Contribution to solar physics. I. A popular account of inquiries into the physical constitution of the sun, with special reference to recent spectroscopic researches; II. Communication to the Royal society of London, and the French Academy of sciences, with notes
None
mcsi NV//8 7* WtUraru u£ ^S^6 m & ss\ X / V: U. b^/TUUL. iuuuu\ JiS<r(trry| "■!?*» • •• ..». i . # • ; • ••.*? • #„- ~\ ft. • ••••-••■• CONTRIBUTIONS SOLAR PHYSICS. mm^ Cv> «<S CONTRIBUTIONS SOLAR PHYSICS. i. A POPULAR ACCOUNT OF INQUIRIES INTO THE PHYSICAL CONSTITUTION OF THE SUN, WITH SPECIAL REFERENCE TO RECENT SPECTROSCOPIC RESEARCHES ; II. COMMUNICATIONS TO THE ROYAL SOCIETY OF LONDON, AND THE FRENCH ACADEMY OF SCIENCES, WITH NOTES. J. NORMAN LOCKYER, F.R.S. MACMILLAN AND CO. 1874. \_7'//t- Right of Translation and Reproduction is reserved, j .,(£ '/yy^,, iJ. ^/-&*W K. CI. AY, SONS, AND TAYLOR, 1' BREAD STREET HILL. LONDON : ; This Book is Dedicated to Balfour Stewart and «a P. C. Jules Janssen. Encouraged by One Friend I undertook the Work which has brought +- me The Other. PREFACE. The present volume has grown out of an intention I formed about a year ago, to publish the- Papers I had communicated to the Royal Society, dealing with the new facts which a new method of inquiry had revealed to us. I formed this resolution because, doubtless owinof to their beinof scattered amone the c> o o publications not easily procurable of a learned body, these Papers were evidently unread by some who were actually engaged, as well as by many who were merely interested, in the inquiry. It next struck me that it would be wise to re- lieve the great — I fear too-great — terseness of these Papers by introducing into the same volume three Lectures I had also published in the Proceedings of the Royal Institution, giving an account of the first out- come of my inquiries, and of the results of the two Eclipse Expeditions which I had accompanied to the viii PREFACE. Mediterranean and India. After I had determined to appeal by the publication of these to a wider public, I chanced to fall upon the notes of a course of four other Lectures on the Sun which I had also given in the Royal Institution, but which remained unpub- lished, and were almost forgotten. This decided me to attempt to make the story of the work more com- plete, and to add to it the information necessary for the general reader both as to the telescopic and spec- troscopic sides of the inquiry, by means of these notes and some of the Essays which I had published at different times during the last ten years in The Reader, Macjnillaris Magazine (these were written in conjunc- tion with my friend Dr. Balfour Stewart), Nature, the Times, Daily News, and other periodicals. This, then, is the origin of the work in its present shape. In the First Part I have attempted to give a sketch of the various inquiries into the Physical Constitution of the Sun, and I have not hesitated to alter the arrangement of the four Lectures, and, in some cases, the Essays to which I have re- ferred, so as to make the story a continuous one. I have not only largely added to the parts dealing with Spectrum Analysis, but I have given a body of PREFACE. ix information on this new science in its special relation to its solar applications which I hope may be found of value. The Second Part, which consists of my Papers communicated to the Royal Society and to the French Academy of Sciences, of course is given verbatim, with the exception of the references to the illustrations, many of which have been transferred to the First Part of the book. To these Papers I have added Notes, on some of the more fundamental outcomes of the research, in which I have attempted to show the relations of my observations to those made by others ; on the objections urged against some of the conclu- sions to which I had been led ; and on the new facts acquired to Science by the labours of my fellow- workers. On this latter point, however, I have been compelled not only to give slight references where I could have wished to have given full details, but to cancel much matter actually written in order to keep the volume within convenient limits. I must, however, thank Professors Young and Respighi for permitting me to largely increase the value of my book by referring at the length I have done to their observations. The faithful translation of the memoirs of the illustrious Italian observer here x PREFACE. given, has been obligingly placed at my disposal by Mrs. G. M. Patmore. Whilst the latter portions of this work have been passing through the press, a very important discussion has been carried on in the French Academy of Sciences, in which nearly every question raised by the new method of research in solar physics has been debated. I much regret that it has been impossible to include a notice of it in the present volume. It may, however, be remarked as satisfactory to English Science, that M. Faye, abandoning the theory of spots of which an account will be found in Chapter IV., has virtually adopted, in the essential point, that pro- posed by the English Observers. My best thanks are due to the Proprietors of the Journals I have named for the readiness with which permission has been granted me to reprint ; while in the matter of illustrations I have to express my obligations to the Council of the Royal Society ; to Dr. Schellen, of Cologne, for the use of several wood- cuts illustrating my own observations, the original drawings of which I sent over to him in 1869 ; to Mr. Westermann, of Leipsig, who added others to them ; and to Professor Roscoe, who has allowed me the use of several which appear in his " Spectrum Analysis." PREFACE. xi I am much indebted to Mr. Cooper for the care he has taken in the preparation of the plates and new- woodcuts. In the compilation of the Notes, and in revision of proof-sheets, my assistant Mr. R. J. Friswell has given me most valuable aid. Up to the present time the spectroscopic exami- nation of the sun has been regarded as the work of the astronomer and physicist, rather than of the chemist ; and in England, though happily not abroad, many professional astronomers and physicists regard it, as a rule, as a matter of tenth-rate importance. I am sanguine enough to hope that, as time goes on, breadth of mind will take the place of the present more than apathy, and that chemists also will more generally interest themselves in, and aid, an inquiry from which, if I am not mistaken, they will learn much. I cannot conclude this Preface without stating that had it not been for the aid afforded me by that admir- able, but too little known, institution, the Government Grant Fund, and by my friend Dr. Frankland, who joined me in a branch of the research and generously •placed his laboratory at my disposal, my observations would probably never have been made. Further, I Aii PREFACE. admit and lament the incompleteness of them and of the book to which I have now consigned them. I know that the work I have attempted to forward requires a man who can give himself entirely up to it, while, less fortunate than many lovers of Science, the only time I have had to devote to these inquiries has consisted of fragments snatched from the leisure left me by my official duties. I have, however, the satis- faction of knowing that the method of observation which. I have had a share in originating is rapidly taking root in other lands, and that it is being recog- nized as national work which the Janssens, Youngs, Respighis, Zollners and Secchis of the future will carry on without break, for the instruction and benefit of mankind. J. Norman Lockyer. Sept. 6t/i, 1873. CONTENTS. PART I. A POPULAR ACCOUNT OF ANCIENT AND MODERN SUN-WORK. THAI'. PAGE I.— What is a Sun ? i Lecture delivered at the Royal Institution in March 1870. II.— Ox the Telescopic Appearance of the Sun .... 13 In part reprinted (with alterations) from the Reader, vol. iii. p. 79, 1864. The remainder consists of a paper communicated to the Royal Astronomical Society in 1865, and printed in Monthly Notices, vol. xxv. p. 236. III.— Mr. Carrington's Researches on Solar Spots ... 52 Reprinted (with additions) from the Reader, vol. iii. p. 58, 1864. IV. — M. Fave's First Theory of the Physical Constitutiox of the Sun 44 Reprinted from the Reader, vol. v. p. 107, 1865. V. — M. Faye's First Theory of the Physical Constitution of the Sun (continued) 51 Reprinted from the Reader, vol. v. p. 140, 1865. VI. — The Sun as a Type of the Material Universe ... 63 An article written in conjunction with Dr. Balfour Stewart, F.R. S. Reprinted from MaC7tiil tan's Magazine, vol. xviii. p 319, 1S68. VII.— The Place of Life in a Universe of Energy .... 85 An article written in conjunction with Dr. Balfour Stewart, F. R.S. Reprinted from Mactnillan's Magazine, vol. xviii. p. 319, 1868. xiv CONTENTS. CHAP. PAGE VIII. — The Place in Science of the New Method. . . . 104 From the Times, Jan. 9th, 1869, with additions from an Article in ftlacmillan's Magazine, vol. xix. p 230, Jan. 1869. IX— The Birth of Spectrum Analysis .... ... 132 Lecture delivered at the Royal Institution in April 1870, revised in the light of recent work. X. — The Modern Spectroscope 154 XI. — Radiation and Absorption 169 XII.— History of the Application of the Principles of Spectrum Analysis to the Solar Spectrum ... 185 XIII. — Results obtained by the Old Method 196 XIV.— The First Results of the New Method 209 A Lecture delivered at the Royal Institution of Great Britain, Friday evening, May 28th, 1869. XV. — The American Eclipse, 1869 240 From Nature, vol. i. p. 14, 1869, and Proc. R. S-, vol. xviii. p. 179. XVI. — The Mediterranean Eclipse, 1870. A Letter from Venice . .' , 258 From Nature, vol. iii. p. 221, 1871. XVII. — The Mediterranean Eclipse, 1870 {continued). More Light 270 From Nature, vol. iii. p. 321, 187L XVIII. — The Story of the Corona in connection with the Mediterranean Eclipse 278 A Lecture delivered at the Royal Institution of Great Britain, Friday evening, March 17, 1871. XIX. — The Atmosphere of the Sun 311 The Rede Lecture, 1871, delivered in the Senate House, Cambridge, on May 24, 1871. XX. — The English Eclipse Expedition, 1871 — I. Preliminary 332 In nn Nature, vol. iv. p. 197, 1872. CONTENTS. CHAT. PAGE XXI. — The English Eclipse Expedition, 1871 {continued)— II. The Bekul Party 339 Extracted from a letter in the Daily News, January 15th, 1872. XXII. — The English Eclipse Expedition, 1871 {continued) — III. A Letter from Ootacamund 347 From Nature, vol v. p. 217, 1872. XXIII. — The English Eclipse Expedition, 1871 {continued) — IV. Intelligence from the Other Parties .... 356 From Nature, vol. v. p. 259. XXIV. — The English Eclipse Expedition, 1871 {continued) — V. General Statement of the Methods used, and Results obtained, By the Expedition .... 362 A Lecture delivered at the Royal Institution of Great Britain, on Monday, March 22, 1872. The chief results obtained by the Expedi- tion have been taken from the ad interim Report presented to the British Association Meeting in Brighton. XXV.— Three Years' Work with the New Method . . . 381 Revised from the short-hand notes of two Lectures delivered before the Literary and Philosophical Society of Newcastle-upon-Tyne, in October 1872. 'J^n^^^^i^. C*4 XXVI. — The Meteorology of the Future 424 From Nature, vol vii. p. 98. PART IT. COMMUNICATIONS TO THE ROYAL SOCIETY OF LONDON AND TO THE FRENCH ACADEMY OF SCIENCES. PACK I. — Observatory Work :— Spectroscopic Observations of the Sun. No. I. . . 435 Spectroscopic Observations of the Sun. No. II. Brief Announcement of. the Success of the New Method. . 439 Complete Account 445 Spectroscopic Observations of the Sun. No. III. . . 477 Spectroscopic Observations of the Sun. No. IV. . . 48' CONTENTS. PAGE Spectroscopic Observations of the Sun. No. V. . . 493 Discussion with Father Secchi in the " Comptes Rendus" of the French Academy of Sciences 500 Spectroscopic Observations of the Sun. No. VI. . . 516 II. — Laboratory Work Preliminary Note of Researches on Gaseous Spectra in Relation to the Physical Constitution of the Sun. (In conjunction with Dr. Frankland) ... 525 Researches on Gaseous Spectra in Relation to the Physical Constitution of the Sun, Stars, and Nebul/e. No. II. (In conjunction with Dr. Frank- land) ' 530 Researches on Gasf.ous Spectra in Relation to the Physical Constitution of the Sun, Stars, and Nebul/e. No. III. (In conjunction with Dr. Frank- land) 532 Researches in Spectrum Analysis in connection with the Spectrum of the Sun. No. 1 534 Researches in Spectrum Analysis in connection with the Spectrum of the Sun. No. II 555 III. — Notes. A. — Spot Phenomena and Theories 561 B. — Mr. Huggins and the New Method 570 C. — Absorption at the Limb 574 D. — Methods of Viewing the Forms of the Promi- nences 578 E. — Structure, Height, and Composition of the Chromosphere 585 F.— Prominences on the Sun 587 G. — Changes of Wave-length 589 CONTENTS. xvii PAGE H.— List of Chromospheric Lines 602 I.— The Line 1474 (Kirchhoff) 621 K.— The Continuous Spectrum at the Base of the Chromosphere 622 L. — The Classification of Prominences 624 M.— Connection between Prominences, Spots, and FACULjE 628 N. — The Expansion of Lines at the Base of the Chromosphere 635 O. — Welling up of Magnesium and Iron 637 P.— Solar Outbursts and Magnetic Storms .... 638 Q. — Simplification of Spectra by Reduction of Pressure 640 APPENDICES.1 L— Instructions to Observers (Eclipse of 187 i) 649 II. — Professor Respighi's Memoir on the frequency and distribution of the Prominences, and their Periodical Variations 654 1 The Appendices referred to on pages 142, 252, and 262 have been omitted for want of space. LIST OF ILLUSTRATIONS. COLOURED AND LITHOGRAPHIC PLATES. Frontispiece.— The Spectroscopic Corona, 1871. Plate I. — The various kinds of Prismatic Spectra Plate II. — Solar Prominences Plate III. — The Photographic Corona, 1869 . .. Plate IV. — The Observatory at Bekul Plate V. — The Photographic Corona, 1871 . .. Plate VI. — Map of the Long and Short Lines of some Metallic Elements to face to face 172 )> 209 ii 240 it 343 1! 375 532 WOODCUTS. PAGE 7 1. Portrait of Galileo . .. 2. Explanation of sun-spots on Wilson's hypothesis ... < 3. Herschel's theory of sun-spots . 1 4. Herschel's theory of sun-spots . I 5. Early sun-work. Schemer's Heliotropium Teleoscopicum . i 6. Cyclonic sun-spot. (Secchi. ) . i 7. Changes in sun-spots. The great sun-spot of 1865. (Howlett. ) i 8. Facuke surrounding a spot, as seen near the sun's edge. (Noble.) 1 9. Sketch of sun-spot, completed about nh. 40m. a.m., April 2, 1865 2 10. The same at I2h. 30m. ... 2 11. The same at 1 2I1. 55m. ... 2 12. The same at 12I1. 30m. ... 2 FIG. 13- 14. IS- 16. 17- 18. 19. 20. 21. 22. 23- the the the the Sun spot. (Secchi.). Sun spot. (Secchi.). The earth, seen from (Vernal equinox) The earth, seen from (Summer solstice). The earth, seen from (Autumnal equinox) The earth, seen from (Winter solstice) The sun, seen from the earth.. Explanation of sun-spots on Kirchhoffs hypothesis. The eclipsed sun Path of moon's shadow during Eclipse of i860 Total Eclipse, Aug. 18, 1868. (Observations at Aden. ) Com- mencement of totality. 29 30 35 35 36 36 37 48 75 no LIST OF ILLUSTRATIONS. FIG. PAGE FIG 24. Total Eclipse (Indian), Aug. 18, 54. 1868, near end of totality. . 113 25. Total Eclipse (Indian), Aug. 18, 55. 1868 : end of totality . . .114 26. Eclipse of 1868. (Corona) . . 120 56. 27. Copy of Tennant's photograph : 57. beginning of totality . . .121 58. 28. Copy of Tennant's photograph : end of totality 122 59. 29. Synoptic view of Tennant's photo- 60. graphs 123 61 30. Tennant's reflector used for photography 124 62. 31. Geometrical form of the prism . 132 32. Prism mounted on a stand . .133 33. Light passing through a plate of 63. glass . 134 64. 34. Images of objects seen through prisms 135 65. 35. Copy of Kepler's diagram . .137 36. Path of the refracted ray . . .138 66. 37. Refraction of light. Apparent ele- vation of the bottoms of vessels 1 38 38. Decomposition of light by the 67. prism 140 39. Unequal refrangibility of two differently coloured lights . . 141 40. Recomposition of white light by means of a second prism . .142 70. 41. Recomposition of white light by 71. means of a lens . . . . .14, 42. Recomposition of white light by 72. means of a rapidly revolving disc, coloured in sectors . .144 43. The first observation of Fraun- 73. hofer's lines 147 44. Reduced copy of Fraunhofer's 74. map of the lines in the solar spectrum , 150 45. Chemical or student's spectro- I 75. scope 157 46. Spectroscope with two prisms .157 76. 47. Steinheil's form of four prism spectroscope 158 77. 48. Bunsen's first form of spectro- scope 158 : 78. 49. Spectroscope with reflected scale. 159 50. Steinheil's slit, showing reflect- ! 79. ing prism 161 51. Path of light through reflecting prism and into the slit . . .161 So. 52. Spectroscope showing the use of comparison spectra and a re- flected scale 161 81. 53. Direct-vision prism with three prisms . . . • 162 PAGE Direct-vision prism with five prisms 162 Telespectroscope of small dis- persion. (Huggins. ) . . . 163 Side view 164 Plan 164 Direct vision star spectroscope (Secchi. ) 165 Sun spectroscope (Lockyer) . . 166 Sun spectroscope (Young) . .166 Young's sun spectroscope, ar- ranged for photography . .167 Automatic arrangement for secur- ing the minimum deviation of the observed ray 168 Electric lamp 170 Electric lamp arranged for throw- ing a spectrum on a screen . 171 Coloured flame of salts in the flame of a Bunsen's burner . 173 Base of ordinary form of Bunsen's burner, showing holes to admit air 173 Mitscherlich's arrangement for flame spectra 1 74 Enlarged view of wick . . .174 Herapath's blow-pipe. View and section 175 Induced current spark . . . .175 Figure showing how the jar is connected with the spark stand. 1 76 Becquerel's arrangement for study- ing the spark spectra of va- pours 177 Geissler's tube, showing electric discharge 178 Wheatstone's map of the spectra of the vapours of some of the elements (1835) x79 Method of observing the absorp- tion of a vapour 180 Vessel with glass sides for study- ing the absorption of liquids . 1S1 Absorption spectra of iodine and nitrous fumes 182 Absorption of magenta and blood 183 Absorption of various thick- nesses of a solution of the salts of chromium. (Gladstone.) . 183 Absorption of various thick- nesses of a solution of potassic permanganate. (Gladstone.) . 183 Coincidence between the bright line given out by sodium vapour and the dark line produced by LIST OF ILLUSTRATIONS. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93- 94. 95- 96. 97- 98. 99. 100. 101. 102. 103. 104. 105. 106. 107, 108 PAGE the absorption of the light of a continuous spectrum by its passage through sodium vapour 192 Coincidence of some of the bright lines of iron with some of the Fraunhofer lines 197 The telluric lines 203 The spectroscope attached to the telescope for solar work . . 214 Enlarged view of spectroscope . 215 Line C (red), with radial slit . 216 216 217 217 222 Line D3 (yellow), with radial slit Line F (blue-green), with radial slit Line C, with tangential slit .. Spectrum of chromosphere, show ing the different lengths of the lines of hydrogen, sodium, and magnesium Contortions of F line in a pro- minence Prominence observed 14th March, 1869, nh. 5m 224 The same prominence, nh. 15m. 225 Spectrum of a sun-spot, show- ing the increased absorption of the sodium vapour .... 226 Sun-spot. (Secchi. ) .... 230 Contortions of F line on disc . 233 Contortions of F line on disc, in connection with spots and up- rushes of bright hydrogen . . 234 Alterations of wave-length in prominences 235 Solar cyclone 236 Non-coincidence of bright and dark F line 237 Comparison of b and adjacent lines 238 The eclipsed sun, August 1869 . 242 Copy of a photograph of the American eclipse of August 7, 1869 General view of the protuber- ances, American eclipse, Aug. 7, 1869 245 Gould's drawing of the corona, near the beginning of totality, at Burlington 248 Gould's drawing of the corona, near the end of totality, at Burlington 249 Why the corona was observed square 276 Total eclipse, 1851. (Dawes) . 279 244 FIG. PAGE 109 Total eclipse, September 1858. (Lias) 280 no. Total eclipse, i860. (Rays ob- served by Feilezsch) . . . 282 in. Rays observed in the total eclipse, 1840 283 112. Why the corona was observed square 285 113. Eclipse of 1 868. The corona as observed at Mantawalek Ke- kee 289 114. Gould's drawing of the corona, near the beginning of totality, at Burlington 290 115. Gould's drawing of the corona, near the end of totality, at Burlington 291 116. Eclipse of 1870. Photograph of the corona taken at Syracuse . 292 1 1 7. Explanation of the feeble or strong polarization of the corona 294 118. Copy of Rayet's diagram . . 299 119. Copy of Pogson's diagram . . 301 120. Track of the moon's shadow, eclipse of 187 1 333 121. The spectrum of hydrogen as seen with a straight, a crooked, and a ring slit 367 122. Young's observation of the rever- sal of the sodium lines over a spot . . . . _ 389 123. Spectroscope, with camera at- tached 392 124. Cirrus mass of prominences observed by Young, Sept. 4, 1869 393 125. Small prominence 393 1 26. A small forked prominence . . 393 127. Cumulus prominences like the fish-mouth in the nebula of Orion. (Young) .... 393 128. Forms of prominences (Zollner) 393 129. Forms of prominences (Zollner) 394 130. Forms of prominences (Young) 394 131. Forms of prominences (Young) 394 132. Rising of a detached fragment . 397 133. Filamentous prominence . . . 397 134. Streamer prominence .... 398 135. Tree-like prominences . . . 399 136. The prominence after the ex- plosion 399 137. The "thunder-head" at 1.40 . 400 138. The " thunder-head " at 1.55 . 400 139. Changes in prominences, Oct. 7, 1869. (Young) 401 LIST OF ILLUSTRATIONS. FIG. PAGE 140. Diagram showing how the pro- minences are daily recorded. (Respighi) 420 141. My first prominence .... 441 142. Secchi's first observation of the chromosphere 461 143. Arrangement for comparison of spectra of different parts of the sun 475 144. Motion forms 490 145. Motion forms 491 146. Uprush of bright hydrogen . . 492 147. Secchi's observations of the C line 503 148. Secchi's observations of the C line 503 149. "Motion forms" and "Lozenges" 523 1 50. Arrangement for throwing image of light-source on the slit . . 537 151. Long and short lines of zinc and cadmium 540 152. Long and short lines of lead and cadmium 541 153. Plan and section of cup used with salts 543 154. Aluminium cup placed in the spark-stand as in use . . . 544 155. Secchi's types of stellar spectra 55S 1 56. Young's observation of the spec- trum of a solar spot between C and D 564 FIG. PAGE 157. Rayet's, Herschel's, and Ten- nant's observations . . . .573 158. Slit for comparing spectra of centre and limb . . . . .576 159. Path of ray 576 160. Mr. Hastings' arrangement for comparison of spectra . . .577 161. Secchi's Merz-prism before the object-glass in cell .... 582 162. The cell without the prism . . 582 163. Diaphragm with annulus used by Lockyer and Seabroke . 583 164. Annulus viewed and brought to focus 584 165. Solar profiles 585 166. Edge of chromosphere (billowy) 585 167. Edge of chromosphere (jagged) 586 168. Young's observation of a pro- minence 588 169. The same 589 1 70. Ferrari's observation of a change of wave-length 596 171. Young's first observations of alterations of wave-length . . 598 1 72. His subsequent observations . 600 173. His subsequent observations . 600 174. Prominence seen by Dx D and D3 light 602 175. Diagrams showing magnetic storms 639 ERRATA. PAGF. 137 line 3 from bottom, for incidently, read incidentally. 155 ,, 7 ,, for 1812, read 1802. 265 ,,2 ,, for its, read our. 299 omit upper horizontal line from diagram. 376 line 2 from bottom, for radial of, read of radial. 411 ,, 12 ,, for phenomena, read phenomenon. 454 , , 6 from top, for solar, read polar. PART I. A POPULAR ACCOUNT OF ANCIENT AND MODERN INQUIRIES INTO THE PHYSICAL CONSTITUTION OF THE SUN, WITH SPECIAL REFERENCE TO RECENT SPECTROSCOPIC RESEARCHES. SOLAR PHYSICS. WHAT IS A SUN? AGES ago, when time was younger than it is now, and chap. i. human curiosity had a whole virgin universe to revel in, when the stars were unmapped and space had been pierced by no sounding-line, what wonder that man, all ignorant of better things, looked upon the Sun — -and wor- shipped it ? Although we know better than this now ; although instinct has given place to reason, superstition to science, and curiosity to inductive philosophy, the mystery of the sun still remains, and we present earth-dwellers wonder at its mighty power with even greater reason than the men of old. For, in fact, modern science, dealing with the whole field of the natural, and leaving the supernatural out of the question, as science must ever do, has almost justified, so to speak, the instinct of those who added adoration to won- derment. The power they felt, we have proved to exist ; the world-supporting, life-sustaining influence with which they invested the glorious orb of day, we now know B in SOLAR PHYSICS. chap. i. belongs to it by right ; man has measured its distance, has weighed it in the balance, and has glimpsed the forces Early ideas. at work on its surface ; and he everywhere finds a stupen- dousness which baffles him and far surpasses anything the imagination of the early inhabitants of this planet could have conceived ; and now that we know so much, rises as of old, towering above the mighty inquiries which have resulted in our present knowledge, the all-embracing question, " What is a Sun ? " Passing over the era of Sun-worship, which has left its mark in every land in which early man dwelt, let us endeavour to learn something as to .the notions concern- ing the sun which were rife when the Western civilization began to dawn. We find as answers to the question " What is a Sun ? " what we must designate as the wildest guesses, although some of them came from some of the greatest minds of antiquity. Thales Thales, one of the first of the Greek astronomers, whose faTz/sutf early life was spent in Egypt, where he deeply studied b.c. the lore of the priests, had ideas concerning the sun which, it will be seen, were in the main sound ; and it is curious that with such a good beginning subsequent philosophers went so far astray. He held that the sun and stars were of an earthy or solid substance, and that their light was due to fires fed by aqueous exhalations. He also explained correctly both solar and lunar eclipses, thereby implying that he thought that the moon shone by reflected light. The anecdote of Thales falling into a well,1 which is related by Plato, with the consequent saying of the Thra- cian female slave, that in trying to discover things in heaven he overlooked those beneath his feet, is widely known, and proves his popular reputation as an astronomer and star-gazer. 1 Plut. Plac. Phil. i. 3, as quoted by Sir G. C. Lewis in his admirable " Historical Survey of the Astronomy of the Ancients," p. 82, to which I am indebted for much information recorded in this chapter. WHAT IS A SUN? But when we come lower down the stream of time, to 580 B.C., and seek what was then the prevailing opinion, we find Anaximander at the head of a school which ex- plained the various solar phenomena by supposing that the sun and stars were bodies of condensed air, containing fire which escaped through certain apertures. He further imaged the sun as a wheel, the rays, forming the spokes, being emitted from the aforesaid apertures. His more de- tailed teachings in solar and lunar physics are thus stated by Sir G. C. Lewis ■} he held that the sun was "of circular form, with an opaque annular band on its exterior, the circumference of which is twenty-eight times that of the earth ; that within this annular band is a fiery central portion, equal in size to the earth ; that the movement of the sun is due to its opaque ring, and that an eclipse of the sun takes place when the central aperture is closed. His hypothesis regarding the moon was similar : he sup- posed the luminous centre to be seen through a tube, like the mouth of a bellows ; that the eclipses (or phases) are caused by the revolutions of the opaque ring Hence Anaximander [unlike Thales] held that the moon shone by her own light." Anaximander's notions, it will be seen, are not over- burdened with clearness ; and, indeed, about this time we get perhaps the greatest number of wild guesses. Zeno- phanes of Colophon taught that the sun was lit and ex- tinguished every day, like coals, accounting for eclipses by the latter process ; he told, moreover, of one eclipse which had lasted a month.2 The fault, however, found with Anaximander, cannot be laid at the door of the next suggestion to be recorded, which wTe owe to Heraclitus of Ephesus. This philosopher made himself noteworthy by the following ingenious ideas. He held that the sun and moon are bowl-shaped hemi- spherical cavities, with a bright side and a dark one. If the whole of the bright side of this hemispherical vessel 1 Op. cit. p. 93. 2 Op. cit. p. $8. B 2 CHAP. I. Anaxi- mander, born 610, died 547 B.C. Zenophaties of Colofhon flonris/ied 540 to 500 B.C. Heraclitus of Ephesns flourished about 504 B.C. SOLAR PHYSICS. chap. i. is turned towards us, we see the uneclipsed sun and full moon. If the whole of the dark part is turned to us, then we see the sun eclipsed, and the moon is then new ; and he further showed how, on this simple hypothesis, by imagining the bowls to be turned now a little one way and now a little the other, it was perfectly easy to account for the phases of the moon and the various kinds of eclipses of the sun.1 The sun, according to this philosopher, was a body of compressed fire fed by exhalations ; it was no bigger than a man's foot, and brighter than the moon in con- sequence of its position in a clearer atmosphere than that near the earth. Anaxa- We must next refer to Anaxagoras of Clazomense, who goras of was seconcj t0 none among the Greek philosophers in his mence, bom intense interest in cosmical speculations. He is stated to alout *o have sa'd that he was born for the contemplation of the b.c. sun, the moon, and the heavens;2 in fact his ideas on physical subjects were so advanced that he was accused of impiety. It is not a little curious that the charge of atheism which was preferred against him should not have been made before, for surely the charge of substituting mechanical force for the direct agency of the Gods would have applied equally to Anaximander or Heraclitus of Ephesus. And it is also curious that his doctrine that the sun was a mass of ignited stone larger than the Pelo- ponnesus, instead of a God who drove his chariot across the sky, was one of the main charges against him. It is consoling to think that Pericles saved his life at all events by his eloquent defence. Anaxi- Next, Anaximenes held that the sun, instead of being bonTl-75 a gl°ke> was Aat 1'ke a leaf; "that the stars, being im- b.c. pelled by the resistance of the condensed air, cause the solstitial movements of the sun ; that the nature of the moon is igneous, and therefore that she shines with her own light ; that the earth is a flat trapezium ; and that on 1 ( )p. (it. p. 96. - Op. cit. p. 103. WHAT IS A SLA7/ account of this form it is supported by the air without chat. i. sinking. He applied the same doctrine to explain the suspension of the sun and moon in space." 1 Empedocles, the contemporary of Herodotus, held that Empedocles r r . , , ... flourished there were two suns, one of fire in the lower hemisphere, yrom 4S5 and another, the upper one, "reflecting its own light ^444n.c from the fiery air upon the earth," as stated by Sir G. C. Lewis. It is but fair to remark that all these ideas were held to be so wild by some of the best men among the Greeks that they rebelled very much against them ; so that we find the whole genus of astronomical men soundly castigated by Socrates, who thought astronomy was desirable for de- termining the day of the month or hour of the night, but that to carry it further was waste of valuable time, and that "speculators on the universe and on the laws of the heavenly bodies were no better than madmen." 2 It is not till the time of Eudoxus of Cnidos, the con- temporary of Plato, that we find any sustained attempt at Plato, real investigation ; and so impressed was he of the extreme 4 }B ^% importance of the sun to the earth, that he is recorded by Plutarch 3 to have declared that he wrould willingly suffer the fate of Phaethon could he only approach the sun near enough to determine its real figure and constitution. By the time of Aristotle the right of free thought in Aristotle, cosmical matters had been thoroughly established, and we 3 4/"c322 find that philosopher defining astronomy as a science founded on observation and calculation, although the idea that the sun and stars are living bodies eminently par- taking of the divine nature occurs in his writings.4 From this time, throughout what we may term the pre- telescopic age, it would appear that the question of the physical constitution of the sun and stars gradually gave 1 Op. cit. p. 96. 2 Op. cit. p. 113 3 Quoted in "Astronomy of the Ancients," p. 148. 4 Op. cit. p. 163. SOLAR PHYSICS. chap. i. way to those which dealt with the various motions of each member of the starry host : " Cycle on epicycle, orb on orb," formed the key-note of a large part of astronomy after this time until we come, nearly two thousand years later, to Galileo and his contemporaries, when we again find ourselves in the midst of a sea of speculation on solar matters. Needless to say that this was no accidental circum- stance. The telescope had been discovered, and with it at the first essay the visible universe had been infinitely expanded. The period of the invention of the telescope is one of the most interesting and momentous in the scientific history of the world : in it the golden age of astronomy maybe said to have dawned. Kepler, We find then living on this planet three immortal men. Brake and First Kepler, who by his theory of elliptic motion at one Galileo, blow swept away the elaborate work of twenty centuries on cycles, epicycles, and excentric motions.1 Secondly, Tycho Brahe, whose admirable observations formed the groundwork of Kepler's investigations. Lastly, Galileo, who was at home in all branches of science ; a man of tremendous mind ; who independently, and as it were by the way, invented the telescope, and who was not only the first man who applied it to celestial objects, but perhaps the most illustrious man who ever did so. We all know his reward. Like Anaxagoras, he offended against current dogmas, and in Christendom or Heathen- dom the penalty for that always was, nay almost is, the same. According to Galileo's own story — and who shall doubt it ? — he began the telescope's work on the sun in the 1 The words with which he sent his discovery forth are too admi- rable not to be quoted in this place : — " I have finished my book. It will be read by the present age or a future one — I care little which. It can well wait for a reader, for has not God waited 6,000 years for a contemplator of His works?" WHAT IS A SUN ? month of October 1610,1 being followed by the Jesuit Scheiner in April 161 1, according to his own statement, and by Fabricius in June of the same year. It is pleasant to think that some of the earliest observa- tions of the sun were made by a countryman of our own, and in the Tower of London. I allude to the eminent mathematician Hariot, who observed the spots as early as December 161 1, though not as early as December 1610, as was once thought by De Zach.2 CHAP. I. Fig. 1. — Galileo. Needless again to say that the old-world answer to our question " What is a Sun ? " was at once changed. It was no longer immaculate ; no longer, as in the thought of Aris- 1 See on the whole question of disputed priority in this matter, " Galile'e, sa Vie, ses Decouvertes, et ses Travaux." Par le Dr. Max Parchappe. Hachette, 1866. P. 92 et seq. 2 Grant's "History of Physical Astronomy," p. 215. Hariot observes spots in 161 1. SOLAR PHYSICS. i. totle, a material image of the spotless purity of the divine mind. Galileo discovered spots on the brightly shining orb, and the thrill that ran through the world of Schoolmen as a result of this announcement may well be imagined ! Galileo's discoveries, thanks to the controversy which was raised by the Jesuit Scheiner, who wished to obtain all the credit for Galileo's work, have come down to us in full detail : for to defend himself, Galileo, in a remarkable series of letters to Welser, the chief magistrate of Augs- burg, gives us the whole history of his work, which here, however, can only be lightly touched upon. Galileos In his first letter, under date May 4, 161 2, he states that first letter, ^g Sp0t-S are real, as bright as the moon, and composed of matter not very dense and differently shaded ; that they are not permanent, but resemble our clouds ; and that it is absurd to think they are planets, which was Scheiner's opinion about this time. Second In his second letter, dated August 14 in the same year, letter. ]ie refers t0 their common movements, and to their limita- tion to zones ; states that they lie near the sun, and move on it and not above it ; and then announces, from obser- vations of them made near the sun's edge, first, that they are deep and of various depths, and secondly, that their blackness diminishes near the edge. Third In his third letter, written on the 10th of December, 161 2, letter. jie demonstrates the sun's rotation, and assigns a period not far from that given by modern observation. Scheiner, who wrote under the signature of Apelles latens post tabulam, to the same Welser, held on his side that the spots were similar to Jupiter's moons, or pro- bably to the strange things which Galileo had then re- cently discovered round Saturn, or that they might be comets. He held that it was impossible they could be on the sun itself, and imagined some to be as far from the sun as the Moon, Venus, or Mercury (on the Ptolemaic system). At the same time he pointed out that they are thin, to account for their oval appearance near the WHAT IS A SUN? limb, adding that they are not fixed stars, although they are as dense as the moon. It is not necessary in this brief sketch to do more than refer to the detailed work of Scheiner recorded in his " Rosa Ursina," or to the subsequent observations of Fabricius and Hevelius.1 It is clear that the answer to our question was now much closer ; indeed we have the spots and no longer the sun itself, due to evaporations and exhalations, the luminary itself being described as a " liquor igneus . . . quasi vastissimum luminum pelagus et mare igneus, q?iod suos Jiabet abyssos, occultos meatus vora- gines atque vortices :" Our next step carries us to 1774; for in De la Hire's memoir presented to the Paris Academy in 1704, there is nothing that calls for remark. Fig. 2. — Explanation of Sun-spots on Wilson's hypothesis — of a solid, dark sun and cloudy envelope. A and c, spots as seen near sun's centre and edge. (See text for explanation.) In 1774, Dr. Wilson of Glasgow communicated a paper Wilson to the Royal Society,2 demonstrating that the spots were work' 1 The reader may with profit consult Grant's " History of Physical Astronomy," p. 213 et seq. " Phil. Trans. 1744. SOLAR PHYSICS. chap. i. cavities in a luminous envelope surrounding the sun, which, ..... , according to him, was a dark globe. His observations were work. made on the great sun-spot of 1769. The reasoning on which he based his idea of the cavernous nature of the spots will easily be gathered from an inspection of the woodcut, Fig. 2 : it will be seen that it depends upon the different appearances put on by the same spot as seen in the centre of the disc and near the sun's limb. In the case of a spot, supposed round, seen in the first position, it is clear that we shall have a round, black shade in the centre, equally surrounded by a half-tone; while when the spot is near the limb it is equally clear that on this hypothesis the central black shade will be almost entirely hidden, and the half-tone quite hidden on one side and largely developed on the other. Now this is exactly what is observed. To this paper of Wilson's, which is now acknowledged to be one of the most important contributions to our Dela knowledge of the sun, in 1776 De la Lande replied in a Lande. paper presented to the Paris Academy. He does not agree with Wilson, although he gives up the idea which he had formerly held, that the spots were masses of scoria?, and then he adds: — "J'ai done pense que les taches etaient • plutot les eminences d'un noyau solide, decouvertes et re- couvertes alternativement par le flux et le reflux de la matiere ignee ou elles sont presque toujours plongees . .. 