When a comet appears it is of importance to determine not only the direction of its CHAP. xxiv.J ORDINARY WORK OF THE EQUATORIAL. 359 motion among the stars, but the position of its axis of figure, and the angles of position and dimensions of its jets. The following diagram gives an example of the manner in which the position of its axis of figure is determined. First the nucleus is made to run along the fixed wire, so that it may be seen that the north vernier truly reads zero under this condition ; if it does jf FIG. 161. Measurement of the Angle of Position of the Axis of Figure of a Comet, a a, positions of fixed wire when the north vernier is at zero ; d d position of movable wire under like conditions ; a' a', d' d', positions of these wires which enable the angle of position of the comet's axis to be measured. The angle a a' or d d' is the angle required. not its index error is noted. The system of wires is then rotated till one of the wires passes through the nucleus and fairly bisects the dark part behind the nucleus. It need scarcely be said that these observations are also of importance with reference to the motion of the binary stars, those compound bodies, those suns re- volving round each other, the discovery of which we owe 360 STARGAZING : PAST AND PRESENT. [BOOK v. to the elder Herscliel. We may thus have two stars a small distance apart ; at another time we may have them closer still ; and at another we may have them gradu- ally separating, with their relative position completely changed. By means of the wire micrometer and the arrangement for turning the system of wires into differ- ent positions with regard to the parallel of declination, FIG. 162. Double Star Measurement, a a, b b, first position of fixed double wire when the vernier reads 0, and the star runs between the wires ; c c, dd, first position of movable wires. a! a', b' b', new position of fixed double wire which determines the angle of position ; c' c', d' d', new positions of the inovable wires which measure the distance. we have a means of determining the positions occupied by the binary stars in all parts of their apparent orbit, as well as their distances in seconds of arc. It is found, however, by experience that the errors of observation made in estimating distances are so large, relatively to the very small quantities measured, that it is absolutely OIIAI-. xxiv.] ORDINARY WORK OF THE EQUATORIAL. 361 necessary to make the determination of the orbit depend chiefly on the positions. And this is done in the following way. It is possible, by knowing the position angles at different dates, to find the angular velocity, and since the areas described by the radius vector are equal in equal times, the length of the radius vector must vary inversely as the square root of the angular velocity, and by taking a number of positions on the orbit of known angular velocity, we can set off radii vectores, and construct an ellipse, or part of one, by drawing a curve through the ends of the radii vectores ; and from the part of the ellipse so constructed it is possible to make a good guess at the remainder. The angular size of this ellipse is obtained from the average of all the measures of distance of the stars. This ellipse is then the apparent ellipse described by the star, and the form and position of the true ellipse can be constructed from it from the con- sideration of the position of the larger star (which must really be the focus), with reference to the foe as of the apparent ellipse ; for if an ellipse be seen or projected on a plane other than its own, its real foci will no longer coincide with the foci of the projected ellipse. The methods adopted in practice, for which we must refer the reader to other works on the subject, are, how- ever, much more laborious and lengthy than the above outline, which is intended merely to show the possibility, or the faint outline of a method of constructing the real ellipse. When the real ellipse or orbit is known, it is then of course possible to predict the relative posi- tions of the two components. Let us consider in some little detail the actual work of measuring a double star. A useful form for entering observations upon, as taken, is the following, which is copied from one actually used. 362 STARGAZING : PAST AND PRESENT. [BOOK v. o 3 SO ^ 01 OS o CM A, 4 < B, 1 < C, 7 >D, the number showing how many tenths of a magnitude the variable is more or less bright than each comparison star, and the magnitude of the latter being known, we get several values of the magni- tude of the variable, a mean of which is taken for the night. In order to show clearly to the eye the variations of a star, and to compute the periods, of maximum and minimum, a graphical method is adopted : a sheet of cross-ruled paper is prepared, on which the dates of observation are represented by the abscissae, and the corresponding observed magnitudes by the ordinates. Dots are then made representing the several observations, and a free-hand curve drawn amongst the dots, which at once gives the probable magnitude at any epoch in the period of observation, the change of the curve from a bend upwards to downwards, or vice versd, indicating a maximum or minimum of magnitude. So much then for the method of determining the intensity of the visible radiation. The next point to consider is the intensity of the thermal radiations we 384 STARGAZING : PAST AND PRESENT. [BOOK vi. pass from photometry to thermometry. The thermopile will in the future be an astronomical instrument of great importance. We need not go into its uses in other branches of physics, we shall here limit ourselves to the astronomical results which have been already obtained. Lord Kosse used a pile of this kind, made of alternate bars of bismuth and antimony. He attacked the moon, and by observing it from new to full, and from full to new, he got a distinct variation of the amount of heat, according as the moon was nearest to the epoch of full moon, or further from that epoch. As the moon was getting full, he found the needle moved, showing heat, and, after the full, it went down again and found its zero again at new. By differential observations Lord Kosse showed that this little instrument, at the focus of his tremendous reflector, was able to give some estimate of the heat of the moon, which may be 500 degrees Fahr. at the surface. It may be said that the moon is very near us, and we ought to get a considerable amount of heat from it ; but the amount is scarcely perceptible without delicate instruments. Still the instrument is so delicate, that the heat of the stars has been estimated. A pile of very similar construction to the one just mentioned has been attached by Mr. Stone to the large equatorial at Greenwich. The instrument consists of two small piles about- one-tenth of an inch across the face ; the wires from each are wound in contrary directions round a galvanometer, so that when equal currents of electri- city are passing they counteract each other, and the needle remains stationary. It only moves when the two currents are unequal ; we have then a differential galvanometer, showing the difference of temperature of CHAP, xxvi.] LIGHT AND HEAT OF THE STARS. 385 the faces of the two piles ; the image of a star is allowed to fall half-way between the two piles then on one pile and then on another ; then matters are reversed, and a mean of the galvanometer readings taken, beginning with zero when the image of a star was exactly between the two piles. The result was this, that the heat received from Arcturus, when at an altitude of 25, was found to be just equal to that received from a cube of boiling water, three inches across each side, at the distance of 400 yards. Arcturus is not the only star which has been observed in this way ; in another star, Vega, which is brighter than Arcturus, it has been demonstrated that the amount of heat which it gives out, when at an altitude of 60, is equal to that from the same cube at 600 yards, so that Mr. Stone shows beyond all question, that Arcturus gives us more heat than Vega. This opens a new field, for if we get heat effects different from the effects on the eye, the stars ought to be catalogued with reference to their thermal relations as well as their visual brightness. Another valuable application of this method is due to Professor Henry, of Washington. Professor Henry imagined that, by means of a thermo-electric pile placed at the eyepiece of the telescope, so that a sun-spot, or a part of the ordinary surface, could be brought on the face of the pile, he could tell whether there was a greater, or less radiation of heat from a spot, than from any other part ; and he was able with the thermopile to show that there was a smaller radiation of heat from the spots than from the other parts of the sun's surface. c c CHAPTER XXVII. THE CHEMISTRY OF THE STARS : CONSTRUCTION OF THE SPECTROSCOPE. IN the addition of chemical ideas to astronomical inquiries, we have one of the most fruitful and interest- ing among the many advances of modern science, and one also which has made the connection between physics and astronomy one of the closest. To deal properly with this part of our book, as the constitution of one of the heavenly bodies can be studied in the laboratory as well as in the observatory, we have to describe physical instruments and methods, as we]l as the more purely astronomical ones. In a now rare book published in London in the year 1653, that is to say, some years before Sir Isaac Newton made his important observations on the action of a prism on the rays of light observations which have been so very rich in results is given Kepler's treatise on Dioptrics. From this one finds that the great Kepler had done all he could to try to investigate the action of a three-cornered piece of glass. It has been considered, that, because Newton was the first to teach us much of its use, he was the first to investigate the properties of the prism. This is not so. Fig. 167 is an illustration taken from this book, by CHAP, xxvn.] THE CHEMISTRY OF THE STARS. 387 which Kepler shows that if we have a prism and pass light through it, we get three distinct results when a ray (F) falls on the prism. He shows that the first surface reflects a certain amount of light, (D i), and that this is uncoloured, because it does not pass through the glass, and that the remainder is refracted by the glass and part emerges at E, coloured like the rainbow. Then he goes on to show that the second surface of the prism also reflects some light internally, and that there is a certain amount of light leaving the prism at M, and going to K. Fio. 167. Kepler's Diagram. By means of a very few experiments Newton was able to show how much knowledge could be got by examination of the prism. The first proposition in Newton's Optics is an attempt to prove that light, which differs in colour, differs also in degree of refrangibility. We frhall recollect from the fifth chapter what this term means, for it was there shown that whenever a ray of light enters obliquely a medium denser than that in which . it had been travelling, it is bent towards the perpendicular to the surface, in fact it is refracted, c c 2 388 STARGAZING : PAST AND PRESENT. [BOOK vi. and those rays which are most refracted by the same substance with the same angle are said to be more refrangible than others. Newton's experiment was very simple. He took a piece of paper, one half of which FIG. 168. Newton's Experiment showing the different Refrangibilities of Colours. was coloured red and the other half blue ; and this was placed on a stand horizontally, in the light from a window, with a prism between it and the eye. He went on to show, that when he allowed the beam of sunlight to fall upon the paper, strongly illuminating CHAP. xxvn.J THE CHEMISTRY OF THE STAHS. ;^ the red and blue portions, making at the same time all the rest of the room as dark as possible (so that the operation was not impeded by extraneous light), when he held a prism in a particular way, he found that the red and the blue occupied different positions when looked at through the prism. When the prism is held as shown, the red is seen below and the blue above. If the prism be turned with the refracting edge downwards, the red is seen above and the blue below. When the. refracting edge is upwards, it is very clear that if the violet is seen uppermost it must be because the violet ray is more refracted, and when the red ray is uppermost, with the refracting edge of the prism down- wards, it is because the red ray is the least refracted. There are other experiments to which he alludes, and by which Sir Isaac Newton considered he had proved that lights which differ in colour differ also in degrees of refrangibility. Newton at one step went to the sun, and his second theorem is "The light of the sun consists of rays of different refrangibility," and then he enters into the proof by experiment. The light from the sun passes through a hole in the window-shutter and through the prism which throws a spectrum on a screen. We now see the full meaning of the different degrees of refrangi- bility. There he had a long band of light of all colours, the red at one end and the blue at the other, showing that the different colours are unequally refracted, or turned from their course. In this way Sir Isaac Newton deter- mined whether the law, that light which differed in colour differed also in refrangibility, held true with regard to the sun ; and he clearly showed that in this case also the light differs in refrangibility, in exactly the same way as 390 STAKGAZING : PAST AND PKESENT. [BOOK vi. the red light and the blue light had done in his experi- ment with the pieces of paper. He was soon able to prove to himself that the circular aperture was not the best thing he could use, because in the spectrum he had a circle of colour representing every ray into which the light could be broken up. If we put a bit of red glass in the path of the rays we get an image of the hole in red; if we use other coloured glasses, we have a circle for each particular colour ; all these images overlap, and the sum total gives us an extremely mixed spectrum, something quite different from what is seen when we introduce a slight alteration, which curiously enough was delayed for a great many years. Sir Isaac Newton recognised the difficulties there were in getting a pure spectrum by means of a circular aperture, but although he used afterwards an oblong opening instead of a circular aperture, in which we had something more or less like what we now use, namely, a " slit " a narrow line of light ; he does not seem to have grasped the point of the thing, because in one of his theorems he says he also tried triangular openings. We shall show how important it is that we should not only have an oblong opening as proposed by Newton, but that that oblong opening should be of small breadth. The moment we exchange the circular aperture for the oblong opening of Newton, we get a spectrum of greater purity, and, as in the case of the circular open- ing the purity depended on the size of the circle, so also in the case of the oblong opening the purity of the spectrum depends very much on the breadth of the oblong opening. We thus sort out the red, orange, yellow, green, blue, and violet ; they are no longer mixed as they are CHAP. XXVIL] THE CHEMISTRY OF THE STARS. 391 when we employ a circular opening. If we attempt the same experiment with red glass interposed we get something more decided than before ; we have no longer a circular patch of light, but an oblong one in the red ; in fact, the exact form of the aperture, or slit, through which we have allowed the light to pass through the prism and lens to form an image. Now although Newton made these important observa- tions on sun-light, he missed one of the things, in fact we may say the thing, which has made sun -light and star- light of so much importance to Astronomy. The oblong FIG. 169. Wollaston's first Observation of the Lines in the Solar Spectrum. opening which Newton used varied from one-tenth to one-twentieth of an inch in width ; but Dr. Wollaston in 1812 we had to wait from 1672 till 1812 to get this apparently ridiculously small extension used such a narrow slit as we have mentioned, and he found that when he examined the light of the sun with a prism before the eye, he got results of which Newton had never dreamt. Dr. Wollaston not only found the light of the sun differing in refrangibility ; but in the different colours of the solar light he found a number of dark lines, which are represented by the black lines across the spectrum in Fig. 169. 392 STARGAZING : PAST AND PRESENT. [BOOK vi. CHAP, xxvii.] THE CHEMISTRY OF THE STARS. 393 In the year 1814 Fraunhofer examined the spectrum by means of the telescope of a theodolite, directing it towards a distant slit, with a prism interposed. In this manner he observed and mapped 576 lines, the appearance of the spectrum to him being represented in Fig. 170. From this time they were called the " Fraunhofer lines." It need scarcely be said that from the time of Wollaston until a few years ago these strange mysterious lines were a source of wonder to all observers FIG. 171. Student's Spectroscope. who attempted to attack the problem. The difference between the simple prism and slit which Newton, Wollaston, and Fraunhofer used to map these lines, and the modern spectroscope, as used with or without the telescope, is due to a suggestion of Mr. Simms in 1830. Let us refer to a modern spectroscope. Fig. 171 repre- sents a form usually used for chemical analysis. The only difference between the spectroscope and the simple prism in Newton's experiment is this, that in the one case the light falls directly from the slit through the 394 STARGAZING : PAST AND PRESENT. [BOOK vr. prism on a screen and is viewed there ;" and in the other the eye is placed where the screen is, and looks through the prism and certain lenses at the slit. The great improvement which Mr. Simms suggested was this simple one. He said, " It would surely be better that the light which passes through the prism or prisms independently of the number I use, should, if possible, pass through them as a parallel beam of light ; and therefore, instead of putting the slit merely on one side of a prism and the eye on the other, I will, FIG. 172. Section of a Spectroscope, showing the Path of the Ray from the Slit. between the slit and the prism, insert an object-glass," as shown in Fig. 172 ; so that the slit of the spectro- scope is the representative of the hole in the shutter. The slit is exactly in the focus of the little object-glass, c, or collimating lens, as it is called; so that naturally the light is grasped by this lens, and comes out in a parallel beam, and travels among the prism or prisms, quite irrespective of course of their number. This parallel beam, in order to be utilized by the eye after it has passed through the system of prisms, is again taken CHAP, xxvn.] THE CHEMISTRY OF THE STARS. 395 up by another object-glass and reduced from its parallel state into a state of convergence, and brought to a focus which can be examined by means of an eyepiece. The red rays from the slit come to a focus at R, and the blue at B, forming there their respective images of the slit, and between B and R are a number of other images of the slit, painted in every colour that is illuminating it, thus forming a spectrum which is viewed by the eyepiece. In fact, the object-glass and eyepiece consti- tute a telescope, through which the slit is viewed, and the coll imat ing lens makes the light parallel, just as if it had come from a distant object, and fit to be utilized in the telescope. This is the principle to be observed in the construction of every spectroscope. "We have now r given an idea of the general nature of the instrument depending on this important addition made by Mr. Simms, which is the basis of the modern spectroscope, and it is obvious that if we want con- siderable dispersion, we can either increase the number of prisms, or increase their dispersive power. We have already shown in a previous chapter that the dispersion depends on the angle of the prisms, and that the calculations necessary for making the object- glass of a telescope were based upon an observation made by passing light through a prism of a particular angle made of the same glass as that of which the pro- posed object-glass was to be constructed. Then, again, we took the opportunity of showing- that with very dense substances greater dispersion could be obtained. We showed how the prism of dense flint glass over- powered the dispersion of the prism of the crown glass, and how the combination gave us refraction without dispersion. 396 STARGAZING : PAST AND PRESENT. [BOOK vi. Fig. 173 is a drawing of a spectroscope containing four prisms. It is a representation of that used by Bunsen and Kirchhoff when they made their maps of the solar spectrum : it is so arranged that the light after passing through the slit goes through the collimating lens, and then through the prisms ; it is afterwards caught by the telescope lens and brought to a focus in front of the eyepiece. It is very important, when we have many FIG. 173. Spectroscope with Four Prisms. prisms, to be able to arrange them so that whether we use one part of the spectrum or the other, each prism shall be in the best condition for allowing the light to traverse it ; that is to say, that it shall be in the position of minimum deviation, when the angles of incidence and emergence are equal, and each surface refracts the ray equally. They can be arranged so, that as the telescope is moved to observe a new part of the spectrum, every prism will be automatically adjusted. CHAP, xxvn.] THE CHEMISTRY OF THE STARS. 397 To insure this the prisms are united to form a chain so that they all move together, and each has a radial bar to a central pin which keeps them at the proper angle. FIG. 174. Automatic Spectroscope (Grubb's form). There is another arrangement which is very simple, in which we get the condition of minimum deviation by merely mounting the prisms on a spring, and then Flo. 175. Automatic Spectroscope (Browning's form). moving the spring with the telescope, in the same way as the telescope moves the other automatic arrangement. For some observations, especially solar observations, 398 STARGAZING : PAST AND PRESENT. [BOOK vr. in which the light is very intense, it is extremely im- portant, in fact essential, to reduce the brilliancy of the spectrum ; and of course this enables us, in the case of the sun especially, to increase the dispersion almost without limit, by having a great number of prisms, or even using the same twice over, in the following manner : On the spectroscope there is a number of prisms so arranged that the light comes from the slit, and travels through the lower portion of the prisms ; it then strikes against the internal reflecting surface of a right-angled prism at the back of the last prism, Fig. 176, and is FIG. 176. Last Prism of Train for returning the Rays. sent up to another reflecting surface, and then comes back again through the same prisms along an upper storey, and then is caught by means of a telescope above the collimator, on the slit of which the sun's image is allowed to fall. This contrivance, 'suggested by the author and Prof. Young independently, is now largely used. Fig. 177 shows an ordinary spectroscope so armed. The light from the slit traverses the upper portions of the prisms ; it is then thrown down by the reflecting prism seen behind the collimator, then, returning along the lower part, it is CHAP, xxvn.] THE CHEMISTRY. OF THE STARS. 399 received by a right-angled prism in front of the object- glass of the observing telescope. Instead of the rays of light being reflected back through the upper storey of the prisms, another method has been adopted ; the last prism is in this case a half prism, and the last surface on which the rays of light FIG 177. Spectroscope with returning Beam. fall is silvered ; the rays then are returned on them- selves, and, when the instrument is adjusted, come to a focus on the inside of the slit plate, forming there a spectrum, any part of which can, by moving the prisms, be made to fall on a small diagonal reflecting prism on one side of the slit, by which it is reflected to the eye- piece. In this arrangement the collimating lens becomes its own telescope lens on the return of the ray. There is another form of spectroscope, called the direct 400 STARGAZING : PAST AND PRESENT. [BOOK vi. vision, which is largely used for pocket instruments. The principle of it is that the light passing through it is dispersed but not turned from its course, just the reverse of the achromatic combination of the object-glass ; a crown-glass prism is cemented on a flint one of sufficient angle that their deviative powers reverse each other but leave a certain portion of the flint-glass dispersion uncorrected ; since, however, the dispersive power of FIG. 178. Direct Vision Prism. the flint-glass is to a great extent neutralized, therefore, in order to make the instrument as powerful as one of the ordinary construction, a number of flint-glass prisms are combined with crown-glass ones, as shown in Fig. 178. There is another form of direct-vision prism, called the Hersch el -Browning, in which the ray is caused to take its original course on emerging by means of two internal reflections. CHAPTER XXVIII. THE CHEMISTRY OF THE STAES (CONTINUED) : PRINCIPLES OF SPECTRUM ANALYSIS. WE have next to say something about the principles on which the use of the spectroscope depends ; if we look through one we can readily observe how each particular ray of light paints an image of the slit. Thus, if we are dealing with a red ray of light, that ray, after passing through the prisms, will paint a red image of the slit ; if the light be violet, the ray will paint a violet image of the slit, and these images will be separated, because one colour is refracted more than the other. Now it follows from this that when the slit is illuminated by white light, white light being white because it contains all colours, we get an infinite number of images of slits touching or overlapping each other, and forming what is called a continuous spectrum. Hence it is that if we examine the light of a match or candle, or even the electric light, we get such a con- tinuous spectrum, because these light sources emit rays of every refrangibility. Modern science teaches us that they do so because the molecules the vibrations of which produce, through the intermediary of the ether, D D 402 STARGAZING : PAST AND PRESENT. [BOOK vi. the sensation of light on our optic nerve are of a certain complexity. In the preceding list of light sources the sun was not mentioned, because its light when examined by Wollaston and Fraunhofer, was found to be discontinuous. Now it is clear that if in a beam of light there be no light of certain particular colours, of course we shall not find the image of the slit painted at all in the corre- sponding regions of the spectrum. This is the whole story of the black lines in the spectrum of the sun and in the spectra of the stars. Here and there in the spectrum of these there are colours, or refrangibilities, of light which are not represented in light which comes from those bodies, arid therefore there is nothing to paint the image of the slit in that particular part of the spectrum ; we get what we call a dark line, which is the absence of the power of painting an image. But then it may be asked, How comes it that the prism and the spectroscope are so useful to astronomers ? In answer we may say, that if we knew no more about the black lines in the spectra of the sun and stars than we knew forty years ago, the spectroscope ought still to be an astronomical instrument, because it is our duty to observe every fact in nature, even if we cannot explain it. But these dark lines have been explained, and it is the very explanation of them, and the flood of know- ledge which has been acquired in the search after the explanation, which makes the spectroscope one of the most valuable of astronomical instruments. Many of us are aware of the magnificent generaliza- tions by which our countrymen, Professors Stokes and Balfour Stewart, and Angstrom, KirchhorT and Burisen, CHAP, xxvni.] THE CHEMISTRY OF THE STARS. 403 were enabled to explain those wonderful lines in the solar spectrum. These lines in the solar spectrum are there because something is at work cutting out those rays of light which are wanting, and they explained this want by showing to us that around the sun and all the stars there are absorb- ing atmospheres containing the vapours of certain sub- stances cooler than the interior of the sun or of the stars. These philosophers also showed us, that we can divide radiation and absorption into four classes, and that we can have general radiation and selective radiation, and general absorption and selective absorption, so that the phenomena that we see in our chemical and physical laboratories and our observatories may all be classed as general and selective radiation, or general and select- tive absorption. Let us explain these terms more fully. Kirchhoff showed us that from incandescent solid and liquid bodies we get a continuous spectrum ; thus from the carbon poles of an electric lamp we get a complete spectrum. That is called a continuous spectrum, and it is an instance of continuous radiation, which we get from the molecular complexity of solids or liquids, and likewise, from dense gases or vapours. When we examine vapours or gasesr which are not very dense we get an indication of selective radiation that is to say, the ]ight one gets from these substances, in- stead of being spread broadcast from the red to the violet, will simply fall here and there on the spectrum ; in the case of one vapour we may get a yellow line a yellow image of the slit and in the case of another vapour, we may get a green one ; the light selects its point of appearance, and does not appear all along the spectrum. D D 2 404 STARGAZING : PAST AND PKESENT. [BOOK vi. This selective radiation is due to a simplification of the molecular structure of the vapours, the simpler states are FIG. 179. Electric Lamp, y, z, wires connecting battery of 50 Grove or Bunsen elements ; G, H, carbon holders ; K, rod, which stops a clockwork move- ment, which when going makes the poles approach until the current passes }- A, armature of a magnet which by means of K frees the clockwork when not in contact ; E, electro-magnet round which the current passes when the poles are at the proper distance apart, causing it to attract the armature A. less rich in vibrations, and therefore instead of getting rays of all refrangibilities we only get rays of some. CHAP. xxviii.J THE CHEMISTRY OF THE STARS. 405 Very striking experiments showing the spectra of bodies may be made with an electric ]amp armed with a condenser and a narrow slit ; by means of a lens Then one or two this slit is magnified on a screen. FIG. 180. Electric Lamp arranged for throwing a spectrum on a screen. D, lens ; E E', bisulphide of carbon prisms. prisms of glass containing bisulphide of carbon are placed in the beam after it has traversed the lens, which draw out the image of the slit into a spectrum. We can then place a piece of sodium on the lower carbon 406 STARGAZING : PAST AND PRESENT. [BOOK vi. CHAP. XXVJIL] THE CHEMISTRY OF THE STARS. 407 pole, and when the poles are brought together it will be volatilized, and its vapour rendered luminous. Its spectrum on the screen will be seen to consist of four lines only, the yellow line being far more brilliant than the rest. Sodium was selected on account of the simplicity of its spectrum. If we put another metal, say calcium, in the place of the sodium, there will appear on the screen the characteristic lines of that metal. A number of distinct FIG. 182. Coloured Flame of Salts in the flame of a Bunsen's Burner. images of the slit in different colours is seen ; if we are well acquainted with the spectrum of any metal, and see it with the spectroscope, it is easy to at once recognise it. Fig. 181 shows at one glance the spectra (1) of iron, (2) of calcium, and (3) of aluminium ; and will clearly indicate the great difference there is between the radiation spectra of the rare vapours of each of the metallic elements. 408 STARGAZING : PAST AND PRESENT. [BOOK vi. The electric light is only required where great brilli- ancy is essential, as for showing spectra on a screen. A Bunsen's burner is the best instrument for studying the spectra of metallic salts. By its means the nature of a salt can be easily studied with a hand spectroscope, and in this way an almost infinitesimal quantity can be detected. These are instances of selective radiation. We will now turn to absorption. If we first get a continuous spectrum from our lantern and then interpose substances in the path of the beam, we can examine their effects on the light. If we first use a piece of neutral- tinted glass, which is a representative of a great many substances which do, for stopping light, what solids and liquids do for giving light namely, it cuts off a portion of every colour ; the spectrum on the screen will be dimmed ; here we have a case of general absorption. If, instead of this, we hold in the beam a vessel containing magenta, a dark band in the spectrum is seen, and if we put a test-tube in its place containing iodine vapour, a number of well-defined lines pervading the spectrum is observed. In these cases clearly, the magenta in one case, and the iodine vapour in the other, have cut off certain colours, and so the slit is not painted in these colours, and dark lines or bands appear. These are instances of selective absorption, certain rays are selected and absorbed, while others pass on unheeded. The easiest method of performing these absorption experiments in the case of liquids is to place the substance in a test-tube in front of the slit of the spectroscope, as shown in Fig. 183, and point the collimator to a strong light. Besides the absorption by liquids, the vapours of the CHAP, xxviii.] THE CHEMISTRY OF THE STARS. 409 metals also absorb selectively, and if a tube containing a piece of sodium and filled with hydrogen (so that the metal will not get oxidized) is placed in the path of the rays, and the sodium heated, the spectrum is at first unaffected, but as the sodium gets hot and its vapour comes off, we can mark its effect on the spectrum. We see a dark line beginning to appear in the yellow, FIG. 18d. Spectroscope arranged for showing Absorption. finally the whole light of that particular colour is ab- sorbed, and we have a dark line in its place. To sum up then : We get from solids, when heated, general radia- tion, and when they act as absorbers, we get general absorption ; from gases and vapours we get selective radiation and selective absorption. Now it at once strikes any one performing these experiments that the dark line of yellow sodium appears in the same place in the spectrum as the bright one, and this is so. When the absorption by sodium vapour is examined by the spectroscope, it is then seen to 410 STARGAZING : PAST AND PRESENT. [BOOK vi. consist of two well-defined lines close together, and when the radiation is examined, it is found to consist of two bright ones, and the absorption and radiation lines, the dark and bright ones, are found to exactly agree in position in the spectrum, showing that the substance that emits a certain light is able to absorb that same light, so that it matters not whether a body is acting as an absorber or radiator, for still we recognize its cha- racteristic lines. In 1814 Fraunhofer strongly suspected the coincidence of the two bright sodium lines with the dark lines in the sun ; afterwards Brewster, Foucault, and Miller showed clearly the absolute coincidence ; and Professor Stokes in 1852 came to the conclusion that the double line D, whether bright or dark, belonged to the metal sodium, and that it absorbed from light pass- ing through it the very same rays which it is able, when incandescent, to emit. The phenomena rendered visible to us by the spectroscope have their origin, as we have said, in molecular vibration, and the reason of the identical position of the light and dark lines, and indeed the whole theory of spectrum analysis, may be shortly stated as follows : The spectroscope tells us that when we break a mass of matter down to its finest particles, or, as some people prefer to call them, ultimate molecules, the vibrations of these ultimate parts of each different kind of matter are absolutely distinct ; so that if we get the ultimate par- ticle, say of calcium, and observe its vibrations we find that the kind of vibration or unrest of one substance of the calcium, for instance is different from the kind of unrest or mode of vibration which is the same tiling o of another substance, let us say sodium. Mark well the expression, ultimate molecule, because the vibrations CHAP, xxviii.] THE CHEMISTRY OF THE STARS. 411 of the larger molecular aggregations are absolutely power- less to tell us anything about their chemical nature. When we bring down a substance to its finest state, and observe, by means of the prism, the vibrations it com- municates to the ether, we find that, using a slit in the spectroscope and making these vibrations paint different images of the slit, we get at once just as distinct a series of images of the slit for each substance as we should get a distinct sequence of notes if we were playing different tunes on a piano. Next, this important consideration comes into play- whenever any element finds itself in this state of fine- ness, and therefore competent to give rise to these phenomena, it will give rise to them in different degrees according to certain conditions. The intensest form is observed when we employ electricity. In a great many cases the vibrations may be rendered very intense by heat. The heat of a furnace or of gas will, for instance, in a great many cases, suffice to give us these pheno- mena ; but to see them in all their magnificence their most extreme cases we want the highest possible tem- peratures, or better still, the most extreme electric energy. What we get is the vibration of these particles rendered visible to our eye by the bright images of the slit or by their bright " lines/' But that is not the only means we have of studying these states of unrest. We can study them almost equally well if, instead of dealing with the radiation of light from the particles themselves, we interpose them between us and a light source of more complicated molecular structure, and hotter or more violently excited than the particles themselves. From such a source the light would come to us absolutely complete ; that is 412 STARGAZING : PAST AND PRESENT. [BOOK vi. to say, a perfectly complete gamut of waves of light, from extreme red to extreme violet. When we deal with these particles between us and a light-source com- petent to give us a continuous spectrum, then we find that the functions of these molecules are still the same, but that their effect upon our retinas is different. They are not vibrating strongly enough to give us effectively light of their own, but they are eager to vibrate, and, being so, they are employed, so to speak, in absorbing the light which otherwise would come to our eyes. So that whether we observe the bright spectrum of calcium or any other metal, or the absorption spectrum under the conditions above stated, we get lines exactly in the same part of the chromatic gamut, with the difference that when we are dealing with radiation we get bright lines, and when dealing with absorption we get dark ones. It was such considerations as these by which the presence of sodium was determined in the sun. Soon followed the discovery of coincidence of other dark lines with the bright lines of numbers of our elements, and we had maps made by Kirch hoff, and Bunsen, and Angstrom, in which almost every dark line is mapped with the greatest accuracy. The dark lines in the spectra of the stars, and the light ones in nebulae, comets, and meteorites have also yielded to us a knowledge more or less accurate of the elements of which these celestial bodies are built up. These radiations and absorptions are the A B C of spectrum analysis, and they have -their application in every part of the heavens which the astronomer studies with the spectroscope. But although it is the A B C it is not quite the whole alphabet. After Kirchhoff and Bunsen had made their experiments showing that we CHAP. XXVIIL] THE CHEMISTRY OF THE STARS. 413 might differentiate between solids, liquids, gases, and vapours, by means of their spectra, and say, here we have such a substance, and there another, either by its spectrum when it is incandescent or from the absorption lines produced by it on a continuous spectrum when it is absorbing, Plucker and Hittorf showed that not only were the spectra very different among themselves, but there were certain conditions under which the spectrum of the same substance was not always the same ; and although they did not make out clearly what it was, they showed that it depended either on the pressure of the gas or vapour, or the density, or the temperature. And other observations since then indicate that we get changes in spectra which are due to pressure, and not to temperature per se ; so that we have another line of research opened to us by the fact, that not only are the spectra of different substances differ- ent, but that the spectra of the same substances are different under different conditions. Fig. 184 represents a hydrogen tube, called a Geissler's tube a glass tube FlG 184 _ Geiss i er > g with hydrogen in it and two platinum Tube - wires, one passing into each bulb, by which a current of electricity can be passed through the gas. In this case we use hydrogen gas in a state of extreme tenuity. If now one of these tubes be connected with a Sprengel pump, we can alter the condition of tenuity at pleasure, 414 STARGAZING : PAST AND PRESENT. [BOOK vi. either reducing the contents of the tube or increasing them by admitting hydrogen from a receiver, by a tap connected to the tubing of the air-pump ; we can thus . considerably increase the amount of gas in the tube and bring it to something like atmospheric pressure. We shall find the colour of the gas through which the spark passes varies considerably as we increase the pressure of the hydrogen in the tube. The hydrogen at starting is nearly as rare as it can be, and if more hydrogen be let in we shall see a change of colour from greenish white to red ; the hydrogen admitted has increased the pressure and the colour of the spark is entirely changed. It is a very brilliant red colour, the colour of the prominences round the sun. It may be asked, probably, whether there are any applications of this experiment to astronomical obser- vation. It is of importance to the astronomer to get the differences of the spectra of the same substance under different conditions, and it is found as important to get these differences between the spectra of the same substance, as those between the spectra of different substances. There is another experiment which will show another outcome of this kind of research. Change of colour in the spark is accompanied by a considerable differ- ence in the spectrum that is to say, it* is clear, to refer back to the colour' of the hydrogen when the light was green, that we should get some green in the spectrum, and when the light became red, there would be some change or increase of light towards the red end of the spectrum. We see that that is perfectly true ; but there is not only a change produced by the different pressures, as shown by the different colours ; CHAP, xxvin.] THE CHEMISTRY OF THE STARS. 415 but if we carry the analysis still further if, instead of dealing with the whole of the spectrum, we examine particular lines, we find in some cases that there are very great changes in them. If, for instance, we examine the bluish-green line given by hydrogen, we shall find it increase in width as the pressure increases. This kind of effect can be shown on the screen by means of the electric lamp. We place some sodium on the carbon poles in the lamp, and have an arrangement by which we can use either twenty or fifty cells at pleasure. The action of a number of cells upon the vapour of sodium in the lamp is this : the more cells we work with, the FIG. 185. Spectrum of Sun-Spot. greater is the quantity of the sodium vapour thrown out, and associated with the greater quantity of vapour is a distinct variation of the light- in fact, an increase in the width and brightness of the yellow lines on the screen. Now just to give an illustration of the profitable application of this : we know, for instance, from other sources, strengthened by this, that in certain regions of the sun, called sun-spots, there are greater quantities of sodium vapour present than in others, or it exists there at greater pressure. If that be so, we ought to get the same sort of result from the sun as we get on the screen 416 STARGAZING : PAST AND PRESENT. [BOOK vi. by varying the density of the sodium vapour. That is so. We do get changes exactly similar to the changes on the screen, only of course it is the dark lines we see, and not the bright ones : the dark lines of sodium are widened out over a sun-spot, Fig. 185, showing its presence in greater quantity, or at greater pressure. Besides the widening of the lines due to pressure, there is something else which must be mentioned. While experimenting with the spark taken between two mag- nesium wires focussed on the slit of the spectroscope by a lens, the lines due to the metal were found to be of unequal lengths. Now, as the lines are simply images FIG. 186. Diagram explaining Long and Short Lines. of the slit, the lengths of the lines depend on the length of the slit illuminated, so that in this case it appeared that the slit was not illuminated to an equal extent by all the colours given out by magnesium vapour, but that the vapour existed in layers round the wires, the lower ones giving more colours, and so also more lines, than the upper ones further from the wire, as is represented in Fig. 186 ; this is only meant to give an idea of the thing, and is not, of course, exactly what is seen, s is the slit of the spectroscope, p the image of one of the magnesium poles ; the other, being at some little distance CHAP, xxvni.] THE CHEMISTRY OF THE STARS. 417 away, does not throw its image on the slit, and therefore does not interfere. The circles shown are intended to represent the layers of vapour giving out the spectrum ; on the right the lower layers give A, B, and c, the next A and B, and the upper ones only B. Now we may reason from this that the layers next the poles are denser than those further off, and give a more com- plicated spectrum than the others ; and also, if the quantity of vapour of any metal is small, we may only get just these longest lines. Of late, experiments have been made in England on other metals for instance, aluminium and zinc, and their compounds ; and it is found that, when the vapour is diluted, as it were, one gets only the longest line or lines ; and in the compounds, where the bands due to the compound compose the chief part of the spectrum, the longest line or lines of the metal only appear. Now what is the application of this t In the sun are found some of the dark lines of certain metals, but not all ; for instance, there are two lines in the solar spectrum corresponding to zinc, but there are twenty-seven bright lines from the metal when volatilized by the electric spark. Why should not these also have their corre- sponding dark lines in the sun ? The answer is, that the non-corresponding lines of the metal are the short ones, and only exist close to the metal where the vapour is dense ; and in the sun the density is not sufficient to give these lines. Here, then, we have at once a means of measuring the quantity of vapour of certain metals composing the sun. It was thought that aluminium was not in the sun, as only two lines of the metal out of fourteen corresponded to black lines in the solar spec- trum. It is now known that these two are the longest E E 418 STARGAZING : PAST AND PRESENT. [BOOK vi. - 13 CHAP. XXVIIL] THE CHEMISTRY OF THE STARS. 419 lines, and that aluminium probably exists in the sun, and zinc, strontium, and barium must also be added. These probably exist in small quantities, in- sufficiently dense to give all the lines seen from a spark in the air. There is also another quite distinct line of inquiry in which the spectroscope helps us. Imagine yourself in a ship at anchor, and the waves passing you, you can count the number per minute ; now let the vessel move in the direction whence the waves come, you would then meet more waves per minute than before ; and if the vessel goes the other way, less will pass you, and by counting the increase or decrease in the number passing, you might estimate the rates at which you were moving. Again, suppose some moving object causes ripples on some smooth water, and you count the number per minute reaching you, then if that object approach you, still moving, and so pro- ducing waves at the same rate, the number of ripples a minute will increase, and they will be of course closer together ; for as the object is approaching you, every subsequent ripple is started, not from the same place as the preceding one, but a little nearer to you, and also nearer to the one preceding, on whose heels it will follow closer. By the increase in the number of ripples, and also the decrease in the distance between them, one can estimate the rate of motion of the object pro- ducing them, for the decrease in distance between the ripples is just the distance the object travels in the time occupied between the production of two waves, which was ascertained when the object was stationary. Now let us apply this reasoning to light. Light, we E E 2 420 / STARGAZING : PAST AND PRESENT. [BOOK vi. now hold, is due to a state of vibration of the particles of an invisible ether, or extremely rare fluid, pervading all space ; and the waves of light, although infmitesi- mally small, move among these particles. Now we know that it is the length of the waves of light which determines their refrangibility or colour, and therefore anything that increases or diminishes their length alters their place in the spectrum; and as waves of water are altered by the body producing them moving to or from the observer, so the waves of light are changed by the motion of the luminous body ; and this change of refrangibility is detected with the spectroscope. By measuring the wave-length of let us say the F line, and the new wave-length as shown by the changed position, we can estimate the velocity at which the light source is approaching or receding from us. This application, as we shall see in the next chapter, enables us to determine the rate at which movements take place in the solar atmosphere. It also gives us the power of measuring the third co-ordinate of the motion of stars. We can,, by the examination of their positions, measure the motion at right angles to our line of sight, and so determine their motion with reference to the two co-ordinates, K.A. and Dec., or Lat. and Long., and just in the same way as we can measure the velocity of the solar gases to or from us, so we can measure the motion of the stars to or from us, thereby giving us the third co-ordinate of motion. It need scarcely be said that by the introduction of the spectroscope a new method of observation, and a new power of gaining facts, has dawned, and the sooner it is used all over the world with the enormous instru- CHAP, xxviii.] THE CHEMISTRY OF THE STARS. 421 ments which are required, the better it will be for science. These then are some of the chief points of spectro- sccpic theory which makes the spectroscope one of the most powerful instruments of research in the hands of the modern astronomer. . CHAPTER XXIX. THE CHEMISTRY OF THE STARS (CONTINUED) : THE TELESPECTROSCOPE. WE have now to speak of the methods of using these spectroscopes for the purpose of astronomical observa- tions. For a certain class of observations of the sun no telescope is necessary, but some special arrangements have to be made. Thus while Dr. Wollaston and Fraunhofer were con- tented with simple prisms, when Kirchhoff observed the solar spectrum, and made his careful maps of the lines, he used an instrument like Fig. 173, and for the purpose of comparing the spectrum of the sun with that of each of the chemical elements in turn, he used a small reflecting prism, covering one-half of the slit, Fig. 188, so that any light thrown sideways on to the slit would be caught by this prism, and reflected on to the slit as if it came from an object near the source of light at which the spectroscope is pointing, so that one-half of the slit can be illuminated by the sun, while the other is illuminated by another light ; and on looking through the eyepiece one sees the two spectra, one above the other; so that we are able to compare the lines in the two spectra. CHAP. XXIX.] THE CHEMISTRY OF THE ST/RS. 423 The sunlight, whether coming from the sun itself or a bright cloud, is reflected into the comparison prism, Fig. 189, of the spectroscope, An instrument called a heliostat can be used for this, reflecting the light either directly into the prism or through the medium of other reflectors. OF c/ FIG. 188. Comparison Prism, showing the path of the Kay. The heliostat is a mirror, mounted on an axis, which moves at the same rate as the sun appears to travel, so that wherever the sun is, the reflector, once adjusted, automatically throws the beam into the instrument, so FIG. 189. Comparison Prism fixed in the Slit. that the light of the moving sun can be observed without moving the spectroscope. An average solar spectrum is thus obtained, and, by means of a prism over one-half of the slit, it was quite possible for Kirchhoff and Bunsen to throw in a spec- trum from any other source for comparison, and so they 424 STARGAZING : PAST AND PEESENT. [BOOK vi. compared the spectra of the metals and other elements with the solar spectrum, and tested every line they could find in the spectra. They first found that the two lines of sodium corresponded with the two lines called D in the spectrum, then that the 460 lines of FIG. 190. Foucault's Heliostat. iron corresponded in the main with dark lines in the solar spectrum ; and so they went on. There is, however, a method of varying the attack on this body altogether, by means of the spectroscope and telescope. "We saw that Kirchhoff and Bunsen contented themselves with an average spectrum of the sun that is to say, they dealt with the general spec- CHAP, xxix.] THE CHEMISTRY OF THE STARS. 425 trum which they got from the general surface of the sun, or reflected from a cloud or any other portion of the sky to which they might direct the reflector ; but by means of some such an arrangement as is shown in Fig. 192, we can arrange our spectroscope so that we shall be able to form an image of the sun by the object-glass of a telescope, on the slit, and allow it to be immersed in any portion of the sun's image we may choose. We then have a delicate means of testing what are the spectroscopie conditions of the spots and of those brighter portions of the sun which are called faculae, and the like. And it is known that, by an arrangement of this kind, it is even possible to pick up, without an eclipse, those strange things which are called the red prominences, or the red flames, which have been seen from time to time during eclipses. If we wish to observe any of the other celestial bodies, we must employ a telescope and form an image on the slit, or else use the heavenly body itself as a slit. In the former case spectroscopes must be attached to telescopes, and hence again they must be light and small, unless a siderostat is employed. In the latter case the prism is placed outside the object-glass, and the true telescope becomes the observing telescope. Fraunhofer, at the beginning of the present century, was the first to observe the spectra of the stars by placing a large prism outside the object-glass, three or four inches in diameter, of his telescope, and so virtually making the star itself the slit of the spectroscope ; and in fact he almost anticipated the arrangement of Mr. Simms, and satisfied the conditions of the problem. The parallel light from the star passed through the prism, 426 STARGAZING: PAST AND PRESENT. [BOOK vi. and by means of the object-glass was brought to a focus in front of the eyepiece, so that the spectrum of the star was seen in the place of the star itself. This system has recently been re-invented, and the accompanying woodcut, Big. 191, shows a prism arranged to be placed in front of an object-glass of four inches aperture. It is seen that the angle of the prism is very small. The objection to this method of procedure is that Fm. 191. Object-glass Prism. the telescope has to be pointed away from the object at an angle depending upon the angle of the prism. In the other arrangement we have the thing managed in a different way : we have the object-glass collecting the light from the star and bringing it to a focus on the slit, and it then passes on to the prisms, through which the light has to pass before it comes to the eye. In this combination of telescope and spectroscope we have what has been called the telespectroscope ; one CHAP. XXIX.] THE CHEMISTRY OF THE STARS 427 method of combination is seen in the accompanying drawing of the spectroscope attached to Mr. Ne wall's great refractor ; but any method will do which unites rigidity with lightness and allows the whole instrument to be rotated with smoothness. For solar observation, as there is light enough to FIG. 192. The Eyepiece End of the Nevvall Refractor (of 25 inches aperture), with Spectroscope attached. admit of great dispersion, many prisms are employed, as shown in Fig. 192; or the prisms may be made so tall that the light may be sent backwards and for- wards many times by means of return prisms, to which reference has been already made. For the observation of those bodies which give a small 428 STARGAZING : PAST AND PRESENT. [BOOK vi. amount of light, fewer prisms must be used, and arrange- ments are made for the employment of reference spectra, i.e., to throw the light coming from different chemical elements into the spectroscope, in order that we may FIG. 193. Solar Spectroscope (Browning's form). test the lines ; whether any line of Sirius, for instance, is due to the vapour of magnesium, as Kirchhoff tested whether any line in the sunlight was referable to iron or the other vapours which he subsequently studied. FIG. 194. Solar Spectroscope (Grubb's form). These are shown in Fig. 195. e is a reflecting prism, and F is another movable reflector to reflect tlie light CHAP, xxix.] THE CHEMISTRY OF THE STARS. 429 from a spark passed between two wires of the metal to be compared, and to throw it on the prism, which FIG. 195. Side view of Spectroscope, showing the arrangement by which the light from a spark is thrown into the instrument by means of the reflecting prism, e, by a mirror F. (Huggins.) FIG. 196. Plan of Spectroscope. T, eyepiece end of telescope, B interior tube, 'carrying A, cylindrical lens ; D, slit of spectroscope ; G, collimating lens ; h h, prisms ; Q, micrometer. (Huggins.) reflects the light through the slit of the spectroscope to the prisms and eye ; if the instrument were in perfect 430 STARGAZING : PAST AND PRESENT. [BOOK vr. adjustment and turned on a star, and a person were to place his eye to the spectroscope, he would see in oiie- FIG. 197. Cambridge Star Spectroscope Elevation. FIG. 198. Cambridge Spectroscope Plan. half of the field of view the spectrum of the star with CHAP, xxix.] THE CHEMISTRY OF THE STARS. 431 > dark lines, and in the other half the spectrum of the vapour with its bright lines ; and if he found the bright lines of the vapour to correspond with any particular dark line of the spectrum of the star, he would know whether the metal exists at that star or not ; so this little mechanical arrangement at once tells him what there is at the star, whether it be iron or anything else. In Figs. 197 and 198 is shown another form of stellar spectroscope, that of the Cambridge (U.S.) observatory ; it is the same in principle as that just described. A direct vision star spectroscope is shown in Fig. 199. Fio. 199. Direct-vision Star Spectroscope. (Secchi.) A new optical contrivance altogether has to be used when star spectra are observed. The image of a star is a point, and if focussed on the slit will of course give only an extremely narrow spectrum ; to obviate this a cylindrical lens is employed, which may be placed either before the slit or between the eyepiece arid the eye. If placed before the slit, it draws out the image of the star to a fine line which just fits the slit, so that a sufficient portion of the slit is illuminated to give a spectrum wide enough to show the lines, or the slit may be dispensed with altogether. 432 STARGAZING : PAST AND PRESENT. [BOOK vi. In stellar observations, when the cylindrical lens is used in front of the slit, special precautions should be taken so as to secure that the position of the cylindrical lens and slit in which the spectrum appears brightest should be used. In any but the largest telescopes the spectra of the stars are so dim that unless great care is used the finer lines will be missed. A slit is not at all necessary for merely seeing the spectra ; indeed they are best seen without one. If a slit be used, it should lie in a parallel and not in a meridian ; under these circumstances slight variations in the rate of the clock axe of no moment. In this and in other observational matters it is good to know what to look for, and there are great generic differences between the spectra of the various stars. In Fig. 200 are represented spectra from the observations of Father Secchi. In the spectrum of Sirius, a repre- sentative of Type I., very few lines are represented, but the lines are very thick ; and stars of this class are the easiest to observe. Next we have the solar spectrum, which is a repre- sentative of Type II., one in which more lines are re- presented. In Type III. fluted spaces begin to appear ; and in Type IV., which is that of the red stars, nothing but fluted spaces is visible, and this spectrum shows that there is something different at work in the atmo- sphere of those red stars to what there is in the simpler atmosphere of the first of Type I. These observations were first attempted, and carried on with some success, by Fraunhofer, and we know with what skill and perse- verance Mr. Huggins has continued the work in later years, even employing reference spectra and determining their chemical constitution as well as their class. We need scarcely say that the same arrangement, CHAP. XXIX.J THE CHEMISTRY OF THE STARS. 433 F F 434 STARGAZING : PAST AND PRESENT. [BOOK vr. minus the cylindrical lens, is good for observing the nebulse and such other celestial objects as comets and planets. For all spectrum work, it has to be borne in mind that the best definition is to be had when the actual colour under examination is focussed on the slit. With re- flectors, of course, there is no difference of focus for the different colours. As the best object-glasses are over- corrected for chromatic aberration, the red focus is generally inside and the blue one outside the visual one. It is not necessary to move the whole spectroscope to secure this ; all collimators should be provided with a rack and pinion giving them a bodily movement back- wards arid forwards. This precaution is of especial importance in the case of solar observations, to which we have next to refer. If in any portion of the sun's image on the plate carrying the slit we see a spot, all we have to do is to move the telescope, and with it of course the sun's image, so that the slit is immersed in the image of the spot ; if, however, we wish to observe a bright portion of the sun, we can immerse this slit in the bright portion. Again, if we wish to examine the chromosphere of the sun, we simply have to cover half the slit with the sun, and allow the other part of the slit to be covered by any surroundings of the sun, and, so to speak, to fish round the edge ; the lower half of the slit, say, is covered by the sun itself, and therefore we shall get from that half the ordinary solar spectrum ; the upper half is, however, immersed in the light reflected from our atmosphere, giving a weak solar spectrum, so that we get a bright and feeble spectrum side by side. But besides the atmo- spheric light falling on the upper part of the slit, the CHAP. xxix.J THE CHEMISTRY OF THE STARS. 435 image of anything surrounding the sun falls there also, and its spectrum is seen with the faint solar spectrum, and we find there a spectrum of several bright lines. Now, as an increase of dispersive power will spread out a continuous spectrum and weaken it, we may almost indefinitely weaken the atmospheric spectrum, and so practically get rid of it, still leaving the bright-line spectrum with the lines still further separated ; so that if it were not for our atmosphere, we should get only the spectrum of the sun and that of its surroundings ; one a continuous spectrum with black lines, and the other consisting of bright lines only. Now if we suppose these observations made if the precaution to which we have alluded be not taken, the spectrum of the sun-spot will differ but little from that of the general surface, and the chromospheric lines will scarcely be visible. If the precaution be taken, in the case of the spot it will be found that every one of the surrounding pores is also a spot ; and if the air be pure the spectrum will be full of hard lines running along the spectrum, just like dust lines, but emphatically not dust lines, because they change with every movement of the sun. The figure of the spot spectrum on p. 415 will show what is meant. Fig. 201 will show the appearance of the chromospheric line when the blue-green light is exactly focussed ; the boundary of the spectrum of the photosphere approaches in hardness that at the end of the slit. By measuring the lengths of the lines we can estimate the height of the vapours producing them ; we find from this that magnesium is usually present to a height of a few hundred miles, and that hydrogen extends to between 3,000 and 4,000 miles ; in some positions of the slit the F F 2 430 STARGAZING : PAST AND PRESENT. [BOOK vi. hydrogen lines are seen to start up to great heights, showing the presence of flames or prominences extend- ing in height to sometimes 100,000 miles. If, without changing the focus, we open the slit wider, and throw the sun's image just off the slit, so that the very bright continuous spectrum no longer dazzles the eye, we shall be able to see these flames whenever they cross the opening, for the image of the slit is focussed on the eye, and the sun and its flames are focussed on FIG. 201. Part of Solar Spectrum near F. the slit, so if we virtually remove the slit by opening it wide, we see the flames ; still the limit of opening is soon approached, and the flood of atmospheric light soon masks them. The red hydrogen line of the spectrum is the best for viewing them, although the yellow or blue will answer. We may also place the sun's image so that the slit is tangential to it, in which case a greater length of the hydrogen layer, or chromosphere, as it is called, is visible, although its height is limited by the opening of the slit. CHAP, xxix.] THE CHEMISTRY OF THE STARS. 437 By these means we are able to view a small part of the chromosphere at a time, and to go all round the sun in order to obtain a daily record of what is going on. If, however, we throw the image of the sun on a disc of metal of exactly the same size, we eclipse the sun, but allow the light of the chromosphere to pass the edge of the disc ; this of course is masked by the atmospheric light, but if the annul us, or ring of chromosphere, be reduced sufficiently small, it can be viewed with a spec- troscope in the place of a slit, in fact it is virtually a circular slit on which the chromosphere rests. By this means nearly the whole of the chromosphere can be seen at once. This is accomplished as follows : The image of the sun is brought to focus on a dia- phragm having a circular disk of brass in the centre, of the same size as the sun's image, so that the sun's light is obstructed and the chromospheric light is allowed to pass. The chromosphere is afterwards brought to a focus again at the position usually occupied by the slit of the spectroscope ; and in the eyepiece is seen the chromo- sphere in circles corresponding to the " C " or other lines. A lens is used to reduce the size of the sun's image, and keep it of the same size as the diaphragm at different times of the year ; and other lenses are used in order to reduce the size of the annulus of light to about -- inch, so that the pencils of light from either side of it ma} 7 not be too divergent to pass through the prisms at the same time, in order that the image of the whole annulus may be seen at once. There are mechanical difficulties in producing a perfect annulus of the required size, so one \ inch in diameter is used, and can be reduced virtually to any size at pleasure. From what has been said it is easy to see that we 438 STARGAZING : PAST AND PRESENT. [BOOK vi. really now get a new language of light altogether, and a language which requires a good deal of interpretation. We have still, indeed, to consider some curious observations which are now capable of being made every day when anything like a sun-storm is going on, by means of the arrangement in which the spectroscope simply deals with the light that comes from a small portion of the sun instead of from all the sun. If we make the slit travel over different portions of the sun on which any up-rushes of heated material, or down- rushes of cold material, or other changes, are going on from change of surface temperature, the Fraunhofer I j m LiJ 1 * i ? 3 4- J FIG. 202. Distortions of F line on Sun. lines, which we have before shown to be straight, instead of being so, appear contorted and twisted in all direc- tions. On the other hand, if we examine the chromo- sphere under the same conditions, we find the bright lines contorted in the same manner. The usually dark lines, moreover, sometimes appear bright, even on the sun itself; sometimes they are much changed in their relative positions with reference to the solar spectrum. The meaning of these contortions has already been hinted at (-p. 420). It was there shown that every colour, or light of every refrangibility, is placed by the prisms in its own particular position, so if a ray of light alters its posi- CHAP, xxix.] THE CHEMISTRY OF THE STARS. 439 tion in the spectrum it must change its colour or refrangibility, so the light producing the F line in the one case, and the absent light producing the dark line in the other, differ slightly in colour, or are rather more or less refrangible than the normal light from hydrogen. In the case when the F line is wafted towards the blue end of the spectrum, the light falling on the slit is rather more refrangible than usual ; and in the middle drawing, Fig. 203, w r here the F line bifurcates, the slit is supplied with two kinds of light differing slightly in refrangibility. Not only does the light radiated FIG. 203. Displacement of F line on edge of Sun. by a substance change in this way, but the light absorbed by that substance also changes, hence the con- tortions of the black lines are due to a similar cause. Here, therefore, we have evidence of a change of refrangibility, or colour, of the light coming from the hydrogen surrounding the sun. This change of re- frangibility is due to the motion of the solar gases, as explained in the last chapter. So we find that the hydrogen producing the light giving us one of the forms of the F line, shown in Fig. 203, is moving towards us at the rate of 120 miles a 440 STARGAZING : PAST AND PRESENT. [BOOK vi. second, while that giving the other form is moving away from us. Let us see how these immense velocities are estimated. By means of careful measurements, Angstrom has shown on his map of the solar spectrum the absolute length of the waves of light corresponding to the lines ; thus the length of the wave of light of hydrogen giving the F line is T ^^o of a millimeter. In Fig. 203 the dots on either side of the F line show the positions, where light would fall, if it differed from the F light by 1, 2, 3, or 4 ten-millionths of a millimeter, so that in the figure the light of that part of the line wafted over the fourth dot is of a wave-length of 4 ten-millionths of a millimeter less than that of the normal F light, which has a wave-length luinnHnnT of a millimeter. The F light therefore has had its wave-] ength reduced by ^^ = T^ part ; and in order that each wave may be decreased by this amount, the source of the light must move towards us with a velocity of TZTS of the velocity of light, which is 186,000 miles per secondhand ^\^ of 186,000 is about 150 ; this then is the velocity, in miles per second, at which the hydrogen gas must have been moving towards us in order to displace the light to the fourth dot, as shown in the figure. CHAPTER XXX. THE TELEPOLAEISCOPE. IN previous chapters we have considered the lessons that we can learn from light from the vibrations of the so- called ether when we put questions to it through various instruments as interpreters. There is still another method of putting questions to these same vibrations, and the instrument we have now to consider is the Polariscope. The spectroscope helped us to inquire into the lengths of the luminiferous waves ; from the polariscope we learn whether there is any special plane in which these waves have their motion. The polariscope is an instrument which of late years has become a useful adjunct to the telescope in examin- ing the light from a body in order to decide whether it is reflected or not, and to ascertain indirectly the plane in which the rays reflected to the eye lie. The action of the instrument depends upon the fact that light which consists solely of vibrations perpendicular to a given plane is said to be completely polarized in that plane. Light that contains an excess of vibrations perpendicular to a given plane is said to be partially polarized in that plane. 442 STARGAZING : PAST AND PRESENT. [BOOK vi. It was Huyghens that discovered the action of Iceland spar in doubly refracting light ; and the light which passed the crystal was called polarized light at the suggestion of Newton, who, it must be remembered, looked upon light as something actually emitted from luminous bodies ; these projected particles were supposed, after passage through Iceland spar, to be furnished with poles analogous to the poles of a magnet, and to be unable to pass through certain bodies when the poles were not pointing in a certain direction. It was not until the year 1808 that Malus discovered the phe- nomenon of polarization by reflection. He was looking through a double-refracting prism at the windows of the Luxembourg Palace, on which were falling the rays of the setting sun. On turning the prism he noticed the ordinary and extraordinary images alternately become bright and dark. This phenomenon he at once saw was in close analogy to that which is observed when light is passed through Iceland spar. At first he thought it was the air that polarized the light, but subsequent experiments showed him that it was due to reflection from the glass. Let us examine some of the phenomena before we proceed to show the use astronomers make of them. It is the property of some crystals, such as tourmaline, when cut parallel to a given direction, called the optic axis of the crystal, to absorb all vibrations or resolved parts of vibrations perpendicular to this line, trans- mitting only vibrations parallel to it. A similar absorption of vibrations perpendicular to a given direction may be effected by various other combinations, of which one, Nicol's prism, is in most common use. Any of these arrangements may be used CHAP, xxx.] THE TELEPOLARISCOPE. 443 as an analyzer with the telescope, for determining whether the light is completely or partially polarized, and in either of these cases which is the plane of polarization. The plane containing the direction of the rays and the line in the analyzer to which the transmitted vibrations are parallel, is called the plane of analyzation : all the light which reaches the eye consists of vibrations in the plane of analyzation. As we rotate the analyzer, we rotate equally the plane of analyzation. If we find a position of the plane of analyzation for which the light received by the eye is a maximum, we know that the light from the object is partially or completely polarized in a plane perpendicular to the plane of analyzation when in this position. To determine whether the polarization is partial or complete, we must turn the analyzer through an angle of 90 from this position : if we now obtain complete darkness, we know that there are no vibrations having a resolved part parallel to the plane of analyza- tion in this position, or that the light is completely polarized in this plane : if there be still some light visible, the polarization is only partial. To explain this a little more fully, we may compare the vibrations or waves of light to waves of more material things : we may have the vibrating particles of the ether moving up and down as the particles do in the case of a wave of water, or the particles may move horizontally as a snake does in moving along the ground. We may consider that ordinary light consists of vibra- tions taking place in all planes, but if it passes through or is reflected by certain substances at certain angles, the vibrations in certain planes are, as it were, filtered out, leaving only vibrations in a certain plane. This light is then said to be polarized, and its plane of 444 STARGAZING : PAST AND PRESENT. [BOOK vi. polarization is found by its power of passing through polarizing bodies only when they are in certain posi- tions. If, for instance, a ray of ordinary light is passed through a crystal of tourmaline, the vibrations of the filtered ray will only lie in one plane ; if then a second crystal of tourmaline be held in a similar position to the first, the ray will pass through it unaffected ; but if it be turned through a quarter of a circle about the ray as an axis, the ray will no longer be able to pass, for being in a position a,t right angles to the first, it will filter out just the rays that the first allows to pass. For illustra- tion, take a gridiron : if we attempt to pass a number of sheets of paper held in all positions through it, only those in a certain plane, viz., that of the rods forming the gridiron, could be passed through, and those that would go through would also go through any number of gridirons held in a similar position. But if another gridiron be placed so that its bars cross those of the first, the sheets of paper could no longer pass, and it is evident that if we could not see or feel the paper, we could tell in what plane it was by the position in which the gridiron must be held to let it pass, and having found the paper to be, say horizontal, we know that the bars of the first grid- iron are also horizontal. So with light, we can analyze a ray of polarized light and say in what plane it is polarized. The example of the gridiron, however, does not quite represent the action of the second crystal ; for if the bars of the second gridiron are turned a very small distance out of coincidence with those of the first, the sheets of paper would be stopped ; but with light, the intensity of the ray is only gradually diminished, until CHAP, xxx.] THE TELEPOLARI8COPE. 445 it is finally quenched when the axes of the crystals are at right angles to each other. Light is polarized by transmission and by reflection. We have already, when we were discussing the principle involved in the double-image micrometer, seen how a crystal of Iceland spar divides a ray into two parts at the point of incidence. Now these two rays are oppo- sitely polarized, that is to say, the vibrations take place in planes perpendicular to each other ; the vibrations of the incident light in one plane are refracted more than the vibrations in the opposite plane, and we have FIG. 204. Diagram showing the Path of the Ordinary and Extraordinary Ray in Crystals of Iceland Spar. therefore two rays, one called the ordinary ray, and the other the. extraordinary ray. Fig. 204 shows a ray of light, s I, incident on the first crystal at I ; it is then divided up into the ordinary ray I R and the extraordi- nary one I R' ; a screen is then interposed, stopping the extraordinary ray and allowing the ordinary one to fall on the second crystal at I. If then this crystal be in a similar position to the first, this ray, vibrating only in one plane, will pass onwards as an ordinary ray, I R ; there being no vibrations in the perpendicular plane to form an extraordinary ray, there will be only one circle 446 STARGAZING : PAST AND PRESENT. [BOOK vi. of light thrown on the screen at o by the lens. But, if the second crystal be turned round the line s s as an axis, the plane of vibration of the ray falling on its surface will no longer coincide with the plane in which an ordinary ray vibrates in the crystal, and it therefore becomes split up into two, one vibrating in the plane as an ordinary ray, and the other in that of an extraordi- nary ray ; we have therefore the ray I R' in addition to the first, and consequently a second circle on the screen at E'. As the crystal rotates, the plane of extraordinary FIG. 205. Appearance of the Spots of Light on the Screen shown in the preceding Figure, allowing the ordinary ray to pass and rotating the second Crystal. refraction becomes more and more coincident with the plane of vibration of the incident ray, until, when it has revolved through 90, it coincides with it exactly ; it then passes through totally as an extraordinary ray, and as the refractive power of the crystal is greater for vibrations in this plane, we get all the light traversing the direction I R and falling on the screen at E', and there being then no light ordinarily refracted, the circle o disappears. Fig. 205 shows the relative bright- ness of the circles E and o as they revolve round the CHAP, xxx.] THE TELEPOLARISCOPE. 447 centre s of the screen, the images produced by the ordinary and the extraordinary ray becoming alternately bright and dark as the crystal is rotated. Fig. 206 shows the images on the screen when the ordinary ray is stopped by the first screen instead of the extraordinary one. A crystal of tourmaline acts in a like manner to Iceland spar, but the ordinary ray is rapidly absorbed by the crystal, so that the extraordinary ray only passes. FIG. 206. Appearance of Spots of Light on Screen on rotating the second Crystal, when the extraordinary ray is allowed to pass through the first Screen. There is an objection to the use of it, as it is not very transparent, and a Nicol's prism is now generally used for polarizing light. It is constructed out of a rhombo- hedron of Iceland spar cut into two parts in a plane passing through the obtuse angles, and the two halves are then joined by Canada balsam. The principle of construction is this : the power of refracting light possessed by Canada balsam is less than that possessed by Iceland spar for the ordinary ray, and greater in the case of the extraordinary ray ; in consequence, the ordi- nary ray is reflected at the surface of junction, while 448 STARGAZING : PAST AND PRESENT. [BOOK vi. the extraordinary ray passes onwards through the crystal. It is manifest then that if two Nicols are used instead of two simple crystals, represented in Fig. 204, FIG. 207. Instrument for showing Polarization by Reflection. there will be only one spot of light on the screen, which is due to the extraordinary ray, and as in certain posi- tions this no longer passes (for the ordinary ray, which appears in the place of the extraordinary when the crystal is used, cannot pass through the Nicol), no light CHAP, xxx.] THE TELEPOLARISCOPE. 449 at all passes in such positions, so that we can use the second Nicol as an analyzer to ascertain in what plane the light is polarized. Light is also polarized by reflection from the surface of a transparent medium. When a ray of ordinary light falls on a plate of glass at an angle of 54 55' with the normal, the reflected ray is perfectly polarized, and at other inclinations the polarization is incomplete. Here then is polarization by reflection. Fig. 207 shows an apparatus for producing this phenomenon. The light falling on the first mirror from E is reflected through the tube as a polarized beam, and this is analyzed by the other mirror (i), whose plane can be rotated round the axis of the tube. The angle of polarization differs with different substances according to their refractive power, for polari- zation of the reflected ray is perfect only when the angle of incidence is such that the reflected ray is at right angles to the refracted one. As a result of what we have said, the light of the sun reflected from the surface of water or from the glass of a window is polarized, and although it may be dazzling to the eye, it is reduced, or even entirely cut off, when falling at the polarizing angle, by looking through the transparent Nicol's prism or plate of glass held in certain positions and acting as an analyzer. On rotating the analyzer there is an alternation of intensity, and by looking at the window through a crystal of Iceland spar as an analyzer, two images would be seen which would alternate in brightness as the crystal is rotated. So also there is a difference in the intensity of the light from the sky when the analyzer is rotated, showing that the light reflected from the watery and dust particles in the air is polarized, and by the position of the analyzer we find G G 460 STARGAZING : PAST AND PRESENT. [BOOK vi. that it is polarized in the plane we should expect if it be, as it is, reflected from the sun. It will be asked, however, what is the astronomical use of determining whether light has an excess of vibrations in any given direction ? To this we may reply that light that is reflected from any body is generally partially polarized in the plane of reflection, and that if we find that the light received from any body is partially polarized in a given plane, we may conclude that it has very likely been reflected in that plane. Hence then in the case of any celestial body the origin of the light of which is doubtful, the polariscope tells us whether the light is intrinsic or reflected. It tells us more than this, it tells us the plane in which the reflection has taken place. As the polarization takes place, when it does take place, at the celestial body, all we have to do is to attach an analyzer to the telescope. A careful application of the above principles has shown that the light from the sun's corona is partially polarized, and in the same plane as it would be if reflected from email particles in the neighbourhood of the sun : so also a portion of the light of Coggia's Comet was found to be polarized, and therefore we say that it reflected sunlight in addition to its own proper light. In what has been hitherto said we have only con- sidered the use of a Nicol, or glass plates, or crystal of Iceland spar as an analyzer, and by the variation of brightness the presence and plane of polarization have been determined ; but unless the polarization is some- what decided, it could not be detected by this method. Advantage is therefore taken of the fact that a plate of CHAP, xxx.] THE TELEPOLARISCOPE. 451 quartz rotates the plane of polarization of a ray passing through it, and it rotates the more refrangible colours more than the others, and some crystals rotate the plane one way, and others in the opposite direction : the crystals are therefore called respectively right- and left- handed quartz ; the thicker the quartz the greater the angle through which the plane of polarization is twisted. This supplies us with a most delicate apparatus, which we next describe. A crystal of right- and a crystal of left-handed quartz are taken and cut to such thickness that a ray of any colour, say green, has its plane turned through 90 on passing through each of them. They are then cut into the form of a semicirole and placed side by side. Any change of the angle of polarization will now affect each plate differently. In one plate the colours will change from red to violet, in the other from violet to red. If now a ray of polarized light, say vibrating in a vertical plane, falls on them, the green rays will have their plane of vibration turned through 90 by each crystal, arid the vibration of the green from both crystals will then be in the horizontal plane. NicoFs prism interposed between the quartz plates and the eye, so as to allow horizontal vibrations to pass, will show the green from both crystals of equal intensity ; the rays of other colours, being turned through a greater or less angle than 90, will not be vibrating horizontally, and will therefore only partially pass through, sa green will be the prevailing colour. If now the plane of vibration of the original ray be turned a little out of the vertical, the ray, on the red side of the green, will appear in one half, and that on the violet side of the green in the other : so that immediately the plane of G G 2 4S2 STARGAZING : PAST AND PRESENT. [BOOK vi. polarization changes, the plates transmit a different colour, and the apparatus must be twisted round through just the same angle as the polarized ray in order to get the crystals of the same colour. It is therefore obvious that the angle made by a polarized ray with a fixed plane is easily ascertained in this manner. There is also another instrument for detecting polari- zation which is perhaps more commonly used than the biquartz : it is generally called Savart's analyser, and is extremely sensitive in its action. On looking through it at any object emitting ordinary light, the white circle of light limited by the aperture of the instrument only is seen ; but if any polarized light should happen to be present, a number of parallel bands, each shaded from red to violet, make their appearance ; on rotating the instrument a point is found when a very slight motion causes the bands to vanish and others to appear in the intermediate spaces, and knowing the position required for the change of bands with light polarized in a known plane, say the vertical plane, it is easy to find how far the plane of polarization of any ray is from the vertical, by the number of degrees through which the instrument must be turned to change the bands. The construction of the instrument, and especially its action, is not easy to understand without a considerable knowledge of optics, but it may be stated that a plate of quartz is cut, in a direction inclined at 45 to its axis, into two parts of the same thickness ; one part is then turned through a right angle and placed with the same surfaces in contact as before ; these are fixed in the instrument so that the light shall traverse them perpendicularly to the plane of section ; the light then passes through a Nicol's prism as an analyser to the eye. The lines observed, " black CHAP. xxx. J THE TELEPOLARISCOPE. 453 centred " in one position, and " white centred " in the position at right angles to this, are always in the direction before referred to. The delicacy of the test supplied by this arrangement increases as this direction is more nearly parallel or perpendicular to the plane of polarization of the ray under examination. CHAPTER XXXI. CELESTIAL PHOTOGRAPHY. THE WAYS AND MEANS. WE come now last of all to that branch of the work of the physical astronomer which bids fair in the future to replace all existing methods of observation. In the introductory chapter we referred to the in- troduction of photographic records of astronomical phenomena as marking an epoch in the development of the science. In the last ones we have to dwell briefly on the modus operandi of the various methods by which the eye is thus being gradually replaced. The point of celestial photography is that it not only enables us to determine form and place, absolutely irre- spective of personal equation so far as the eye is con- cerned, but that, properly done, it gives us a faithful and lasting record of the operation, so that it is not forgotten ; Mr. De La Rue has called the photographic plate the retina which does not forget, and an excellent name it is. We may pass over altogether the ordinary photo- graphic processes, which have been carried on with a degree of skill and patience which is beyond all praise, and confine our attention exclusively to the instrumental processes. Be it remembered, we have no longer to CHAP, xxxi.] CELESTIAL PHOTOGRAPHY. 455 consider the visual rays, but the so-called chemical rays, which lie at the violet end of the spectrum. We must also recollect that, in a former chapter, we have seen that the optician's business was to throw aside the violet rays altogether to discard them, caring nothing for them, because, so far as the visible form of the objects is concerned, they help very little. But we shall see in a moment that, if we wish to use refrac- tors for photographing, we must abolish this idea, and undo everything we did to get a perfect telescope to see the body, because in the case of the photographic processes employed at present, the visible rays have as little to do with building up the image on the photo- graphic plate as the blue rays have to do with building up the image on the retina of the eye. We shall see presently how admirably this has been done by Mr. Eutherfurd. If, however, we use reflectors instead of refractors, we are able to utilize all the rays by means of the same mirror without alteration, as the focus is the same for all rays, so that a reflector is equally good for all classes of observation. Let us first consider the cases in which the plate is made to replace the retina with the ordinary telescope. We shall see in the sequel that whether the spectro- scope, polari scope, or other physical instrument be added to the telescope when we pass, that is to say, from mechanical to physical astronomy the plate can still replace the eye with advantage. The body of the telescope, with the object-glass or mirror at one end and the plate at its focus in place of the eyepiece, forms the camera, corresponding to those we find in photographic studios. The plate-holder shown iu section in the accompanying figure is therefore the 456 STARGAZING : PAST AND PRESENT. [BOOK vi. only addition required to make a telescope into a camera for ordinary work. Fig. 208. A is a screw of such a size that it can be inserted into the eye-piece end of the telescope ; the sensitive plate is held between a lid at the back, which opens for the plate to be inserted, and a slide in front, which is drawn out so as to expose the face of the plate to the object. A piece of ground glass of extreme fineness is inserted in the slide, on which the object is focussed before the sensitive plate is put in. It is easy then by FIG. 208. Section of Plate-holder. the eye-piece focussing-screw to put this nearer or further away from the object-glass, so that the image is thrown sharply on the ground glass. When that is done the ground glass is taken away, and the sensitive plate put there in its place, and then exposed as required, so that the methods are similar to the ordinary photographic process. We have here an arrangement that enables us to photograph the moon, stars, and planets. M. Faye has proposed that for the transit circle also the photographic method should be applied, the chronograph registering CHAP. xxxi. J CELESTIAL PHOTOGRAPHY. 457 the time of the instantaneous opening of the slide, instead of the time the star is seen to transit, so that the position of the star with respect to the wires is registered at a certain known time ; therefore, not only for physical astronomy have we the means of making observations without an observer at all, but also for position observations. Every one knows sufficient of photography to be aware that, if we wish to secure the image of a faint object, such as a faint star or a faint part of the moon, we must expose the plate for some little time, as we have to do in ordinary photography if the day is dull, and there- fore the larger the aperture of the telescope the more light passes ; and the shorter the focus is, and the more rapid the process, the shorter will be the exposure ; if the focus is short, the image will be small ; but as we can magnify the image afterwards, rapidity becomes of greater moment, as the shorter the time of exposure is the less atmospheric and other disturbances and errors in driving the telescope come into play. Still, if we photograph the moon or other object, we do not wish to limit ourselves to the size of the original negative obtained at the focus. If the negative is well defined that is, if it possesses the quality of enlargeableness there is no difficulty in getting enlarged prints. The method of enlarging photographs is very simple ; all that is required is a large camera, the negative to be copied being placed nearer the lens than the prepared paper, so that the image is larger than the original. Fig. 209 shows an enlarging camera: the body, A, can be made of wood, or better still, of a soft material, bellows-fashion, so that the length can be altered at pleasure. In the end, at B, is fixed a lens an ordinary 458 STARGAZING : PAST AND PRESENT. [BOOK vi. portrait lens will do, but a proper copying lens is preferable ; and E is a piece of wood with a hole in its centre, over which the negative is placed, the distance of E to B being also adjustible ; then, by altering the lengths of B E and B c, the image of the negative can be made to appear of suitable size. At the end, c, a piece of sensitive paper is placed, and the light of the sun being allowed to fall through the negative and lens, the paper soon becomes printed, and can be toned and fixed as an ordinary paper positive. The camera may be carried on a rough equatorial mounting, con- sisting of an axis pointing to the pole, and pulled round FIG. 209. Enlarging Camera. F, heliostat for throwing beam of sunlight on the reflector, which throws it into the camera ; E, negative ; B, focussing- lens ; c, plate- or paper-holder ; D, focussing-screw. with the sun by attaching a string to an equatorial telescope, moved by clockwork ; or a heliostat can be used with more advantage, thereby allowing the camera to be stationary ; a good enlarging lens is a very de- sirable thing, for most lenses seem to distort the image considerably. If we wish to obtain a large direct image of the moon, we must, as said before, employ a telescope of as long a focal length as possible ; for reasons just CHAP, xxxi.j CELESTIAL PHOTOGRAPHY. 459 mentioned, this is not always desirable. If, however, large images can be obtained as good as small ones, they can of course be enlarged to a much greater size. The primary image of the moon taken by Mr. De La Kue's exquisite reflector is not quite an inch in diameter. In one of Mr. Kutherfurd's telescopes of fifteen feet focus, the image of the moon is somewhat larger about one and a half inch in diameter. In Mr. Newall's magnificent refractor, the focal length of which is thirty feet, the diameter is over three inches. In the Mel- bourne reflector the image obtained is larger still. In celestial photography we have not only to deal with faint objects. With the sun the difficulty is of no ordinary character in the opposite direction, because the light is so powerful that we have to get rid of it. Now there are two methods of doing this, and as in a faint object we get more light by increasing the aper- ture, so with a bright light like that of the sun we can get rid of a large amount of it by reducing the aperture of our telescope ; but it is found better to reduce infini- tesirnally the time of exposure, and methods have been adopted by which that has been brought down to the one-hundredth part of a second. Let us show the simple way in which this can be done by the means of an addition to an ordinary plate- holder. Fig. 208 shows the ordinary plate-holder, like those used generally for photography. What is termed . the instantaneous slide, B, Fig. 210, consists of a plate with an adjustible slit in it inserted between the object itself and the focus. This can be drawn rapidly across the path of the rays by means of a spring, D ; we can bring it to one side, and fix it by a piece of cotton, E, and 460 STARGAZING : PAST AND PRESENT. [BOOK vi. then we can release it by burning the cotton, when the spring draws it rapidly across. The velocity of the rush of the aperture across the plate, and the time of expo- sure, can be determined by the strength of the spring and the aperture of the slit. If the velocity is too great, we can alter the size of the slit, c. If we absorb some of the superabundant light by means of yellow glass, or some similar material, we can keep the opening wide enough to prevent any bad effects of diffraction coming into play. The light of the sun is so intense that another method may be employed. Instead of having the plate at the FIG. 210. Instantaneous Shutter. focus of the object-glass we may introduce a secondary magnifier in the telescope itself, and thus obtain an enlarged image, the time necessary for its production being still so short (irVth of a second) that nothing is lost from the disturbances of the air. A telescope with this addition is called a photohelio- graph. The first instrument of this kind was devised by Mr. De La Rue, and for many years was regularly em- ployed in taking photographs of the sun at Kew. Some astronomers object to this secondary magnifier, and to obtain large images use very long focal lengths, CHAP, xxxi.] CELESTIAL PHOTOGRAPHY. 461 and of course a siderostat is employed. In this way FIG. 211. Photoheliogr.iph as erected in a Temporary Observatory for Photographing the Transit of Venus in 1874. Professor Winlock obtained photographs of the sun which have surpassed the limits of Mr. NewalFs re- 462 STARGAZING : PAST AND PRESENT. [BOOK vi. fractor ; the negatives have a good definition, and show a considerable amount of detail about the spots ; they were taken by a lens, inserted at the end of a gaspipe forty feet long. The pipe was fixed in a horizontal position, facing the north, and at the extreme north part of it was the lens, a single one of crown glass, with no attempt to correct it. In front of it was a siderostat, moved by a clock, reflecting the light down the tube, so that the image of the sun could be focussed on the ground glass at the opposite end. One will see the importance of shortening the time for even the brightest object. Those who are favoured with many opportunities of looking through large tele- scopes know that the great difficulty we have to deal with is the atmosphere ; because we have to wait for definition, and the sum total of the photograph of any one particular thing depends upon these atmospheric fits. If we require to photograph an object, it will be obvious that the more fits we have, the worse it will be, because we get a number of images partially superposed which would otherwise give as good an effect as we could get by an ordinary eye observation. It is therefore most important to reduce the interval as much as possible. CHAPTER XXXII. CELESTIAL PHOTOGRAPHY (CONTINUED). SOME RESULTS. THE process used should therefore be the most rapid attainable ; any work on photography will give a number of processes of different degrees of rapidity, but a pro- cess that suits one person's manipulation may prove a failure in another's, and the general principles are the only rules suitable for all. First, the glass plate should be carefully cleaned, the collodion lightly coloured, the bath strong and neutral, certainly not acid, and the deve- loper fairly strong. Pyrogallic acid and silver should not be used for intensifying ; a good intensifier is made by adding to a solution of iodide of potassium, strength one grain to the ounce of water, a saturated solution of bichloride of mercury, drop by drop, until the precipi- tate at first formed ceases to be re-dissolved ; use this after fixing. Now let us inquire what has been done by this impor- tant adjunct to ordinary means of observing. We may say that celestial photography was founded in the year 1850 by Professor Bond, who obtained a daguerrotype of the moon about that date. An immense advance has been made, but not so great as there might have 464 STARGAZING : PAST AND PRESENT. [BOOK vi. been if the true importance of the method had been recognized as it ought to have been ; and if we study the history of the subject we find that till within the last few years we have to limit ourselves to the works of two men who, after Bond, set the work rolling. Several observers took it up for a time ; but the work requires much both of time and money, and different men dropped off from time to time. There remained always steadfast one Englishman and one American Mr. De La Eue and Mr. Rutherfurd. The magnificent work Mr. De La Eue has done was begun in 1852. He was so anxious to see whether England could not do something similar to what had been done in America, that, without waiting for a driving clock, he thought he would see whether photographs of the moon could be taken by moving the telescope by hand. He soon found that he was working against nature that nature refused to be wooed in this way ; the moon in quite a decided manner declined to be photographed, and we waited five years till Mr. De La Rue was armed with a perfect driving clock. Mr. Rutherfurd was waiting for the same thing in America. At last, in 1857, Mr. De La Rue got a driving clock to his reflector of thirteen inches aperture, and began those admirable photographs of the moon which are now so well known. Since the above date the moon has been photographed times without number, and Mr. De *La Rue has made a series which shows the moon in all her different phases. They are remarkable for the beautiful way in which the details come out in all parts of the surface. We must recollect that these pictures of which we have spoken, some of them a yard in diameter, were first taken on glass about three inches across, the CHAP, xxxii.] CELESTIAL PHOTOGRAPHY. 465 image covering the central inch. At the same time the British Association granted funds for the photographic registration of sun-spots at the Kew Observatory, where the sun was photographed every day for many years. Encouraged by success, Mr. De La Eue, in 1858, attacked the planets Jupiter and Saturn, and some of the stars. He discovered that photographs of the moon can be combined in the stereoscope so that the moon shows itself perfectly globular. To accomplish this result it was necessary to photo- graph her at different epochs, so that the libration, which gives it the appearance of being turned round slightly and looking as it would do to a person several thousand miles to the right or left of the telescope, should be utilized. These two views when combined give the appearance of solidity just as the image of a near object combined by the two eyes gives that ap- pearance. The reason of this appearance of solidity is easily seen by looking at an orange or ball first with one eye and then with the other, when it is noticed that each eye sees a little more of one side than the other ; and it is the combination of these slightly dissimilar images that gives the solid appearance. If we examine two of these photographs combined for the stereoscope, we see that they have the appear- ance of being taken from two stations a long distance apart. One shows a little more of the surface on one side than the other. They are 'obtained in different lunations, when the moon, in the same phase, has turned herself slightly round, showing more of one side. In this way we have a distinct effect due to libration. In the year 1859 Mr. De La Rue found that sun- pictures could be combined stereoscopically in the same manner. H H 456 STARGAZING : PAST AND PRESENT. [BOOK vi. When we turn to the labours of Mr. Kutherfurd, we find him in 1857 armed with a refractor of Hi inches aperture ; the actinic focus, or rather the nearest approach to a focus, was yVths of an inch from the visual focus. With this telescope, without any correction whatever, he, in 1857 and 1858, obtained photographs of the moon which, when enlarged to five inches in diameter, were well defined. He also obtained impressions of stars down to as far as the fifth magnitude, and also of double stars some 3" apart for instance, 7 Virginis was photographed double. The ring of Saturn and belts of Jupiter were also plainly visible, but ill-defined. The satellites of Jupiter failed to give an image with any exposure, while their primary did so in five or ten seconds. The actinic rays, instead of coming to a point and producing an image of a satellite, were spread over a certain area and thereby rendered too weak to impress the plate. In the summer of 1858 Mr. Eutherfurd combined his first stereograph of the moon independently of Mr. De La Rue's success in England. Mr. Rutherfurd then commenced an inquiry of the greatest importance, which will in time bring about a revolution in the processes employed. In 1859 he attempted, by placing lenses of different curvatures between the object-glass and the focus, to bring the chemical rays together, leaving the visual rays out of the question ; this had the effect of shortening the focus considerably and improving the photographs ; but he found that, except for the middle of the field, this method would not answer. He therefore in 1860 attempted another arrangement, and one which he found answered extremely well for short telescopes. CHAP, xxxii.] CELESTIAL PHOTOGRAPHY. 467 Between the lenses of the object-glass of a 4^-inch refractor he put a ring which separated the lenses by three-quarters of an inch, and reduced the power of the flint-glass lens, which corrects the crown-glass for colour, so that the combination became achromatic for the violet rays instead of for the yellow. With this lens he was successful to a certain extent : he obtained even better results than with the 1 1 J inch ; but eventually he re- jected this method, which we may add has recently been tested by M. Cornu, who thinks very highly of it. He next attempted a silver-on-glass mirror in 1861 ; in the atmosphere of New York it only lasted ten days; he gave it up ; and he then very bravely, in 1864, at- tacked the project de novo, and began an object-glass of a telescope which should be constructed so as to give best definition with the actinic rays, just as ordinary object-glasses are made to act best with the visual rays. He found that in order to bring the actinic portion of the rays to a perfect focus, it was necessary that a given crown-glass lens should be combined with a flint, which will produce a combined focal length of about rV shorter than would be required to satisfy the condi- tions of achromatism for the eye. This combination was of course absolutely worthless for ordinary visual observation ; his new lens when finished was 11^ inches aperture and a little less than 14 feet focal length. With this he obtained impressions of ninth magnitude stars, and within the area of a square degree in the Proesepe in Cancer twenty-three stars were photographed in three minutes' exposure. Castor gave a strong im- pression in one second, and stars of 2" distance showed as double. But even with this method Mr. Rutherfurd was not satisfied. Coming back to the 11^-inch object- H H 2 468 STARGAZING : PAST AND PRESENT. [BOOK vi. glass which he had used at first, he determined to see whether or not the addition of a meniscus lens outside the front lens would not give him the requisite short- ness of the focus and bring the actinic rays absolutely together. By this arrangement he got a telescope which can be used for all purposes of astronomical research, and he has also eclipsed all his former photographic efforts. CHAPTER XXXIII. CELESTIAL PHOTOGRAPHY (CONTINUED) RECENT RESULTS. HAVING in the previous chapter dealt with some of the pioneer work, we come finally to consider some of the applications which in the last years have occupied most attention. With regard to the sun, we need scarcely say that Messrs. De La Eue and Stewart have been enabled, by the photographic method, to give us data of a most remarkable character, showing the periodicity of the changes on the sun's surface, and so establishing their correlation with magnetic and other physical phenomena. These photographic researches, following upon the eye observations of Schwabe, Sporer, Carrington and others, have opened up to us a new field of inquiry in con- nection with the meteorology of the globe ; and it is satisfactory to learn that photoheliographs are now daily at work at Greenwich, Paris, Potsdam, and the Mauritius, and that shortly India will be included in the list. Quite recently, the importance of these permanent records of the solar surface has been demonstrated by Dr. Janssen, the distinguished director of the Physical Observatory at Meudon, in a very remarkable manner. It seems a paradox that discoveries can be made 470 STARGAZING : PAST AND PRESENT. [BOOK vi. depending on the appearance of the sun's surface by observations in which the eye applied to the telescope is powerless ; but this is the statement made by Dr. Janssen himself, and there is little doubt that he has proved his point. Before we come to the discovery itself let us say a little concerning Dr. Janssen's recent endeavours. Among the six large telescopes which now form a part of the equipment of the new Physical Observatory recently established by the French government at Meudon, in the grounds of the princely Chateau there, is one to which Dr. Janssen has recently almost exclusively confined his attention. It is a photoheliograph giving images of the sun on an enormous scale compared with which the pictures obtained by the Kew photoheliograph are, so to speak, pigmies, while the perfection of the image and the photographic processes employed are so exquisite, that the finest mottling on the sun's surface cannot be overlooked by those even who are profoundly ignorant of the interest which attaches to it. This perfection of size and image have been obtained by Dr. Janssen by combining all that is best in the principles utilised in one direction by Mr. De La Rue, and in the other by Mr. Rutherfurd, to which we have before referred. In the Kew photoheliograph, which has done such noble work in its day that it will be regarded with the utmost veneration in the future, we have first a small object-glass corrected after the manner of photo- graphic lenses, so as to make the so-called actinic and the visual rays coincide, and then the image formed by this lens is enlarged by a secondary magnifier con- structed, though perhaps not too accurately, so as to make the actinic and visual rays unite in a second image on a CHAP, xxxin.] CELESTIAL PHOTOGRAPHY. 471 prepared plate. Mr. Rutherfurd's beautiful photographs of the sun were obtained in a somewhat different man- ner. In his object-glass, as we have seen, he discarded the visual rays altogether and brought only the blue rays to a focus, but when enlargements were made, an ordinary photographic lens that is, one in which the blue and yellow rays are made to coincide was used. Dr. Janssen uses a secondary magnifier, but with the assistance of M. Pragmowski he has taken care that both it and the object-glass are effective only for those rays which are most strongly photographic. Nor is this all ; he has not feared largely to increase the aperture and focal length, so that the total length of the Kew in- strument is less than one-third of that in operation in Paris. The largely-increased aperture which Dr. Janssen has given to his instrument is a point of great importance. In the early days of solar photography the aperture used was small, in order to prevent over-exposure. It was soon found that this small aperture, as was to be ex- pected, produced poor images in consequence of the diffraction effects brought about by it. It then became a question of increasing the aperture while the exposure was reduced, and many forms of instantaneous shutters have been suggested with this end in view. With these, if a spring be used, the narrow slit which flashes across the beam to pay the light out into the plate changes its velocity during its passage as the tension of the spring changes. Of this again Dr. Janssen has not been un- mindful, and he has invented a contrivance in which the velocity is constant during the whole length of run of the shutter. By these various arrangements the plates have now 472 STARGAZING : PAST AND PRESENT. [BOOK vi been produced at Meudon of fifteen inches diameter, showing details on the sun's surface subtending an angle of less than one second of arc. So much for the modus operandi. Now for the branch of solar work which has been advanced. It is more than fifteen years ago since the question of the minute structure of the solar photosphere was one of the questions of the day. The so-called " mottling " had long been observed. The keen-eyed Dawes had pointed out the thatch-like formation of the penumbra of spots, when one day Mr. Nasmyth announced the discovery that the whole sun was covered with objects resembling willow-leaves, most strangely and effectively interlaced. We may sum up the work of many careful observers since that time by stating that the mottling on the sun's surface is due to dome-like masses, and that the ' ' thatch " of the penumbra is due to these dome-like masses being drawn, either directly or in the manner of a cyclone, towards the centre of the spot. In fact the " pores " in the interval between the domes are so many small spots, while the faculse are the higher levels of the cloudy surface. The fact that faculse are so much better seen near the limb proves that the absorp- tion of the solar atmosphere rapidly changes between the levels reached by the upper faculse and the pores. Thus much premised, we now come to Dr. Janssen's discovery. An attentive examination of his photographs shows that the surface of the photosphere has not a constitution uniform in all its parts, but that it is divided into a series of figures more or less distant from each other, and presenting a peculiar constitution. These figures have contours more or less rounded, often very rectilinear, CHAP. XXXIIL] CELESTIAL PHOTOGRAPHY. 473 and generally resembling polygons. The dimensions of these figures are very variable ; they attain sometimes a minute and more in diameter. While in the interior of the figures of which we speak the grains are clear, distinctly terminated, although of very variable size, in the boundary the grains are as if half effaced, stretched, stained; for the most part, indeed, they have disappeared to make way for trains of matter which have replaced the granulation. Everything indi- cates that in these spaces, as in the penumbrse of spots, the photospheric matter is submitted to violent move- ments which have confused the granular elements. We have already referred to the paradox that the sun's appearance can now be best studied without the eye applied to the telescope. This is what Dr. Janssen says on that point. " The photospheric network cannot be discovered by optical methods applied directly to the sun. In fact, to ascertain it from the plate, it is necessary to employ glasses which enabled us to embrace a certain extent of the photographic image. Then if the magnifying power is quite suitable, if the proof is quite pure, and espe- cially if it has received rigorously the proper exposure, it will be seen that the granulation has not everywhere the same distinctness ; that the parts consisting of well- formed grains appear as currents which circulate so as to circumscribe spaces where the phenomena present the aspect we have described. But to establish this fact, it is necessary to embrace a considerable portion of the solar disc, and it is this which it is impossible to realise when we look at the sun in a very powerful instrument, the field of which is, by the very fact of its power, very small. In these conditions we may very easily conclude 474 STARGAZING : PAST AND PRESENT. [BOOK vi. that there exist portions where the granulation ceases to be distinct or even visible ; but it is impossible to suppose that this fact is connected with a general system." But it is not alone with the uneclipsed sun that the new method enables us to make discoveries. The extreme importance of photography in reference to eclipse obser- vations cannot be over estimated. Most of our best observations of eclipses have been wrought by means of FIG. 212. Copy of Photograph taken during the Eclipse of 1869. photography. The time of an eclipse is an exciting time to astronomers ; and it is important that we should have some mechanical operation which should not fail to record it. The first eclipse photograph was taken in 1851. In 1860, chiefly owing to the labours of Mr. De La Kue, our CHAP, xxxni.] CELESTIAL PHOTOGRAPHY. 475 knowledge was enormously increased. The Kew photo- heliograph was the instrument used, and the series of pictures obtained showed conclusively that the promi- nences belonged to the sun. In 1868 the prominences were again photographed. In 1869 the Americans at- tacked the corona, and their suggestion that the base of it was truly solar has been confirmed by other photo- graphs taken in 1870, 1871, and 1875. Although to the eye the phenomena changed from place to place, to the camera it was everywhere the same with the same duration of exposure. It is not to be wondered at, then, that on the occasion of the last transit of Venus, which may be regarded as a partial eclipse of the sun, photography was sug- gested as a means of recording the phenomena. Science is largely indebted to Dr. Janssen, Mr. De La Eue, and others for bringing celestial photography to aid us in this branch of work also. While on the one hand astronomers have to deal with precious moments, to do very much in very little time, in circumstances of great excitement ; the photographer on the other goes on quietly preparing and exposing his plates, and noting the time of the exposure, and thus can make the whole time taken by the planet in its transit over the sun's disc one enormous base line. His micrometrical measures of the position of the planet on the sun's disc can be made after all is over. It was suggested by Dr. Janssen that a circular plate of sufficient size to contain sixty photo- graphs of the limb of the sun, at the points* at which Venus entered and left it could be moved on step by step round its centre, and so expose a fresh surface to the sun's image focussed on it, say every second. In this 476 STARGAZING : PAST AND PRESENT. [BOOK vi. way the phenomena of the transit were actually recorded at several stations. With reference to the moon, we have said enough to show that if we wish to map her correctly, it is now no longer necessary to depend on ordinary eye observations *^85 -, ri ^, _ ; ;^;, 'ifS^I FIG. 213. Part of Beer and Madler's Map of the Moon. alone ; it is perfectly clear that by means of an image of the moon, taken by photography, we are able to fix many points on the lunar surface. Still, although we can thus fix these and use the.m as so many points of the first order, as one might say, in a triangulation, there is much that photography cannot do ; the work of the eye CHAP, xxxiii.] CELESTIAL PHOTOGRAPHY. 477 observer would be essential in filling in the details and giving the contour lines required to make a map of the moon. The accompanying drawings on the same scale show that up to the present, for minute work, the eye beats the camera. FIG. 214. The same Region copied from a Photograph by De La Rue. The light of the moon is so feeble in blue rays that a long exposure is necessary for a large image, and during the exposure all the errors in the rate of the clock are magnified. We need not enlarge on the extreme importance of what Mr. Rutherfurd has been doing in photographing 478 STARGAZING : PAST AND PRESENT. [BOOK vi. star clusters and star groups. It is doubly important to astronomy, and starts a new mode of using the equa- torial and the clock ; in fact, it gives us a method by which observations may be photographically made of the proper motion of stars, and even the parallax of stars may be thus determined independently of any errors of observers. Mr. Eutherfurd shows that the places of stars can be measured by a micrometer on a plate in the same way as by ordinary observation ; hence photography can be made use of in the measurement of position and distance of double stars. As an instance of the extreme beauty of the photo- graphs of stars produced by a proper instrument, it may be stated that with the full aperture of the ll^-inch object-glass corrected only for the ordinary rays, Mr. Rutherfurd found that he required an exposure of more than ten seconds to get an image of the bright star Castor ; but now, instead of requiring ten seconds, he can get a better image in one. The reason of this is, that, with the object-glass corrected only for the visual rays, the chemical ones are spread over a certain small area instead of coming to a point, and so, of course, the intensity is reduced ; but when the chemical rays all come to one point the intensity is greater, since the image of the star is smaller and the action more intense. Let us follow Mr. Rutherfurd a little in his actual work. First, a wet plate is exposed for four minutes. This gives stars down to the tenth magnitude. But there may be points on the plate which are not stars, hence a second impression is taken on the same plate after it has been slightly moved. All points now doubled are true stars. Now for measures of arc. Another photograph is taken, and the driving clock is stopped ; the now CHAP, xxxin.] CELESTIAL PHOTOGRAPHY. 479 moving stars down to the fourth magnitude are bright enough to leave a continuous line, the length of this in a very accurately known interval, say two minutes, enables the arc to be calculated. Next comes the mapping. The negative is fixed on a horizontal divided circle on glass illuminated from below. Above it is a system of two rails, along which travels a carrier with two microscopes, magnifying fifty diameters. By the one in the centre, with two cross wires in the field of view, the photograph is observed ; by the other, armed with a wire micrometer, a divided scale on glass which is fixed alongside the rail is read. Suppose we wish to measure the distance between two stars on the plate. The plate is rotated, so that the line which joins them coincides with that which is described by -the optical axis of the central microscope marked by the cross wires when the carrier runs along the rails. This microscope is then brought successively over the two stars, and the other microscope over the scale reads the nearest divi- sion, while the fractions are measured by the micrometer. Hence, then, the fixed scale, and not a micrometer screw, is depended upon for the complete distance. In this way the distance between the stars on the plate can be measured to the iriW part of a millimetre. So far then we have shown how photography has been called in to the aid of the astronomer, and how, by means of photography, pictures of the different celestial bodies have been obtained of surpassing ex- cellence. Now, photography is also the handmaiden to the spectroscope in the same way as it is the handmaiden to the telescope. Not only are we able to determine and register the appearance of the moon and planets, 480 STARGAZING : PAST AND PRESENT. [BOOK vi. but, day by day, or hour by hour, we can photograph a large portion of the solar spectrum ; and not only so, but the spectrum of different portions of the sun : nay, even the prominences have been photographed in the same manner ; while more recently still, Drs. Huggins and Draper have succeeded in photographing the spec- trum of some of the stars. We owe the first spectrum of the sun, showing the various lines, to Becquerel and Draper ; the finest hitherto published we owe to Mr. Euthcrfurd. (1) KlRCHHOFFS MAP. F SET WS 05 814 213 02 FIG. 215. Comparison between Kirchhoff's Map and Rutherfurd's Photograph. This magnificent spectrum extends from the green part of the spectrum right into that part of the spec- trum called the ultra-violet. Of course it had to be put together from different pictures, because there is a dif- ferent length of exposure required for the different parts ; the exposure of any particular part of the spectrum must be varied according to the amount of chemical intensity in that part. If the line G- was exposed, say for fifteen seconds, the spectrum near the line F would CHAP, xxxn.] CELESTIAL PHOTOGRAPHY 481 I I 482 STARGAZING : PAST AND PRESENT. [BOOK vi. require to be exposed for eight minutes, and at the line H, which is further away from the luminous part of the spectrum than G-, there the exposure requisite would be two or three minutes. In order to obtain a photograph of the average solar spectrum, the camera replaces the observing telescope, and a heliostat is used, as in the ordinary way. The FIG. 217. Telespectroscope with Camera for obtaining Photographs of the Solar Prominences. beam, however, should be sent through an opera-glass in order to condense it, and thereby to render the exposure as short as possible. Further, -if an electric lamp be mounted as shown in Fig. 216, observations, similar to those originally made by Kirchhoff, of the coincidence on the various metallic CHAP, xxxii.] CELESTIAL PHOTOGRAPHY. 483 lines with the Fraunhofer ones, can be permanently recorded on the photographic plate. The lens between the lamp and the heliostat is for the purpose of throw- ing an image of the sun between the carbon poles. The lens between the lamp and spectroscope then focuses both the poles and the image of the sun on to the slit. The spectrum of the sun is first obtained by un- covering a small part of the slit and allowing the image of the sun to fall on this uncovered portion, the lamp not being in action. 'When this has been done the light of the sun is shut off. The metal to be studied is placed in the lower pole ; the adjacent portion of the slit is uncovered, that at first used being closed in the process. The current is then passed to render the metal incandescent. After the proper exposure the plate is developed and the spectra are seen side by side. Fig. 187 is a woodcut of a plate so obtained. If the spectrum of any special part of the sun, or the prominences, has to be photographed, then either a siderostat must be employed, or a camera is adjusted to the telespectroscope, as shown in Fig. 2.1 7. For the stars, of course, much smaller dispersion must be used, but the method is the same ; and what has already been said by way of precaution about the obser- vation of stellar spectra applies equally to the attempt to obtain spectrum photographs of these distant suns. I i 2 INDEX. A. ABERRATION (see Chromatic Aberration, Spherical Aberration) Absorption, general and selective, 403, 408 ; spectroscope arranged for showing, 409 Adjustment of the transit instrument, 238 ADJUSTMENTS OF THE ^EQUATORIAL (Chap, xxi.), 328 Achromaticity of Huyghen's eyepiece, 110 Achromatic lenses, 84, 86 Achromatism, 126 Airy's transit circle, 284 Alexandrian Museum, astronomical observations, 19 Alt-azimuth, 287, 289 Altitudes, instrument used by Ptolemy for measuring, 35 Aluminium ; line spectrum of, 406 ; the sun, 417 Analyser for polarization of light, 443, 450 Anaximander, his theory of the form of the earth, 6 ; invention of the gnomon ascribed to him, 16, 17 ; meridian observations by, 25 Anchor escapement, 197 Angles of position, measurement of, 358366, 372 Angstrom, spectrum analysis, 402, 412 ; wave-lengths, 406 Annealing of lenses and specula, 121 Archimedes, clocks used by, 176 Arcturus, heat of, 385 Argelander, magnitudes of stars, 382 Aries, its position in the zodiac, 34 Aristillus, his observations in the Alexandrian Museum, ly Armillce ^Equatorice of Tycho Braliw, 26, 41, 45 ; his Armillce Zodiacalcs, 28 Ascension, Right (see Right Ascension) Arctic circle, Euclid's observations of stars in the, 10 Astrolabe, invented by. Hipparchus, 25 ; engraving of Tycho Brahe' s, 26, 41 ; his ecliptic astrolabe, 28 Astronomical clock, 240 (see Clock) ASTRONOMICAL PHYSICS (Book VI.), 371 ASTRONOMY OF PRECISION, INSTRU- MENTS USED IN (Chap, xix.), 284 290 Astrophotometer, Zdllner's, 379 Autolycus, first map of the stars by, 8, 9 Automatic spectroscope, 397 Auzout, invention of micrometer as- cribed to, 219, 221 Axis of collimation, 218, 220 B, BARIUM, in the sun, 419 Barlow, correction of aberration in lenses, 88 j "Barlow lenses," 89, 229 Barometrical pressure, its effect on the pendulum, 193 Berthon's dynameter, 116 Bessel's transit instrument, 284 Binary stars, 351, 359, 360 Blair (Dr.), object-glasses, 88 Bloxam's improved gravity escape- ment, 201 Bond (Prof.), spring governor, 320, 321 ; celestial photography, 463 Bouguer's photometer, 379 Brahe, Tycho (see Tycho Brahe) 480 INDEX. Brewster (Sir David), his list of Tyclio Brahe's instruments, 38 ; spectrum analysis, 410 British Horological Institute, time signals, 280 Browning's method of silvering glass specula, 137 ; of mounting specula, 144 ; automatic spectroscope, 397 ; solar spectroscope, 428 Bunsen (Ernest de), on ancient astrono- mical observations, 6 Bunsen (Prof.) spectroscope, 396 ; his burner, flame of, 407 ; his work in spectrum analysis, 402, 412, 423 C. CALCIUM, line spectra of, 406, 418 Cambridge Observatory (U.S.), equa- torial at, 339 ; star spectroscope, 430 ; transit circle, 247, 248, 251 Camera, enlarging, for celestial photo- graphy, 458 Canada balsam, its power of refracting light, 447 Candles used to measure time, 176 Canopus, observations of, by Posi- donius, 8 Cassegrain's reflecting telescope, 103, 149, 169 ; with Mr. Grubb's mount- ing, 301 Casting lenses and specula, 121 Castor, photograph of, 478 Catalogues of stars (see Stars) Celestial globe, 23 CELESTIAL PHOTOGRAPHY (Chap, xxxi., XXXIL), 454 Chair, observing, for equatorial tele- scopes, 339 Chaldeans, their observations of the motions of the moon, 4 ; early use of the gnomon, 16 Chance and Feil, manufacture of glass discs, 119, 305 CHEMISTRY OF THE STARS (Chap, xxvu. xxx.), 386453 Chinese, observations of conjunctions of planets, 4, 5 ; early use of the gnomon, 16, 17 Chromatic aberration of object-glasses and eyepieces, 87, 109, 123 CHRONOGRAPH, THE (Chap, xvn.), 253270 " Chronographic method" of transit observation, 259 Chronograph at Greenwich Observa- tory, 260264 CHRONOMETER, THE (Chap, xni.), rise and progress of time-keeping, 206 210 ; compensating balance, 207 ; detached lever escapement, 208 ; . chronometer escapement fusee, 209 Chronometers used for determining "local time," 281 Chronophers, for distributing ' ' Green- wich time," 275, 276 Cincinnati Observatory, 338 Circle, the ; its first application as an astronomical instrument, 6, 7, 8, 10 ; division into degrees, 8, 17, 21 Circles, great, denned by Euclid, 12 CIRCLE READING (Chap, xiv), 211 217 ; Digges' diagonal scale, 213 ; the vernier, 214 CIRCLE, TRANSIT (see Transit Circle) Circle, meridian, at Cambridge (U.S.), 248 ; mural, 241, 242 Circumpolar stars, 239 Clarke (Alvan), improvement in telescope lenses, 305 ; great equa- torial at Washington, 309, 319 Clement, inventor of the anchor es- capement, 197 Clepsydras, 36 CLOCK, THE (Chap, xni.), 175205 ; ancient escapement, 177 ; crown wheel, 178 ; clock train, 180 ; wind- ing arrangements, 181 ; pendulum, 183; cycloidal pendulum, 185; compensating pendulums, 187 ; Graham's, .Harrison's, and Green- wich pendulums, 188 ; clock at Royal Observatory, Greenwich, 194 ; escapements, 196 ; anchor escapement, 197 ; Graham's dead- beat, 199 ; Mudge's gravity escape- ment, 200 ; escapement of clock at Greenwich, 203 ; arrangements at Edinburgh Observatory, 269 ; astro- nomical, 240, 244, 245.346; sidereal, 254, 256, 266 ; solar, 254 ; standard, at Greenwich, 194, 203, 204, 271, 274 Clock, driving, for large telescopes, 318 Clocks driven and controlled by elec- tricity, 272 Clock stars, 267 Clock tower at Westminster, 277 Coggia's comet, its light polarized, 450 Collimation and collimation-error in the transit instrument and equa- torial, 238, 247, 328 Colour, amount produced by a lens, k 81, 84, 86 ; spectrum analysis, 407, INDEX. 487 4U8, 414, 416 ; of stars, 165, 351, 433 ; of waves of light, 420 ; re- frangibility of, 387 Comet of 1677, discovered by Tycho Brahe, 47 Comet, measurement of the angle of position of its axis, 359 Comparison prism of the spectroscope, 423 Compensating balance, 207 Compensating pendulums, 187 193 Composite mounting of large tele- scopes, 310 Concave lenses (see Lenses) Concave mirrors (see Mirrors) Conjugate images, 64 Conjunctions of planets, first observa- tions, 4 Constellations, first observations, 5, 9 ; Orion and its neighbourhood, 156 Convex lenses (see Lenses) Convex mirrors (see Mirrors) Cooke, adjustment of object-glasses, 141 ; improvement in telescope lenses, 305 ; equatorial refractor, 300 ; driving clock for large tele- scopes, 321 ; illuminating lamp for equatorial telescopes, 326 Copernicus, parallactic rules of, 41 Copernicus lunar crater), 354 Cross wires for circle reading, 212, 216, 218 ; in transit eyepiece, 234, 257 Crown -glass prisms, 83, 84; lenses, 86, 88 Crystals of Iceland spar, double refrac- tion by (see Iceland Spar) ' Culmination of stars, first observations of, 5 Cycloidal pendulum, 185 D. DAWES, solar eyepiece, 114, 115, 349 ; photometry, 378 Day, solar and sidereal, 253, 254, 256 Day eyepiece, 113 Days, first reckoning of, 19; measure- ment of, 176 Dead-beat escapement, 198 Deal time-ball, 275, 279 Declination, 24, 234, 241, 243, 251 ; measured by Tycho Brahe, 45 Declination axis of the equatorial, 299, 308, 327, 328 Defining power of the modern tele- scope, 160, 164 ; stars in Orion a test of, 165 Degrees, division of the circle into, 8, 17, 21 De La Eue (Warren, F.R.S.), his re- flecting telescope, 108 ; improve- ments in polishing specula, 134 ; celestial photography, 454, 459, 460, 464, 465, 475 Denderah, the zodiac of, 7 Dent (E. & Co. ), clock at Royal Ob- servatory, Greenwich, 194, 203, 204, 271, 274 Detached lever escapement, 208 Deviation of light, 79, 82 Deviation error in the transit instru- ment, 240, 248 Dials of ancient clocks, 257 Diagonal scale, Digges', 213 Differential observations made with the equatorial, 367 Digges' diagonal scale, 213 Diogenes Laertes, on the invention ot the gnomon, 16 Dioptrics, Kepler's treatise on, 386 Direct vision spectroscope, 431 Dispersion of light by prism, 79, 80, 82 Dividing power of telescopes, 165 Dollond, experiments with lenses, 85 ; correction of chromatic aberration 89 ; on manufacture of flint-glass discs, 118 ; pancratic eyepiece, 113 Dome form of observatory, 338, 339 Double stars, 351, 359 ; measurement of, 360 Double-image micrometer, 225, 229 Double refraction by crystals of Iceland spar (see Iceland Spar) Driving clock, for large telescopes, 318, 346 Drum form of observatory, 338 Dundee time signal, 278 E. EARTH, The, its position in Ptolemy's system, 3 ; early theories of its form, 6 ; circumference measured by Posidonius, 8 : Euclid's theory of its position, 12 ; inclination of its axis, 14, 17 ; size measured by Eratosthenes, 19 ; position in Tycho Brahe's system, 46 Eclipses, first observations of, 4 ; eclipses of Jupiter's moons ; eclipses, solar, photograph of, 474 Ecliptic, plane of the, 13, 14 ; discovery of its inclination, 17 ; inclination measured by Eratosthenes, 19 Ecliptic astrolabe of Tycho Brahe, 28 488 INDEX. Edinburgh Observatory, clock arrange- ments at, 269 ; standard clock, 272 ; time signals, 278 Egyptians, their record of eclipses, 4 ; zodiac of Denderah, 7 Eichens, his equatorial telescope at Paris, 314, 315 ; siderostat con- structed by him, 344 Electricity, its application to the chronograph, 265 ; to driving and controlling clocks, 272 Electric lamp, 404 ; arranged for spectrum analysis, 405 Emery used in grinding lenses and specula, 127 English mounting of large telescopes, 310 Equation of time, 254 EQUATORIAL, THE (Book V.), 293 368 (see Telescopes) EQUATORIAL OBSERVATORY, THE (Chap, xxii.), 337 342 (see Obser- vatories) EQUATORIAL, THE ; its ordinary work, (Chap, xxiv.), 349368 Equinoctial circle, observations of, by Euclid, 11 Equinoxes, first observations of, 15, 16, 17, 22 ; precession of the, 33 Eratosthenes, observations of, 17 ; his measurement of the earth, and in- clination of the ecliptic, 19 ; meri- dian circle invented by, 20 Erecting eyepiece, 113 Errors, collimation and deviation, in the transit instrument, 238, 240, 247, 328 Errors ; personal equation, 259 ; adjust- ments of the equatorial, 329 Ertel, vertical circle designed by, 290 Escapements of clocks, 196 205 ; ancient, 177; anchor, 197; Gra- ham's, 199 ; Mudge's, 200 ; Green- wich clock, 203 ; detached lever, 208 ; chronometer escapement, 209 Ethereal vibrations, 373, 401, 410, 420, 449, 450 Euclid, his observations of the stars, 8, 9, 10 ; of great circles, horizon, meridian and tropics, 11, 12 ; theory of the earth's position, 12 ; pole star, 14 Extra-meridional observations, first employment of, 23, 25 "Eye and ear" method in transit observations, 259 Eyeball, section of the, 66 Eyepieces, Huyghen's, 110 ; Eamsden's, Dollond's, 112; erecting or "day eyepiece," 112 ; Dawes's solar eye- piece, 114; magnifying power of, 116 Eyepiece of Greenwich transit circle, 246 ; of transit instrument, 257 F. FAYE, M., celestial photography, 456 Feil and Chance, manufacture of nii.t glass discs, 119, 305 Fixed stars (see Stars) Flame of salts in a Bunsen's burner, 407 Flint-glass prisms, 83, 84; lenses, 86, 170 Flint-glass, improvements in the manu- facture of discs of, 118, 119, 305 Focal length of telescopes, 82, 458 ; of lenses, 62, 63 ; of convex mirrors, 94 Foucault. ; his reflecting telescope, 108 ; improvement of specula, 117; mode of polishing specula, 134, 136 ; mounting of his telescope, 311 ; governor of driving clo.ck for large telescopes, 323 ; siderostat, 343 ; spectrum analysis, 410 ; heliostat, 424 Fraunhofer ; manufacture of flint-glass discs, 118 ; large telescopes, 303 ; lines in the solar spectrum, 392 ; spectrum analysis, 402, 410, 422, 425, 432, 438 ' Frederick II. of Denmark, his patron- age of Tycho Brahe, 38 Fusee for chronometers, 209 G, GALILEO ; Ms telescopes, 73, 78 ; their magnifying power, 77 ; the pen- dulum, 183, 184 Gascoigue, eyepieces and circle reading, 212; cross wires for "telescopic sight," 219 Gateshead, Mr. Newall's refractor, 302 Geissler's tubes, 413 German mounting of large telescopes, 299 Gizeh, great pyramid of, an astrono- mical instrument, 6 Glasgow, electric time-gun, 278 Glass, injurious effects of the duty on, 305 Glass specula, methods of silvering, 137 Globe, celestial, 23 ; terrestrial, 23 Gnomon ; its invention and early use, 16 ; improvements in, 18, 175 INDEX. 489 Graham ; dead-beat escapement, 192, 199 ; mercurial pendulum, 188 Gravity escapement, 200, 202 Greeks, their early use of the gnomon, 16 Greenwich, Royal Observatory ; per- spective view and plan of transit circle, 243, 245, 251 ; transit room, 251, 257 ; meridian of, 252 ; chro- nograph, 260 264 ; computing room, 267 ; standard sidereal clock, 267 ; mean solar time clock, 268 ; standard clock, 274 ; pendulum, 188 ; reflex zenith tube, 286 ; alt- azimuth, 290 ; equatorial, 310 ; ther- mopile, 384 ; photoheliograph, 469 "GREENWICH TIME" AND THE USE MADE or IT (Chap. XVIIL), 271 283 Gregorian telescope, 149 Gridiron pendulum, 188, 189, 192 Grinding of lenses and specula, 127 Grubb ; production and polishing of metallic specula, 121, 134 ; adjust- ment of object-glasses, 141 ; Casse- grainian and Newtonian reflectors, 102, 108, 301, 303 ; great Mel- bourne equatorial telescope, 108, 314, 315, 317, 324, 327 ; mode of mounting its speculum, 145 149 ; automatic spectroscope, 397 ; solar spectroscope, 428 Guinand, manufacture of flint-glass discs, 118 Guns fired as time-signals, 278 H. HALIBURTON, on ancient astronomical observations, 6 Hall ; experiments with lenses, 85 ; manufacture of flint-glass discs, 118 Harcourt, Vernon, experiments with phosphatic glass, 123 Harrison's gridiron pendulum, 188 HEAT OF STARS, DETERMINATION OF (Chap. XXVL), 377385 Heliometer, 224 Heliostat, 423, 458 Henry (Prof), radiation of heat from sun-spots, 385 Herschel (Sir John), lenses corrected for aberration, 88 ; table of reflective powers, 169 ; star magnitudes, 381 Herschel, Sir "William, his reflecting telescopes, 103, 108 ; his mode of polishing specula, 129 ; great tele- scope at Slough, 169, 294 Herschel-Browning direct- vision prism, 400 Hipparchus, trigonometrical tables con- structed by, 17 ; discoveries of, 25 35 ; his measurement of space, 21 3 Hittorf ; spectrum analysis, 413 Holmes (N. J.), his proposal of the electric time-gun, 278 Hooke, improvement in clock escape- ments, 196 ; micrometer, 221, 222 ; zenith sector invented by, 285 ; siderostat suggested by, 343 Horizon, the first astronomical instru- ment, 4, 7, 8 ; defined by Euclid, 12 Horological Institute, time - signals, 280 Hours, first reckoning of, 19 ; measure- ment of, 176 Hour circle of the equatorial telescope, 328, 335 Huen, island of, granted to Tycho Brahe, 38 Huggins (Dr.), telespectroscope, 429,432 Huyghens ; telescopes used by, 81 ; eyepiece, 110, 116, 212 ; application of the pendulum to clocks, 183 ; his measurements of space, 219, 223, 343 ; polarized light, 442 Hydrogen in the sun, 435 I. ICELAND spar crystals ; double refrac- tion by, 226, 228 ; polarization of light, 442, 445, 447, 449, 450 Illuminating power of the telescope, 158, 166, 168, 169 ; stars in Orion, a test of, 164 Images, double, seen through Iceland spar, 2-27 Inclination of the earth's axis, 14, 17 Inclination of the ecliptic, 17 ; mea- sured by Eratosthenes, 19 Index error ; adjustments of the equa- torial, 330 Iron, line spectrum of, 406, 418 Irrationality of the spectrum, 87 J. JANSSEN (Dr.), solar photography, 471; discoveries in solar physics, 472 Jupiter, in Ptolemy's system, 3 ; in Tycho Brahe's, 46 ; as a telescopic object, 351 ; photographs of, 465, 466 Jupiter's moons, observation of their eclipses to deteimiiie "local time*" 282 INDEX. K. KEPLER'S treatise on dioptrics, 386 Kew Observatory ; photographs of the sun and sun-spots, 460. 465, 470, 475 Kirchhoff ; spectroscope, 396 ; spectrum analysis, 402, 403, 412, 422, 428 Kitchener (Dr.), improved eyepiece, 113 ; stars in Orion, 164 Knohel's photometer, 378 Knott, star magnitudes, 381 L. LAMP for equatorial telescope, 325 Lamp, electric (see Electric Lamp) Lassell ; his Newtonian telescope, 108, 311 ; production, polishing, and mounting metallic specula, 121, 132, 144 Latitude ; observations of Posidonius, 8 ; parallels of, 23 Lattice-work for tubes of telescopes, 172 Lenses ; action of, 55, 58, 85 ; concave and convex, 61, 71, 75 ; amount of colour produced by, 81 ; achromatic, 84 ; Hall and Dollond's experiments, 85 ; correction for colour, 87 ; cor- rection for aberration in eyepieces, 109, 116; production of, 117 Lens, crystalline, of the eye, 67 Lewis (Sir G. C.), his "Astronomy of the Ancients," 9 Liebig, improvement in specula, 117 Light ; refraction, 55 72 ; deviation and dispersion, 79, 80, 82, 83 ; de- composition and recomposition, 83 ; reflection, 90 99 ; action of a re- flecting surface, 91 ; angles of in- cidence and reflection, 92 ; concave and convex mirrors, 94 98 ; velo- city of, 159 ; loss due to reflection, 168; effective, in reflectors, 169; vibration of particles, 373, 401 ; polarization, 441 453 LIGHT OF STARS, DETERMINATION or _ (Chap, xxvi.), 377-385 Lindsay (Lord), siderostat at his obser- vatory, 347 Local time, 281 Longitude, meridians of, 23 ; as de- termined by Hipparchus and Tycho Brahe, 44 ; determined by clock and transit instrument, 280 ; expressed in degrees and time, 280 M. Magnesium vapour; colour of, 416; in the sun, 435 Magnifying power of large telescopes, 154, 155 ; stars in Orion, a test of, 163 Magnitude of stars, 377 Malus, discovery of polarization by re- flection, 442, 448 Malvasia (Marquis), his micrometer, 219, 221 Manlius, gnomon erected by him at Eome, 18 Maps of the stars (see Stars) Mars, in Ptolemy's system, 3 ; in Tycho Brahe 's, 46 ; as a telescopic object, 350 Martin's method of silvering glass specula, 138 Mauritius, photoheliograph at, 469 Mean time, 254 Mean solar time clock at Greenwich, 268 Melbourne Observatory ; great reflect- ing telescope, 312, 313, 337 ; com- position and production of specula, 120, 121,' 129 ; view of optical part, 143 ; mode of mounting speculum, 144 149; photographs of the moon, 459 Mercurial pendulum, 187, 188, 192 Mercury, in Ptolemy's system, 3 ; m Tycho Brahe's, 4d ; as a telescopic object, 350 Meridian, denned by Euclid, 12 Meridional observations, first employ- ment of, 20 Meridian of Greenwich, 252 Meridian circle, the first, 20 ; at Cam- bridge (U.S.), 248 Meridians of longitude (see Longitude) MERIDIONAL OBSERVATIONS, MODERN (Book IV.) 233290 Merz (M.), manufacture of flint-glass discs, 119 ; cost of large object- glasses, 172 ; large telescopes, 303 Metallic specula, 120, 171 Meton, meridian observations by, 25 Meudon Observatory, solar photography at, 470 MICROMETER, THE (Chap, xv.), 218 232 ; wire micrometer, 221, 352 ; heliometer, 224 ; double image, 229 ; position, 353 ; measurements made by, 355, 359366, 368 Microscopes, for reading transit circles, 247 ; for Ne wall's telescope, 307 Middlesborough, time signal, 278 Milky Way, observations of Euclid, 11 INDEX. 491 Miller, spectrum analysis, 410 Mirrors, concave and convex, 94 98 Mirrors for reflecting telescopes (see Specula) MODERN MERIDIONAL OBSER- VATIONS (Book IV.), 233290 Molecular vibration, 373, 401, 410, 429, 449, 450 Months, first observations of, 5 Moon, The, in Ptolemy's system, 3 ; motions observed by the Chaldeans, 4 ; parallax observed by Ptolemy, 35 ; used by Hipparchus to deter- mine longitude, 44 ; as a telescopic object, 350 ; the lunar crater, Copernicus, 354 ; measurement of shadow thrown by a lunar hill, 355 ; photographs and stereographs, 459, 464, 465, 466 ; part of Beer and Madler's map, 476 ; of De La Rue's photograph, 477 MOUNTING OF LARGE TELESCOPES (Chap, xx.), 293327 Mounting of specula for reflecting tele- scopes, 144, 149, 169 Mudge, grinding and polishing specula, 129 ; gravity escapement, 200 Mural circle, 241, 242 Mural quadrant, Tycho Brahe's, 233, 235 Multiple stars, 351 N. NEBULA, 351 Nebula of Orion, 157, 158 Neptune, as a telescopic object, 351 Newall's equatorial refractor, 302 ; with spectroscope, 427 ; flint-glass discs for, 119 ; production of discs for object-glass, 128 ; photographs of the moon, 459 Newcastle, time signals, 278 Newton (Sir Isaac), on refracting tele- scopes, 82 ; his reflecting telescope, 101, 102 ; use of pitch in polishing specula, 128 ; refrangibility of light, 387 ; polarized light, 442 Newtonian reflector, 149 ; view of optical part, 143 ; effective light, 169 ; Grubb's form, 303 ; Brown- ing's form, 304 ; mounting of, 310 Nicols' prism, 115 ; measurement of the light of stars, 380 ; polarization of light, 443, 447, 448, 419, 450 North pole, diagram illustrating how it is found, 249, 251 0. OBJECT-GLASSES, production of, 118, 119 ; correction of colour, 88 j correction for spherical aberration, 126 ; mode of polishing, 128 ; mode of centring, 140 ; illustrations of defective adjustment, 141 ; ad- justment of, 163 ; its perfection in modern telescopes, 166, 305 ; cost of production, 172 ; divided, for duplication of image, 225 Object-glass prism, 426 Observatories [see Alexandrian Mu- seum, Cambridge (U.S.), Cincin- nati, Edinburgh, Greenwich, Huen (Tycho Brahe's), Kew, Lord Lind- say's, Mauritius, Melbourne, Meu- don, Paris, . Potsdam, Vienna, Washington] Observing chair for equatorial tele- scopes, 339 Optical action of the eye, 67 ; long and short sight, 69, 71 Optical qualities of telescopes, per- manence of, 170 Optic axis in crystals of Iceland spar 228 "Optick tube," telescope so first called, 55, 139151 Orion, first observations of, 5 ; Orion and the neighbouring constellations, 156 ; nebula of, 157, 158 ; stars in, a test for power of telescopes, 164 166; facilities for ob&erving, 164 P. PARALLACTIC rules, 51 ; used by Pto- lemy, 35 ; by Tycho Brahe, 38, 41 Parallax of the moon, observed by Ptolemy, 35 Paris Observatory, reflecting equatorial telescope, 314, 315, 337 ; sidero- stat, 344 ; photoheliograph, 469 Pendulum, 183, 185, 187, 188 Personal equation, 259 Phosphatic glass for lenses, 123 PHOTOGRAPHY, CELESTIAL (Chap, xxxi., xxxii.), 454483 Photography, stellar, 172 Photoheliograph, for photographs of the sun, 460, 470 ; for transit of Venus (1874), 461 Photometry, 373, 377 PHYSICS, ASTRONOMICAL (Book VI.), 371 492 INDEX. PHYSICAL INQUIRY, GENERAL FIELD OF (Chap, xxv.), 371376 Picard, transit circle, 284 Pisces, its position in the zodiac, 34 Pitch employed in polishing lenses and specula, 128, 132 Plane of the ecliptic, 13, 14 Planets, in Ptolemy's system, 3 ; first observations of conjunction, 4, 5 ; motions observed by Autolycus, 9 ; in Tycho Brahe's system, 46 ; Saturn seen with object-glasses of 3f and 26 inches, 160, 161 ; as telescopic objects, 350; photographs of, 465 Pleiades, the first observations of, 5 Pliicker, spectrum analysis, 413 Pogson, star magnitudes, 381, 382 Pointers ot pre- telescopic instruments, 35, 49, 214, 216 Polar axis of the equatorial, 299, 302, 308, 311, 312, 324, 328, 329, 346 Polariscope, 441 453 Polarization of light, 441453 Pole, North, 238 ; diagram illustrating how it is found, 249 Pole star, first observations of, 6 ; observations of Euclid, 10, 14 ; its position, 238 Polishing lenses and! specula, 128, 171 : Lord Rosse's polishing machine, 181; Mr. Lassell's, 132 Posidonius, measurement of the earth's circumference, 8 Position circle, 353 Position micrometer, 353, 358 Post Office Telegraphs, for distribution of Greenwich time, 275 Potsdam, photoheliograph at, 469 Precession of the equinoxes, 33 Prime-vertical, 285 Prime-vertical instrument, 287 Primum mobile of Ptolemy, 3 Prisms, action of, 55 ; crown and flint- glass, 83, 84 ; water, 85 ; doubly refracting, for the micrometer, 226 ; direct vision, 400 ; in the spectro- scope, 393 400 ; object-glass prism, 426 Ptolemy, the Heavens according to, 3 ; trigonometrical tables, 17 ; sun's altitude, 21 ; his discoveries, 35 ; parallax of the moon, 35 ; his measurement of time, 36 ; paral- lactic rules, 38, 51 Purbach, observation of altitudes by, 36 Pyramids, the first constructed astro- nomical instruments, 5, 6 Q. QUADRANTS used by Tycho Brahe, 38 ; his quadrana maximus, 48 Quadrant, mural, 233, 235 Quartz crystals for polarizing light, 450, 452 K. RADIATION of stars, visual, 383 ; thermal, 385 Radiation, general and selective, 403, 408 Rauisden's eyepiece, 112, 212 Reading microscopes, for Greenwich and Cambridge (U.S.) transit circles, 247 ; for Newall's telescope, 307 Red stars (see Colour of Stars) Reflection of light (see Light) Reflecting telescopes (see Telescope) Reflective powers, Sir John Herschel's table of, 168 Reflector, diagonal, for solar observa- tions, 114 Reflecting and refracting telescopes compared, 170 Reflex zenith-tube at Greenwich, 286 Refracting telescopes (see Telescopes) Refracting and reflecting telescopes compared, 170 Refraction of light (see Light) Refraction, double, by crystals of Ice- land spar (see Iceland Spar) Refrangibility of colours, 387; of light, 420 Regiomontanus, altitudes measured by, 36 Regulation of clocks by electricity, 272 Rising of stars (see Stars) Right ascension, 24, 234, 241, 249, 257 ; measured by Hipparchus, 44 ; by Tycho Brahe, 45 Ring micrometer, 368 Robinson (Dr.), specula of Melbourne telescope, 129; apertures of object- glasses, 168 Rockets fired as time signals, 281 Romer, wires in a transit eyepiece, 220 ; transit circle and transit in- strument, 284 Rosse (Lord), his reflecting telescope, 108, 294, 311, 312; composition of reflector, 120 ; production of metallic specula, 121, 131 ; nebula of Orion as seen by his reflector, 157, 158 ; illuminating power of INDEX. 493 his telescope, 1.59 ; effective light 1 69 ; thermopile observations, 384 Royal Observatory, Greenwich (see Greenwich) Rudolph II. (Emperor), his patronage of Tycho Brahe, 42 Rumford's photometer, 377 Rutherfurd, his work in celestial photography, 455, 464, 466, 471, 477, 480 SALTS, flame of, in a Bimsen's burner, 407 Sand clocks and sand glasses, 176 Saturn, in Ptolemy's system, 3 ; in Tycho Brahe's, 46 ; as seen with a 3f inch and 26 inch object-glass, 160, 161 ; as a telescopic object, 351 ; mode of measuring its rings, 357 ; photographs of, 465, 466 Savart's analyser for polarization of light, 452 Scarphie, employed by Eratosthenes, 19 Schemer's telescope, 78 Seasons, The, 15, 16 Secchi (Father), direct - vision star spectroscope, 431 ; stellar spectra, 433 Setting of stars (see Stars) Sextants used by Tycho Brahe, 38, 50 Sidereal clock, 254, 266 (see Clock) Sidereal day, 256 Sidereal time, 240, 254, 324 SIDEROSTAT, THE (Chap, xxiii.), 313 348, 461 ; at Lord Lindsay's Ob- servatory, 347 Signals for distributing "Greenwich time," 278 Signals, time, 281, 283 Signs of the zodiac (see Zodiac) Silver-on-glass reflector at the Paris Observatory, 316 Silvering glass specula, modes of, 137 ; silvered glass reflectors, 171 Simms, his introduction of the colli- mator in the spectroscope, 393, 425 Sirius, first observations of, 5 ; spec- trum of, 432 Slough, Sir Wm. Herschel's telescope at, 294 Smyth (Admiral), stars in Orion, 165 ; colours of stars, 351 ; star magni- tudes, 381 Smyth (Prof. Piazzi), on the pyramids as astronomical instruments, 6 ; position of the venial equinox, 34 ; clock arrangements at Edinburgh Observatory, 269 Sodium, discovery of its presence in the sun, 412 Solar photography, 459, 465 Solar spectroscope, 435 ; Browning's and Grubb's forms, 428 Solar spectrum, 390, 391, 392, 423, 433, 436, 438, 439 ; photographs of, 479, 480 Solar time, 253, 255 Solstices, first observations of the, 15, 16, 17, 22 Southing of stars, 234 SPACE MEASURERS (Book III.), 135 232 ; circle reading, 211 ; Digges' diagonal scale, 213 ; the vernier, 214 ; micrometers, 218 Space-penetrating power of the tele- scope, 1 54 ; stars in Orion, a test of, 165 Spectroscope, construction of the, 393 400 ; automatic, 397 ; arranged for showing absorption, 409 ; at- tached to Newall's refractor, 427 ; solar, Browning's and Grubb's forms, 428 Spectrum produced by prisms, irra- tionality of the, 86, 87 Spectrum, solar, 390, 391, 392 Spectrum analysis, principles of, 401 421 Specula, production of, 117, 120 ; cast- ing, annealing, 121 ; curvature, 122 ; grinding, 127 ; polishing, 128 ; silvering, 137 ; mounting, 142, 169, 172 ; effective light, 169 ; repolishing, 171 ; cost as compared with object-glasses, 172 Spherical aberration, 87 ; diagram illus- trating, 104, 105 ; its correction in eyepieces, 109, 111 ; of specula, 123,' 1 24 Sprengel pump, 413 Spring governor of driving-clock for large telescopes, 319, 320 " Spurious disc " of fixed stars, 163 Standard clock at Edinburgh Observa- tory, 272 Standard sidereal clock of Greenwich Observatory, 267 Standard solar time clock of Green- wich Observatory, 267 STARS, CHEMISTRY OF THE (Chap. xxvn. xxx.), 386 453 STARS, LIGHT AND HEAT OF (Chap. xxvi.), 377 ; variable, 377385 Stars, first observations of the, 4, 5, 6, 7 ; first maps of, 8 ; observations of 494 INDEX. Autolycus, Euclid, and Posidonius, 8, 10 ; first catalogues of, 19 ; lati- tude and longitude of, 24, 30 ; posi- tions tabulated by Hipparclms, 30 ; Tycho Brahe's catalogue and map of, 42, 44; stars in Gemini seen through a large telescope, 155 ; nebula of Orion, 157; Orion and its neighbourhood, 156 ; double, as de- fined by telescopes of different power, 162, 164, 167, 167 ; distance of stars from the earth, 159 ; facili- ties for observing Orion ; its stars, a test for power of telescopes, 164 ; stellar photography, 172, 465, 466, 467, 478 ; their rising and setting as measurers of time, 176 ; double, measurement of, 359, 361, 362 ; spectrum of red star, 433 Star-clusters, double and multiple stars, 351 Star-spectra, from Father Secchi's ob- servations, 433 j photographs of, 479 Star spectroscopes, at Cambridge (U.S.), 430 ; direct vision, 431 Star-time (see Sidereal Time) Steinheil, improvement of specula, 117 SteUar day, 256 Stereographs of the moon, 465, 466 Sternberg, Tycho Brahe's Observatory, 38 Stewart (Prof. Balfour), spectrum analysis, 402 ; solar photography, Stokes (Prof.), experiments with phos- phatic glass, 123; spectrum analysis, 402, 410 Stone, thermopile at Greenwich, 384 Strontium in the sun, 419 Struve, transit instrument, 285 ; double stars, 362 ; star magnitudes, 381 Sun, The ; in Ptolemy's system, 3 ; first determination of its yearly course, 8, 15 ; course in the zodiac, described by Autolycus, 9 ; altitude determined by the gnomon, 16, 18 ; and the Scarphie, 19, 20 ; telescopes for observing, 114 ; " mean sun," 256 ; as a telescopic object, 349 ; presence of sodium in, 412, 415 ; vapour of other metals, 417 ; ab- sorption spectrum, 418 ; telespectro- scopic observations, 436 ; of the chromosphere, 437 ; sun-storms, 438, 439; photographs, 459, 469, Sun-dials, 18 Sun-spots observed by Galileo and Scheiner, 78 ; examined by the position micrometer, 358 ; spectra of, 415, 435 Sunderland time signals, 278 T. TALCOTT, zenith telescope designed by, dOU Taurus, its position in the zodiac, 34 Telegraph wires, their application in determining "local time," 281 TELEPOLARISCOPE, THE (Chap. xxx. ), 441453 Telespectroscope, 426 TELESCOPE, THE (Book II.), 55-- 172 TELESCOPE, THE EQUATORIAL (Book V.), 293368 TELESCOPE : -VARIOUS METHODS OF MOUNTING LARGE TELESCOPES (Chap, xx.), 293327; refracting, 7389 ; Galilean, 73 ; magnifying power of the telescope, 76, 79 ; Schemer's telescope, 78 ; focal length of early telescopes, 79 ; achromatic, 86 ; reflecting, 100 108 ; Gregory's telescope, 101 ; Newton's, 102 ; Cassegrain's, 103 ; Sir W. Herschel's 103, 108 ; Lord Rosse's, De La Rue's, Lassell's, Foucault's, Grubb's, 108 ; eyepieces, 109 116 ; Huyghen's eyepiece, 110; Rarnsden's eyepiece, 112; magnifying power of eyepieces, 116 ; lenses and specula, 117 138 ; flint glass for lenses, 119; the "optick tube," 139151 ; the modern tele- scope, 152 172 ; magnifying and space penetrating power, 154, 155 ; illuminating power, 158 ; defining .power, 160 ; reflecting and refract- ing compared, 170 ; permanence of optical qualities, 370; "telescopic sight," 219 ; Sir Wm. Herschel's at Slough, 294 ; Lord Rosse's re- flector, 294, 311, 312 ; refractor on alt-azimuth tripod, 296 ; simple equatorial mounting, 298 ; the German mounting, 299 ; "Wash- ington great equatorial, 309 ; Eng- lish mounting, 310 ; forked mount- ing, 310 ; Greenwich equatorial, 310 ; Melbourne reflector, 312, 313 ; Paris reflector, 314 ; driving clock, 318 ; Newall's refractor with INDEX. 495 spectroscope, 427 ; De La Rue's, 459; Rutherfurd's, 466; Newall's, 459 ; Melbourne, 459 Telescope, zenith (see Zenith Telescope) Temperature, its effect on the pendulum, 187, 193 Terrestrial globe, 23 Thales, his employment of the gnomon, 17 Theodolite, 288 Theodolite, astronomical, 287 Thermometry, 374, 384 Thermopile, 374 Time ; first reckoning of, 19 ; early measurements, 36, 44, 175 ; modern measurement of, 253 ; sidereal, solar, and mean, 254, 256 TIME AND SPACE MEASURERS (Book III.), 175232 Time, Greenwich (see Greenwich Time) Time, local, 281 Time balls for distributing Greenwich time, 275 Time signals, 278, 281, 283 Timocharis, his observations in the Alexandrian museum, 19 Tourmaline, in polarization of light, 443 TRANSIT CIRCLE, THE ( Chap, xvi.), 233 252 ; system of wires in eyepiece, 220 ; at Greenwich and Cambridge (U.S.), 247, 248, 251; mode of using, 253, 284 TRANSIT CLOCK, THE (Chap, xvn.), 253270 Transit instrument, 171, 234, 236, 237 ; mode of using, 253 ; Romer's, 584 ; Strave's, 285 Transit of Venus, photographic obser- vations, 475 Trigonometrical tables, first construc- tion of, 17 Tropics, defined by Euclid, 12 Trouvelot, ring of Saturn observed i with the Washington refractor, 161 Tube of the telescope, ] 39151 Tycho Brahe ; astrolabe, 26 ; ecliptic astrolabe, 28 ; discoveries of, 37 52 ; biography of, 37 ; list of his instruments, 38 ; portrait, 39 ; cata- logue of stars, 42 ; observatory (en- graving), 43, 287 ; his solar system, 46 ; discovery of comet of 1677, 47 ; instruments for measuring distances and altitudes of stars, 51 ; clocks, 179, 184, 196; diagonal scale for measuring space, 213 ; mural quad- rant, 233 ; transit circle, 284 U. UNITED STATES Naval Observatory, 341 Uranus, as a telescopic object, 351 Uraniberg, Tycho Brahe's Observatory, 38 V. VARIABLE stars, 377 Velocity of gases in sun-storms, 440 Venice, ancient clock dials, 257 Venus, in Ptolemy's system, 3 ; in Tycho Brahe's, 46 ; employed by Tycho Brahe in determining longi- tude, 44 ; as a telescopic object, 350 ; transit of ; instrument used in the expedition of 1874, 236 ; pho- tographic observations, 475 Vibrations, ethereal, 373, 401, 410, 449, 450 Vienna, refracting telescope, 141 Villarceau, Yvon, driving clocks, 324 Vega, heat of, 385 Vernal equinox, its position in the constellations, 34 Vernier, the, 214 Vertical circle, Ertel's, 290 W. WALTHER, altitudes measured by, 36 Washington Observatory ; great re- fracting telescope, 302, 309 ; flint glass discs, 119 ; ring of Saturn seen through it, 161 Watches, detached lever escapement for, 207 Water clocks, 176 Wave-lengths of light of solar gases, 440 Westminster clock-tower, 277 Wheatstone (Sir C. ) ; " chronographic method" of transit observation, 259 ; apparatus for controlling clocks, 271 Winlock (Prof.), photographs of the sun, 461 Wires, cross, for circle reading, 212, 216 ; system of wires in a transit eyepiece, 220, 234, 257 ; in eyepiece of Greenwich transit circle, 246 ; wires of the transit instrument* 234 Wire micrometer, 221, 352 496 INDEX. Wolfius, correction of chromatic aber- ration in lenses, 89 Wollaston (Dr.), lines in the solar spec- trum, 391 ; spectrum analysis, 402, 422 Wyck (Henry de), clock made in 1364 by, 178 Y. Ys of the transit instrument, 238, 284 Years, first observation of, 5 ; deter- mination of their length, 22 ZENITH, zenith sector, zenith telescope, reflex zenith tube, at Greenwich, 285 Zenith distances, measurement of, 51 Zodiac, first defined, 8, 9; observations of Euclid, 11, 12 ; of Denderah, 7 Zollner's astrophotometer, 379 Zero of right ascension, 249 Zinc in the sun, 419 THE END. LONDON: R. CLAY, SONS, AND TAYLOU, ERKAD STREET HILL, K.C. RETURN CIRCULATION DEPARTMENT 198 Main Stacks LOAN PERIOD 1 HOME USE 2 3 4 5 6 ALL BOOKS MAY BE RECALLED AFTER 7 DAYS. Renewls and Recharges may be made 4 days prior to the due date. Books may be Renewed by calling 642-3405. DUE AS STAMPED BELOW APR 2 9 2000 FORM NO. DD6 UNIVERSITY OF CALIFORNIA, BERKELEY BERKELEY CA 94720-6000