" '' '''' 
 
 
 REESE LIBRARY 
 
 
 UNIVERSITY OF CALIFORNIA. 
 
 Received; . 
 Accessions Ni 
 
 Shelf No. __. 
 
. v 
 
 \. 
 x 
 
 ; 
 
 ** 
 
 - 
 
 . :.-y 
 
AURORA: 
 
 THEIR CHARACTERS AND SPECTRA. 
 
 BY 
 
 J. RAND CAPRON, F.R.A.S. 
 
 " And now the Northern Lights begin to bum, faintly at first, like sunbeams playing in the 
 waters of the blue sea. Then a soft crimson glow tinges the heavens. There is a blush on the 
 cheek of night. The colours come and go ; and change from crimson to gold, from gold to 
 crimson. The snow is stained with rosy light. Twofold from the zenith, east and west, 
 flames a fiery sword ; and a broad band passes athwart the heavens, like a summer sunset. Soft 
 purple clouds come sailing over the sky, and through their vapoury folds the winking stars shine 
 white as silver. With such pomp as this ia Merry Christmas ushered in, though only a single 
 star heralded the first Christmas." LONGFELLOW. 
 
 LONDON : 
 E. & F. N. SPON, 46 CHARING CROSS. 
 
 NEW YORK: 
 446 BROOM STREET. 
 
 1879. 
 
 " 
 
 
PRINTED BY TAYLOR AND FRANCIS, 
 BED LION COURT, FLEET STREET. 
 
 
TO 
 
 PROF. CHAELES PIAZZI SMYTH, F.R.S.E, 
 
 ASTBONOMEB ROYAL FOR SCOTLAND. 
 
 (INK OF THK EARLIEST SPECTROSCOPIC OBSERVERS 
 
 > OP 
 
 THE AURORA AND ZODIACAL LIGHT, 
 
 THIS VOLUME 
 
 18 
 
 RESPECTFULLY DEDICATED 
 
 
 
 BY 
 
 . 
 
 THE AUTHOR. 
 
PREFACE. 
 
 PROBABLY few of the phenomena of Nature so entirely charm and interest 
 scientific and non-scientific observers alike as the Aurora Borealis, or 
 " Northern Lights " as it is popularly called. Whether contemplated as 
 the long low quiescent arc of silver light illuminating the landscape with a 
 tender radiance, as broken clouds and columns of glowing ruddy light, or as 
 sheaves of golden rays, aptly compared by old writers to aerial spears, such a 
 spectacle cannot fail at all times to be a subject of admiration, in some cases 
 even of awe. 
 
 Hence it is no wonder that the Aurora has always received a considerable 
 amount of attention at the hands of scientific men. Early explorers of the 
 Arctic Regions made constant and important observations of it and its 
 character ; and the list of references to works given in the Appendix will 
 show how often it formed the subject of monographs and communications to 
 learned Societies. The early contributions seem relatively more numerous 
 than those of a later date ; and the substance of them will be found well 
 
vi PKEFACE. 
 
 summed up in Dr. Brewster's 'Edinburgh Encyclopaedia' (1830), article 
 "Aurora." A most complete and able epitome of our more recent experience 
 and knowledge of the Aurora and its spectrum has been contributed by my 
 friend Mr. Henry R. Procter to the present (9th) edition of the 'Encyclopaedia 
 Britannica,' article " Aurora Polaris." It is, however, a drawback to Ency- 
 clopaedic articles that their matter is of necessity condensed, and that they 
 rarely have the very desirable aid of drawings and engravings to illustrate 
 their subjects. In spite, therefore, of the exhaustive way, both as to fact and 
 theory, in which the contributor to the ' Encyclopaedia Britannica ' has realized 
 his task, it seemed to me there was still room left for a popular treatise, having 
 for its object the description of Aurorae, their characters and spectra. The 
 question of the Aurora spectrum seems the more worthy of extended dis- 
 cussion in that it still remains an unsolved problem. In spite of the obser- 
 
 o 
 
 vations and researches of Angstrom, Lemstrom, and Vogel abroad, and of 
 Piazzi Smyth, Herschel, Procter, Backhouse, and others at home, the goal is 
 not yet reached; for while the faint and more refrangible lines are but 
 doubtfully referred to air, the bright and sharp red and green lines, which 
 mainly characterize the spectrum, are as yet unassociated with any known 
 analogue. 
 
 With these views, and to incite to further and closer observations, I have 
 been induced to publish the present volume as a sort of Auroral Guide. For 
 much of the history of the Aurora I am indebted to, and quote from former 
 articles and records, including the two excellent Encyclopaedic ones before 
 referred to. Mr. Procter, Mr. Backhouse, and my friend Mr. W. H. Olley 
 have each kindly furnished me with much in the way of information and 
 suggestion. Dr. Schuster has lent me tubes showing the true oxygen 
 spectrum ; while Herr Carl Bock, the Norwegian naturalist, has enabled me 
 
PREFACE. vii 
 
 to reproduce a veritable curiosity, viz. a picture in oil painted by the light of 
 a Lapland Aurora. The experiments detailed in Part III. were suggested by 
 the earlier ones of De la Rive, Varley, and others, and demonstrate the effect 
 of the magnet on electric discharges. For assistance in these I am indebted 
 to my friend Mr. E. Dowlen. 
 
 The illustrations are mainly from original drawings of my own. Those 
 from other sources are acknowledged. Messrs. Mintern have well reproduced 
 in chromo-lithography the coloured drawings illustrating the Aurorse, moon- 
 patches, &c. 
 
TABLE OF CONTENTS. 
 
 PART I. 
 
 THE AURORA AND ITS CHARACTERS. 
 
 CHAPTER I. 
 
 The Aurora as known to the Ancients : pp. 1-5. 
 
 CHAPTER II. 
 
 Some general Descriptions of Aurora : pp. 6-15. 
 By Sir John Franklin : pp. 6, 7. 
 By Rev. James Farquharson : pp. 7, 8. 
 By Mons. Lottin (Lardner) : pp. 8-11. 
 By Lieut. Weyprecht (Payer) : pp. 11-14. 
 Article in ' Edinburgh Encyclopaedia : ' pp. 14, 15. 
 
 CHAPTER III. 
 
 \i Some specific Descriptions of Auroras : pp. 16-30. 
 Capt. Sabine's Auroras : p. 16. 
 
 Aurora seen at Sunderland, February 8th, 1817 : p. 16. 
 Dr. Hayes's Aurora, 6th January, 1861 : pp. 16, 17. 
 Prof. Lemstrom's Aurora, 1st September, 1868: pp. 17, 18. 
 Mr. J. R. Capron's Aurora, October 24th, 1870 : pp. 18, 19. 
 Mr. Barker's red and white Aurorae, 9th November (1870?) : p. 19. 
 Mr. J. R. Capron's Aurora, February 4th, 1872 : pp. 19, 20. 
 Cardiff, Aurora seen at: p. 21. 
 
 b 
 
x CONTENTS. 
 
 Mr. J. R. Capron's Aurora, February 4th, 1874 : pp. 21, 22. 
 
 Mr. Herbert Ingall's Aurora, July 18th, 1874: pp. 22, 23. 
 
 Mr. J. R. Capron's white Aurora, September llth, 1874: pp. 23, 24. 
 
 Dr. Allnatt's Aurora, June 9th, 1876 : p. 24. 
 
 Herr Carl Bock's Lapland Aurora, October 3rd, 1877 : p. 25. 
 
 Rev. T. W. Webb's Aurora : pp. 25, 26. 
 
 English Arctic Expedition, 1875-76 : p. 26. 
 
 Aurora Australis : pp. 2629. 
 
 Prof. Piazzi Smyth's typical Aurora : pp. 29, 30. 
 
 CHAPTER IV. 
 
 Phenomena simulating Auroras : pp. 31-32. 
 Auroric Lights (Kinahan) : pp. 31, 32. 
 Luminous Arch : p. 32. 
 
 CHAPTER V. 
 
 J Some qualities of the Aurora: pp. 33-51. 
 Noises attending Auroras : pp. 33, 34. 
 Colours of Aurora : pp. 35-37. 
 Height of Aurora : pp. 37-40. 
 Phosphorescence attending Aurora : pp. 41-44. 
 Aurora and Ozone : pp. 44, 45. 
 Polarization of Aurora light : pp. 45, 46. 
 Number of Auroras : p. 46. 
 Duration of Aurora : p. 47. 
 Travelling of Aurora (Donati) : pp. 47, 48. 
 
 Geographical distribution of Auroras (Fritz and Loomis) : pp. 48, 49. 
 Extent and principal zone of the Aurora : pp. 50, 51. 
 
 CHAPTER VI. 
 
 Aurora in connexion with other Phenomena : pp. 52-69. 
 Auroras and Clouds : pp. 52-54. 
 Aurora and Thunderstorms : pp. 54, 55. 
 Aurora and Magnetic Needle : pp. 55-58. 
 Auroras, Magnetic Disturbances, and Sun-spots : pp. 58-62. 
 Aurora and Electricity : pp. 62-64. 
 Aurora and Meteoric Dust : pp. 64, 65. 
 Aurora and Planets Venus and Jupiter : pp. 66, 67. 
 Aurora and Zodiacal Light : pp. 67-69. 
 
CONTENTS. x i 
 
 CHAPTER VII. 
 
 Aurora-like patches on the partially-eclipsed Moon : pp. 70-77. 
 
 CHAPTER VIII. 
 
 Aurora and Solar Corona : pp. 78-81. 
 
 CHAPTER IX. 
 
 Supposed Causes of the Aurora : pp. 82-87. 
 Prof. Lemstrom's Theory : pp. 87, 88. 
 Theories of MM. Becquerel and De la Rive : p. 88. 
 M. Plante's electric experiments : pp. 89, 90. 
 
 PART II. 
 
 THE SPECTRUM OF THE AURORA. 
 
 CHAPTER X. 
 
 Spectroscope adapted for the Aurora : pp. 91-93. 
 Spectrum of Aurora described : pp. 94-100. 
 Flickering of the green line : p. 100. 
 
 Mr. Backhouse's graphical spectra of four Aurora : p. 101. 
 Lord Lindsay's Aurora-spectrum, October 21, 1870 : p. 102. 
 Spectrum of the Aurora Australis : pp. 103, 104. 
 Professor Piazzi Smyth's Aurora-spectra : p. 104. 
 . Author's Catalogue of the Auroral lines : pp. 104-106. 
 Theories in relation to the Aurora and its spectrum : pp. 106, 107. 
 
 CHAPTER XI. 
 
 The comparison of some Tube and other Spectra with the Spectrum of the Aurora 
 
 pp. 108-120. 
 
 Hydrogen-tube: pp. J08-110. 
 Carbon- and oxygen-tubes : pp. 110-114. 
 Geissler mercury-tube and barometer mercurial vacuum : pp. 114, 115. 
 
xii CONTENTS. 
 
 Air-tubes : pp. 115, 116. 
 Phosphorescent tube : pp. 116, 117. 
 Spark in air: p. 117. 
 Spark over water : p. 117. 
 Phosphoretted-hydrogen flame: pp. 117, 118. 
 Iron-spectrum: p. 118. 
 Spectrum of mercury: p. 119. 
 Table of coincidences : p. 119. 
 
 CHAPTER XII. 
 
 J 
 Notes on Professor Angstrom's Theory of the Aurora- spectrum : pp. 121-127. 
 
 CHAPTER XIII. 
 
 \/The Oxygen-spectrum in relation to the Aurora (Procter and Schuster) : pp. 128-131. 
 
 PART III. 
 MAGNETO-ELECTRIC EXPERIMENTS IN CONNEXION WITH THE AURORA. 
 
 INTRODUCTION : pp. 133-135. 
 
 CHAPTER XIV. 
 
 Examination of Geissler-tubes under action of the Magnet : pp. 136-146. 
 Nitrogen-tubes : pp. 136, 137. 
 Oxygen-tubes: p. 138. 
 Hydrogen-tubes : pp. 138, 139. 
 Water-gas tube : p. 139. 
 Ammonia-tube : pp. 139, 140. 
 Carbonic-acid tube : p. 140. 
 Chlorine-tubes : pp. 140, 141 . 
 Iodine-tubes : pp. 141-143. 
 Bromine-tubes : pp. 143, 144. 
 Silicic fluoride-tubes : p. 144. 
 Sulphuric-acid tubes : pp. 144, 145. 
 Sulphur-tube : pp. 145, 146. 
 
CO 
 
 INTENTS. xiii 
 
 CHAPTER XV. 
 
 Effect of Magnet on a capillary Glass Tube : pp. 147, 148. 
 Action of magnet on a bar of heavy glass : pp. 147, 148. 
 
 CHAPTER XVI. 
 
 Effect of Magnet on wide Air (Aurora) tube : pp. 149-153. 
 Stratification (note on) : pp. 149, 150. 
 Effect of magnet on Pliicker (air-) tube : pp. 150-152. 
 Effect of magnet on Pliicker tube (tin chloride) : pp. 152, 153. 
 Effect of magnet on tin-chloride Geissler tube : p. 153. 
 
 CHAPTER XVII. 
 
 Effect of Magnet on bulbed Phosphorescent Tube : pp. 154-156. 
 
 Effect of magnet on small phosphorescent (powder) tubes : pp. 156, 157. 
 Lighting-up tubes with one wire only (Marquis of Salisbury's observations) : pp. 
 157, 158. 
 
 CHAPTER XVIII. 
 
 Action of the Magnet on the Electric Spark : pp. 159, 160. 
 
 CHAPTER XIX. 
 
 The Discharge in vacua in Larger Vessels, and Magnetic Effects thereon : pp. 161-165. 
 Some of Baron Reichenbach's magnetic researches tested : pp. 165, 166. 
 
 Summary of the foregoing Experiments and their Results : p. 167. 
 
 CHAPTER XX. 
 Some concluding Remarks: pp. 168-171. 
 
xiv CONTENTS. 
 
 APPENDICES. 
 
 A. References to some Works and Essays on the Aurora : pp. 173, 174. 
 
 B. Extracts from the Manual and Instructions for the (English) Arctic Expedition, 
 
 1875 : pp. 175-181. 
 
 C. Extracts from Parliamentary Blue-book, containing the " Results derived from the 
 
 Arctic Expedition, 1875-76 : " pp. 182-188. 
 
 D. Aurora and Ozone : pp. 189-193. 
 
 E. Dr. Vogel's Inquiries into the Spectrum of the Aurora : pp. 194-207. 
 
LIST OF PLATES. 
 
 Plate. 
 
 I. The Aurora during the Ice-pressure To face page 14 
 
 II. Aurora seen by Dr. Hayes, 6th January, 1861 ,,16 
 
 III. Aurora, Guildford, Oct. 24, 1870 18 
 
 IV. Aurora, Guildford, Feb. 4, 1872; Eclipsed Moou, Aug. 23, 
 
 24, 1877 % ,,20 
 
 V. Corona, Graphical Auroras, Zodiacal Light, &c ,,21 
 
 VI. Aurora, Guildford, Feb. 4, 1874 ; Spectrum des Nordlichts 
 
 (Vogel) a 22 
 
 VII. Aurora, Kyle Akin, Isle of Skye, Sept. 11, 1874 .... ,,24 
 
 VIII. Herr Carl Bock's Lapland Aurora, Oct. 3, 1877 .... ,,25 
 IX. Compared Aurora and other Spectra. Loomis's curves of 
 
 Auroras, Magnetic Declination, and Solar Spots ... ,,59 
 
 X. Spectroscope, Micrometer, Tubes ,,91 
 
 XI. Aurora-spectra, Candle-spectrum ,,102 
 
 XII. Aurora-spectrum, Solar spectrum, and Candle-spectrum . 104 
 
 XIII. V'ogcl's Aurora-lines, Aurora-lines near G, and in the red 
 
 and green ,,108 
 
 XIV. Aurora, Hydrocarbons, Oxygen ,,110 
 
 XV. Aurora and Air-tubes, &c ,,115 
 
 XVI. Aurora, Phosphoretted Hydrogen, Iron, &c 117 
 
 XVII. Effect of Magnet on Tubes and Spark . . ' , ,,134 
 
 \\III. Same, and Oxygen-spectrum ., 154 
 

 
 /?/> 
 
 ((UNIVERSITY 1 
 
 /^ * 
 
 SSJFC?^ 
 PABT I. 
 
 THE AURORA AND ITS CHARACTERS. 
 
 CHAPTER I. 
 
 THE AUEOEA AS KNOWN TO THE ANCIENTS. 
 
 IN Seneca's ' Qusestiones Naturales,' Lib. I. c. xiv., we find the following : 
 " Tempus est, alios quoque ignes percurrere, quorum diversae figurae sunt. 
 Aliquando emicat stella, aliquando ardores sunt, aliquando fixi et hserentes, 
 nonnunquam volubiles. Horum plura genera conspiciantur. Sunt Sothynoe 1 , 
 quum velut corona cingente introrsus igneus cceli recessus est similis effossae 
 in orbem speluncae. Sunt PitMtce 2 , quum magnitude vasti rotundique ignis 
 dolio similis, vel fertur vel in uno loco flagrat. Sunt Chasmata 3 , quum ali- 
 quod coeli spatium desedit, et flammam dehiscens, velut in abdito, ostentat. 
 Colores quoque omnium horum plurimi sunt. Quidam ruboris acerrimi, 
 quidam evanidse ac levis flammae, quidam Candidas lucis, quidam micantes, 
 quidam aequaliter et sine eruptiohibus aut radiis fulvi. 
 
 ************ 
 
 C. xv. " Inter hsec ponas licet et quod frequenter in historiis legimus, 
 ccclum ardere visum : cujus nonnunquam tarn sublimis ardor est ut inter 
 ipsa sidera videatur, nonnunquam tarn humilis ut speciem longinqui incendii 
 praebeat. 
 
 "Sub Tiberio Caesare cohortes in auxilium Ostiensis colonise cucurrerunt, 
 tanquam conflagrantis, quum coali ardor fuisset per magnam partem noctis, 
 parum lucidus crassi fumidique ignis." 
 
 B 
 
 Seneca's 
 ' Quaestiones 
 Naturales,' 
 Lib. I. c. xiv. 
 Description 
 of Aurora. 
 
 1 ftodvvos, 
 a hollow. 
 
 " iriOos, a 
 
 cask. 
 
 * X'"/ 10 ' a 
 chasm. 
 
 Seneca, 
 
 C. XV. 
 
THE AUEOEA AND ITS CHAEACTEES. 
 
 Translation. 
 
 Aurora) fre- 
 quently read 
 of in history. 
 
 A spurious 
 Aurora. 
 
 We may translate this : " It is time other fires also to describe, of which 
 there are diverse forms. 
 
 " Sometimes a star shines forth ; at times there are fire-glows, sometimes 
 fixed and persistent, sometimes flitting. Of these many sorts may be distin- 
 guished. There are Bothynoe, when, as within a surrounding corona, the 
 fiery recess of the sky is like to a cave dug out of space. There are Pithitse, 
 when the expanse of a vast and rounded fire similar to a tub (dolium) is either 
 carried about or glows in one spot. 
 
 " There are Chasmata, when a certain portion of the sky opens, and gaping 
 displays the flame as in a porch. The colours also of all these are many. 
 Certain are of the brightest red, some of a flitting and light flame-colour, 
 some of a white light, others shining, some steadily and yellow without 
 eruptions or rays. 
 
 ************ 
 
 " Amongst these we may notice, what we frequently read of in history, the 
 sky is seen to burn, the glow of which is occasionally so high that it may be 
 seen amongst the stars themselves, sometimes so near the Earth (humilis) that 
 it assumes the form of a distant fire. Under Tiberius Caesar the cohorts ran 
 together in aid of the colony of Ostia as if it were in flames, when the 
 glowing of the sky lasted through a great part of the night, shining dimly 
 like a vast and smoking fire." 
 
 From the above passages many striking particulars of the Aurora may be 
 gathered; and by the division of the forms of Aurora into classes it is 
 evident they were, at that period, the subject of frequent observation. 
 The expression " et quod frequenter in historiis legimus " shows, too, that the 
 phenomena of Auroral -displays were a matter of record and discussion with 
 the writers of the day. 
 
 Various forms of Aurora may be recognized in the passages from Chap. xiv. ; 
 while in those from Chap. xv. a careful distinction is drawn between the 
 Auroree seen in the zenith or the upper regions of the sky, and those seen on 
 the horizon or apparently (and no doubt in some cases actually) near the 
 Earth's surface. 
 
 The description of the cohorts running to the fire only to find it an Aurora, 
 calls to mind the many similar events happening in our own days. Not, 
 however, but that a mistake may sometimes occur in an opposite direction. 
 In the memoirs of Baron Stockmar an amusing anecdote is related of one 
 Herr von Radowitz, who was given to making the most of easily picked up 
 information. A friend of the Baron's went to an evening party near Frank- 
 
THE AUKORA AS KNOWN TO THE ANCIENTS. 3 
 
 fort, whore he expected to meet Herr von Radowitz. On his way he saw a 
 barn burning, stopped his carriage, assisted the people, and waited till the 
 flames were nearly extinguished. When he arrived at his friend's house he 
 found Herr von Radowitz, who had previously taken the party to the top of 
 the building to see an Aurora, dilating on terrestrial magnetism, electricity, 
 and so forth. Radowitz asked Stockmar's friend, " Have you seen the beautiful 
 Aurora Borealisl" He replied, " Certainly; I was there myself; it will soon 
 be over." An explanation followed as to the barn on fire : Radowitz was 
 silent some ten minutes, then took up his hat and quietly disappeared. 
 
 It is probable that many of the phantom combats which are recorded to 
 have appeared in forms of fire in the air on the evenings preceding great 
 battles might be traced to Aurora, invested with distinct characteristics by 
 the imagination of the beholders. Auroras are said to have appeared in the 
 shape of armies of horse and foot engaged in battle in the sky before the Aurora as 
 death of Julius Caesar, which they were supposed to foretell. For more than P ortents - 
 a year before the siege and destruction of Jerusalem by Titus Vespasian, the 
 Aurora was said to have been frequently visible in Palestine. 
 
 Josephus, in his 'Wars of the Jews' (Whiston's Translation, Book VI. 
 chap. v. sect. 3), in referring to the signs and wonders preceding the destruc- 
 tion of Jerusalem, speaks of a star or comet, and that a great light shone 
 round about the altar and the holy house, which light lasted for half an hour, 
 and that a few days after the feast of unleavened bread a certain prodigious 
 and incredible phenomenon appeared " for before sunsetting chariots and 
 troops of soldiers in their armour were seen running about among the clouds, 
 and surrounding of cities." (This, if an Aurora, must have been an instance 
 of a daylight one.) 
 
 We find in Book II. of Maccabees, chap. v. verses 1, 2, 3, 4 (B.C. about 
 176 years): 
 
 " 1. About this same time An tiochus prepared his second voyage into Egypt : 
 
 " 2. And then it happened that through all the city, for the space almost of 
 forty days, there were seen horsemen running in the air, in cloth of gold, and 
 armed with lances like a band of soldiers. 
 
 " 3. And troops of horsemen in array, encountering and running one against 
 another, with shaking of shields and multitude of pikes, and drawing of 
 swords and casting of darts, and glittering of golden ornaments and harness 
 of all sorts. 
 
 "4. Wherefore every man prayed that that apparition might turn to 
 good." 
 
 B2 
 
4 THE AUEOEA AND ITS CHAEACTEES. 
 
 Early de- In Aristotle's ' De Meteoris,' Lib. I. c. iv. and v., the Aurora is described 
 
 scriptaons of ag an a pp earance resembling flame mingled with smoke, and of a purple red 
 or blue colour. Pliny (Lib. II. c. xxvii.) speaks of a bloody appearance of 
 the heavens which seemed like a fire descending on the earth, seen in the 
 third year of the 107th Olympiad, and of a light seen in the nighttime 
 equal to the brightness of the day, in the Consulship of Caecilius and Papi- 
 rius (Lib. II. c. xxxiii,), both of which may be referred to Aurora?. 
 
 In the ' Annals of Philosophy,' vol. ix. p. 250, it is stated that the Aurora 
 among English writers is first described by Matthew of Westminster, who 
 relates that in A.D. 555 lances were seen in the air (" quasi species lan- 
 cearum in aere visse sunt a septentrionali usque ad occidentem "). 
 
 In the article in the 'Edinb. Encyc.' vol. iii. (1830), the Aurora (known to the 
 vulgar as " streamers " or " merry dancers ") is distinguished in two kinds 
 the " tranquil " and the " varying." Musschenbroek enumerates as forms : 
 trabs, " the beam," an oblong tract parallel to the horizon ; sagitta, " the 
 arrow ;" faces, "the torch;" capra saltans, "the dancing goat;" bothynoe, 
 " the cave," a luminous cloud having the appearance of a recess or hollow in 
 the heavens, surrounded by a corona ; pithice, " the tun," an Aurora re- 
 sembling a large luminous cask. The two sorts of Aurora? distinguished as 
 the " bothynoe " and "pithia? " are evidently taken from the passage in Seneca's 
 ' Qusestiones ' before quoted. In ' Liberti Fromondi Meteorologicorum ' 
 (London, 1656), Lib. II. cap. v. "De Meteoris supremse regionis aeris," 
 art. 1. De Capra, Trabe, Pyramide, &c., these and other fantastic forms 
 attributable to Aurora? are more fully -described. 
 
 In the article " Aurora Polaris," Encyc. Brit. edit, ix., we find noted that 
 from a curious passage in Sirr's ' Ceylon and the Cingalese,' vol. ii. p. 117, it 
 would seem that the Aurora, or something like it, is visible occasionally in 
 Ceylon, where the natives call it "Buddha Lights," and that in "many parts 
 of Ireland a scarlet Aurora is supposed to be a shower of blood. The earliest 
 mentioned Aurora (in Ireland) was in 688, in the 'Annals of Cloon-mac-noise,' 
 after a battle between Leinster and Munster, in which Foylcher O'Moyloyer 
 was slain. 
 
 In the article in the Edinb. Encyc. before referred to it is stated that it was 
 not much more than a century ago that the phenomenon had been noticed to 
 occur with frequency in our latitudes. 
 
 Dr. Halley had begun to despair of seeing one till the fine display of 
 1716. 
 
 The first account on record in an English work is said to be in a book 
 
THE AURORA AS KNOWN TO THE AXCIEXTS. 5 
 
 entitled ' A Description of Meteors by \V. F. D. D.' (reprinted, London, 1G54), 
 
 which speaks of " burning spears " being seen January 30, 1560. The next Auroras not 
 
 is recorded by Stow as occurring on October 7, 1564 ; and, according to Stow ^jj'. 1 "^ 1 * "' 
 
 and Camden, an Aurora was seen on two nights, 14th and 15th November, 
 
 1574. 
 
 Twice, again, an Aurora was seen in Brabant, 13th February and 28th 
 September, 1575. Cornelius Gemma compared these to spears, fortified 
 cities, and armies fighting in the air. Aur^l were seen in 1580 and 1581 
 in Wirtemberg, Germany. 
 
 Then we have no record till 1621, when an Aurora, described by Gassendi 
 in his ' Physics,' was seen all over France, September 2nd of that year. 
 
 In November 1623 another, described by Kepler, was seen all over 
 Germany. 
 
 From 1666 to 1716 no appearance is recorded in the ' Transactions of the 
 French Academy of Sciences;' but in 1707 one was seen in Ireland and at 
 Copenhagen; while in 1707 and 1708 the Aurora was seen five times. 
 
 The Aurora of 1716, occurring after an interval of eighty years, which <f 
 Dr. Halley describes, was very brilliant and extended over much country, 
 being seen from the west of Ireland to the confines of Russia and the east of 
 Poland, extending nearly 30 of longitude, and from about the 50th degree of 
 latitude, over almost all the north of Europe, and in all places exhibiting at 
 the same time appearances similar to those observed in London. An Aurora 
 observed in Bologna in 1723 was stated to be the first that had ever been 
 seen there ; and one recorded in the ' Berlin Miscellany ' for 1797 is called a 
 very unusual phenomenon. Nor did Aurone appear more frequent in the 
 Polar Regions at that time, for Cselius states that the oldest inhabitants of 
 Upsala considered the phenomenon as quite rare before 1716. Anderson, of 
 Hamburg, -writing about the same time, says that in Iceland the inhabitants 
 themselves were astonished at the frequent Aurorse then beginning to take 
 place ; while Torfaeus, the Icelander, who wrote in 1706, was old enough 
 to remember the time when the Aurora was an object of terror in his native 
 country. 
 
 According to M. Mairan, 1441 Aurora were observed between A.D. 583 
 and 1751, of which 972 were observed in the winter half-years and 469 
 during the summer half-years. In our next Chapter we propose to give some 
 general descriptions of Aurora? from comparatively early sources. 
 
THE AUEOEA AND ITS CHAEACTEES. 
 
 CHAPTER II. 
 
 Sir John 
 
 Franklin's 
 
 description. 
 
 Parts of 
 the Aurora : 
 beams, 
 flashes, and 
 arches. 
 
 Formation 
 of the 
 Aurora. 
 
 Arches of 
 the Aurora. 
 
 SOME GENEEAL DESCEIPTIONS OF ATJEOE^. 
 
 SIR JOHN FRANKLIN (' Narrative of a Journey to the Shores of the Polar Sea 
 in the years 1819, 1820, 1821, 1822 ') describes an Aurora in these terms: 
 
 " For the sake of perspicuity I shall describe the several parts of the 
 Aurora, which I term beams, flashes, and arches. 
 
 " The beams are little conical pencils of light, ranged in parallel lines, 
 with their pointed extremities towards the earth, generally in the direction 
 of the dipping-needle. 
 
 " The flashes seem to be scattered beams approaching nearer to the earth, 
 because they are similarly shaped and infinitely larger. I have called them 
 flashes, because their appearance is sudden and seldom continues long. 
 When the Aurora first becomes visible it is formed like a rainbow, the light 
 of which is faint, and the motion of the beams undistinguishable. It is 
 then in the horizon. As it approaches the zenith it resolves itself into 
 beams which, by a quick undulating motion, project themselves into wreaths, 
 afterwards fading away, and again and again brightening without any visible 
 expansion or contraction of matter. Numerous flashes attend in different 
 parts of the sky." 
 
 Sir John Franklin then points out that this mass would appear like an arch 
 to a person situated at the horizon by the rules of perspective, assuming its 
 parts to be equidistant from the earth ; and mentions a case when an Aurora, 
 which filled the sky at Cumberland House from the northern horizon to the 
 zenith with wreaths and flashes, assumed the shape of arches at some distance 
 to the southward. He then continues : " But the Aurora does not always 
 make its first appearance as an arch. It sometimes rises from a confused mass 
 of light in the east or west, and crosses the sky towards the opposite point, 
 exhibiting wreaths of beams or coronse boreales on its way. An arch also, 
 which is pale and uniform at the horizon, passes the zenith without displaying 
 any irregularity or additional brilliancy." Sir John Franklin then mentions 
 seeing three arches together, very near the northern horizon, one of which 
 exhibited beams and even colours, but the other two were faint and uniform. 
 (See example of a doubled arc Aurora observed at Kyle Akin, Skye, Plate 
 VII.) 
 
GENERAL DESCRIPTIONS OF AUROR.K. 
 
 He also mentions an arch visible to the southward exactly similar to one in 
 the north. It appeared in fifteen minutes, and he suggests it probably had 
 passed the zenith before sunset. The motion of the whole body of the 
 Aurora from the northward to the southward was at angles not more than 
 20 from the magnetic meridian. The centres of the arches were as often in 
 the magnetic as in the true meridian. A delicate electrometer, suspended 
 50 feet from the ground, was never perceptibly affected by the Aurora. 
 
 Sir John Franklin further remarks that the Aurora did not often appear 
 immediately after sunset, and that the absence of that luminary for some 
 hours was in general required for the production of a state of atmosphere 
 favourable to the generation of the Aurora. 
 
 On one occasion, however (March 8th, 1821), he observed it distinctly 
 previous to the disappearance of daylight ; and he subsequently states that 
 on four occasions the coruscations of the Aurora were seen very distinctly 
 before daylight had disappeared. 
 
 [In the article "Aurora Polaris," Encyc. Brit. edit, ix., the Transactions of 
 the Royal Irish Academy, 1788, are referred to, where Dr. Usher notices that 
 the Aurora makes the stars flutter in the telescope ; and that, having re- 
 marked this effect strongly one day at 11 A.M., he examined the sky, and saw 
 an Auroral corona with rays to the horizon. 
 
 Instances are by no means rare of the principal Aurora-line having been 
 seen in waning sunlight, and in anticipation of an Aurora which afterwards 
 appeared.] 
 
 The Rev. James Farquharson, from the observation of a number of Aurorse 
 in Aberdeenshire in 1823 ('Philosophical Transactions,' 1829), concluded: 
 that the Aurora follows an invariable order in its appearance and progress ; 
 that the streamers appear first in the north, forming an arch from east to 
 west, having its vertex at the line of the magnetic meridian (when this 
 arch is of low elevation it is of considerable breadth from north to south, 
 having the streamers placed crosswise in relation to its own line, and all 
 directed towards a point a little south of the zenith) ; that the arch moves 
 forward towards the south, contracting laterally as it approaches the zenith, 
 and increasing its intensity of light by the shortening of the streamers and the 
 gradual shifting of the angles which the streamers near the east and west 
 extremities of the arch make with its own line, till at length these streamers 
 become parallel to that line, and then the arch is seen in a narrow belt 3 or 
 4 only in breadth, stretching across the zenith at right angles to the magnetic 
 meridian ; that it still makes progress southwards, and after it has reached 
 
 Aurora does 
 uot often 
 appear until 
 some hours 
 after sunset. 
 
 Aurora- 
 seen in day- 
 light. 
 
 The Rev. 
 James Far- 
 quharson's 
 observa- 
 tions. 
 
 Auroral 
 arch. 
 
8 
 
 THE AUEOEA AND ITS CHAKACTEKS. 
 
 Passage 
 across the 
 zenith . 
 
 M. Lottiu's 
 observa- 
 tions. 
 
 Formation 
 of the 
 auroral 
 bow. 
 
 It ascends to 
 the zenith. 
 
 several degrees south of the zenith again enlarges its breadth by exhibiting 
 an order of appearances the reverse of that which attended its progress 
 towards the zenith from the north ; that the only conditions that can explain 
 and reconcile these appearances are that the streamers of the Aurora are 
 vertical, or nearly so, and form a deep fringe which stretches a great way from 
 east to west at right angles to the magnetic meridian, but which is of no great 
 thickness from north to south, and that the fringe moves southward, preserving 
 its direction at right angles to the magnetic meridian. 
 
 Dr. Lardner, in his ' Museum of Science and Art,' vol. x. p. 189 et seq., 
 alludes to a description of " this meteor " (sic) supplied by M. Lottin, an officer 
 of the French Navy, and a Member of the Scientific Commission to the North 
 Seas. Between September 1838 and April 1839, being the interval when 
 the sun was constantly below the horizon, this savant observed nearly 150 
 Auroras. During this period sixty-four were visible, besides many concealed 
 by a clouded sky, but the presence of which was indicated by the disturbances 
 they produced upon the magnetic needle. 
 
 The succession of appearances and changes presented by these " meteors " 
 is thus graphically described by M. Lottin : 
 
 " Between four and eight o'clock P.M. a light fog, rising to the altitude of 
 six degrees, became coloured on its upper edge, being fringed with the light 
 of the meteor rising behind it. This border, becoming gradually more 
 regular, took the form of an arch, of a pale yellow colour, the edges of which 
 were diffuse, the extremities resting on the horizon. This bow swelled slowly 
 upwards, its vertex being constantly on the magnetic meridian. Blackish 
 streaks divided regularly the luminous arc, and resolved it into a system of 
 rays. These rays were alternately extended and contracted, sometimes slowly, 
 sometimes instantaneously, sometimes they would dart out, increasing and 
 diminishing suddenly in splendour. The inferior parts, or the feet of the 
 rays, presented always the most vivid light, and formed an arc more or less 
 regular. The length of these rays was very various, but they all converged 
 to that point of the heavens indicated by the direction of the southern pole 
 of the dipping-needle. Sometimes they were prolonged to the point where 
 their directions intersected, and formed the summit of an enormous dome of 
 light. 
 
 " The bow then would continue to ascend toward the zenith. It would 
 suffer an undulatory motion in its light that is to say, that from one ex- 
 tremity to the other the brightness of the rays would increase successively in 
 intensity. This luminous current would appear several times in quick sue- 
 
GENERAL DESCRIPTIONS OF AURORA. 9 
 
 cession, and it would pass much more frequently from west to east than in 
 the opposite direction. Sometimes, but rarely, a retrograde motion would 
 take place immediately afterward ; and as soon as this wave of light had run 
 successively over all the rays of the Aurora from west to east, it would return 
 in the contrary direction to the point of its departure, producing such an 
 effect that it was impossible to say whether the rays themselves were actually 
 affected by a motion of translation in a direction nearly horizontal, or if this 
 more vivid light was transferred from ray to ray, the system of flays them- 
 selves suffering no change of position. The bow, thus presenting the ap- 
 pearance of an alternate motion in a direction nearly horizontal, had usually 
 the appearance of the undulations or folds of a ribbon or flag agitated by the 
 wind. Sometimes one, and sometimes both of its extremities would desert 
 the horizon, and then its folds would become more numerous and marked, 
 the bow would change its character and assume the form of a long sheet of 
 rays returning into itself, and consisting of several parts forming graceful 
 curves. The brightness of the rays would vary suddenly, sometimes sur- 
 passing in splendour stars of the first magnitude ; these rays would rapidly 
 dart out, and curves would be formed and developed like the folds of a 
 serpent ; then the rays would affect various colours, the base would be red, 
 the middle green, and the remainder would preserve its clear yellow hue. 
 Such was the arrangement which the colours always preserved. They were 
 of admirable transparency, the base exhibiting blood-red, and the green of 
 the middle being that of the pale emerald ; the brightness would diminish, 
 the colours disappear and all be extinguished, sometimes suddenly and some- 
 times by slow degrees. After this disappearance fragments of the bow 
 would be reproduced, would continue their upward movement and approach 
 the zenith ; the rays, by the effect of perspective, would be gradually 
 shortened ; the thickness of the arc, which presented then the appearance of 
 a large zone of parallel rays, would be extended; then the vertex of the 
 bow would reach the magnetic zenith, or the point to which the south pole Reaches the 
 of the dipping-needle is directed. At that moment the rays would be seen 
 in the direction of their feet. If they were coloured they would appear as 
 a large red band, through which the green tints of their superior parts could 
 be distinguished, and if the wave of light above mentioned passed along 
 them their feet would form a long sinuous undulating zone ; while throughout 
 all these changes the rays would never suffer any oscillation in the direction 
 of their axis, and would constantly preserve their mutual parallelisms. 
 
 "While these appearances are manifested new bows are formed, either 
 
 c 
 
10 
 
 THE AURORA AND ITS CHAEACTEES. 
 
 Multiple 
 
 bows. 
 
 Corona 
 formed. 
 
 Duration of 
 corona. 
 
 commencing in the same diffuse manner or with vivid and ready formed rays ; 
 they succeed each other, passing through nearly the same phases, and arrange 
 themselves at certain distances from each other. As many as nine have been 
 counted having their ends supported on the earth, and in their arrangement 
 resembling the short curtains suspended one behind the other over the scene 
 of a theatre, and intended to represent the sky. Sometimes the intervals 
 between these bows diminish, and two or more of them close upon each 
 other, forming one large zone traversing the heavens and disappearing towards 
 the south, becoming rapidly feeble after passing the zenith. But sometimes 
 also, when this zone extends over the summit of the firmament from east to 
 west, the mass of rays appear suddenly to come from the south, and to form, 
 with those from the north, the real boreal corona, all the rays of which 
 converge to the zenith. This appearance of a crown, therefore, is doubtless 
 the mere effect of perspective ; and an observer placed at the same instant at 
 a certain distance to the north or to the south would perceive only an arc. 
 
 " The total zone, measuring less in the direction north and south than in 
 the direction east and west, since it often leans upon the corona, would be 
 expected to have an elliptical form ; but that does not always happen : it has 
 been seen circular, the unequal rays not extending to a greater distance than 
 from eight to twelve degrees from the zenith, while at other times they reach 
 the horizon. 
 
 " Let it then be imagined that all these vivid rays of light issue forth 
 with splendour, subject to continual and sudden variations in their length 
 and brightness ; that these beautiful red and green tints colour them at in- 
 tervals ; that waves of light undulate over them ; that currents of light 
 succeed each other ; and in fine, that the vast firmament presents one immense 
 and magnificent dome of light, reposing on the snow-covered base supplied 
 by the ground, which itself serves as a dazzling frame for a sea calm and 
 black as a pitchy lake. And some idea, though an imperfect one, may be 
 obtained of the splendid spectacle which presents itself to him who witnesses 
 the Aurora from the Bay of Alten. 
 
 " The corona when it is formed only lasts for some minutes ; it sometimes 
 forms suddenly, without any previous bow. There are rarely more than two 
 on the same night, and many of the Auroras are attended with no crown 
 at all. 
 
 " The corona becomes gradually faint, the whole phenomenon being to the 
 south of the zenith, forming bows gradually paler and generally disappearing 
 before they reach the southern horizon. All this most commonly takes place 
 
GENERAL DESCRIPTIONS OF AURORA. 11 
 
 in the first half of the night, after which the Aurora appears to have lost its 
 intensity ; the pencils of rays, the bands, and the fragments of bows appear 
 and disappear at intervals. Then the rays become more and more diffused, 
 and ultimately merge into the vague and feeble light which is spread over 
 the heavens, grouped like little clouds, and designated by the name of 
 auroral plates (plaques aurorales). Their milky light frequently undergoes 
 striking changes in the brightness, like motions of dilatation and contraction, 
 which are propagated reciprocally between the centre and the circumference, 
 like those which are observed in marine animals called Medusae. The phe- 
 nomena become gradually more faint, and generally disappear altogether on Disappear- 
 the appearance of twilight. Sometimes, however, the Aurora continues 
 after the commencement of daybreak, when the light is so strong that a 
 printed book may be read. It then disappears, sometimes suddenly ; but it 
 often happens that, as the daylight augments, the Aurora becomes gradually 
 vague and undefined, takes a whitish colour, and is ultimately so mingled 
 with the cirro-stratus clouds that it is impossible to distinguish it from them." 
 
 Lieutenant Weyprecht has grandly described forms of Aurora in Payer's 
 ' New Lands within the Arctic Circle ' (vol. i. p. 328 et seq.) as follows : 
 
 " There in the south, low on the horizon, stands a faint arch of light. It Lieut. Wey- 
 looks as it were the upper limit of a dark segment of a circle ; but the stars, 
 which shine through it in undiminished brilliancy, convince us that the 
 darkness of the segment is a delusion produced by contrast. Gradually the 
 arch of light grows in intensity and rises to the zenith. It is perfectly 
 regular ; its two ends almost touch the horizon, and advance to the east and 
 west in proportion as the arch rises. No beams are to be discovered in it, 
 but the whole consists of an almost uniform light of a delicious tender colour. 
 It is transparent white with a shade of light green, not unlike the pale green 
 of a young plant which germinates in the dark. The light of the moon 
 appears yellow contrasted with this tender colour, so pleasing to the eye and 
 so indescribable in words, a colour which nature appears to have given only 
 to the Polar Regions by way of compensation. The arch is broad, thrice the 
 breadth, perhaps, of the rainbow, and its distinctly marked edges are strongly 
 denned on the profound darkness of the Arctic heavens. The stars shine 
 through it with undiminished brilliancy. The arch mounts higher and 
 higher. An air of repose seems spread over the whole phenomenon ; here 
 and there only a wave of light rolls slowly from one side to the other. It 
 begins to grow clear over the ice ; some of its groups are discernible. The 
 arch is still distant from the zenith, a second detaches itself from the dark 
 
 c2 
 
12 
 
 THE AUKOBA AND ITS CHAKACTEES. 
 
 Formation 
 of arches. 
 
 Band of 
 light ap- 
 pears. 
 
 Second band 
 and rays. 
 
 Corona 
 formed. 
 
 segment, and this is gradually succeeded by others. All now rise towards 
 the zenith ; the first passes beyond it, then sinks slowly towards the northern 
 horizon, and as it sinks loses its intensity. Arches of light are now stretched 
 over the whole heavens ; seven are apparent at the same time on the sky, 
 though of inferior intensity. The lower they sink towards the north the 
 paler they grow, till at last they utterly fade away. Often they all return 
 over the zenith, and become extinct just as they came. 
 
 " It is seldom, however, that an Aurora runs a course so calm and so 
 regular. The typical dark segment, which we see in treatises on the subject, 
 in most cases does not exist. A thin bank of clouds lies on the horizon. 
 The upper edge is illuminated ; out of it is developed a band of light, which 
 expands, increases in intensity of colour, and rises to the zenith. The colour 
 is the same as in the arch, but the intensity of the colour is stronger. The 
 colours of the band change in a never-ceasing play, but place and form 
 remain unaltered. The band is broad, and its intense pale green stands out 
 with wonderful beauty on the dark background. Now the band is twisted 
 into many convolutions, but the innermost folds are still to be seen distinctly 
 through the others. Waves of light continually undulate rapidly through 
 its whole extent, sometimes from right to left, sometimes from left to right. 
 Then, again, it rolls itself up in graceful folds. It seems almost as if breezes 
 high in the air played and sported with the broad flaming streamers, the 
 ends of which are lost far ofF on the horizon. The light grows in intensity, 
 the waves of light follow each other more rapidly, prismatic colours appear 
 on the upper and lower edge of the band, the brilliant white of the centre 
 is enclosed between narrow stripes of red and green. Out of one band have 
 now grown two. The upper continually approaches the zenith, rays begin 
 to shoot forth from it towards a point near the zenith to which the south 
 pole of the magnetic needle, freely suspended, points. 
 
 " The band has nearly reached it, and now begins a brilliant play of rays 
 lasting for a short time, the central point of which is the magnetic pole 
 a sign of the intimate connexion of the whole phenomenon with the mag- 
 netic forces of the earth. Round the magnetic pole short rays flash and flare 
 on all sides, prismatic colours are discernible on all their edges, longer and 
 shorter rays alternate with each other, waves of light roll round it as a centre. 
 What we see is the auroral corona, and it is almost always seen when a band 
 passes over the magnetic pole. This peculiar phenomenon lasts but a short 
 time. The band now lies on the northern side of the firmament, gradually 
 it sinks, and pales as it sinks ; it returns again to the south to change and 
 
GENERAL DESCRIPTIONS OF AUKOR/E. 
 
 13 
 
 Single-rayed 
 band. 
 
 play as before. So it goes on for hours, the Aurora incessantly changes 
 place, form, and intensity. It often entirely disappears for a short time, 
 only to appear again suddenly, without the observers clearly perceiving how 
 it came and where it went ; simply, it is there. 
 
 " But the band is often seen in a perfectly different form. Frequently it 
 consists of single rays, which, standing close together, point in an almost 
 parallel direction towards the magnetic pole. These become more intensely 
 bright with each successive wave of light ; hence each ray appears to flash 
 and dart continually, and their green and red edges dance up and down as 
 the waves of light run through them. Often, again, the rays extend through 
 the whole length of the band, and reach almost up to the magnetic polej 
 These are sharply marked, but lighter in colour than the band itself, and in 
 this particular form they are at some distance from each other. Their colour 
 is yellow, and it seems as if thousands of slender threads of gold were 
 stretched across the firmament. A glorious veil of transparent light is spread 
 over the starry heavens ; the threads of light with which this veil is woven 
 are distinctly marked on the dark background ; its lower border is a broad 
 intensely white band, edged with green and red, which twists and turns in 
 constant motion. A violet-coloured auroral vapour is often seen simul- 
 taneously on different parts of the sky. 
 
 " Or, again, there has been tempestuous weather, and it is now, let us 
 suppose, passing away. Below, on the ice, the wind has fallen; but the 
 clouds are still driving rapidly across the sky, so that in the upper regions its 
 force is not yet laid. Over the ice it becomes somewhat clear ; behind the 
 clouds appears an Aurora amid the darkness of the night. Stars twinkle 
 here and there ; through the opening of the clouds we see the dark firmament, 
 and the rays of the Aurora chasing one another towards the zenith. The 
 heavy clouds disperse, mist-like masses drive on before the wind. Fragments Fragments. 
 of the northern lights are strewn on every side : it seems as if the storm had 
 torn the Aurora bands to tatters, and was driving them hither and thither 
 across the sky. These threads change form and place with incredible 
 rapidity. Here is one ! lo, it is gone ! Scarcely has it vanished before it 
 appears again in another place. Through these fragments drive the waves of 
 light : one moment they are scarcely visible, in the next they shine with 
 intense brilliancy. But their light is no longer that glorious pale green ; it 
 is a dull yellow. It is often difficult to distinguish what is Aurora and what 
 is vapour ; the illuminated mists as they fly past are scarcely distinguishable 
 from the auroral vapour which comes and goes on every side. 
 
 Aurora in 
 stormy wea- 
 ther. 
 
14 
 
 THE AUEOEA AND ITS CHAEACTEES. 
 
 Bands. 
 
 Eays reach 
 the pole. 
 
 No noise. 
 
 Descriptions 
 of Aurorae 
 in high 
 Northern 
 latitudes. 
 
 In Siberia. 
 
 " But, again, another form. Bands of every possible form and intensity 
 have been driving over the heavens. It is now eight o'clock at night, the 
 hour of the greatest intensity of the northern lights. For a moment some 
 bundles of rays only are to be seen in the sky. In the south a faint, scarcely 
 visible band lies close to the horizon. All at once it rises rapidly, and spreads 
 east and west. The waves of light begin to dart and shoot, some rays mount 
 towards the zenith. For a short time it remains stationary, then suddenly 
 springs to life. The waves of light drive violently from east to west, the 
 edges assume a deep red and green colour, and dance up and down. The 
 rays shoot up more rapidly, they become shorter; all rise together and 
 approach nearer and nearer to the magnetic pole. It looks as if there were 
 a race among the rays, and that each aspired to reach the pole first. And 
 now the point is reached, and they shoot out on every side, to the north and 
 the south, to the east and the west. Do the rays shoot from above down- 
 wards, or from below upwards I Who can distinguish 1 From the centre 
 issues a sea of flames : is that sea red, white, or green ? Who can say "? It is 
 all three colours at the same moment ! The rays reach almost to the horizon : 
 the whole sky is in flames. Nature displays before us such an exhibition of 
 fireworks as transcends the powers of imagination to conceive. Involuntarily 
 we listen : such a spectacle must, we think, be accompanied with sound. 
 But unbroken stillness prevails; not the least sound strikes on the ear. 
 Once more it becomes clear over the ice, and the whole phenomenon has 
 disappeared with the same inconceivable rapidity with which it came, and 
 gloomy night has again stretched her dark veil over everything. This was the 
 Aurora of the coming storm the Aurora in its fullest splendour. No pencil 
 can draw it, no colours can paint it, and no words can describe it in all its 
 magnificence." 
 
 A reproduction of the woodcut in Payer's ' Austrian Arctic Voyages,' 
 illustrating some of the features of the above description, will be found on 
 Plate I. 
 
 In the ' Edinburgh Fjicyclopsedia,' article " Aurora," we find : 
 
 " In high Northern latitudes the Aurorae Boreales are singularly resplen- 
 dent, and even terrific. 
 
 " They frequently occupy the whole heavens, and, according to the testimony 
 of some, eclipse the splendour of stars, planets, and moon, and even of the 
 sun itself. 
 
 " In the south-eastern districts of Siberia, according to the description of 
 Gmelin, cited and translated by Dr. Blagden (Phil. Trans, vol. Ixxiv. p. 228), 
 
Mate I. 
 
 [To face pnf>.' I I | 
 
 THE AURORA DURING THE ICE-PRESSURE. 
 
 [Payer's 'Austrian Arctic Voi/ages.'] 
 
GENERAL DESCRIPTIONS OF AURORA. 
 
 15 
 
 the Aurora is described to begin with single bright pillars, rising in the 
 north, and almost at the same time in the north-east, which, gradually 
 increasing, comprehend a large space of the heavens, rush about from place 
 to place with incredible velocity, and finally almost cover the whole sky up 
 to the zenith, and produce an appearance as if a vast tent were expanded in 
 the heavens, glittering with gold, rubies, and sapphires. A more beautiful 
 spectacle cannot be painted ; but whoever should see such a northern light 
 for the first time could not behold it without terror." 
 
 Maupertius describes a remarkable Aurora he saw at Oswer-Zornea on the 
 18th December, 1876. An extensive region of the heavens towards the 
 south appeared tinged of so lively a red that the whole constellation of Orion 
 seemed as if dyed in blood. The light was for some time fixed, but soon 
 ' became movable, and, after having successively assumed all the tints of violet 
 and blue, it formed a dome of which the summit nearly approached the 
 zenith in the south-west. 
 
 Maupertius adds that he observed only two of the red northern lights in 
 Lapland, and that they are of very rare occurrence in that country. 
 
 The observations of Carl Bock, the Norwegian naturalist, kindly com- 
 municated by him to me, and detailed in Chapter III., quite confirm this 
 observation of Maupertius as to the rare occurrence of red Aurorse in 
 Lapland, he having only seen one. 
 
 Maupertius's 
 description 
 ut Oswer- 
 Zornea. 
 
 Red Auroras 
 rare in Laj>- 
 land. 
 
 (UNIVERSITY 
 
 - 
 
16 
 
 THE ATJKOKA AND ITS CHAEACTEES. 
 
 Captain 
 Sabine's 
 Aurora. 
 
 Curvature of 
 arches to- 
 wards each 
 other. 
 
 Aurora seen 
 at Sunder- 
 land, Feb. 8, 
 1817. 
 
 Formation of 
 corona. 
 
 Dr. Hayes's 
 Aurora, 6th 
 January, 
 1861. 
 
 CHAPTER III. 
 
 SOME SPECIFIC DESCRIPTIONS OF AUEOE.E, INCLUDING EESULTS OF 
 THE ENGLISH AECTIC EXPEDITION, 1875-76. 
 
 Captain Sabine's Aurora. 
 
 CAPTAIN SABINE describes Aurora seen at Melville Island (Parry's first 
 voyage, January 15). Towards the southern horizon an ordinary Aurora 
 appeared. The luminous arch broke into masses streaming in different 
 directions, always to the east of the zenith. 
 
 The various masses seemed to arrange themselves in two arches, one 
 passing near the zenith and a second midway between the zenith and the 
 horizon, both north and south, but curving towards each other. At one 
 time a part of the arch near the zenith was bent into convolutions like a 
 snake in motion and undulating rapidly. 
 
 Aurora seen at Sunderland, February 8th, 1817. 
 ('Annals of Philosophy,' vol. ix. p. 250.) 
 
 It began about 7 P.M. during a strong gale from the N.W., with single 
 bright streamers in the N, and N.W., which covered a large space and rushed 
 about from place to place with amazing velocity, and had a fine tremulous 
 motion, illuminating the hemisphere as much as the moon does eight or nine 
 days from change. About 11 o'clock part of the streamers appeared as if 
 projected south of the zenith and looked like the pillars of an immense 
 amphitheatre, presenting a most brilliant spectacle and seeming to be in a 
 lower region of the atmosphere, and to descend and ascend in the air for 
 several minutes. (This appears to have been the formation of a corona.) 
 One streamer passed over Orion, but neither increased nor diminished its 
 splendour. 
 
 Description of Aurora by Dr. Hayes, Qth January, 1861. 
 
 ' Recent Polar Voyages ' contains a narrative of the voyage of Dr. Hayes, 
 who sailed from Boston on the 6th of July, 1860, and wintered at Port 
 Foulbe. He witnessed a remarkable display of the Aurora Borealis on the 
 morning of the 6th January, 1861. 
 
Plate 11. 
 [To face page 16.] 
 
 AURORA SEEN BY Dr. HAYES, 6th JANUARY, 1861. 
 
 ['Resent Poltir Voyages.'] 
 
JIT 
 
 
SPECIFIC DESCRIPTIONS OF AURORJE. 
 
 17 
 
 The darkness was so profound as to be oppressive. Suddenly, from the 
 rear of the black cloud which obscured the horizon, flashed a bright ray. 
 Presently an arch of many colours fixed itself across the sky, and the Aurora 
 gradually developed. 
 
 The space within the arch was filled by the black cloud ; but its borders 
 brightened steadily, though the rays discharged from it were exceeding 
 capricious, now glaring like a vast conflagration, now beaming like the glow 
 of a summer morn. More and more intense grew the light, until, from 
 irregular bursts, it matured into an almost uniform sheet of radiance. 
 Towards the end of the display its character changed. Lurid fires flung 
 their awful portents across the sky, before which the stars seemed to recede 
 and grow pale. 
 
 The colour of the light was chiefly red ; but every tint had its turn, and 
 sometimes two or three were mingled ; blue and yellow streamers shot across 
 the terrible glare, or, starting side by side from the wide expanse of the 
 radiant arch, melted into each other, and flung a strange shade of emerald 
 over the illuminated landscape. Again this green subdues and overcomes 
 the red ; then azure and orange blend in rapid flight, subtle rays of violet 
 pierce through a broad flash of yellow, and the combined streams issue in 
 innumerable tongues of white flame, which mount towards the zenith. 
 
 The illustration which accompanies this description in the work is repro- 
 duced on Plate II., and forcibly reminds one of the " curtains " of the 
 Aurora described in the preceding Chapter by Mons. Lottin. 
 
 Prof. Lemstrom' s Aurora of 1st September, 1868. 
 
 In the first Swedish Expedition, 1868, some remarkable observations were 
 made on the appearance of luminous beams around the tops of mountains, 
 which M. Lemstrom showed by the spectroscope to be of the same nature as 
 Aurora. 
 
 On the 1st September, 1868, on the Isle of Amsterdam in the Bay of 
 Sweerenberg, there was a light fall of snow, and the snowflakes were observed 
 falling obliquely. All at once there appeared a luminous phenomenon 
 which, starting from the earth's surface, shot up vertically, cutting the 
 direction of the falling snowflakes, and this appearance lasted for some 
 seconds. On examination with a spectroscope the yellow-green line was 
 found by Lemstrom (but of feeble intensity) when the slit of the instrument 
 was directed towards a roof or other object covered with snow, and even in 
 the snow all round the observer. 
 
 Develop- 
 ment of 
 Aurora. 
 
 Rays 
 
 changed to 
 glow. 
 
 Mixed 
 colours. 
 
 Colours 
 change. 
 
 Tongues of 
 white flame 
 formed. 
 
 Prof. Lem- 
 strom's 
 Aurora, 1st 
 September, 
 1868. 
 
 Aurora from 
 earth's sur- 
 face. 
 
 Yellow- 
 green line 
 seen. 
 
18 
 
 THE AUEOEA AND ITS CHAEACTERS. 
 
 /" Lemstrom's 
 conclusions. 
 
 Mr. J. E. 
 
 Capron's 
 Aurora, Oct. 
 24, 1870. 
 
 Silver glow 
 in north. 
 
 Phosphores- 
 cent clbud- 
 streamers. 
 
 Crimson 
 masses on 
 horizon. 
 
 Coloured 
 streamers. 
 
 Corona 
 formed. 
 
 Aurora fades 
 away. 
 
 M. Lemstrom concluded that an electric discharge of an auroral nature, 
 which could only be detected by means of the spectroscope, was taking place 
 on the surface of the ground all around him, and that, from a distance, it 
 would appear as a faint display of Aurora. 
 
 [It should, however, here be noted that the reflection of an Aurora from a 
 white or bright surface would give, in a fainter degree, the spectrum of the 
 Aurora itself; and, apart from the phenomena seen by the eye, the case 
 fails to be conclusive that an Aurora on the surface of the ground was 
 examined.] 
 
 Mr. J. E. Capron's Aurora of October 2ith, 1870. 
 
 The description, from my notes made at the time of this fine display, is as 
 follows : " Last evening (October 24) the Aurora Borealis was again most 
 beautifully seen here (Guildford). At 6 P.M. indications of the coming 
 display were visible in the shape of a bright silver glow in the north, which 
 contrasted strongly with the opposite dark horizon. For two hours this 
 continued, with the addition from time to time of a crimson glow in the 
 north-east, and of streamers of opaque-white phosphorescent cloud, shaped 
 like horse-tails (very different from the more common transparent auroral 
 diverging streams of light), which floated upwards and across the sky from 
 east and west to the zenith. At about 8 o'clock the display culminated ; and 
 few observers, I should think, ever saw a more lovely sky-picture. Two 
 patches of intense crimson light about this time massed themselves on the 
 north-east and north-west horizon, the sky between having a bright silver 
 glow. The crimson masses became more attenuated as they mounted up- 
 wards ; and from them there suddenly ran up bars or streamers of crimson 
 and gold light, which, as they rose, curved towards each other in the north, 
 and, ultimately meeting, formed a glorious arch of coloured light, having at 
 its apex an oval white luminous corona or cloud of similar character to the 
 phosphorescent clouds previously described, but brighter. At this time the 
 spectator appeared to be looking at the one side of a cage composed of 
 glowing red and gold bars, which extended from the distant parts of the 
 horizon to a point over his head. Shortly after this the Auroral display 
 gradually faded away, and at 9 o'clock the sky was of its usual appearance, 
 except that the ordinary tint seemed to have more of indigo, probably by 
 contrast with the marvellous colours which had so lately shone upon it." 
 
 T. F., describing the same Aurora from Torquay, says it showed itself at 
 sundown, attained its maximum at 8, and lasted until 11. At 8 o'clock more 
 

 / , 
 
SPECIFIC DESCRIPTIONS OF AUKO1LE. 
 
 19 
 
 than half the visible heavens was one sea of colour ; the general ground 
 gnvnish yellow and pale rose, with extensive shoals of deep rose in the east 
 and west; while from the north, streaming upwards to and beyond the 
 zenith, were tongues and brushes of rosy red, so deep that the sky between 
 looked black. Mr. Gibbs reported that in London, at about 8 o'clock, bril- 
 liant crimson rays shot up to the zenith, and the sky seemed one mass of fire. 
 A facsimile of my water-colour sketch of this fine discharge is given on 
 I'kto III. 
 
 T. F.'s de- 
 scription of 
 same at 
 Torquay. 
 
 Mr. Gibbs's 
 report in 
 London. 
 
 Mr. Barker's (superposed) red and white Aurora?, 9th November (1870?). 
 
 On the 9th November (1870 \) Mr. Barker saw at New Haven (U. S.) a most 
 magnificent crimson Aurora. At about a quarter to 6 P.M. it consisted of a 
 brilliant streamer shooting up from the north-western horizon. This was 
 continued in a brilliant red, but rather nebulous, mass of light passing 
 upwards and to the north. Its highest points were from 30 to 40 in 
 altitude. A white Aurora, consisting of bright streamers, appeared simul- 
 taneously and extended round to the north-east. Prof. Newton informed 
 Mr. Barker that he had observed an equally brilliant red patch of auroral 
 light in the north-east five or ten minutes earlier. 
 
 Since the lower end of the red streamers was much lower than that of the 
 white, it would seem as if the red were seen through the white, the red being 
 most remote. 
 
 Spectroscopic observations of this Aurora were made. The crimson Aurora 
 lasted less than half an hour, and then disappeared. In the white Aurora, 
 which remained, the crimson line could not be seen. 
 
 It may be here noted that during the Aurora seen by Carl Bock in Lapland, 
 and painted by him by its own light (described, p. 25), he had the impression 
 of sets of vibrating rays behind each other, and in the drawing it looks as if 
 streamers were seen behind an arc. 
 
 Mr. Barker's 
 Auroras, 9th 
 November 
 (1870 ?). 
 
 Bed Aurora. 
 
 White 
 Aurora. 
 
 Red seen 
 
 through 
 
 white. 
 
 Crimson line 
 not seen in 
 white 
 Aurora. 
 
 Carl Bock's 
 
 vibrating 
 
 rays. 
 
 Mr. J. R. Caproris Aurora of February ith, 1872. 
 
 My description of this Aurora as seen at Guildford, and as given at the time, 
 is as follows : " Last evening, returning from church a little before 8 P.M., 
 the sky presented a weird and unusual aspect, which at once struck the eye. 
 A lurid tinge upon the clouds which hung around suggested the reflection of 
 a distant fire ; while scattered among these, torn and broken masses of vapour, 
 having a white and phosphorescent appearance, and quickly changing their 
 
 D2 
 
 Mr. J. K. 
 Capron's 
 Aurora, 
 Feb. 4, 1872. 
 
 Masses of 
 phospho- 
 rescent 
 vapour. 
 
20 
 
 THE AUKOKA AND ITS CHAEACTEES. 
 
 Eose tints 
 appeared. 
 
 Aurora 
 from behind 
 clouds. 
 
 Formation 
 of corona. 
 
 Duration of 
 corona. 
 
 Streamers 
 from corona. 
 
 Eain during 
 Aurora. 
 
 Wind during 
 night. 
 
 forms, reminded me of a similar appearance preceding the great Aurora of 
 24th October, 1870. Shortly some of these shining white clouds or vapours 
 partly arranged themselves in columns from east to west, and at the same 
 time appeared the characteristic patches of rose-coloured light which are 
 often seen in an auroral display. About 8 o'clock the clouds had to a certain 
 extent broken away, and the Aurora shone out from behind heavy banks of 
 vapour, which still rested on the eastern horizon, the north-west horizon 
 being free from cloud and glowing brightly with red light. And now, at 
 about 8.15, was presented a most beautiful phenomenon. While looking 
 upwards, I saw a corona or stellar-shaped mass of white light form in the 
 clear blue sky immediately above my head *, not by small clouds or rays 
 collecting, but more in the way that a cloud suddenly forms by condensation 
 in the clear sky on a mountain top, or a crystal shoots in a transparent 
 liquid, having too, as I thought, an almost traceable nucleus or centre, from 
 which spear-like rays projected. From this corona in a few seconds shot 
 forth diverging streamers of golden light, which descended to and mingled 
 with the rosy patches of the Aurora hanging about the horizon. The spaces 
 of sky between the streamers were of a deep purple (probably an effect of 
 contrast). The display of the -corona, though lasting a few minutes only, 
 was equal to, if not excelling in beauty, the grand display of October 1870, 
 before described, in which case, however, a ring or disk of white light of con- 
 siderable size took the place of the stellar-shaped corona. What struck me 
 particularly was the corona developing itself as from a centre in the clear 
 sky, and the diverging streamers apparently shooting downwards, whereas in 
 general the streamers are seen to shoot up from the horizon and converge 
 overhead. The effect may have been an illusion ; but, if so, it was a remark- 
 able one. The general Aurora lasted for some time, till it was lost in a 
 clouded sky ; and, in fact, rain was descending at one time while the Aurora 
 was quite bright. Strong wind prevailed during the night f . The Aurora 
 was probably very extensive, as the evening, notwithstanding the clouds, was 
 nearly as bright as moonlight. The peculiar clouds referred to must have 
 
 * M. Lemstrom (Swedish Expedition, 1868) concludes that the corona of the Aurora Borealis 
 is not entirely a phenomenon of perspective, but that the rays have a true curvature, that they 
 are currents flowing in the same direction and attract each other. There is also an account 
 [ante, p. 16] of an Aurora at Melville Island (Parry's first voyage), in which two arches were 
 seen curving towards each other. 
 
 t A brilliant display in December 1870, on the east coast of Sicily, was followed by very 
 violent storms, with the overflow of the Tiber and the flooding of Eome. 
 
Plate IV. 
 
 Fi g. 1. 
 
 AURORA. GUILDFORD. FEB. 4-. 1872. 
 
 FROM A WATER COLOUR DRAWING. 
 
 Fig. 3. 
 
 ECLIPSED MOON. GUILDFORD. AUG. 23. 24. 1877. 
 
 FROM WATE.R COLOUR DRAWINGS. 
 
xgp 
 
 ,IT 
 .^ 
 

Plate V. 
 
 CORONA. GRAPHICAL AURORA. ZODIACAL LIGHT &c. 
 
 PLUCKERS TUBE: BETWEEN. 
 POLES OF MAGNET. 
 
 Fig: 2. 
 CORONA AND RAYS. 
 
 Pig: 4. 
 M BACKHOUSE. GRAPHICAL AUROR/E 
 
 I 2 3 
 
 NUMBER OF AURORAS IN EACH 
 MONTH OF THE YEAR (KXEMTZ.) 
 
 RED 
 
 RED 
 
 Pig. 6. 
 Oct. 24- 1S7O. CAPRON. 
 
 
 PIAZZI SMYTH & ANGSTROM. 
 
 VIOLET 
 
 VIOLET 
 
 MinternBros li 
 
SPECIFIC DESCRIPTIONS OF AUROB.E. 
 
 21 
 
 preceded the Aurora in daylight, as I recollect seeing them at 6.30 as we 
 went to church." 
 
 They had even then a peculiarly wild, ragged, and phosphorescent appear- 
 ance, and so much resembled some I had seen to accompany the Aurora 
 of October 1870, that I predicted (as came to pass) a display later in the 
 evening. A facsimile of my water-colour sketch of this Aurora is given on 
 Plate IV. fig. 1, while the corona and rays are represented (with rather too 
 hard an outline) on Plate V. fig. 2. 
 
 Phospho- 
 rescent 
 clouds pre- 
 ceded tin- 
 Aurora in 
 daylight. 
 
 Aurora 
 predicted. 
 
 Description of an Aurora seen at Cardiff. 
 
 An Aurora was seen at Cardiff. A dusky red aspect of the sky towards the 
 north, and extending itself across the zenith westward, made its appearance 
 about half-past 5 P.M. The lights reached their greatest intensity at 6 o'clock, 
 when the sky was suffused with a rich crimson glow, a broad band of colour 
 reaching from N.E. to W. A corona of deep hue, having rugged sharply 
 defined edges, stood out prominently in the zenith, apparently on a parallel 
 plane to the earth, and having its centre almost immediately over the head of 
 the spectator. 
 
 From this corona, elliptic in form, and in its broadest diameter about four 
 times the size of the moon, there were thrown out brilliant silvery blue radii, 
 extending to the N.E. and W. horizon, and presenting the appearance of a 
 vast cupola of fire. 
 
 At half-past 6 the lights died completely out, leaving masses of cloud 
 drifting up from the south, and a shower of rain fell. The corona was 
 remarked upon as unusual. At Edinburgh the sky was brilliant for several 
 hours. (The date of this Aurora is uncertain, as the account is from an 
 undated newspaper cutting. It is supposed to be in February 1872, but 
 could hardly have been on the 4th, as the Aurora of that date did not reach 
 its maximum development at Edinburgh till 8 P.M.) 
 
 Aurora seen 
 at Cardiff. 
 
 Formation 
 of corona. 
 
 Ksidii 
 
 thrown out 
 from corona. 
 
 Kain fell 
 when Aurora 
 died out. 
 
 Mr. J. E. Caprons Aurora, seen at Guildoivn, Guildford, 
 
 February tth, 1874. 
 
 About 7 P.M. my attention was drawn to a silvery brightness in the north- 
 east. Above, and still more to the east, was a bright cloud of light, which 
 looked dense and misty, and gave one the impression of an illuminated fog- 
 cloud. The edges were so bright that the adjacent sky, but for the stars 
 shining in it, might, by contrast, have been taken for a dark storm-cloud. 
 
 Silvery 
 brightness 
 in N.E. 
 
22 
 
 THE ATJKOKA AND ITS CHARACTERS. 
 
 Light-cloud, 
 which moved 
 from E. to 
 W. 
 
 Formation 
 of arc in N. 
 
 Streamers. 
 
 Horizontal 
 clouds of 
 misty light. 
 
 Spectrum of 
 the Aurora 
 described. 
 
 The light-cloud expanded upwards until its apex became conical, and then 
 moved rapidly from east, along the northern horizon, until it reached the 
 due west, where it rested, and formed for some time a luminous spot in 
 the sky. About the same time a long low arch of light formed along the 
 northern horizon, having a brighter patch at each extremity ; and these being 
 higher in the sky, the arch and turned-up ends were in shape like a Tartar 
 bow. This bow was permanent ; and later on a cloud of rose-coloured light 
 formed in the east, looking like the reflection of a distant fire. From the 
 bow also shot up curved streamers of silver light towards the zenith, which 
 at one time threatened to form a corona. This, however, did not happen, 
 and the Aurora gradually faded away, until, when the moon rose about 8, 
 a silver tinge in the east alone remained. I should also mention that fleecy 
 horizontal clouds of misty light floated in the north above the bow across the 
 streamers. 
 
 Mr. H. Taylor informed me he saw a similar Aurora some three weeks 
 before, in which the bright horizontal light and short white streamers were 
 the main characters. I am not sure that the horizontal light-clouds were not 
 actual mist-clouds illuminated by reflection of the Aurora ; not so, however, 
 I think, the first-mentioned cloud, which had more the appearance of the 
 aura in the large end of an illuminated Geissler tube. 
 
 I examined the Aurora with a Browning direct-vision spectroscope, and 
 
 o 
 
 found Angstrom's line quite bright, and by the side of it three faint and misty 
 bands towards the blue end of the spectrum upon a faintly illuminated 
 ground. I could also see at times a bright line beyond the bands towards the 
 violet. There was not light enough to take any measurements of position of 
 the lines. 
 
 I made a pencil sketch of this Aurora, at the time when the light-cloud 
 had moved W. and the arc formed, and of the spectrum. These drawings are 
 reproduced on Plate VI. figs. 1 and la. 
 
 Mr. Herbert 
 
 In gall's 
 Aurora, July 
 
 18, 1874. 
 
 Haze canopy 
 formed. 
 
 Mr. Herbert Ingall's Aurora, July I8th, 1874. 
 
 An Aurora of July 18th, 1874, seen by Mr. Herbert Ingall at Champion 
 Hill, S.E., was described by him as an extraordinary one. About 11 the 
 sky was clear ; at midnight the sky was covered by a sort of haze canopy, 
 sometimes quite obscuring the stars, and then suddenly fading away. Mr. 
 Ingall was shortly after remarking the sky in the S.E. and S. horizon as being 
 more luminous than usual, when his attention was drawn to a growing 
 
Plate. VI. 
 
 Fig.l. 
 
 AURORA, GUILDFORD. FEB. 4. 1874. 
 
 FAC-SIMILE. OF PENCIL SKETCH. 
 
 F i 4 1* 
 
 n 
 
 rltw NorvLUchts. (Vogel.j 
 Fig. 2. 
 
 MintrnBroblh. 
 

 [UITIVTSKSITY 
 
SPKCIFIC DESCRIPTIONS OF AUBOR.E. 
 
 23 
 
 brightness in the S.W., and a moment afterwards bright bluish flames 
 "swept over the S.W. and W. horizon, as if before a high wind. They were 
 not streamers, but bright blue flames." They lasted about a minute and 
 faded ; but about two minutes afterwards a glowing luminosity appeared in 
 the W.S.W., and broke into brilliant beams and streamers. The extreme 
 rays made an angle of 90 with each other, the central ray reaching an 
 altitude of 50. The extreme divergence of the streamers (indicating their 
 height above the earth's surface), and their direction (from W.S.W. toE.N.E.) 
 at right angles to the magnetic meridian, suggested to Mr. Ingall a disturb- 
 ance of an abnormal character. The rays had an oscillatory motion for about 
 fifteen seconds, and then disappeared, " as if a shutter had suddenly obscured 
 the source of light." 
 
 Mr. Ingall's remarks were corroborated by an observer in lat. 54 46' 6"'2 N., 
 long. 6 h 12 m 19 s- 75 W. The display, however, was more brilliant, and the 
 intensity of light at midnight illuminated the whole district as with an electric 
 light. The rays, too, bore tints differing from one another ; the largest 
 seemed to partake of the nature of the blue sky, while the smaller ones, 
 running parallel with the horizon, were ever changing from blue to orange-red. 
 
 On June 25 (same year 1 ?), between 9 and 10 o'clock, the Rev. Chas. Gape 
 saw at Rushall Vicarage, Scole, Norfolk, in the E.S.E., very frequent flashes 
 or streaks of a pale blue colour darting from the earth towards the heavens 
 like an Aurora. The day had been dull and close, with distant thunder. 
 In the E.S.E. it was dark, but overhead and everywhere else it was clear 
 and starry. 
 
 Bright 
 hluinh flame* 
 appeared. 
 
 Beams and 
 
 streamers 
 
 appeared. 
 
 Oscillatory 
 motion of 
 rays. 
 
 Mr. Ingall's 
 remarks cor- 
 roborated. 
 
 Km. C. 
 
 Gape saw 
 flashes or 
 streaks of a 
 pale blue 
 colour. 
 
 Mr. J. E. Capron's White Aurora of September* llth, 1874. 
 
 On September 11, 1874, we were at Kyle Akin, in the Isle of Skye. The 
 day had been wet and stormy, but towards evening the wind fell and the 
 sky became clear. About 10 P.M. my attention was called to a beautiful 
 Auroral display. 
 
 No crimson or rose tint was to be seen, but a long low-lying arc of the 
 purest white light was formed in the north, and continued to shine with more 
 or less brilliancy for some time. The arc appeared to be a double one, by the 
 presence of a dark band running longitudinally through it. 
 
 Occasional streamers of equally pure white light ran upwards from either 
 end of the bow. The moon was only a day old, but the landscape was lighted 
 up as if by the full moon ; and the effect of Kyle Akin lighthouse, the 
 
 Mr. J. E. 
 Capron's 
 white 
 Aurora of 
 Sept. 11, 
 1874. 
 
 Double arc 
 of pure 
 white lifrlii 
 in the N. 
 
 White 
 streamers. 
 
24 
 
 THE AUEORA AND ITS CHAEACTEES. 
 
 Auroral bow 
 believed to 
 be near the 
 earth. 
 
 No trace of 
 brown colour 
 in segment 
 of sky below 
 the arc. 
 
 Dr. Allnatt's 
 
 Aurora, 
 
 June9,1876. 
 
 Band of 
 auroral light 
 appeared. 
 
 Streaks of 
 cirro-stratus 
 divided the 
 Aurora. 
 
 Want of 
 electric 
 manifesta- 
 tions attri- 
 buted to 
 absence of 
 sun-spots. 
 
 numerous surrounding islands, and the still sea between was a true thing of 
 beauty. The display itself formed a great contrast to the more brilliant 
 but restless forms of Aurorse generally seen. I particularly noticed a some- 
 what misty and foggy look about the brilliant arc, giving it almost a solid 
 appearance. The space of sky between the horizon and the lower edge of 
 the arc was of a deep indigo colour, probably the effect of contrast. I had a 
 strong impression that the bow was near to the earth, and was almost con- 
 vinced that the eastern end and some fleecy clouds in which it was involved 
 were between myself and the peaks of some distant mountains. 
 
 I have not seen any other account of this Aurora, of which I was able at 
 the time to obtain a sketch. This is reproduced on Plate VII. It was a 
 lovely sight, and wonderfully unlike the cloud-accompanied and crimson 
 Aurorse which I had seen in the South. 
 
 It is noticed in Parry's ' Third Voyage ' that the lower edge of the auroral 
 arch is generally well defined and unbroken, and the sky beneath it so exactly 
 like a dark cloud (to him often of a brownish colour), that nothing could 
 convince to the contrary, if the stars, shining through with undiminished 
 lustre, did not discover the deception. 
 
 I saw no trace of brown colour. The segment below the arch resting on 
 the horizon was of a deep indigo colour. 
 
 Dr. Allnatfs Aurora, June 9th, 1876. 
 
 Dr. Allnatt, writing to the ' Times ' from Abergele, North Wales, near the 
 coast of the Irish Channel, reported an Aurora on the night of the 9th 
 June, 1876. After a cool and gusty day, with a strong N.E. wind and a 
 disturbed sea, there appeared at 11 P.M. in the N. horizon a broad band of 
 vivid auroral light, homogeneous, motionless, and without streamers. About 
 midnight a long attenuated streak of black cirro-stratus stretched parallel 
 with the horizon, and divided the Aurora into nearly symmetrical sections. 
 On the preceding day the sky was covered with dark masses of electric cloud 
 of weird and fantastic forms. The season had been singularly unproductive 
 of high electric manifestations, which Dr. Allnatt thought might be attri- 
 butable to the comparative absence of spots on the solar disk. [It may here 
 be noted how conspicuous the years 1877 and 1878 have been for absence of 
 Sun-spots and of Auroras.] 
 
I 
 
 (X 
 
 . 
 
K 
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 Pd 
 
 H 
 
 H 
 
 o c~ 
 
 CO 
 
 H ^ 
 
 gs 
 
 J o 
 
 1-1 o 
 
 gt- 
 
 H ^ 
 
 - - 
 PP O 
 
 o 
 
 p 
 
 w 
 
 H CO 
 
 IS 
 
 ru ix 
 
 v 
 
 o 
 
 I 
 rt 
 
SPECIFIC DESCEIPTIONS OF AtTKOEJE. 
 
 25 
 
 Herr Carl Bock's Lapland Aurora, 3rd October, 1877. 
 
 In January 1878 I had the pleasure to meet, at the Westminster Aquarium, 
 Herr Carl Bock, the Norwegian naturalist, who accompanied four Laplanders, 
 two men and two women, with sledges, tents, &c., on their visit to this 
 country. The Laplanders (as mentioned elsewhere) did not confirm the 
 accounts of noises said to have been observed by Greenlanders and others 
 during the Aurora. Carl Bock mentioned to me that the displays he saw in 
 Lupland were most brilliant, but generally of the yellow type (the Lap- 
 landers called the Aurora " yellow lights "). He saw only one red Aurora. 
 He kindly lent me a picture (probably in its way unique), an oil-painting of 
 an Aurora Borealis, entirely sketched by the light of the Aurora itself. 
 
 The painting is remarkable for the tender green of the sky, an effect 
 probably due to a mixture of the ordinary sky colour with the yellow light of 
 the Aurora. This picture was taken at Porsanger Fjord, in lat. 71 50', on 
 3rd October, 1877. It lasted from 9 P.M. till about 11 P.M. The rays kept 
 continually moving, and certain of them seemed in perspective and behind 
 the others. It will be noticed that the inner edge of the arc is fringed with 
 rays, contrary to the sharp and definite margin which is usually presented. 
 Probably two Auroras or auroral forms were seen a quiescent arc in front, 
 and a set of moving streamers beyond. Two larger and brighter patches of 
 light are seen at each extremity of the arc, as in the case of the Aurora seen 
 by me at Guildown, February 4th, 1874, which, indeed, the display much 
 resembles. A reduced facsimile of Herr Bock's excellent picture is given on 
 Plate VIII. 
 
 Herr Bock also acquainted me that on the following day he saw an Aurora 
 in which the lines of light, instead of being vertical, were longitudinal, and 
 were continually swept along in several currents. They were not so strong 
 as in the former case. 
 
 Herr Carl 
 Bock's Lap- 
 land Aurora, 
 3rd Oct. 
 
 1877. 
 
 Lapland 
 Aurora 
 generally of 
 the yellow 
 type. 
 
 A picture 
 painted by 
 light of the 
 Aurora. 
 
 Movement 
 of the rays. 
 
 Inner edge 
 of arc 
 
 fringed with 
 rays. 
 
 Aurora of 
 
 longitudinal 
 
 rays. 
 
 Eev:T. W. Webb's Aurora. 
 
 The Rev. T. W. Webb has described to me in a letter an Aurora very like 
 that seen by Carl Bock in Lapland, and apparently the prevailing type in 
 those regions. An arc similar to that figured by Carl Bock appeared in the 
 N.W., and seemed to resolve itself into two sets of streamers moving in 
 opposite directions (or the one set might be fixed and the other moving), like 
 the edges of two great revolving toothed wheels. This lasted but for a few 
 
 Eev. T. W. 
 
 Webb's 
 Aurora. 
 
 Arc resolved 
 into sets of 
 streamers 
 moving in 
 opposite 
 directions. 
 
 * 
 
2G 
 
 THE ATJKOEA AND ITS CHAEACTEES. 
 
 English 
 Arctic 
 Expedition, 
 1875-76. 
 
 Instructions 
 for use of 
 officers. 
 
 Appendix B. 
 
 Capt. Sir 
 G. Nares's 
 report. 
 
 True Auroras 
 seldom ob- 
 served, and 
 displays 
 faint. 
 
 Citron-line 
 observed on 
 only two 
 displays. 
 
 Appendix C. 
 
 Aurora 
 Australis. 
 
 Mr.Forster's 
 description. 
 
 Long 
 
 columns of 
 white light 
 spreading 
 over the 
 whole sky. 
 
 seconds ; but during that interval the tints were varied and brilliant, including 
 blue and green. 
 
 The English Arctic Expedition 1875-76, under Capt. Sir George Nares. 
 
 In anticipation of the starting of this Expedition, some instructions for the 
 use of the officers in connexion with the hoped for display of brilliant Aurorse 
 were prepared : as to general features of the Aurora, by Professor Stokes ; as 
 to Polarization, by Dr. William Spottiswoode ; and as to Spectrum work, by 
 Mr. Norman Lockyer and myself. As these instructions were somewhat 
 elaborate, and will apply to all Auroral displays, I have supplied a copy 
 of them in Appendix B. They were unfortunately not brought into 
 requisition, for want of the Aurora themselves. Capt. Sir George Nares 
 has reported to the Admiralty, under date 5th December, 1877, as follows: 
 " Although the auroral glow was observed on several occasions between 
 25 October, 1875, and 26 February, 1876, true Auroras were seldom observed ; 
 and the displays were so faint, and lasted so short a time, and the spectrum 
 observations led to such poor results, that no special report has been con- 
 sidered necessary. Although the citron-line was observed occasionally, on 
 only two displays of the Aurora was it well defined, and then for so short a 
 time that no measure could be obtained." (For Sir George Nares's further 
 Report see Appendix C, containing extracts from blue-book, ' Results derived 
 from the Arctic Expedition, 1875-6.') 
 
 Aurora Australis. 
 
 In an article on Aurora? in high Southern latitudes (Phil. Trans. No. 461, 
 and vol. liv. No. 53), we find that Mr. Forster, who as naturalist accom- 
 panied Capt. Cook on his second voyage round the world, says : " On 
 February 17th, 1773, in south latitude 58, a beautiful phenomenon was 
 observed during the preceding night, which appeared again this, and several 
 following nights. It consisted of long columns of a clear white light 
 shooting up from the horizon to the eastward almost to the zenith, and 
 gradually spreading over the whole southern part of the sky. These columns 
 were sometimes bent sideways at their upper extremities ; and though in most 
 respects similar to the northern lights of our hemisphere, yet differed from 
 them in being always of a whitish colour, whereas ours assume various tints, 
 especially those of a fiery and purple hue. The sky was generally clear when 
 they appeared, and the air sharp and cold, the thermometer standing at the 
 freezing point." This account agrees very closely in particulars with Capt. 
 
V T>T/- 
 
 ' w i 
 
 SPECIFIC DESCRIPTIONS OF AURORA. 
 
 Maclear's notice of Aurora Australia [after referred to], and especially in the 
 marked absence of red Aurora. 
 
 The height of the barometer does not appear to be mentioned, the tempe- 
 rature being apparently much the same as in the more recent cases. 
 
 In a letter dated from H.M.S. 'Challenger,' North Atlantic, April 10th, 
 1876, Capt. Maclear was good enough to communicate to me some parti- 
 culars of an Aurora Australis seen 3rd March 1874, in lat. 54 S., long. 108 E. 
 The letter is mainly descriptive of the spectrum (which will be described in 
 connexion with the general question of the spectrum of the Aurora). It 
 states that the red line was looked for in vain, and that the light appeared 
 of a pale yellow, and had none of the rosy tint seen in the northern 
 displays. 
 
 Capt. Maclear has since contributed to ' Nature,' of 1st November 1877, a 
 description of four Aurorse seen from the 'Challenger' in high southern 
 latitudes (including the one communicated to me). He speaks of the oppor- 
 tunity of observing as not frequent, either from the rarity of the phenomena, 
 or because the dense masses of cloud prevalent in those regions prevented 
 their being seen except when exceptionally bright. There were four appear- 
 ances described : 
 
 (1.) At 1.30 on the morning of February 9th, 1874, preceded by a watery 
 sunset, lat. 57 S. and long. 75 K, bar. 29-0 in., ther. 35 ; brilliant streaks 
 to the westward. Day broke afterwards with high cirrus clouds and clear 
 horizon. 
 
 (2.) At 9.30 P.M., February 21, 1874, lat. 64 S., long. 89 E., bar. 28-8 in., 
 ther. 31 ; one bright curved streamer. The Aurora preceded a fine morning 
 with cumulo-stratus clouds, extending from Jupiter (which appeared to be 
 near the focus) through Orion and almost as far beyond. Under this a black 
 cloud, with stars visible through it. Real cumuli hid great part of the 
 remainder of the sky, but there were two vertical flashing rays which moved 
 slowly to the right (west). Generally the Aurora was still bright. 
 
 (3.) At midnight, March 3rd, 1874, lat. 53 30' S., long. 109 E., bar. 29-1, 
 ther. 36, after some days' stormy weather, a brilliant sunset, followed by a 
 fine morning. Soon after 8 P.M. the sky began to clear and the moon shone 
 out. Noticing the light to the southward to be particularly bright, Capt. 
 Maclear applied the spectroscope, and found the distinguishing auroral line. 
 
 About midnight the sky was almost clear, but south were two or three 
 brilliant light clouds, colour very white-yellow, shape cumulo-stratus. From 
 about west to near south extended a long feathery light of the same colour, 
 
 E2 
 
 Capt. 
 
 Maclear's 
 Aurora 
 Australis, 
 3rd March, 
 1874. 
 
 Light of 
 pale yellow 
 tint only. 
 
 Capt. 
 Maclear's 
 Aurora 
 described in 
 ' Nature.' 
 
 Feb. 9, 1874. 
 
 Feb. 21, 
 1874. 
 
 March 3, 
 1874. 
 
 Auroral Hue 
 found in 
 light to 
 southward. 
 
 Brilliant 
 white clouds 
 ueen. 
 
28 
 
 THE ATJEOKA AND ITS CHAEACTEES. 
 
 March 6, 
 
 1874. 
 
 Capt. 
 Maclear 
 suggests 
 whether a 
 low baro- 
 meter has 
 to do with 
 the absence 
 of red. 
 
 Barometer 
 falls after 
 the Aurora, 
 and strong 
 gale from 
 theS.orS.W. 
 follows. 
 
 Pale yellow 
 glow rare in 
 the Aurora 
 Borealis. 
 
 Spectrum 
 of Auroras 
 Australes 
 extends 
 more into 
 the violet. 
 
 Capt. 
 
 Howes's de- 
 scription of 
 a Southern 
 Aurora. 
 
 parallel with the horizon, and between south and west there appeared occa- 
 sionally brilliant small clouds. The upper edges seemed hairy, and gave one 
 the idea of a bright light behind a cloud. The forms changed, but no par- 
 ticular order was noticed. 
 
 (Here follows a description of the spectrum, and the mode in which a 
 delineation by the lines was obtained.) 
 
 (4.) At 8 P.M., March 6th, 1874. This was a slight Aurora, seen to the 
 southward ; f after this the clouds changed to high cirrus. Capt. Maclear 
 suggests whether a low barometer has any thing to do with the absence of red 
 in the spectrum, the normal state of the barometer being an inch lower in 
 those regions than in more temperate latitudes. 
 
 Edin. Encyc. vol. iii. article " Aurora." Dr. Kirwan observed that the 
 barometer commonly falls after the Aurora. Mr. Winn, in the seventy-third 
 volume of the Phil. Trans., makes the same remark, and says that in twenty- 
 three instances, without fail, a strong gale from the south or south-west 
 followed the appearance of an Aurora. If the Aurora were bright, the gale 
 came on within twenty-four hours, but was of no long continuance ; if the 
 light was faint and dull, the gale was less violent, longer in coming, and longer 
 in duration. 
 
 The pale yellow-coloured glow referred to by Capt. Maclear is, in my expe- 
 rience, rare in the Aurora Borealis. It is probably the " sequaliter et sine 
 eruptionibus aut radiis fulvi" described by Seneca (ante, p. 1), and may pro- 
 bably belong to more southern climes. 
 
 We shall see too, by-and-by, that these Aurorae Australes as to spectrum 
 extend more into the violet than the Aurora Borealis. The yellow, as com- 
 plementary to violet, is likely thus to make (in the absence of the red) its 
 appearance. 
 
 It is, however, somewhat singular that Carl Bock found almost exclusively 
 yellow Aurorse in Lapland. 
 
 In Proctor's ' Borderland of Science,' article " The Antarctic Regions," we 
 find quoted a passage from a letter by Capt. Howes, of the ' Southern Cross,' 
 in which a graphic description is given of a Southern Aurora : 
 
 " At about half-past one on the 2nd of last September the rare pheno- 
 menon of the Aurora Australis manifested itself in a most magnificent manner. 
 Our ship was off Cape Horn, in a violent gale, plunging furiously into a 
 heavy sea, flooding her decks, and sometimes burying her whole bows beneath 
 the waves. The heavens were as black as death, not a star was to be seen, 
 when the brilliant spectacle first appeared. 
 
SPECIFIC DESCRIPTIONS OF ATJROILE. 
 
 29 
 
 " I cannot describe the awful grandeur of the scene ; the heavens gradually 
 changed from murky blackness till they became like vivid fire, reflecting a 
 lurid glowing brilliancy over every thing. The ocean appeared like a sea of 
 vermilion lashed into fury by the storm, the waves dashing furiously over our 
 side, ever and anon rushed to leeward in crimson torrents. Our whole ship 
 sails, spars, and all seemed to partake of the same ruddy hues. They were 
 as if lighted up by some terrible conflagration. Taking all together the 
 howling, shrieking storm, the noble ship plunging fearlessly beneath the 
 crimson-crested ways, the furious squalls of hail, snow, and sleet, drifting 
 over the vessel, and falling to leaward in ruddy showers, the mysterious 
 balls of electric fire resting on our mast-heads, yard-arms, &c., and, above all, 
 the awful sublimity of the heavens, through which coruscations of auroral 
 light would shoot in spiral streaks, and with meteoric brilliancy, there was 
 presented a scene of grandeur surpassing the wildest dreams of fancy." 
 
 The foregoing picture presents a singular contrast to the yellow-white 
 Aurorse described as seen in high southern latitudes by Capt. Maclear, and is 
 interesting as a southern Aurora of a red or ruddy tint. Looking, however, 
 at the extreme rarity of red Auroras in those latitudes, and the description of 
 " mysterious balls of electric fire resting on our mast-heads, yard-arms, &c." (a 
 phenomenon not often noticed in connexion with thd Aurora), it suggests 
 itself that the case in question may have been an instance not of a true 
 Aurora, but of an electric display, with conditions approaching those expe- 
 rienced by travellers who have found themselves in mountainous districts 
 surrounded by storm-clouds charged with electricity *. 
 
 Prof. Piazzi Smyth's Typical Aurora. 
 
 Prof. Piazzi Smyth was kind enough lately to send me the fourteenth 
 volume of the ' Astronomical Observations made at the Royal Observatory, 
 Edinburgh, during the years 1870-1877.' This volume, amongst its other 
 interesting matter, affords some valuable information on the subject of the 
 Aurora Borealis. The Aurora plates are five in number, three comprising 
 some well-executed chromo-lithographs of typical Aurorae, from sketches 
 made by Prof. Smyth, the other two plates being of the Aurora spectrum. 
 The Aurorae delineated are thus described : 
 
 * Some curious instances have been recently (January 1879) given in the ' Times ' of such 
 electric phenomena, comprising, amongst others, gas lighted by the finger in Canada, points 
 of flame seen on the ironwork of Teignniouth Bridge, and similar points seen on the alpenstocks 
 and axes of a party making a mountain ascent in Switzerland. 
 
 Balls of 
 electric fire 
 resting on 
 mast-heads 
 &c. 
 
 Prof. Piazzi 
 
 Smyth's 
 
 typical 
 
 Aurora. 
 
30 
 
 THE AUKOKA AND ITS CHAEACTEES. 
 
 
 Aug.6,1871, 
 quiescent 
 arc. 
 
 August 21, 
 1871, 
 active arc. 
 
 Sept.7,1871, 
 arc strea- 
 mers and 
 clouds. 
 May 8, 1871. 
 double arc 
 (longi- 
 tudinal). 
 April 28, 
 1871, mul- 
 tiple arc. 
 Oct. 25, 
 1870, 
 coloured 
 Aurora. 
 
 Plate 5. (August 6, 1871.) An example of a mild quiescent kind of 
 auroral arc, with dark cavernous substratum. (August 21, 1871.) An ex- 
 ample of a bright large active arc darting out rays. 
 
 Plate 6. (September 7, 1871.) An auroral arc, with streamers and dark 
 clouds, and maintaining a bright appearance though in proximity to the 
 moon. (May 8, 1871.) A double-arched auroral arc (the arches are longi- 
 tudinally arranged). 
 
 Plate 7. (April 28, 1871.) A multiple-arched arc of Aurora with moon- 
 light. (October 25, 1870.) A case of grandest coloured Auroras, or Aurora 
 superb and almost universal. 
 
 All the foregoing drawings are very vivid and striking, and form a most 
 interesting set of typical forms of Aurorse. 
 
 According to my own experience, the Aurora with arches arranged longi- 
 tudinally, thus, s ^^~N, is the rarest of all the forms. I have not met 
 with it myself, nor do I recollect an illustration of one other than Prof. 
 Smyth's. 
 
PHENOMENA SIMULATING AURORA. 
 
 31 
 
 CHAPTER IV. 
 
 PHENOMENA SIMULATING AURORA. 
 
 Auroric Lights (Kinahan). 
 
 MB. G. HENRY KINAHAK writes to ' Nature,' from Ovoca, under date 
 January 27th, 1877, and speaks of two distinct kinds of light so classed one 
 brilliant and transparent, of a white yellowish-blue or yellowish-red colour, 
 while the other is semi-opaque and of a bloody red colour, the latter being 
 considered in Ireland a forerunner of bad weather. The first kind generally 
 appears as intermittent pencils of light that suddenly appear and disappear. 
 
 Usually they proceed or radiate from some point near the north of the 
 horizon ; but Mr. Kinahan has frequently seen them break from a point in 
 the heavens, not stationary, but jumping about within certain limits. Some- 
 times these lights occur as suddenly flashing clouds of light of a white colour, 
 but at other times of blue and reddish yellow. 
 
 If this class of lights is watched into daylight, they appear somewhat like 
 faint rays of a rising sun. One morning, while travelling in West Galway in 
 the twilight, they were very brilliant, and quite frightened Mr. Kinahan's 
 car-driver, who thought the sun was going to rise in the north instead of the 
 east. The second, or bloody red light, usually occurs in clouds floating in 
 one direction up into the heavens, but often diffused over a portion of the sky. 
 Mr. Kinahan has never seen them coming from the east, and on only a few 
 occasions from the south, but generally from the west, north-west, or north. 
 
 If both kinds of light appear at the same time, the second while passing 
 over the first dims it. If the second class is watched into daylight, they 
 appear as dirty misty clouds that suddenly form and disappear without the 
 spectator being able to say where they come from or where they go to, or as 
 a hazy mist over a portion of the sky, that suddenly appears and disappears, or 
 as misty rays proceeding from a point in the horizon. Generally, when 
 these clouds occur, there is a bank of black clouds to the westward. 
 
 Mr. Kinahan then speaks of the season as having been prolific in auroric 
 light, as there had been few nights since the 1st October then last (1876) in 
 which they did not appear. On many occasions they were late in the night, 
 
 Mr. Kina- 
 han's 
 Auroric 
 Lights. 
 White and 
 red. 
 
 White light 
 appears in 
 pencils 
 radiating 
 from a 
 point. 
 
 Frequently 
 not sta- 
 tionary, but 
 jumping 
 about. 
 In daylight 
 like sun- 
 rays. 
 
 Red light 
 appears in 
 clouds float- 
 ing upwards 
 or diffused. 
 
 Red light 
 appears as 
 dirty misty 
 clouds in 
 daylight, 
 or as a mist 
 or misty 
 rays. 
 
 Season since 
 October 
 1876 pro- 
 lific in auro- 
 ric light. 
 
32 
 
 THE ATJEOBA AND ITS CHAEACTEES. 
 
 Mr. 3. Allan 
 Broun ques- 
 tions nature 
 of these 
 lights, as 
 Aurora is 
 seldom seen 
 at 5 A.M. in 
 this country. 
 
 On 77 occa- 
 sions seen 
 only twice 
 so early. 
 
 Season was 
 of marked 
 infrequency 
 elsewhere. 
 
 being very common and brilliant during the dark days of December, a few 
 hours before dawn (about 5 o'clock). Each time there was a fine day they 
 appeared also, and the weather broke again. 
 
 Mr. Jno. Allan Broun refers to this graphic account of Mr. Kinahan's, 
 and concludes there must have been some mistake as to the nature of these 
 " auroric lights," as the Aurora Borealis is very rarely seen at 5 A.M. in this 
 country. In the years 1844 and 1845, during which the Aurora was sought for 
 at Makerstown every hour of the night, it was observed in 77 nights on an 
 average of nearly three hours each night ; but it was seen only twice so early, 
 and that with a bright or brilliant Aurora, which remained during five hours 
 on the first occasion, and from 6 P.M. to 6 A.M. on the second. Parts of the 
 phenomenon seen by Mr. Kinahan, Mr. Broun also could not say he had ever 
 seen ; and if Mr. Kinahan's observations could have been confirmed it would 
 have been most important, especially as made so frequently at the epoch of 
 minimum. The description is in many respects a sufficiently recognizable 
 one of auroral discharges ; but the frequent appearance in early morning is 
 certainly unusual, and few if any Auroras seem to have been recorded as 
 appearing elsewhere in Great Britain during the time which Mr. Kinahan refers 
 to as so prolific (see, however, Dr. Allnatt's, ante, p. 24). In fact, the season 
 in question was one of marked infrequency (see English Arctic Expedition 
 Report, ante, p. 26). Mr. Buchan furnished Mr. Broun with a note of Auroras 
 seen in the stations of the Scottish Meteorological Society during the year 
 1876, and they were 42 in number, 26 in the first half, and 16 in the second 
 half of the year. The greater part were seen in the most northerly stations, 
 including the Orkney, Shetland, and Faroe Islands, and only 9 south of 
 the Forth. 
 
 Luminous 
 arch, Sept. 
 11, 1814. 
 
 Height 
 above hori- 
 zon 6 to 9 
 miles. 
 
 It moved 
 southward, 
 and was 
 assumed to 
 differ from 
 the Aurora. 
 
 Luminous Arch. 
 
 In the ' Annals of Philosophy,' vol. iv. p. 362, there is a minute description 
 of a luminous arch which appeared in the sky on the night of Sunday, 
 September llth, 1814, and, was seen in the west of England opposite the 
 Irish Sea, the west part of the south of Scotland, and part of the west of 
 Ireland. It was described as a part of either a body of dense greyish-white 
 light, or a mass of luminous matter in the shape of an arch. Its height above 
 the horizontal line was estimated at not more than 9 nor less than 6 miles. 
 
 Its direction when first seen was N. 80 E., and S. 80 W. It moved to the 
 southward. It was assumed to differ from the Aurora Borealis in wanting 
 coruscations, and in its having a much paler light. 
 
SOME QUALITIES OF THE ATKOBA. 
 
 33 
 
 CHAPTER V. 
 
 
 
 SOME QUALITIES OF THE AUKOBA. ' 
 
 Noises attending Aurora. 
 
 Ix the Edinb. Encyc., Gmelin is stated, in continuation of his description of 
 an Arctic Aurora, to add : " For however fine the illumination may be, it is 
 attended, as I have heard from the relation of many persons, with such a 
 hissing, cracking, and rushing noise through the air, as if the largest fire- 
 works were playing off." To describe what they then heard, the natives are 
 said to use the expression, " Spolochi chodjat " that is, The raging host is pass- 
 ing. The hunter's dogs, too, are also described as so much frightened when 
 the Auroras overtake the hunters, that they will not move, but lie obstinately 
 on the ground till the noise has passed. This account of noises seems to be 
 confirmed by other testimony. They are stated to have been heard at Hud- 
 son's Bay and in Sweden ; and Musschenbroek mentions that the Greenland 
 whale-fishers assured him they had frequently heard the noise of the Aurora 
 Borealis, but adds that " no person in Holland had ever experienced this 
 phenomenon." Mr. Cavallo declares he " has repeatedly heard a crackling 
 sound proceeding from the Aurora Borealis " (Elements of Nat. or Exper. 
 Phil. vol. iii. p. 449). Mr. Nairne mentions that in Northampton, when the 
 northern lights were very bright, he is confident he perceived a hissing or 
 whizzing sound. Mr. Belknap of Dover, New Hampshire, North America, 
 testifies to a similar fact (American Trans, vol. ii. p. 196). 
 
 Sir John Franklin mentions, in his ' Journey to the Shores of the Polar 
 Sea ' : " Nor could we distinguish its (the Aurora's) rustling noise, of which, 
 however, such strong testimony has been given to us that no doubt can remain 
 of the fact." 
 
 In detail, he mentions he never heard any sound that could be unequivo- 
 cally considered as originating in the Aurora, although he had had an oppor- 
 tunity of observing that phenomenon for upwards of 200 nights (the Aurora 
 was registered at Bear Lake 343 times without any sound being heard to 
 attend its motions) ; but the uniform testimony of the natives and all the 
 older residents in the country induced him to believe that its motions were 
 sometimes audible. On the llth March, at 10 P.M., a body of Aurora rose 
 
 t 
 
 Noises at- 
 tending 
 Aurorw. 
 
 Gmelin af- 
 firms them. 
 
 Other testi- 
 mony to 
 them. 
 
 ' Musschen- 
 
 \ broek. 
 
 Cavallo. 
 
 Nairne 
 
 Belknap. 
 
 Sir John 
 Franklin 
 negativ - 
 them. 
 
 March llth. 
 
34 
 
 THE AUEOEA AND ITS CHAEACTEES. 
 
 Hissing 
 noise heard 
 during Au- 
 rora's pas- 
 
 Explained 
 to arise from 
 the snow. 
 
 Capt. Sabine 
 also nega- 
 tives noise. 
 
 Article 
 " Aurora 
 Polaris," 
 Encyc.Brit., 
 suggests 
 noises as 
 not impro- 
 bable. 
 
 Payer nega- 
 tives and 
 discredits 
 noises. 
 
 As also 
 Weyprecht. 
 
 Herr Carl 
 Bock nega- 
 tives noises 
 in the case 
 of Lapland 
 Aurorae. 
 
 Auroral / 
 noises in 
 telephone. 
 
 Kinging 
 sound in 
 vacuum-tube 
 under influ- 
 ence of 
 magnet. 
 
 Adverse 
 conclusion 
 
 noises 
 anpany- 
 ing Aurora. 
 
 N.N.W. ; and after a mass had passed E. by S., the remainder broke away 
 in portions, which crossed about 40 of the sky with great rapidity. A hissing 
 noise, like that of a bullet passing through the air, was heard, which seemed 
 to proceed from the Aurora ; but Mr. Wentzel assured the party the noise 
 was occasioned by severe cold succeeding mild weather, and acting upon the 
 surface of the snow previously melted in the sun's rays. A similar noise was 
 heard the next morning. 
 
 In Parry's first voyage, Captain Sabine describes an Aurora seen at Melville 
 Island, and adds that the Aurora had the appearance of being very near the 
 party, but no sound could be heard. 
 
 In the article " Aurora Polaris," Encyc: Brit, edition ix., the writer admits 
 the evidence of scientific Arctic voyagers having listened in vain for such 
 noises ; but, referring to the statements of Greenlanders and others on the 
 subject, concludes there is no a priori improbability of such sounds being 
 occasionally heard, since a somewhat similar sound accompanies the brush- 
 discharge of the electric machine. 
 
 Payer, of the Austrian Polar Expedition (1872-1874), states that the 
 Aurora was never accompanied by noise, and discredits the alleged accounts 
 of noises in the Shetlands and Siberia. 
 
 Lieut. Weyprecht, of the same expedition, says (ante, p 14): " Involuntarily 
 we listen ; such a spectacle must, we think, be accompanied with sound, but 
 unbroken silence prevails, not the least sound strikes on the ear." 
 
 Herr Carl Bock, who accompanied the Laplanders visiting this country (at the 
 Westminster Aquarium) in 1877-78, and who witnessed many brilliant auroral 
 displays in Lapland, assured me he could trace no noise, except on one occa- 
 sion, when he heard a sort of rustling, which he attributed to the wind. The 
 Laplanders themselves did not associate any special noise with the Aurora. 
 
 It has been recently stated, in an article on the Telephone in ' Nature,' that 
 Professor Peirce "has observed the most curious sounds produced from a 
 telephone in connexion with a telegraph wire during the Aurora Borealis ;" 
 but no further details are given. In experimenting with a silicic fluoride 
 vacuum-tube between the poles of an electro-magnet. I found, on the magnet 
 being excited, that the capillary stream of blue light was decreased in volume 
 and brightness, and at the same time from within the tube a peculiar whistling 
 or slightly metallic ringing sound was heard. 
 
 I certainly have never met with an instance of noise accompanying an 
 Aurora and traced to it. On the whole the balance of evidence seems quite 
 adverse to any proof of noises proper ordinarily accompanying an Aurora. 
 
SOME QUALITIES OF THE AUEOEA. 
 
 Colours of the Aurora. 
 
 Sir John Franklin considered the colours in the Polar Aurora did not 
 depend on the presence of any luminary, but were generated by the motion 
 of the beams, and then only when that motion was rapid and the light bril- 
 liant. The lower extremities, he says, quivered with a fiery red colour, and 
 the upper with orange. He also saw violet in the former. Other observers 
 have, in their various descriptions of Aurora, mentioned the colours of the 
 rays or beams as red, crimson, green, yellow, &c. ; in fact, comprising the 
 range of the spectrum. Violet seems less frequently mentioned. The red 
 or crimson colour is frequently the first indication of the coming Aurora, and 
 is usually seen on or near the horizon. The colours have frequently been 
 observed to shift or change. 
 
 Prof. Piazzi Smyth, in a letter to 'Nature,' describing the Aurora of 
 February 4th, 1872, as seen at Edinburgh, says that when the maximum de- 
 velopment was reached all the heavens were more or less covered with pink 
 ascending streamers, except towards the N., which was dark and grey first by 
 means of a long low arch of blackness, transparent to large stars, and then by 
 the streamers which shot up from this arch, which were green and grey only 
 for several degrees of their height, and only became pink as they neared the 
 zenith. The red streamers varied from orange to rose-pink, red rose, and 
 damask rose. 
 
 The Professor pointed out that the spectroscope knew no variety of reds 
 giving one red line only, and attributed this to the mixing up of rays and 
 streamers of blackness out of the long low arch. When the Aurora faded 
 away a true starlight-night sky appeared ; so that evidently the dark arch and 
 streamers were a,s much part of the Aurora as the green and red lights. 
 
 Dr. Allnatt, at Frant, found in the case of the same Aurora the south-western 
 part of the heavens tinged by a bright crimson band. A dark elliptical cloud 
 extending from S. to S.E. was illuminated at its upper edge with a pale yellow 
 light, and sent up volumes of carmine radii interspersed with green and the 
 black alternating matter characteristic of elemental electricity. Almost due 
 E., and of about 25 degrees elevation, was a bright insulated spot of vivid 
 emerald-green, which appeared almost sufficiently intense to cast a faint 
 shadow from intercepting objects. At 7 o'clock the Aurora had passed the 
 zenith, and the sky presented a weird and wonderful appearance. A dark 
 rugged cloud, some 8 degrees E. of the zenith, was surrounded by electric 
 
 P2 
 
 Colours of 
 the Aurora. 
 Sir John 
 Franklin's 
 views. 
 
 Other ob- 
 servers have 
 described all 
 colours of 
 spectrum. 
 
 Violet rare. 
 
 Crimson 
 
 indicates 
 
 coining 
 
 Aurora. 
 
 Prof. Piazzi 
 Smyth de- 
 scribes co- 
 lours of Au- 
 rora of Feb. 
 4, 1872, as 
 seen at 
 Edinburgh. 
 
 Dr. Allnatt 
 at Frant 
 describes 
 vivid colours 
 of same 
 Aurora. 
 
36 
 
 THE AUEOEA AND ITS CHAEACTEES. 
 
 Descriptions 
 at Black- 
 burn and 
 Cambridge. 
 Lapland 
 Aurora 
 yellow. 
 
 Hydrogen 
 vacuum-tube 
 suggestive 
 of Aurora 
 colours. 
 Variation of 
 tints in. 
 
 Variation of 
 colour in ni- 
 trogen tube 
 under influ- 
 ence of mag- 
 net. 
 
 Change from 
 rosy to violet 
 hue. 
 
 Photogra- 
 phic plates 
 taken. 
 
 Difference 
 in. 
 
 light of all hues carmine, green, yellow, blood-red, white, and black ; and 
 the bright spot still existed in the south. 
 
 At Blackburn, in Lancashire, the rays were described as glowing in the 
 N.E. from silvery white to deepest crimson; and at Cambridge the same 
 Aurora was described as of a brilliant carmine tint. The Auroras seen in 
 Lapland by Herr Carl Bock, were, he informed me, almost invariably yellow ; 
 he saw only one red one. 
 
 The behaviour of a hydrogen Geissler vacuum-tube will be subsequently 
 referred to in the Chapter on the comparison of some tubes with the Aurora 
 spectrum, and is suggestive as to Aurora colours. 
 
 The capillary part of this tube, when lighted by a small coil, was found 
 to vary in tint silver-white, bright green, and crimson being each in suc- 
 cession the dominant colour, according to the working of the break of the 
 coil. When a spectroscope was used, the red, blue, and violet lines of the 
 gas were seen to change in intensity in accordance with the light colour seen 
 in the tube. 
 
 A Geissler nitrogen vacuum-tube was also so arranged that the capillary part 
 of it should be vertically between the conical extremities of the armatures of 
 a large electro-magnet, the armatures just being clear of the outside of the 
 tube. The tube was then lighted up by a small coil, and the magnet excited 
 by four large double-plate bichromate cells. 
 
 The stream of light was steady and brilliant, and, except at the violet pole, 
 of the rosy tint peculiar to a nitrogen vacuum-tube. On excitation of the 
 electro-magnet, the discharge was seen to diminish in volume, with an ap- 
 parent increase in impetuosity ; and not only the capillary part, but in a less 
 degree the bulbs also of the tube, changed from a rosy to a well-marked violet 
 hue. 
 
 We several times connected and disconnected the magnet with its batteries, 
 but always with the same result. Of the spectrum of the capillary part of 
 this tube we took photographic plates with quartz prisms and lenses, taking 
 care that all things should be as equal as possible, the apparatus undis- 
 turbed, and the time of exposure exactly the same. One plate was taken with 
 the tube in its normal condition, the other while it was under the influence of 
 the magnet. The spectra were identical, except that the plate of the tube 
 influenced by the magnet was decidedly the brightest, and was found to 
 penetrate more into the violet region (the Author's ' Photographed Spectra,' 
 p. 60, plate xxv.). These plates effectually corroborated the change of colour, 
 as the violet ray would have more photographic effect than the rosy. The 
 
SOME QUALITIES OF THE AURORA. 
 
 37 
 
 identity of the spectra of the capillary part proved that the change in colour 
 could not have proceeded from an extension of the violet glow. (A similar 
 experiment will be found also detailed in Part III. Chapter XII.) 
 
 Height of the Aurora. 
 
 Sir John Franklin (Narrative of a Journey on the Shores of the Polar Sea 
 in the years 1819, '20, '21, '22) says: "My notes upon the appearance of 
 the Aurora coincide with those of Dr. Richardson in proving that that pheno- 
 menon is frequently seated within the region of the clouds, and that it is 
 dependent in some degree upon the cloudy state of the atmosphere." And 
 further : " The observations of Dr. Richardson point particularly to the 
 Aurora being formed at no great elevation, and that it is dependent upon 
 certain other atmospheric phenomena, such as the formation of one or other 
 of the various modifications of cirro-stratus." 
 
 Sir John Franklin also refers to notes from the Journal of Lieut. Robert 
 Hood, R.N., on an Aurora: 
 
 The observations were made at Basquian House, and at the same time by 
 Dr. Richardson at Cumberland House, quadrants and chronometers having 
 been prepared for the purpose. On the 2nd April the altitude of a brilliant 
 beam was 10 0' 0" at 10 h l m s at Cumberland House. Fifty-five miles 
 S.S.W. it was not visible. It was estimated that the beam was not more than 
 7 miles from the earth, and 27 from Cumberland House. On the 6th April 
 the Aurora was for some hours in the zenith at that place, forming a confused 
 mass of flashes and beams; and in lat. 53 22' 48" N., long. 103 7' 17", it 
 appeared in the form of an arch, stationary, about 9 high, and bearing 
 N. by E. It was therefore 7 miles from the earth. 
 
 On the 7th April the Aurora was again in the zenith before 10 P.M. at 
 Cumberland House, and in lat. 53 36' 40" N., long. 102 31' 41". The 
 altitude of the highest of two concentric arches at 9 h P.M. was 9, at 9 h 30 m 
 it was 11 30', and at 10 h O m s P.M. 15 0' 0", its centre always bearing 
 N. by E. During this time it was between 6 and 7 miles from the earth. 
 [The bearings are true, not magnetic.] 
 
 Sir J. Franklin says this was opposed to the general opinion of meteorolo- 
 gists of that period : he also noticed he had sometimes seen an attenuated 
 Aurora flashing across the sky in a single second, with a quickness of motion 
 inconsistent with the height of 60 or 70 miles, the least that had hitherto been 
 ascribed to it. 
 
 Height of 
 Aurora. 
 
 Sir John 
 Franklin 
 considers it 
 within the 
 region of the 
 clouds. 
 
 At no great 
 elevation. 
 
 Observa- 
 tions of 
 Lieut. Ro- 
 bert Hood 
 and Dr. 
 Richardson. 
 
 A beam not 
 more than 7 
 miles from 
 the earth. 
 
 An arch 7 
 miles from 
 the earth. 
 
 An arch be- 
 tween 6 and 
 7 miles from 
 the earth. 
 
 Sir John 
 
 Franklin's 
 
 remarks. 
 
38 
 
 THE AUEOEA AND ITS CHAEACTEES. 
 
 Dr. Bichard- 
 
 .son's conclu- 
 
 Captain 
 Parry ob- 
 served Au- 
 rorse near 
 the Earth's 
 surface. 
 
 Sir W. R. 
 
 Grove's ob- 
 servation at 
 Chester. 
 
 Mr. Ladd's 
 observation 
 at Margate. 
 
 The author's 
 observation 
 at Kyle 
 Akin, Skye. 
 
 Dalton's cal- 
 culation of 
 100 miles. 
 
 Backhouse's 
 50 to 100 
 miles. 
 
 Prof. New- 
 ton, mean 
 130 miles. 
 
 The needle was most disturbed, February 13, 1821, P.M., at a time when 
 the Aurora was distinctly seen passing between a stratum of cloud and the 
 earth ; and it was inferred from this and other appearances that the distance 
 of the Aurora from the earth varied on different nights. Dr. Richardson con- 
 cludes that his notes prove, independent of all theory, that the Aurora is 
 occasionally seated in a region of the air below a species of cloud which is 
 known to possess no altitude ; and is inclined to infer that the Aurora Borealis 
 is constantly accompanied by, or immediately precedes, the formation of one 
 or other of the forms of cirro-stratus. 
 
 Captain Parry observed Aurorse near the earth's surface ; and records that 
 he and two companions saw a bright ray of the Aurora shoot down from the 
 general mass of light between him and the land, which was distant some 
 3000 yards. Sir W. R. Grove (' Correlation of Physical Forces ') saw an 
 Aurora at Chester, when the flashes appeared close, so that gleams of light 
 continuous with the streamers were to be seen between him and the houses 
 " he seemed to be in the Aurora." Mr. Ladd, of Beak Street, Regent Street, 
 has related to me an appearance he was struck with, and examined carefully. 
 Standing in the evening in Margate Harbour, he saw a white ray of the 
 Aurora, which, apparently shooting downwards, was clearly placed between 
 his eye and the opposite head of the pier, which projected into the sea. Mr. 
 Ladd also informed me that Prof. Balfour assured him that such an appear- 
 ance was not unusual. In the double-arc Aurora seen by me in the Isle of 
 Skye, September 11, 1874 (described ante, p. 23), I had a strong impression 
 that the bow was near the earth, and thought that the eastern end, and some 
 fleecy clouds in which it was involved, were between myself and the peaks of 
 the distant mountains. 
 
 In the article " Aurora Polaris," Encyc. Brit., edition ix., Dalton is 
 instanced as having calculated the height of an Aurora in the north of Eng- 
 land at 100 miles ; and Backhouse as having made many calculations, with the 
 result of an average height of 50 to 100 miles. Prof. Newton, too, is quoted 
 for the height of 28 Aurorse (calculated by one observation of altitude and 
 amplitude of an arch) as ranging from 33 to 281 miles, with a mean of 
 130 miles. It is, however, pointed out that a height of 62 miles above the 
 earth's surface would imply a vacuum attainable with difficulty, even with 
 the Sprengel pump. This difficulty is then met by a reference to the observed 
 altitude of some meteors, and to a suggestion of Prof. Herschel's that electric 
 repulsion may carry air or other matter up to a great height. Dr. Lardner 
 (' Museum of Science and Art,' vol. x. p. 192) speaks of the height of Auroras 
 
SOME QUALITIES OF THE AUKOEA. 
 
 39 
 
 as not certainly ascertained ; but considers them atmospheric phenomena 
 scarcely above the region of the clouds, and does not think it probable that 
 their elevation in any case can exceed a few miles. 
 
 M'Clintock, after noticing that the beams of the Aurora were most frequently 
 seen in the direction of open water, says that in some cases patches of light 
 could be plainly seen a few feet above a small mass of vapour over an opening 
 in the ice. Captain Ross, in his Antarctic voyage, saw the bright line of the 
 Aurora forming a range of vertical beams along the top of an ice-cliff; and 
 suggested this was produced by electrical action taking place between the 
 vaporous mist thrown upwards by the waves against the berg, and the colder 
 atmosphere with which the latter was surrounded. 
 
 Bergman, from a mean of 30 computations, makes the height of the phe- 
 nomenon to be 72 Swedish (about 468 English) miles. 
 
 Father Boscovich calculated the height of an Aurora Borealis observed on 
 the 16th December, 1727, to have been 825 miles. 
 
 Mairan supposed the far greater number of Aurorse to be at least 600 miles 
 above the surface of the earth. Euler assigned them an elevation of several 
 thousands of miles. Dr. Blagden, however, limited their height to about 
 100 miles, which he supposed to be the region of fireballs remarking that 
 instances were upon record in which northern lights had been seen to join 
 and form luminous balls, darting about with great velocity, and even leaving 
 a train behind them like common meteors (Phil. Trans, vol. Ixxiv. p. 227). 
 
 Mr. Dalton, from an observation of the luminous arches on a base of 
 22 miles, found the altitude of the Aurora to be about 150 miles (Dal ton's 
 ' Meteorological Observations and Essays,' 1793, pp. 54, ]53). 
 
 Dr. Thompson, 'Annals of Philosophy,' vol. iv. p. 429 (1814), assumes that 
 the height of the beams above the surface of the earth was much greater than 
 that of most other meteorological appearances, and gives (p. 430) a table of 
 Aurorse, mainly taken from Bergman, Opusc. v. p. 291, of 31 Aurorse 
 observed in the years 1621 to 1793, with heights in English miles. The 
 lowest is, 23rd February, 1784, London (Cavendish), 62 miles; the highest, 
 23rd October, 1751, Fournerius, 1006 miles ! The average of the 31 esti- 
 mated observations gives a height of about 500 miles. It is not stated 
 how these observations were obtained, though methods are mentioned how 
 they might be. 
 
 Prof. Heis, of Miinster, exhibited at the recent Scientific Loan Collection 
 at South Kensington (' Official Catalogue,' 3rd edit. p. 296, No. 1231) an 
 instrument for the determination of the position of the point of convergence 
 
 Elevation of 
 Aurora) can- 
 not exceed a 
 few miles. 
 
 M'Clin- 
 tock' s obser- 
 vations. 
 
 Capt. Boss 
 saw Aurora? 
 on an ice- 
 cliff, which 
 he attributed 
 to electric 
 action. 
 
 Bergman 
 estimates 
 height as 
 468 miles. 
 Boscovich 
 825 miles. 
 
 Mairan 600 
 miles. 
 Euler seve- 
 ral thou- 
 sand miles. 
 
 Dr. Blagden 
 about 100 
 miles. 
 
 Dalton 150 
 miles. 
 
 Dr. Thomp- 
 son assumes 
 considerable 
 height. 
 
 His table. 
 
 Average of 
 31 observa- 
 tions, 500 
 miles. 
 
40 
 
 THE AUEOKA AND ITS CHAEACTEES. 
 
 Prof. Heis's 
 instrument 
 for deter- 
 mining 
 height of 
 Auroras. 
 
 Professor 
 Newton's 
 method of 
 calculating 
 height. 
 
 Gave a 
 height from 
 33 to 281 
 miles, and a 
 mean of 130 
 miles. 
 
 of the rays of the Aurora, and for determining the height of the Aurora. ' .A 
 ball resting in a pan was to be brought into position, so that several diverging 
 pencils of Aurora, when properly viewed, were covered by the rod which 
 passed through the centre of the ball. The point of the rod (which could be 
 moved up and down in the ball), when the instrument was set to the astrono- 
 mical meridian, showed the azimuth and altitude of the converging point of 
 the pencils of light. This point of convergence does not coincide with the 
 point to which the inclination-needle directs. From the deviation of the two 
 points, the height of the Aurora could be calculated. 
 
 Professor H. A. Newton (Sil. Journ. of Science, 2nd ser. vol. xxix. p. 286) 
 has proposed a method of calculating the height of Aurora by one observation 
 of altitude and amplitude of an arch. It assumes that the auroral arches 
 are arcs of circles, of which the centre is the magnetic axis of the earth, or at 
 least that they are nearly parallel to the earth's surface, and probably also to 
 the narrow belt or ring surrounding the magnetic and astronomical poles. 
 Professor Newton finds that, d being the distance from the observer to the 
 centre of curvature of the nearest part of this belt (for England, situated about 
 75 N. lat., 50 W. long.), h the apparent altitude of the arch, 2a its amplitude 
 on the horizon, x its height, E, the earth's radius, and c the distance of the 
 observer from the ends of the arch : 
 
 sin < = sin d cos a cosec (d-\-h) 
 tan c=z sin h sin < sec 2 < . 
 #=R (sec c 1) . . . . 
 
 (1) 
 (2) 
 (3) 
 
 This method with 28 Aurora? gave a height from 33 to 281 miles and a mean 
 of 130 miles. 
 
 Galle has suggested (Pogg. Ann. cxlvi. p. 133) that the height of Aurorae 
 might be calculated from the amount of divergence between the apparent 
 altitude of the auroral corona and that indicated by the dipping-needle, a 
 principle which has been adopted in Prof. Heis's apparatus before described. 
 The results do not differ materially from Professor Newton's. 
 
 The conclusions to be arrived at from the foregoing instances and opinions 
 are certainly very puzzling. The terrestrial character of some Aurorae seems 
 well established. The height to which these phenomena may ascend is left 
 almost a matter of conjecture, and further observations are very desirable. 
 
SOME QUALITIES OF THE AUROKA. 
 
 41 
 
 Phosph orescetice. 
 
 In the voyage of the ' Hansa ' (' Recent Polar Voyages,' p. 420), on the 9th 
 September, 1869, at 10 P.M., Aurora gleams appeared in the west, shooting 
 towards the south. " Radiant sheaves and phosphorescent bands mounted 
 towards the zenith," but the phantasmagoria quickly vanished. M. Silbermann 
 (' Comptes Rendus,' Ixviii. p. 1120) mentions storm-clouds which threw out 
 tufts of cirri from their tops, which extended over the sky, and resolved into, 
 h'rst, fine, and afterwards more abundant rain. (I saw a fine day example of this 
 on the Lago di Guarda, ending in a copious discharge of rain attended with 
 loud thunder and vivid lightning.) Usually the fibres were sinuous ; but in 
 much rarer cases they became perfectly rectilinear and surrounded the cloud 
 like a glory, and occasionally shone with a sort of phosphorescence. On the 
 night of 6th September, 1865, at 11 P.M., a stormy cloud was observed in the 
 N.N.W., and lightning was seen in the dark cumulous mass. Around this mass 
 extended glories of a phosphorescent whiteness, which melted away into the 
 darkness of the starry sky. Round the cloud was a corona, and outside this 
 two fainter corona?. After the cloud had sunk below the horizon the glories 
 were still visible. 
 
 Sabine mentions a cloud frequently enveloping Loch Scavaig, in Skye, as 
 being at night perfectly self-luminous, and that he saw rays, similar to those 
 of the Aurora, but produced in the cloud itself. Sabine also refers to 
 luminous clouds mentioned in Gilbert's Annals, and to observations by 
 Beccaria, Deluc, the Abbe Rozier, Nicholson, and Colla; and to luminous 
 mists as observed by Dr. Verdeil at Lausanne in 1753, and by Dr. Robinson 
 in Ireland. 
 
 He also describes (Parry's First Voyage) an Aurora seen at Melville 
 Island, and says the light was estimated as equal to that of the moon when a 
 week old. Besides the pale light, which resembled the combustion of phos- 
 phorus, a slight tinge of red was noticed when the Aurora was most vivid ; 
 but no other colours. This Aurora was repeatedly seen on the following day. 
 
 Mr Procter, in a letter to me, suspects that the Aurora is generally formed 
 in a sort of " mist or imperfect vapour ; " and this mist or imperfect vapour 
 seems in many instances to form part of the Aurora, and to partake of its 
 self-luminous character. M'Clintock does not imagine that the Aurora is 
 ever visible in a perfectly clear atmosphere. He has often observed it just 
 silvering or rendering luminous the upper edge of low fog or cloud-banks, 
 and with a few vertical rays feebly vibrating. 
 
 G 
 
 Phosphores- 
 cence. 
 
 Phosphores- 
 cent bands. 
 
 Storm- 
 clouds which 
 threw out 
 cirri. 
 
 Shone with 
 a sort of 
 phosphores- 
 cence. 
 
 Storm-cloud 
 surrounded 
 by glories of 
 a phospho- 
 rescent 
 whiteness. 
 
 Sabine's 
 luminous 
 cloud at 
 Loch Sca- 
 vaig, Skye. 
 Other obser- 
 vations of 
 luminous 
 clouds. 
 
 Aurora at 
 
 Melville 
 
 Island. 
 
 Procter 
 
 suspects 
 Aurora is 
 formed in n 
 mist. 
 
 M'Clintock : 
 Aurora is 
 never visible 
 in a per- 
 fectly < 
 atmosphere. 
 
42 
 
 THE ATJEOKA AND ITS CHAEACTEES. 
 
 Aurora of 
 Feb. 4, 1874. 
 
 Illuminated 
 fog-cloud. 
 
 Capt. Oli- 
 ver's meteor- 
 cloud. 
 
 Auroral dis- 
 play, 24th 
 Oct., 1870. 
 
 Streamers 
 of phospho- 
 rescent 
 cloud. 
 
 Aurora of 
 Feb. 4, 1872, 
 at Frant. 
 
 Eadii of 
 phosphores- 
 cent light. 
 
 The author's 
 description 
 of same 
 Aurora. 
 Masses of 
 phosphores- 
 cent vapour. 
 
 Day Auroras 
 must have a 
 phosphores- 
 cent glow. 
 
 Angstrom 
 considers 
 yellow- 
 green line 
 due to fluo- 
 rescence or 
 phosphores- 
 cence. 
 
 Oxygen and 
 some of its 
 compounds 
 phosphores- 
 cent. 
 
 A phospho- 
 retted hy- 
 drogen 
 spectrum- 
 band is close 
 to yellow- 
 green 
 auroral line. 
 
 An instance of apparent phosphorescence is supplied by the Aurora of the 
 4th February, 1874 (ante), when a bright cloud of light was seen which gave 
 the impression of an " illuminated fog-cloud." Captain S. P. Oliver saw at 
 Buncrana, Co. Donegal, on February 4, 1874, what he describes as a meteor- 
 cloud, viz. " a broad band of silvery white and luminous cloud." This 
 appearance, as described by another correspondent, was evidently an imper- 
 fectly formed (perhaps actually forming) Auroral arc. The great Auroral 
 display of the 24th of October, 1870, as seen by me, included, according to 
 my notes made at the time, " streamers of opaque white phosphorescent 
 cloud, very different from the more common transparent Auroral diverging 
 streams of light." 
 
 Describing the Aurora of February 4, 1872, at Frant, Dr. Allnatt says : 
 " At a later hour of the night the canopy of cirro-stratus had separated, and 
 was transformed into luminous masses of radiant cumulus. At 10.40 the 
 Aurora reappeared in the N., and sent luminous radii of white phosphorescent 
 light from the periphery of a segment of a perfectly circular arch " *. 
 
 Again, February 4th, 1872, as described by me, the first signs of the 
 Aurora were (in dull daylight) a lurid tinge upon the clouds, which suggested 
 the reflection of a distant fire ; while scattered among these, " torn and broken 
 masses of white vapour having a phosphorescent appearance " reminded me 
 of a similar observation in October 1870. 
 
 The day Aurorse, which are elsewhere described, and are not very un- 
 common, could, we may presume, hardly be seen without the presence of 
 
 O 
 
 some phosphorescent glow. Professor Angstrom, in his Aurora Memoir 
 (discussed elsewhere), in discussing the yellow-green line, considers the only 
 probable explanation to be that it owes its origin to fluorescence or phos- 
 phorescence. He says that some fluorescence is produced by the ultra-violet 
 rays ; and adds, " an electric discharge may easily be imagined, which, though 
 in itself of feeble light, may be rich in ultra-violet light, and therefore in a 
 condition to cause a sufficiently strong fluorescent light." And he refers to 
 the fact that oxygen and some of its compounds are phosphorescent. 
 
 In the examination of certain spectra connected with the Aurora, detailed 
 in Part II., I have shown that the bright edge of one of the phosphoretted 
 hydrogen bands is in close proximity to the yellow-green Auroral line. I 
 have also referred to the peculiar brightening by reduction of temperature of 
 one of the bands in the red end of the spectrum of phosphoretted hydrogen, 
 
 * On the occasion of the Aurora of September 24, 1870, Dr. Allnatt says, " the air seemed 
 literally alive w ith the unwonted phosphorescence." 
 
SOME QUALITIES OF THE AURORA. 
 
 43 
 
 so that from almost invisible it became bright, and to the peculiar brightening 
 of a line in the yellow-green in certain " Aurora " and phosphorescent tubes. 
 It has also been observed that the electric discharge has a phosphorescent or 
 fluorescent after-glow (isolated, I believe, by Faraday). It seems difficult to 
 avoid in some way connecting all these circumstances with the yellow-green 
 line of the Aurora, if not also with the line in the red. 
 
 Mr. Sorby, in his experiments on the connexion between fluorescence and 
 absorption (' Monthly Microscopical Journal '), found in the spectrum of a 
 solution in alcohol of a strongly fluorescent substance called bonelleine (the 
 green colouring-matter found in the Aurelia Bonellia-viridis) two bright 
 bands, the one red and the other green, with centres respectively at 6430 
 and 5880, and their limits towards the blue end at 6320 and 5820. On 
 adding an acid the red band changed its place to 6140. The superficial 
 membranous coloured layer of the fungi Eussula nitida and vesca in alcohol 
 gave an absorption band with centre at 5540, while the spectrum of fluo- 
 rescence extended to 4400. A solution of Oscillatorioe in water gave a 
 spectrum of absorption with bands at 6200 and 5690 ; while the spectrum of 
 fluorescence showed two bright bands having their centres at 6470 and 5800, 
 and their limits towards the blue end at 6320 and 5710. 
 
 These instances of course cannot be connected with the Aurora except as 
 showing the spectrum region and lines of fluorescence. The sea phospho- 
 rescence, according to Professor Piazzi Smyth, has a continuous spectrum 
 extending from somewhat below E to near F (Plate V. fig. 3). 
 
 Angstrom, on the occasion of the starry night when he found traces of the 
 green line in all parts of the heavens, speaks of the sky as being " almost 
 phosphorescent." 
 
 The author of the Aurora article in the Encyc. Brit, suggests that the 
 phosphorescent or fluorescent light attributed to the Aurora may be due to 
 chemical action. He also questions Angstrom's assumption that water- 
 vapour is absent in the higher atmosphere, and thinks that it and other 
 bodies may, by electric repulsion, be carried above the level they would 
 attain by gravity. He then continues that if discharges take place between 
 the small sensible particles of water or ice in the form of cirri (as Silber- 
 mann has shown to be likely) surface decomposition would ensue, and it is 
 highly probable the nascent gases would combine with emission of light. 
 He adds " that it has been almost proved that in the case of hydrogen 
 phosphide the very characteristic spectrum (light?) produced by its com- 
 bustion is due neither to the elements nor to the products of combustion, but 
 
 Phosphores- 
 cent or fluo- 
 rescent 
 after-glow 
 of electric 
 discharge. 
 
 Sorby's ex- 
 periments 
 on fluores- 
 cence and 
 absorption. 
 
 Bonelleine, 
 spectrum of. 
 
 Coloured 
 layer of 
 fungi. 
 
 Spectrum of 
 OsciUatoriee. 
 
 Sea phos- 
 phorescence, 
 a continuous 
 spectrum. 
 
 Angstrom 
 finds the 
 sky almost 
 phosphores- 
 cent. 
 
 Author of 
 article in 
 Encyc. Brit. 
 suggests 
 that the 
 phosphores- 
 cent or fluo- 
 rescent light 
 may be due 
 to chemical 
 action. 
 
44 
 
 THE AUEOEA AND ITS CHAEACTEES. 
 
 Herschel's 
 observation 
 of phospho- 
 rescence in 
 Geissler and 
 " garland " 
 tubes. 
 
 to some peculiar action at the instant of combination ; and it is quite pos- 
 sible that under such circumstances as above described water might also give 
 an entirely new spectrum." Professor Herschel has referred to the phos- 
 phorescent light which remains glowing in Geissler tubes after the spark has 
 passed, and to the fact that one of the globes of a " garland " tube which 
 was heated did not shine after the spark had passed, apparently because of 
 the action of heat on the ozone to which the phosphorescence might be due. 
 (See experiments on Mr. Browning's bulbed tube, Part III. Chap. XV.) 
 
 Aurora and 
 Ozone. 
 
 Smells of 
 sulphur 
 during 
 Aurora at- 
 tributed to 
 
 Question 
 whether the 
 oxygen of 
 the air may 
 be changed 
 into ozone. 
 
 Ozone de- 
 stroyed by 
 heat. 
 
 Ozone in a 
 large bell- 
 receiver not 
 manifested 
 in spectrum. 
 
 Professor 
 Dewar de- 
 monstrates 
 that ozone 
 is condensed 
 oxygen. 
 
 Aurora and Ozone. 
 
 Accounts are given by travellers in Norway of their being enveloped in the 
 Aurora, and perceiving a strong smell of sulphur, which was attributed to the 
 presence of ozone. M. Paul Rollier, the aeronaut, descended on a mountain 
 in Norway 1300 metres high, and saw brilliant rays of the Aurora across a 
 thin mist which glowed with a remarkable light. To his astonishment, an 
 incomprehensible muttering caught his ear ; when this ceased he perceived a 
 very strong smell of sulphur, almost suffocating him (' Arctic Manual,' p. 726). 
 
 In the case of the Aurora, the question naturally arises whether the 
 oxygen of the air may be changed into ozone, perhaps also whether the 
 nitrogen may not be modified in some similar manner. 
 
 The absorption spectra of oxygen, and of the same gas in its form of ozone, 
 may possibly differ ; but this can hardly happen in the case of incandescent 
 oxygen, for ozone is at once destroyed by heat at 300, and slowly at 100, 
 and must be partially at least destroyed by the heat of the discharge. If 
 any lines were due to ozone in such a spectrum, we should expect they would 
 be weakened by heat and brightened by cold. 
 
 In the case of a continued discharge in a large exhausted bell-receiver, the 
 presence of ozone in considerable quantities was manifested to us by its 
 odour when the receiver was removed from the pump ; but the spectrum of 
 the stream of light did not appear to differ from that in Geissler tubes. 
 
 In a course of lectures at the Royal Institution in March 1878, on the 
 Chemistry of the Organic World, Prof. Dewar appears to have demonstrated, 
 by Prof. Andrews' apparatus, that ozone is really condensed oxygen, and, 
 further, that during this condensation heat is absorbed, which is evolved 
 during the decomposition or re-expansion. 
 
 He also exhibited the oxidizing power of ozone in its action on mercury, 
 and commented on its similar action upon organic matter in forming nitrates, 
 
SOME QUALITIES OF THE AURORA. 
 
 45 
 
 ;uul on its remarkable bleaching properties, but added there was as yet no 
 proof of its combining with free nitrogen. That peroxide of hydrogen 
 ;u companies the formation of ozone by the slow combustion of phosphorus, 
 and that this peroxide acts with ozone in decomposing organic bodies, though 
 in an inexplicable manner, the Professor considered to be proved. He also 
 referred to the silent discharge probably perpetually going on between the 
 upper and lower strata of the atmosphere, and also between these and the 
 earth, accounting, as the Professor considered, for some of the chemical 
 actions whereby nitrogenous compounds are formed in the soil. 
 
 As far as I am aware, no information as to a possible spectrum of ozone, 
 or a modification of the oxygen or other spectra by its presence, has, up to 
 the present time, been obtained *. 
 
 It has been suggested by Mr. Procter and myself that the electric discharge 
 in an exhausted moist tube, if subjected to a considerable degree of cold, 
 might produce a modification of the air-spectrum, perhaps even a spectrum 
 .analogous to that of the Aurora. 
 
 For some further notes on this subject see Appendix D (Aurora and Ozone). 
 
 Refers to the 
 silent dis- 
 charge 
 between 
 the atmo- 
 sphere and 
 the earth. 
 
 No spec- 
 trum of 
 ozone ob- 
 tained. 
 
 Suggestion 
 to subject 
 electric dis- 
 charge to 
 influence of 
 cold. 
 
 Polarization of the Aurora Light. 
 
 In ' Nature,' vol. vii. p. 201, is contained an account of observations of the 
 polarization of the zodiacal light and of the Aurora, by Mr. A. Cowper 
 Ranyard, who, using both a double-image prism and a Savart on the great 
 Aurora of February 4th, 1872, detected no trace of polarization. He also 
 examined a smaller one of 10th November, 1871, with a like result. 
 
 Mr. Fleming (who refers to these observations) remarks that the only other 
 account he had met with was contained in Prof. Stephen Alexander's Report 
 on his Expedition to Labrador, given in Appendix 21 of the U.S. Coast Survey 
 Report for 1860, p. 30. Professor Alexander found strong polarization with 
 a Savart's polariscope, and thought that the dark parts of the Aurora gave 
 the strongest polarization. This was in latitude about 60, at the beginning 
 of July, and near midnight. It is not stated whether there was twilight or 
 air-polarization at the time, nor is the plane of polarization given. 
 
 The question naturally arises, especially as the darkest parts of the Aurora 
 are usually situated low down near the horizon, whether the polarization in 
 the latter case did not proceed from the atmosphere and not from the Aurora 
 
 See, however, Dr. Schuster's article " On the Spectra of Lightning," Phil. Mag. May 1879, 
 p. 316. 
 
 Polarization 
 of the Au- 
 rora light. 
 
 Mr .Ranyard 
 found none. 
 
 Prof. Alex- 
 ander found 
 strong pola- 
 rization in 
 latitude CO . 
 
46 
 
 THE AUEOEA AND ITS CHAEACTEES. 
 
 Mr. Shroe- 
 der found 
 no polariza- 
 tion. 
 
 Polarization 
 not found in 
 the zodiacal 
 light ; ex- 
 cept faint 
 traces by 
 Mr. Burton. 
 
 Number of 
 Auroras. 
 
 Sir John 
 
 Franklin's 
 
 observations. 
 
 Periodicity 
 as to days 
 not esta- 
 blished. 
 
 Maxima and 
 minima. 
 
 Kaemtz's 
 table. 
 
 Ur. Hayes's 
 observations 
 in winter of 
 1860-61. 
 
 Captain 
 Maguire's 
 observations 
 at Point 
 Barrow as 
 to number 
 and time of 
 appearances. 
 
 itself. Mr. Shroeder found no traces of polarization in the Aurora of 
 February 4th, 1872. Further examinations of the Aurora with some delicate 
 form of polariscope would seem very desirable. 
 
 The evidence of polarization in the case of the zodiacal light seems also 
 almost entirely negative Mr. Eanyard pointing out observations of his own, 
 of Captain Tupman, and of Mr. Lockyer with this result. Mr. Burton, 
 using a Savart set so as to give a black centre when the bands were parallel 
 to the plane of polarization, believed he detected faint traces of polarization 
 in the brightest parts of the zodiacal light (as seen in Sicily), the bands being 
 black-centred when their direction coincided with the axis of the cone of 
 light. Mr. Burton saw no trace of bands when examining the slight remain- 
 ing twilight apart from the zodiacal light. Mr. Ranyard was not able to 
 confirm Mr. Burton's observations on the same evening and with the same 
 instrument. 
 
 Number of Aurora. 
 
 Sir John Franklin saw in the Arctic Regions, in the years 1819, 1820, 
 1821, 1822 : In the month of September two Aurora, in October three, in 
 November three, in December two, in January five, in February seven, in 
 March sixteen, in April fifteen, and in May eleven. 
 
 Periodicity as to days seems to have no certain law ; and though certain 
 days in February and March are marked as those of fine returning displays, 
 they must be looked on as accidental. 
 
 Two well-marked annual maxima seem to occur in March and October (the 
 latter the greater), and two minima in June and January, the greater in 
 June (Encyc. Brit.). The 4th of February, 1872, and same day 1874, are, 
 however, curious instances of a recurring remarkable display. 
 
 A table by Ksemtz, showing the number of Auroras in each month of 
 the year, with the maxima and minima as above stated, will be found on 
 Plate V. fig. 5. 
 
 Dr. Hayes has observed that in the winter of 1860-61 (when the ten or 
 eleven years' inequality was at its maximum) only three Aurorse were seen and 
 recorded, and they were feeble and short in duration. 
 
 Captain Maguire, at Point Barrow (1852-54), reports that the Aurora was 
 seen six days out of seven, and on 1079 occasions, being nearly one third of 
 the hourly observations. It was seldom seen between 9 A.M. and 5 P.M., not 
 at all between 10 A.M. and 4 P.M. It increased regularly and rapidly from 
 5 P.M. until 1 A.M., and then diminished in the same way until 9 A.M. 
 
 The winters of 1877 and-1878 and the springs of 1878 and 1879 have been 
 singularly deficient in Aurorse. I have seen none at Guildown. 
 
SOME QUALITIES OF THE AURORA. 
 
 47 
 
 Duration of Aurora. 
 
 In the article in the ' Edinb. Encyc.' before referred to some remarks are 
 made on the duration of the Aurora. Sometimes it is formed and disappears 
 in the course of a few minutes. At other times it lasts for hours or during 
 the whole night, or even for two or three days together. Musschenbroek 
 observed one in 1734 which he considered to have lasted ten days and nights 
 successively, and another in 1735 which lasted from the 22nd to the 31st 
 March. 
 
 With respect to Captain Maguire's observations (ante) it may be remarked 
 that Aurora? may doubtless frequently run on into and through the day 
 without their being noticed (instances, however, are known of Aurora? seen 
 in daylight) ; and hence it is difficult to judge of the limit of duration of a 
 particular Aurora unless indications are sought for during the day (by the 
 shapes of clouds, action of the magnet, &c.) as well as during the night. 
 Probably Aurora? seen during successive nights may be parts of a continuous 
 discharge. 
 
 The Travelling of Auroras. 
 
 Donati undertook to study the Aurora with reference to the mode of its 
 extension ; and he arrived at the result that the Aurora of February 4, 1872, 
 was not observed in different regions of the earth in the same physical 
 moment ; but everywhere at the same local hour, as in the case of celestial 
 phenomena, which do not share in the earth's rotation. 
 
 The Minister of Foreign Affairs sent a circular to all Italian Consuls, 
 asking them the necessary questions ; and in reply received reports from 
 forty-two places in our hemisphere and from four in the southern, the 
 places embracing in one latitude the considerable extent of 240 degrees of 
 longitude. 
 
 An epitome of the tables (in which the results are divided into three zones) 
 is as follows : 
 
 Zone. 
 
 Mean longitude 
 of zone. 
 
 No. of 
 stations. 
 
 Mean hour 
 of maximum. 
 
 Mean hour 
 of end. 
 
 Eastern .... 
 
 2 hrs. 5 inins. E. 
 
 9 
 
 9ihrs. 
 
 124 hrs. 
 
 Middle .... 
 
 hr. 20 mins. E. 
 
 17 
 
 8 hrs. 
 
 lljnrs. 
 
 Western .... 
 
 5 hrs. 38 mins. W. 
 
 13 
 
 8J hrs. 
 
 9| hrs. 
 
 Duration of 
 
 Aurora. 
 
 Sometimes a 
 few minutes ; 
 at other 
 times the 
 whole night 
 or even days. 
 
 Aurora) may 
 run on into 
 the day 
 without 
 being 
 noticed. 
 
 Travelling 
 of Auroras. 
 
 Donati's in- 
 vestigations. 
 
 Questions 
 sent to 
 Italian Con- 
 suls. 
 
 Table of 
 results. 
 
48 
 
 THE AURORA AND ITS CHARACTERS. 
 
 Extensions 
 of the 
 Aurora. 
 
 The Aurora 
 passed 
 through 
 four periods. 
 First period 
 of origin, 
 light weak. 
 Second 
 period, in- 
 crease of in- 
 tensity. 
 Third period, 
 continuous 
 brightness. 
 Fourth 
 period, de- 
 crease. 
 
 Donati's 
 conclusions. 
 
 Explanation 
 of mode of 
 propagation 
 of same 
 Aurora. 
 
 Donati summed up the facts : That the light phenomena of this Aurora 
 began to show themselves in the extreme east of the southern hemisphere in 
 Eden and Melbourne ; shortly after, they were observed in the east of our 
 hemisphere in China (but not in Japan) ; from China the Aurora passed 
 over the whole of Asia and Europe, and crossed the Atlantic and the 
 American Continent as far as California. It was invisible in Central and 
 South America. During these immense extensions it passed through four 
 periods. In the first (called by Donati the period of origin) the light of the 
 Aurora was pretty weak, and spread from Shanghai to Bombay ; in the 
 second period, during which it passed on from Bombay to Taganrog, it 
 acquired a sudden increase of intensity ; in the third period (called by Donati 
 the normal) the Aurora passed over Europe from east to west with regularity 
 and a continuous brightness; the fourth period, that of decrease, was ob- 
 served in America. The Aurora had a tendency to end earlier in reference 
 to the local hour in the western stations than in the eastern. The accele- 
 ration on an average of the end of the phenomenon was twenty minutes for 
 every hour of longitude. 
 
 Donati concluded that these facts were not reconcilable with the theory of 
 the Aurora depending on meteorological and electro-magnetic phenomena of 
 the globe. Since, too, we have not a yearly, but a ten-yearly period of the 
 Aurora, which coincides with that of sun-spots and terrestrial magnetism, 
 Donati supposed that the cosmic causes of the polar lights were electro- 
 magnetic currents between the sun and the earth. This would explain the 
 mode of propagation of the Aurora of 4th February. Conceive an electric 
 current going from the earth to the sun, or vice versa ; certain phenomena 
 of the Aurora could only be observed in those parts of the atmosphere which 
 have a determinate position or direction with reference to this current ; and 
 consequently these phenomena would be successively visible on the different 
 meridians, as these meridians, by reason of the earth's rotation, assume the 
 same position to the current. For the Aurora to be visible certain meteoro- 
 logical and telluric circumstances must, however, doubtless work together 
 with the cosmical cause. 
 
 Geographi- 
 cal Distri- 
 bution of 
 Aurorse. 
 
 Geographical Distribution of Aurora; (Fritz and Loomis). 
 
 Professors Fritz and Loomis have investigated this subject; and Peter- 
 mann's ' Mittheilungen,' vol. xx. (1874), contains a paper by the former, 
 from which it appears that the northern limit of Auroras chosen by Professor 
 
SOME QUALITIES OF THE AURORA. 
 
 49 
 
 Loomis nearly coincided, except in England, with a line of frequency in 
 Professor Fritz's paper. This line nearly passes through Toronto, Man- 
 chester, and St. Petersburg. Professor Loomis places it as far north as 
 Edinburgh. On a line across Behring's Straits, and coming down below 
 60 X. in America and the Atlantic, and just north of the Hebrides, to 
 Driintheim, and including the most northern points of Siberia, the frequency 
 is represented by 100. 
 
 Within this is another zone of greatest frequency and intensity, which 
 passes just south of Point Barrow, in lat. 72 N., on the northern coast of 
 America, and by the Great Bear Lake to Hudson's Bay, where it reaches a 
 latitude of 60, then on to Nain, on the coast of Labrador, and to the south 
 of Cape Farewell ; then bending sharper to the northward, it passes between 
 Iceland and the Faroe Islands, near to the North Cape, on by the northern 
 ice-sea to Nova-Zembla and Cape Tschejuskin, and on just to the north of 
 the Siberian coast to the south of Kellett Land, thence returning to Point 
 Barrow. 
 
 More or less parallel with this line are the lines on which annually nearly 
 the same number of Aurora are seen. The line for one Aurora annually 
 went from Bordeaux, through Switzerland, past Krakau, south of Moscow 
 and Tobolsk, to the northern end of Lake Baikal, on, to the Sea of Ochotsk 
 and to the Southern Aleutes, thence through Northern California to the 
 mouth of the Mississippi and to Bordeaux. The line for five Aurora 
 annually went from Brest through Belgium, Stettin, Wologda, between 
 Tobolsk and Beresow, parallel to the previous line to Ochotsk, and on to 
 Brest, &c. Almost exactly with the line of greatest frequency coincides 
 the line forming the boundary of the direction of visibility of the Northern 
 Light towards the Pole or towards the Equator; while northwards of this 
 line the Polar Light is seen in the direction towards the Equator ; and from 
 all stations the Northern Lights are seen in directions which are pretty much 
 normal to that curve and the entire system of isochasms. 
 
 Professor Fritz has remarked that the curves of greater frequency tend 
 towards the region of atmospheric pressure, and also that they bear some 
 relation to the limit of perpetual ice tending most southward where, as in 
 North America, the ice limit comes further south. He also endeavours to 
 show a connexion between the periods of maximum of Aurora and those of 
 ice-formation, and considers ice to be an important local cause influencing 
 their distribution. These being most frequently seen over open water in 
 the Arctic regions, has been referred to as noticed by Franklin and others. 
 
 H 
 
 Prof. Fritz's 
 and Prof. 
 Loomis's 
 line of fre- 
 quency. 
 
 Within this 
 another zone 
 of greatest 
 frequency 
 and in- 
 tensity. 
 
 Lines on 
 which annu- 
 ally nearly 
 the same 
 number of 
 Aurora; are 
 seen. 
 
 Assumed 
 
 connexion 
 
 between 
 
 Aurora 
 
 and ice- 
 
 forinatiou. 
 
50 
 
 THE AUEOEA AND ITS CHAEACTEES. 
 
 Extent and 
 principal 
 zone of 
 Aurora. 
 
 M.Moberg's 
 Finland ob- 
 servations 
 (1846-55) 
 compared by 
 Prof. Fritz 
 with those 
 in other re- 
 gions. 
 
 Number 
 limited to 
 Finland 
 only small. 
 
 One third of 
 Auroras seen 
 in America 
 and Europe 
 simultane- 
 ously. 
 
 Extent and principal Zone of the Aurora. 
 
 The Finland observations, published by M. Moberg in his ' Polarlichter 
 Katalogue ' of Northern Lights in the years 1846-55, numbering 1100, have 
 been compared by Prof. Fritz, in his paper in the ' Wochenschrift fiir Astro- 
 nomie,' with the auroral phenomena of the same period in all other regions. 
 The Table shows that of 2035 days of the mouths August to April on 
 which Northern Lights were seen, 1107 days were those of Northern Lights 
 for Finland. On 794 they were visible simultaneously in America, and 
 mostly also in Europe ; on 101 days in Europe only, and on 212 days in Fin- 
 land only. On 958 days Northern Lights were visible in Europe and America 
 which were not visible in Finland. All these numbers refer only to the 
 months August to April, as in the remaining months the brightness of the 
 night in Finland makes such observations impossible. 
 
 The conclusion is arrived at that a large portion of Auroras have no very 
 great extension, or that the causes producing the phenomena must often be 
 of a very local character ; while in another portion of the phenomena the 
 extent, or the regions of simultaneous appearance are very considerable. 
 
 The number limited to Finland, for which hitherto corresponding observa- 
 tions from other lands are wanting, is very small 212, or 19 per cent, of the 
 whole number seen in Finland. With the increase of frequency of the 
 phenomena at the time of maximum, the number observed in Finland and 
 America on the same day increases ; while those observed in Finland and 
 Europe only, or in Finland only, decreased, in accordance with the known 
 law that with the frequency the intensity and extent also increase. 
 
 Between 1826 and 1855, of 2878 days on which, in America, the Northern 
 Lights were seen, there were 1065 on which they were also visible in Europe; 
 so that at least every third day of Aurora? was common to both these portions 
 of the globe. In the years 1846 to 1855, and 1868 to 1872, there were in 
 the first period 657 Northern-Light days common to America and Europe out 
 of 1691, and in the second 397 out of 715. 
 
 The comparison by Prof. Fritz of M. Moberg's Finland observations has 
 been lately reviewed in ' Nature' (March 8, 1878) and the result arrived at 
 that, "After ten years, in spite of the vastly accumulated material of careful 
 observations, there appears no necessity to change Herr Fritz's system of 
 curves in any essential detail ; indeed certain parts of the same, which were 
 at first only based on probability and supposition (the part of the principal 
 zone between the north of Norway and Nishen Kolynisk as an instance), we 
 
SOME QUALITIES OF THE AUKORA. 
 
 51 
 
 now know with perfect certainty to be correct." It has been remarked that 
 the local occurrence of Aurorae is not in accordance with the hypothesis of 
 the phenomenon being one of a cosmical nature. 
 
 The winter of 1870 was remarkable for brilliant displays ; and the displays 
 of October 24th and 25th, 1870, were remarkably brilliant in England and 
 in America also, and the Aurora Australis was seen on the same days at 
 Madras. These displays were seen in England and America in the day- 
 time as patches or corona? of white light, with streamers stretching upward 
 from them. 
 
 Local oc- 
 currence of 
 the Aurora 
 not in favour 
 of its as- 
 sumed cos- 
 mical 
 nature. 
 
 H 2 
 
52 
 
 THE ATJKOEA AND ITS CHAEACTEES. 
 
 Aurora) and 
 clouds. 
 
 Dr.Eichard- 
 son's obser- 
 vations. 
 
 Aurora con- 
 stantly ac- 
 companied 
 by or imme- 
 diately pre- 
 cedes the 
 formation 
 of cirro- 
 stratus. 
 
 Polarity dis- 
 cerned in 
 cirro-stratus 
 clouds. 
 
 Apparent 
 polarity of 
 Aurora 
 might per- 
 haps be 
 ascribed to 
 the clouds 
 themselves. 
 
 CHAPTER VI. 
 THE ATJEOEA IN CONNEXION WITH OTHEE PHENOMENA. 
 
 Aurorce and Clouds. 
 
 DR. RICHARDSON (' Sir John Franklin's Narrative '), so long ago as the years 
 1819-1822, made many recorded observations on the connexion of clouds 
 with the Aurora Borealis in the Polar regions. Some of these are alluded 
 to in Chapter V., section " Height of the Aurora," for the purpose of showing 
 the moderate distance he found it to be above the earth ; and his inference is 
 there mentioned, " that the Aurora Borealis is constantly accompanied by or 
 immediately precedes the formation of one or other of the various kinds of 
 cirro-stratus." On the 13th November and 18th December, 1820, the con- 
 nexion of an Aurora with a cloud intermediate between cirrus and cirro- 
 stratus is mentioned. It is, however, also mentioned that the most vivid 
 coruscations of the Aurora were observed when there were only a few 
 attenuated shoots of cirro-stratus floating in the air, or when that cloud was 
 so rare that its existence was only known by the production of a halo round 
 the moon. (An instance of attenuated streaks of cirro-stratus in connexion 
 with an auroral arc will be found in the Aurora seen at Guildown on the 4th 
 February 1874, a sketch of which is reproduced on Plate VI. fig. 1.) 
 
 Dr. Richardson goes on to express his opinion that he, on some occasions, 
 discerned a polarity in the masses of clouds belonging to a certain kind of 
 cirro-stratus (approaching cirrus), by which their long diameters, having all 
 the same direction, were made to cross the magnetic meridian nearly at right 
 angles. 
 
 Dr. Richardson further suggests that if it should be thereafter proved that 
 the Aurora depends upon the existence of certain clouds, its apparent polarity 
 might perhaps be ascribed to the clouds themselves which emit the light ; or, 
 in other words, the clouds might assume their peculiar arrangement through 
 the operation of one cause (magnetism, for instance), while the emission of 
 light might be produced by another a change in their internal constitution 
 perhaps connected with a motion of the electric fluid. 
 
 Dr. Richardson further remarks that, generally speaking, the Aurora 
 appeared in small detached masses for some time before it assumed that con- 
 
THE AURORA IN CONNEXION WITH OTHER PHENOMENA. 
 
 53 
 
 vergency towards the opposite parts of the horizon which produced the arched 
 form. 
 
 Sir John Franklin says in his Polar expeditions he often perceived the clouds 
 in the daytime disposed in streams and arches such as the Aurora assumes. 
 
 Dr. Low (' Nature,' iv. p. 121) considers he witnessed a complete display of 
 auroral motions in cirrus cloud, and considers all clouds subject to magnetic 
 or diamagnetic polarization ; he states that when the lines converge towards 
 the magnetic pole fine weather follows, and when at right angles to it wet and 
 stormy. 
 
 In the Encyc. Brit, edition 9, article " Aurora Polaris," after referring to 
 the evidence of Franklin, Richardson, and Low, M. Silbermann (' Comptes 
 Rendus,' Ixviii. p. 1051) is quoted in detail for observed connexion between 
 the Aurora and cirrus cloud. 15th April, 1869, at ll h 16 m , an Aurora 
 appeared and disappeared ; but it seemed as if the columns were still visible, 
 and it soon became obvious that fan-like cirrus clouds, with their point of 
 divergence in the north, had taken the place of the Aurora. Between 1 and 
 2 A.M. the clouds had passed the zenith, and let fall a little fine frozen rain. 
 At 4 A.M. the cirrus of the false Aurora was still visible, but deformed towards 
 the top, and presenting a flaky aspect. The cirrus never appeared to replace 
 the Aurora either from right or left, but to substitute itself for it like the 
 changes of a dioramic view. 
 
 Payer, in his 'Austrian Arctic > Voyages,' thinks that the occurrence of the 
 Aurora during the day (i. e. light clouds with its characteristic movement) had 
 been rather imagined than actually observed, and that the transition of white 
 clouds into auroral forms at night has never been satisfactorily proved. He, 
 however, mentions the mist-like appearance of the Aurora. 
 
 Dr. Allnatt observed the splendid Aurora of 4th February, 1872, at Frant, 
 and noticed the weird and wonderful appearance of the phenomena. At 6 P.M. 
 the Aurora commenced by the S.W. portion of the heavens being tinged with 
 a bright carmine hue, and in a short time the whole visible hemisphere was 
 lighted up. A dark elliptical cloud extending from S. to S.E. and S.W. sent 
 up volumes of coloured radii. At 7 the Aurora had passed the zenith, and a 
 dark, broken, rugged cloud some 8 E. of zenith was surrounded by electric 
 light of all hues. At 7.40 the Aurora began to wane, and passed into a 
 homogeneous cirro-stratus of sufficient density to obscure the stars, disappearing 
 at 7.45. 
 
 At a later hour of the night the canopy of cirro-stratus had separated and was 
 transformed into luminous masses of radiant cumulus ; so that, as Dr. Allnatt 
 
 Sir John 
 Franklin's 
 observa- 
 tions. 
 
 Dr. Low's. 
 
 M. Silber- 
 mann's 
 obser- 
 vations, 
 15th April. 
 1869. 
 
 Cirrus 
 clouds took 
 the place of 
 the Aurora. 
 
 Payer 
 thinks the 
 transition of 
 Aurora into 
 clouds not 
 proved. 
 
 Dr.Allnatt's 
 observa- 
 tions, 4th 
 February. 
 1872, at 
 Frant. 
 
 Aurora 
 passed into 
 cirro- 
 stratus. 
 
 Later, cirro- 
 stratus was 
 
54 
 
 THE AUEOEA AND ITS CHABACTEES. 
 
 transformed 
 into lumi- 
 nous cu- 
 mulus. 
 
 Aurora and 
 thunder- 
 storms. 
 Silber- 
 mann's 
 theory. 
 
 Prof. Piazzi 
 Smyth on 
 monthly 
 frequency of 
 Aurora and 
 storms. 
 
 His table 
 of observa- 
 tions. 
 
 Silber- 
 
 mann's 
 
 observations 
 
 15th April, 
 
 1869. 
 
 30th April, 
 
 1865. 
 
 observes, there were called in requisition almost all the most prominent cloud- 
 modifications during the progress of the phenomena. The succession of 
 formation, transformation, and reformation from Aurora to cloud and from 
 cloud to Aurora was, Dr. Allnatt concluded, conclusive of the theory before 
 advanced of the electric origin of the recurrent rayed cloud-modifications in 
 the place of the magnetic meridian, over which so much mystery had been 
 cast. 
 
 Aurora and Thunder-storms. 
 
 Silbermann asserts that Aurora? are produced by the same general phenomena 
 as thunder-storms, and concludes that the Aurora? of 1859 and 1869 assumed 
 the character of thunder-storms which, instead of bursting in thunder, had 
 been drawn into the upper parts of the atmosphere, and their vapour being 
 crystallized in tiny prisms by the intense cold, the electricity became luminous 
 in flowing over these icy particles. 
 
 Professor Piazzi Smyth has observed that the monthly frequency of Aurorae 
 varies inversely with that of thunder-storms. His Table of comparisons is as 
 follows : 
 
 January 
 
 Februa 
 
 March 
 
 April 
 
 May 
 
 June 
 
 July 
 
 August 
 
 Septeml 
 
 October 
 
 onth. Lightning. 
 
 y ...... 24-0 
 
 Auroras. 
 29-7 
 
 iry 14-4 
 7-0 
 
 42-5 
 35-0 
 
 15-4 
 
 27-5 
 
 . . - 37-4 
 
 4-8 
 
 48-0 
 
 o-o 
 
 55-2 
 
 0-5 
 
 t 38-4 
 
 12-6 
 
 iber 22-4 
 :r 20-8 
 
 36-6 
 
 49-4 
 
 iber 15-0 
 
 32-4 
 
 ber 15-0 
 
 28-8 
 
 Mean of whole year 
 
 24-0 
 
 20-1 
 
 Silbermann, on 15th April, 1869, observed a fall of rain (tiny crystals of ice) 
 on the disappearance of an Aurora and its change into cloud forms (see section, 
 "Aurora? and Clouds," p. 53). He also observed a rain of little sparkling ice- 
 prisms on 30th April, 1865, at Paris, the city being then enveloped in a cirrus 
 of vertical fibres similar to that which frequently accompanies the Aurora. 
 
THE AURORA IN CONNEXION WITH OTHER PHENOMKN \. 
 
 55 
 
 On the occasion of the Aurora seen by me at Guildown, 4th February, 
 1872, rain fell immediately succeeding the formation of the corona. 
 
 The falling of rain as an immediate sequence of an Aurora seems, however, 
 to be rather the exception than the rule ; but possibly this may vary with the 
 character of the Aurora itself whether it be of the crimson class, passing 
 into cloud and accompanied with much electric disturbance, or of the more 
 quiescent white. 
 
 A falling barometer following a display of Aurora? has been noticed by Sir 
 John Franklin and others ; and in some cases (notably one in Sicily before 
 referred to) storms and floods have accompanied this. 
 
 In a paper read before the Royal Society of Edinburgh in 1868, Prof. 
 Christison mentioned, as a fact of importance to agriculturists, that the first 
 great Aurora after autumn is well advanced, and following a period of fine 
 weather, is a sign of a great storm of rain and wind in the forenoon of the 
 second day afterwards. 
 
 Mr. C. L. Prince, in his ' Climate of Uckfield,' p. 218, remarks that displays 
 of Aurora? are almost invariably followed by very stormy weather, after an 
 interval of from 10 to 14 days. 
 
 Aurora and the Magnetic Needle: 
 
 Sir John Franklin, in his ' Narrative ' (before referred to), gives Lieutenant 
 Robert Hood, R.N., the credit of being "the first who satisfactorily proved, by 
 his observations at Cumberland House (before mentioned), the important fact 
 of the action of the Aurora upon the compass-needle," and also " to have 
 proved the Aurora to be an electrical phenomenon, or at least that it induces 
 a certain unusual state of electricity in the atmosphere." Sir John Franklin 
 then mentions that the motion communicated to the needle was neither sudden 
 nor vibratory. Sometimes it was simultaneous with the formation of arches, 
 prolongation of beams, or certain other changes of form or of activity of the 
 Aurora. But generally the effect of these phenomena upon the needle was 
 not visible immediately ; but in about half an hour or an hour the needle had 
 obtained its maximum of deviation. From this its return to its former position 
 was very gradual, seldom regaining it before the following morning, and fre- 
 quently not until the afternoon, unless it was expedited by another arch of 
 the Aurora operating in a direction different from the former one. The mag- 
 netic needle in the open air was disturbed by the Aurora whenever it approached 
 the zenith. Its motion was not vibratory (as observed by Mr. Dalton), perhaps 
 owing to the weight of the card. It moved slowly to the E. or W. of the 
 
 A falling 
 barometer 
 observed to 
 follow 
 Aurone. 
 Professor 
 Christison's 
 observa- 
 tions. 
 
 Aurora and 
 the mag- 
 netic needle. 
 
 Sir .John 
 Franklin's 
 observa- 
 tions. 
 
 Motioneom- 
 municated 
 to the needle 
 was neither 
 sudden nor 
 vibratory. 
 Return of 
 needle to its 
 former posi- 
 tion vi -r\ 
 gradual. 
 
56 
 
 THE ATJEOKA AND ITS CHAEACTEES. 
 
 Different 
 positions of 
 the Aurora 
 had consi- 
 derable in- 
 fluence on 
 the direc- 
 tion of the 
 needle. 
 
 Needle dis- 
 turbed when 
 Aurora not 
 visible. 
 
 Quiescent 
 
 yellow 
 
 Aurora 
 
 produced no 
 
 perceptible 
 
 effect on 
 
 needle. 
 
 Eeturn of 
 needle more 
 speedy after 
 formation 
 of a second 
 arch. 
 
 Slow when 
 disturbance 
 was consi- 
 derable. 
 
 magnetic meridian, and seldom recovered its original direction in less than 
 eight or nine hours. The greatest extent of its aberration was 45'. The 
 arches of the Aurora were remarked commonly to traverse the sky nearly at 
 right angles to the magnetic meridian ; but deviation was not rare, and it was 
 considered that the different positions of the Aurora had considerable influence 
 on the direction of the needle. When an arch was nearly at right angles to 
 the magnetic meridian, the motion of the needle was towards the W. This 
 motion was greater when the extremity of the arch approached from the west 
 towards the magnetic north. A westerly motion also took place when the 
 extremity of an arch was in the true north, or about 36 to the west of the 
 magnetic north. The motion of the needle was towards the east when the 
 same end of an arch originated to the southward of the magnetic west, and 
 when of course its opposite extremity approached nearer to the magnetic 
 north. In one case only a complete arch was formed in the magnetic meridian. 
 In another the beam shot up from the magnetic north to the zenith. In both 
 these cases the needle moved towards the west. The needle was most 
 disturbed on February 13, 1821, at a time when an Aurora was distinctly 
 seen passing between a stratum of clouds and the earth. Sometimes the 
 needle deviated though no Aurora was visible ; but it was uncertain whether 
 there might riot have been a concealed Aurora at the time. Clouds were 
 sometimes observed during the day to assume the form of the Aurora, and 
 deviations of the needle were occasionally remarked at such times. An 
 Aurora sometimes approached the zenith without producing any change of 
 position of the needle ; while at other times a considerable alteration took 
 place, though the beams or arches did not come near the zenith. The Aurora 
 was frequently seen without producing a perceptible effect on the needle. At 
 such times it was generally an arch or a horizontal stream of dense yellowish 
 light with little or no internal motion. The disturbance of the needle was 
 not always proportionate to the agitation of the Aurora, but was always 
 greater when the quick motion and vivid light were observed to take place 
 in a hazy atmosphere. In a few instances the needle commenced at the instant 
 a beam started from the horizon upwards ; and its return was according to 
 circumstances. If an arch formed immediately afterwards, having its extremities 
 placed on opposite sides of the magnetic north and south to the former one, 
 the return of the needle was more speedy, and it generally went beyond the 
 point from which it first started. When the disturbance was considerable, it 
 seldom regained its usual position before 3 or 4 P.M. on the following day. 
 On one occasion oly the needle had a quick vibratory motion (between 
 
THE AURORA IX CONNEXION WITH OTHER PHENOMENA. 
 
 57 
 
 343 50' and 344 40'). The disturbance produced by the Aurora was so great 
 that no accurate deductions as to diurnal variation could be made. 
 
 Payer, in his 'New Lands within the Arctic Circle' (vol. i. pp. 327, 328), 
 gives the result of the magnetic observations on board the Austrian ship 
 ' Tegetchoff ' in the years 1872-74, made by means of a magnetic theodolite, 
 a dipping-needle, and three variation instruments. The extraordinary distur- 
 bances of the needle rendered the determination of exact mean values for the 
 magnetic constants impossible. The following were the principal results of 
 these observations : 
 
 (1) The magnetic disturbances were of extraordinary magnitude and 
 frequency. 
 
 (2) They were closely connected with the Aurora, and they were greater as 
 the motion of the rays was more rapid and fitful and the prismatic colours 
 more intense. Quiescent and regular arches, without changing rays or 
 streamers, exercised mostly no influence on the needle. 
 
 (3) In all the disturbances the declination-needle moved towards the east, 
 and the horizontal intensity decreased while the inclination increased. 
 
 Sir John Franklin sums up his information as to the needle to much 
 the same effect, viz. that brilliant and active coruscations cause a deflec- 
 tion almost invariably if they appear through a hazy atmosphere and if 
 the prismatic colours are exhibited in the beams or arches. On the contrary, 
 when the air is clear and the Aurora presents a steady dense light of a 
 yellow colour and without motion, the needle is often unaffected by its 
 appearance. 
 
 Parry (Third Voyage) found his variation-needle (extremely light and 
 delicately suspended) in no instance affected by the Auroras ; but he seems to 
 have principally met with the quiescent form of that phenomenon. 
 
 M. Lottin, the French savant (whose description of an Auroral display has 
 been given in Chapter II.), observed in the North Sea, between September 
 1838 and April 1839, while the sun was below the horizon, 150 Aurora. 
 During this period 64 were visible, "besides many which a cloudy sky 
 concealed, but the presence of which was indicated by the disturbances they 
 produced upon the magnetic needle " (Lardner's ' Museum of Science and Art,' 
 vol. x. p. 189). 
 
 It has been remarked by some observers that grand displays of the Aurora 
 are frequently preceded or accompanied by an extraordinary motion of the 
 needle to the westward. 
 
 Captain Maguire found at Point Barrow (1852-54) that the appearance of 
 
 I 
 
 Magnetic 
 observations 
 on board the 
 'Tegetehoff.' 
 
 Distur- 
 bances 
 great. 
 Greater 
 as the rays 
 were rapid. 
 Quiescent 
 arches exer- 
 cised no 
 influence. 
 Declination- 
 needle, 
 effects on. 
 
 Parry's ex- 
 perience. 
 
 Grand dis- 
 plays ac- 
 companied 
 by motion 
 of needle to 
 the west. 
 
58 
 
 THE AUEOEA AND ITS CHARACTEES. 
 
 Solar dis- 
 turbances 
 and Aurora. 
 
 Cipoletti's 
 observation. 
 
 Dr. Thomp- 
 son con- 
 cludes that 
 cylinders of 
 Aurora 
 cannot be 
 doubted to 
 be magnets. 
 
 the Aurora in the south was connected with the motion of the magnet to the 
 east of the magnetic north, and if in the north to the west of the same. 
 
 On an occasion in 1859 great solar disturbances were observed, the 
 Greenwich magnets were much disturbed, and a fine Aurora was visible. 
 
 Cipoletti, of Florence, remarks on the strong magnetic disturbances at 
 Vienna and Munich during the Aurorse of 4th February, 1872, and 4th 
 February, 1874. 
 
 Dr. Thompson, in his 'Annals of Philosophy,' vol. iv. p. 431 (1814), mentions 
 as an authenticated fact that during the prevalence of the Aurora the magnetic 
 needle was frequently observed to become unsteady, and (p. 432) concludes 
 that cylinders of Aurora cannot be doubted to be magnets. The only three 
 bodies capable of assuming magnetic properties are iron, nickel, and cobalt. 
 When meteors are considered, it is not altogether extravagant to conjecture 
 that bodies similar in their nature to some of the solid bodies which constitute 
 our globe may exist in some unknown state in the atmosphere. 
 
 During the Aurora of 13th May, 1869, the declination at Greenwich varied 
 1 25', while the vertical force experienced four successive maxima, and 
 the greatest oscillation amounted to 0-04 of the total mean value. The hori- 
 zontal force varied only 0-014 of its mean value. 
 
 During the Aurora of 15th April, 1869, the declination at Stony hurst varied 
 2 23' 14" in nine minutes. 
 
 Aurorse, 
 magnetic 
 distur- 
 bances, and 
 sun-spots 
 in Italy. 
 
 AurorcB, Magnetic Disturbances, and Sun-spots. 
 
 Aurorse were frequent in Italy in April 1871. On the 10th a remarkable 
 one was seen, with declinometer deflected towards the east, and 63 sun-spots 
 were counted. On the morning of the 10th the deflection continued, and at 
 midday 97 sun-spots were counted. 
 
 On the 18th a brilliant Aurora lasted to 10 o'clock at night. From this 
 time till the 23rd the Aurora appeared constantly, giving a reddish tinge 
 in the north and north-west. A brilliant display took place on the evening 
 of the 23rd. On the evenings when the Aurora appeared the magneto- 
 meters were disturbed throughout Italy, and ended by a violent agitation 
 during the whole of the 24th. Sun-spots were observed at Eome, Palermo, 
 and Moncalieri, but the greater number on the days of the Aurorse. A 
 brilliant display at Moncalieri on June 18 was accompanied by very violent 
 magnetic disturbance. 
 
 September 25, 1870, Mr. Proctor counted 102 spots on the solar disk ; and 
 
Plate IX 
 
 Vi.t-.-L 
 
 THREE. AURORA SPECTRA AND EIGHT OTHERS FOR COMPARISON. 
 
 1) K b Y 
 
 I I I 
 
 1 1 
 
 Li 
 
 T 
 
 I 
 
 _!L 
 
 I i 
 
 G 
 
 _L 
 
 CH 
 
 Cl 
 
 rc 
 
 i 
 
 AV 
 
 4 5 6 7 8 a 1O 11 
 
 S. Solar Spectrum. N, Nitrogen (Air) \Va1ts. O, Oxygen (Air) Walls. 
 C.H, CarVHyd. v.tuLe Watts. Cl,Carb.Hyi flarno C.C.Ciuidlr ^apr.u 
 O.P, Oxygen v, tube Prorter. IJron Walls. A , Aurora (iype) AV, Aurora Voge] 
 B73. Aurnru by liarkcr in 1873. 
 
 B73 
 
 CHART 
 
 178O 
 
 F i : 2 . 
 
 LOOM IS 
 
 790 18OO 1810 1S2O 1B3O J84-O 185O 1UGO 
 
 1870 
 
 i Brua -lit 
 
THE AUKOKA IN CONNEXION WITH OTHER PHENOMENA. 
 
 59 
 
 on the night of the 24th and morning of the 25th an Aurora of unwonted 
 magnificence was visible at various stations in England, France, and Germany. 
 
 With respect to sun-spots and the magnet, the frequency of magnetic 
 storms, causing oscillation of the needle, gradually increased from a minimum 
 in 1843 to a maximum in 1848, giving a variation of something near 11 
 years altogether. Schwabe observed the sun-spots for 24 years, and found 
 they had a regular maximum and minimum every five years, and that the 
 years 1843 and 1S|8 were minimum and maximum years coinciding with the 
 magnetic variation at those periods. 
 
 Professor Loomis (' American Journal of Science,' vol. v. April 1873) 
 considers that a comparison between the mean daily range of the magnetic 
 declination and the number of Auroras observed in each year, and also with 
 the extent of the black spots on the surface of the sun, establishes a con- 
 nexion between these phenomena, and indicates that auroral displays (at 
 least in the middle latitudes of Europe and America) are subject to a law 
 of periodicity, that their grandest displays are repeated at intervals of 
 about CO years, and that there are also other fluctuations, less distinctly 
 marked, which succeed each other at an average interval of about 10 or 11 
 years, the times of maxima corresponding quite remarkably with the maxima 
 of solar spots. 
 
 An illustration of the result of these observations is given on Plate IX. 
 fig. 2. The curves are in close correspondence, and the coincidence at the 
 times of maximum and minimum is remarkable. The auroral maximum 
 
 | 
 
 generally occurs a little later than the magnetic maximum ; and the con- 
 nexion between the auroral and magnetic curves appears somewhat more 
 intimate than between the auroral and sun-spot curves. 
 
 Professor Loomis contends " that the black spot is a result of a distur- 
 bance of the sun's surface, which is accompanied by an emanation of some 
 influence from the sun, which is almost instantly felt upon the earth in an 
 unusual disturbance of the earth's magnetism, and a flow of electricity, deve- 
 loping the auroral light in the upper regions of the earth's atmosphere." 
 
 This connexion between the sun's spots and the earth's magnetism has 
 been considered as proved ; and one instance at least of an intense disturbance 
 and outbreak of the sun's surface having been observed simultaneously with 
 the occurrence of a terrestrial magnetic storm is a matter of record. This 
 will be found detailed in the ' Monthly Notices of the Royal Astronomical 
 Society,' vol. xx. pp. 13 and 15, and is so interesting in its character that it 
 may be briefly referred to here. Mr. R. C. Carrington, September 1, 1859, 
 
 i2 
 
 Proctor's 
 sun-spots 
 and Aurora. 
 
 Sun-spots 
 and the 
 magnet. 
 11 years' 
 period. 
 Schwabe's 
 sun-spot 
 period. 
 
 Prof. 
 
 Loomis 
 considers 
 connexion 
 established 
 between 
 magnetic 
 declination, 
 auroral dis- 
 plays, and 
 sun-spots. 
 
 Illustrative 
 table of co- 
 incidences. 
 
 Prof. 
 Loomis 
 considers 
 a sun-spot 
 a solar dis- 
 turbance 
 affecting the 
 earth's mag- 
 netism. 
 
60 
 
 THE AUEOEA AND ITS CHAEACTEES. 
 
 Carrington 
 and Hodg- 
 son's obser- 
 vations of 
 bright spots 
 on the sun, 
 accompa- 
 nied by 
 magnetic 
 disturbance 
 at Kew, and 
 followed by 
 wide-spread 
 Auroras. 
 
 Mr. John 
 Allan 
 Broun's 
 magnetic 
 oscUlation- 
 curves ; 
 showing 
 that the 
 
 ll h 18 m , while observing and drawing a group of solar spots, saw suddenly 
 two patches of intense bright light break out in the middle of the group. 
 The brilliancy was fully equal to that of direct sunlight. Seeing the outbreak 
 was on the increase, Mr. Carrington left the telescope, to call some one to 
 witness it. On his return within sixty seconds it was nearly concluded. The 
 spots travelled from their first position, and vanished as two rapidly fading 
 dots of white light. In five minutes the two spots traversed a space of about 
 35,000 miles. Mr. Carrington found no change in the group itself. His 
 impression was that the phenomena took place at an elevation considerably 
 above the general surface of the sun, and above and over the great group of 
 spots on which it was seen projected. It broke out at ll h 18 m , and 
 vanished at ll h 23 m . Mr. R Hodgson independently on the same day, and at 
 close upon the same time, saw a very brilliant star of light, much brighter 
 than the sun's surface, most dazzling to the protected eye, illuminating the 
 upper edges of the adjacent spots and streaks. The rays extended in all 
 directions, and the centre might be compared to a Lyrse when seen in a large 
 telescope. It lasted for some five minutes. 
 
 At the very moment of this solar disturbance the instruments at Kew indi- 
 cated a magnetic storm; and Proctor, in his volume on the Sun, page 206, 
 details how this magnetic storm was accompanied by very widely-spread 
 indications of electrical disturbance in many parts of the globe. Vivid 
 Auroras were seen not only in both hemispheres, but in latitudes and places 
 where they are seldom witnessed. Eome, Cuba, and the West Indies, the 
 tropics within 18 of the equator, and even South America and Australia, are 
 thus referred to for displays. At Melbourne, on the night of September 2nd, 
 the greatest Aurora ever seen there made its appearance. 
 
 It was observed, too, that magnetic communication was at the same time 
 disturbed all over the earth. Strong currents, continually changing their 
 direction, swept along the telegraphic wires. At Washington and Phila- 
 delphia the signal-clerks received severe shocks, and the wires had to give up 
 work. At a station in Norway the transmitting apparatus was set fire to ; 
 and at Boston, in North America, a flame of fire followed the pen of Baine's 
 electric telegraph. 
 
 In an interesting communication to ' Nature ' (January 3rd, 1878), entitled 
 " The Sun's Magnetic Action at the Present Time," Mr. John Allan Broun 
 has contributed some magnetic oscillation-curves, deduced from observa- 
 tions made in the Trevandrum Observatory (nearly on the magnetic equator), 
 by which, if confirmed by other observations, it would appear that the sun's 
 
THE AURORA IN CONNEXION WITH OTHER PHENOMENA. 
 
 Gl 
 
 magnetic action has lately become gradually more constant. The curves are 
 three in number, no. 1 for the years 1855-58, no. 2 for the years 1865-68, 
 no. 3 for the years 1874-77. In no. 1 curve the minimum is very clearly 
 marked by two points corresponding to April 1 and May 1, 1856, and there 
 is little difference in the rapidity with which the curve descends to and 
 ascends from the minimum. In no. 2 curve the epoch of minimum is by no 
 means so well marked ; it occurs between the points for April 1 and Sep- 
 tember 1, 18G6. There is also a considerable difference in the rapidity of 
 variation in the descending and ascending branches of the curve. The 
 descent is nearly as rapid as in curve no. 1 ; but the ascent is very much 
 slower. In curve no. 3 the lowest point is that for December 1, 1875 ; but 
 it is even now, with points a year and a half later, difficult to say whether 
 this is the minimum or not, the point for January 1, 1877, being only 0-02 
 (two hundredths of a minute of arc) higher. In this curve the change of 
 range in diurnal oscillation is quite insignificant from November 1, 1874, to 
 April 1, 1877, an interval of three years and five months. In the diagram 
 given by Mr. Broun the curves show themselves gradually flattening, no. 3 
 being almost a straight line. 
 
 Mr. Broun remarks upon the report of Sir George Nares as to the insig- 
 nificant nature of the Auroras seen in the Arctic Expedition in the winter of 
 1875-76, and the accompanying statement that, as far could be discovered, 
 they were totally unconnected with any magnetic or electric disturbance ; and 
 states, as the result of his own experience in the south of Scotland, that several 
 of the Aurorse observed by him were of the very faintest kind, " were traces " 
 which he could never have remarked had he not been warned by very slight 
 magnetic irregularities to examine the sky with the greatest attention. 
 Again, in no case had he seen the faintest trace of an Aurora without finding 
 at the same time a corresponding irregularity in the movement of the force or 
 declination-magnet. 
 
 Prof. Piazzi Smyth, commenting on this article, makes the inquiry how the 
 sun-spot 'cycle and the terrestrial magnetic oscillation cycle can be considered 
 as agreeing, the sun-spot cycle, according to Prof. Wolf, being ll'lll years, 
 and the magnetic cycle 10-5 years according to Mr. Broun. 
 
 Another correspondent writes and quotes M. Faye, in 'La Meteorologie 
 Cosmique,' for the remark, " La periode des taches portee all ans '1 par M. 
 Wolf n'etant pas egale a celle des variations magnetiques (10 ans '45), ces 
 deux phenomenes n'ont aucun rapport entre eux." 
 
 Mr. Broun, in a further letter, rejoins that if we could accept Dr. Wolfs 
 
 sun s mag- 
 netic a. 
 has lately 
 become more 
 constant. 
 
 In diagram, 
 curves gra- 
 dually 
 flatten. 
 
 Mr. Broun 
 never found 
 an Aurora 
 without a 
 correspond- 
 ing irregu- 
 larity in the 
 declination- 
 needle. 
 
 Prof. Pia/zi 
 Smyth com- 
 ments on 
 variance in 
 the cycles. 
 
 M. Faye's 
 remarks to 
 a similar 
 effect. 
 
62 
 
 THE AITEOEA AND ITS CHAEACTEES. 
 
 Mr. Broun's 
 rejoinder 
 and expla- 
 nation. 
 
 Mr. Jen- 
 kins's expla- 
 nation of 
 Prof. 
 Loomis's 
 chart. 
 
 Jupiter's 
 suspected 
 connexion 
 with sun- 
 spots. 
 
 Infre- 
 quency of 
 Auroras and 
 absence of 
 sun-spots in 
 1876-78. 
 
 view we should find that the mean duration of a cycle for both phenomena 
 since 1787 would be 11-94 years, while the sun-spot results for eight cycles 
 determined by Dr. Wolf during eighty years before 1787 give 10-23 or, if we 
 take nine cycles, 10'43 years for the mean duration. It is by mixing these 
 two very different means that the Zurich philosopher finds 11-1 years, a mean 
 which Mr. Broun considers can evidently have no weight given to it. On 
 the other hand, if Dr. Wolf is in error (as Mr. Broun believes he is) as to 
 the existence of a maximum in 1797, the mean durations for the eighty years 
 after and for the eighty years before 1787 agree as nearly as the accuracy of 
 the determinations for the beginning of the eighteenth century will permit. 
 Mr. Broun then repeats his conviction that the sun-spot maxima and minima 
 are really synchronous with those of the magnetic diurnal observations. 
 
 Mr. B. G. Jenkins, in a letter to 'Nature,' refers Prof. Smyth to Prof. 
 Loomis's chart of magnetic oscillations given in Prof. Balfour Stewart's paper 
 in ' Nature ' (vol. xvi. p. 10), for the purpose of showing that there are exactly 
 seven minimum periods from 1787 to 1871, the mean of which is twelve 
 years, the mean of the seven corresponding maximum periods being ITS 
 years. The true magnetic declination-period is, then, the mean of these, viz. 
 11'9 years. In exactly the same manner he finds that the mean period 
 of sun-spots is !! 9 years. 
 
 The auroral displays also have the same period. Mr. Jenkins also refers 
 to Wolf, De La Rue, Stewart, and Loewy, as having stated their belief that 
 Jupiter is the chief cause in the production of sun-spots, and draws attention 
 to the period of 11'9 years as being Jupiter's anomalistic year, or the time 
 which elapses between two perihelion passages. 
 
 The infrequency of Aurora during the years 1876-78, and a corresponding 
 comparative absence of sun-spots, may be added to the evidence on the 
 subject. I have seen no account of important Auroras during the years 
 mentioned, and day after day has recently (1878) passed with a perfectly 
 clean sun-disk. 
 
 Aurora and 
 electricity. 
 
 Sir John 
 Franklin's 
 experience 
 with elec- 
 trometer. 
 
 Aurora and Electricity. 
 
 Sir John Franklin failed to get indications of electricity connected with the 
 Aurora with a pith-ball electrometer ; but with another form of electrometer 
 specially constructed for the purpose he seems to have got some, though not 
 very strong or regular, indications of repulsion between the needle of the 
 instrument and the conductor when Aurorae were seen. He does not decide 
 
THE AUKORA IN CONNEXION WITH OTIIEE PHENOMENA. 
 
 63 
 
 whether the electricity was received from or summoned into action by the 
 Aurora. 
 
 Parry, at Fort Bowen, with a gold leaf electroscope connected with a chain 
 attached by glass rods to the skysail mast-head, 115 feet above sea-level, found 
 no effect. 
 
 Dr. Allnatt, at Frant, during the display of 4th February, 1872, found the 
 earth's electricity so powerful that the gold leaves of the electrometer remained 
 divergent for a considerable time. 
 
 M'Clintock observes that on six occasions of Aurora in Baffin's Bay, the 
 electroscope was strongly affected, and on three occasions of Aurora at Port 
 Kennedy. The electricity was always positive. 
 
 Dec. 18. Dr. AValker called him to see the electroscope. The charge was 
 at first weak, but afterwards strong enough to keep the leaves diverged. 
 Dr. Walker found two periods of minimum electrical disturbance about 9 P.M. 
 and noon. 
 
 Electric currents have been reported as produced in telegraph wires during 
 Aurora. Though transient they are said to be often very powerful, and to 
 interrupt the ordinary signals. Loomis (Sillim. Journ. vol. xxxii.) mentions 
 cases where wires have been ignited, brilliant flashes produced, and com- 
 bustible materials kindled by their discharge. 
 
 Here, too, we may note the account of electric phenomena in the case of the 
 Aurora Australis described (ante, p. 28) by Mr. Proctor. 
 
 Mr. George Draper, of the British-Indian Submarine Telegraph Company, 
 speaking of the Aurora of February 4, 1872 (and writing to the ' Times ' 
 under date February 5th), states that the Aurora visible in London was also 
 visible at Bombay, Suez, and Malta, and that the Company's electrician at 
 Suez reported that the earth-currents there were equal to 170 cells (Daniell's 
 battery), and that sparks came from the cable. The electrical disturbances 
 lasted until midnight, and interrupted the working of both sections of the 
 British Indian Cable between Suez and Aden, and Aden and Bombay. For 
 some days previously the signals on the British Indian cables had been much 
 interfered with by electrical and atmospheric disturbances. 
 
 At Malta there was a severe storm on the morning of the 4th, so that it 
 was necessary to join the cable to earth for some hours, and the Aurora was 
 very large and brilliant there. 
 
 The electrical disturbances on the cables in the Mediterranean and on those 
 between Lisbon and Gibraltar, and Gibraltar and the Guadiana, were also 
 very great. The signals on the land line between London and the Land's End 
 
 Parry's 
 experience. 
 
 Dr.Allnatt's 
 experience, 
 February 4, 
 1872. 
 
 M'Clintock 
 found elec- 
 troscope 
 affected in 
 Baffin's Bay 
 and Port 
 Kennedy. 
 
 Electric 
 currents in 
 telegraphic 
 wires during 
 Aurora. 
 
 Mr. George 
 Draper's 
 report as to 
 disturbed 
 condition of 
 the Indian 
 Submarine 
 and other 
 cables 
 during 
 Aurora of 
 February 4, 
 1872. 
 
64 
 
 THE AURORA AND ITS CHARACTERS. 
 
 Aurora and 
 meteoric 
 dust. 
 
 Theories of 
 Dr. Zeyfuss 
 and M. 
 Groneman. 
 
 Ferruginous 
 dust in the 
 Polar 
 Regions. 
 
 Baum- 
 hauer's pro- 
 position. 
 
 were interrupted for several hours on the night of the 4th by atmospheric 
 currents. Similar effects accompanied the displays of Oct. 24 and 25, 1870. 
 
 Aurora and Meteoric Dust. 
 
 A theory has been propounded independently by Dr. Zeyfuss and by 
 M. Groneman, of Groningen, according to which the light of the Aurora is 
 caused by clouds of ferruginous meteoric dust ignited by friction with the 
 atmosphere. Groneman shows that these might be arranged along the mag- 
 netic curves by the action of the earth's magnetic force during their descent, 
 and that their influence might produce the observed magnetic disturbances. 
 
 The arches might be accounted for by the effects of perspective ; and the 
 iron spectrum shows correspondence with some of the lines of the Aurora. 
 Ferruginous particles have been found in the dust of the Polar regions accord- 
 ing to Professor Nordenskiold, but whether derived from stellar space or from 
 volcanic eruption is uncertain. A difficulty has been suggested that while 
 meteors are more frequent in the morning, or on the face of the earth which 
 is directed forward on its orbit, the reverse prevails in the case of Aurorse. 
 Groneman meets this by supposing that in the first case the velocity may be 
 too great to allow of arrangement by the earth's magnetic force. He accounts 
 for the infrequency of the Aurora in equatorial regions by the weakness of the 
 earth's magnetic force, and the fact that when it does occur the columns must 
 be parallel to the earth's surface. 
 
 Baumhauer (Compt. Kend. vol. Ixxiv. p. 678) advances, as regards Polar 
 Aurora?, the proposition, that not only solid masses large and small, but clouds 
 of " uncondensed " (meteoric) matter probably enter our atmosphere. 
 
 If from our knowledge of the meteoric stones which fall to the earth's 
 surface we may draw any conclusion respecting the chemical constitution of 
 these clouds of matter, it would appear that they may contain a considerable 
 portion of the magnetic minerals iron and nickel. Let such a cloud approach 
 our earth, regarded as a great magnet, it would be attracted towards the 
 Pole, and, penetrating our atmosphere, the particles which have not been 
 oxidized, and are in a state of extremely fine division, would by their oxidation 
 generate light and heat, producing the polar Aurora?. Baumhauer suggests 
 it would be interesting in support of this theory to detect in the soil of polar 
 areas the presence of nickel. The presence of iron and nickel in meteoric 
 masses in considerable quantities is frequent ; and cases are also on record by 
 Eversmann of hailstones containing crystals of a compound of iron and sulphur, 
 by Pictet of hailstones containing nuclei which proved to be iron, and by 
 
THE AUEORA IN CONNEXION WITH OTHER PHENOMENA. 
 
 65 
 
 Cozari of hailstones containing nuclei of an ashy-grey colour, the larger ones 
 of which were attracted by the magnet, and found to contain iron and nickel. 
 Nickel was found by Reichenbach in parts of Austria on hills consisting of 
 beds of sandstone and limestone, and quite free from metallic veins. 
 
 Mr. J. W. N. Lefroy, in ' Nature,' describes a phenomenon seen by him at 
 Fremantle, West Australia, in the month of May, which he designates 
 " A Lunar Rainbow, or an Intra-lunar convergence of Streams of slightly 
 illuminated Cosmic DustV 
 
 It lasted about three quarters of an hour, and consisted of one grand central 
 feather, of very bright white cloud, springing out of the horizon at W.N.W., 
 and crossing the meridian at about 20 north of the zenith, with a width of 
 7 to 8. 
 
 On either side of this was a system of seven or eight minor beams of light, 
 extending from the W. to the E. horizon, subtending a chord common to them- 
 selves and to the main stream, and converging towards the axis of the central 
 stream so as to intersect it at a point about 30 or 40 below the western horizon, 
 at which the whole system subtended an azimuth of about 20. Near the zenith, 
 where its transverse section was a maximum, that section subtended an angle 
 of about 40. 
 
 The idea strongly suggested itself to Mr. Lefroy of converging streams of 
 infinitely minute particles of matter passing through space at a distance from 
 the earth at which its aerial envelope may have still a density sufficient by 
 its resistance to give cosmic dust passing through it that illumination which 
 it possessed. In about twenty minutes the streams of light had attained their 
 maximum brightness. Their apparent figure was that of a nearly circular 
 (slightly flattened) arc of an amplitude of 15 or 20, as viewed from the middle 
 point of its chord. 
 
 The brightness and the convergence of the streams were both more marked 
 towards the western horizon than the eastern. This same phenomenon was 
 described in the ' South-Australian Register ' as a beautiful lunar rainbow 
 visible in the western heavens. 
 
 Mr. Lefroy and other observers concurred in the impression that the minor 
 lateral streams on the N. side of the main one intervened between the eartli 
 and the moon, and that one or more of them in their slow vibrations swept the 
 surface of the moon and sensibly obscured its light. There can be hardly any 
 question that the phenomenon observed was in fact an Aurora. 
 
 It may be a question whether iron and nickel particles of meteoric origin 
 do not ordinarily exist in the atmosphere in a greater degree than we suspect, 
 
 K 
 
 Mr. Lef rev's 
 description 
 of a pheno- 
 menon 
 ascribed by 
 him to 
 streams of 
 cosmic dust. 
 
 Suggestion 
 as to collect- 
 
THE ATJKOEA AND ITS CHAEACTEES. 
 
 ing iron and 
 nickel parti- 
 cles from the 
 atmosphere. 
 
 The Aurora 
 and planets 
 Venus and 
 Jupiter. 
 
 The planet 
 Venus's halo 
 during 
 Aurora. 
 
 Dr. Miles's 
 observation 
 of Venus 
 during an 
 Aurora. 
 
 Brightness 
 of stars 
 during 
 Auroras. 
 
 Aurora of 
 Oct. 24, 
 1870, and 
 Jupiter. 
 
 and might be detected if special means, such as magnets, plates of glass 
 covered with glycerine, &c., were adopted for the purpose of collecting and 
 examining the cosmic dust. Larger gatherings than usual of iron and nickel 
 particles during the presence of Aurorae would be in support of Mr. Lefroy's 
 theory. 
 
 The Aurora and the Planets Venus and Jupiter. 
 
 During a brilliant Aurora seen at Sunderland, February 8, 1817 ('Annals 
 of Philosophy,' p. 250), about 8 o'clock, Venus was about 8 above the horizon, 
 and displayed a very peculiar appearance. Her rays passed through a thin 
 mist or cloud, probably electric, of a deep yellow tint. Her apparent mag- 
 nitude seemed increased, and a halo was formed round her as sometimes 
 appears round the moon in moist weather ; but the stars that were in that 
 part of the heavens shone with their accustomed brilliancy. 
 
 The Rev. T. W. Webb, in his ' Celestial Objects ' (1859), p. 43, quoting 
 from the Philosophical Transactions, mentions that, " January 23rd, 1749-50, 
 there was a splendid Aurora Borealis about 6 P.M. The Rev. Dr. Miles, at 
 Tooting, had been showing Jupiter and Venus to some friends with one of 
 Short's reflectors, greatest power 200, when a small red cloud of the Aurora 
 appeared, rising up from the S.W. (as one of a deeper red had done before), 
 which proceeded in a line with the planets and soon surrounded both. Venus 
 appearing still in full lustre, he viewed her again with the telescope without 
 altering the focus, and saw her much more distinctly than ever he had done 
 upon any occasion. His friends were of the same opinion. They all saw her 
 spots plain (resembling those in the moon), which he had never seen before, 
 and this while the cloud seemed to surround it as much as ever." 
 
 I think this effect might perhaps have arisen from the Aurora acting as a 
 screen, and removing the glare with which so bright an object as Venus is 
 always accompanied ; but the case is a singular one, and one would be glad of 
 further experience. I suggested observations on this head during Sir Geo. 
 Nares's Arctic Expedition ; but the suggestion, for some reason of which I am 
 not aware, was not included in the official instructions issued. 
 
 Remarks are frequent of the brightness of stars as seen through Aurorae. 
 Payer, of the Austrian Expedition, remarks that falling stars passed through 
 the Aurora without producing any perceptible effect or undergoing any change. 
 
 A grand display of the Aurora took place 24th October, 1870. About this 
 time the belts of Jupiter were observed to be highly coloured. As observed 
 by me on the night of November 2, 1870, at 9 P.M., with an SJ-in. Browning 
 
SITY 
 
 / 
 
 THE AURORA IN CONNEXION WITH OTHEE PHENOAfENA. 
 
 67 
 
 reflector, achromatic eyepieces 144, 305, and 450, the equatorial zone was of 
 a distinctly dark ochre colour, deepening to red-brown as it approached the 
 lower (N.) edge. Two thin belts above were slate-purple, and a darker belt 
 below was of a deep purple colour. 
 
 I.assell, Proctor, and others have reported Jupiter's belts to exhibit the 
 brightest colours at the period of Aurora. Mr. Browning gives a drawing of 
 Jupiter as seen on January 31, 1870 (a year noted for Aurora?), with the belts 
 brightly coloured. The finest view of Jupiter I ever had was on the 
 8th February, 1872 (a fine Aurora was on the 4th), when, with the 8^-inch 
 Browning reflector, I saw the whole surface of the planet (by glimpses) cloud- 
 mottled. The equatorial belt was, however, then slightly tinted only. In 
 Dr. Miles's observation (p. 66) he does not seem to have noticed the colouring 
 of Jupiter's belts. 
 
 The three past years, 1876, 1877, and 1878, have been distinguished by the 
 iufrequency of Aurorse ; and Jupiter's equatorial zone and belts have been 
 mainly reported of light tints. 
 
 The subject apparently deserves more attention than it has hitherto received. 
 
 According 
 to Lassell 
 and others, 
 Jupiter's 
 belts exhibit 
 the brightest 
 colours at 
 period of 
 Aurora. 
 
 Infrequency 
 of Auroras 
 and light- 
 ness in tint 
 of Jupiter's 
 belts. 
 
 The Aurora and the Zodiacal Light. 
 
 Angstrom in 1867 found the spectrum of the Zodiacal Light to be mono- 
 chromatic, consisting of a single line in the green, to which he assigned 
 approximately the position 1259 on Kirchhoff's scale, the same that he had 
 determined for the green line of the Aurora Borealis ; and Respighi, on the 
 Red Sea, on the evening of the llth and the morning of the 12th January 
 1872, perceived in the Zodiacal Light not only this green line, but near it, 
 towards the blue, a band or zone of apparently continuous spectrum. 
 
 At the Observatory of the Royal University of Campidoglio, February 5th, 
 1872, Respighi, at 7 P.M., was able to discern the same spectrum ; and on 
 directing the spectroscope to other points he found that this spectrum showed 
 itself in all parts of the heavens from the horizon to the zenith, more or less 
 defined in different parts, but everywhere as bright as in the Zodiacal Light. 
 The Observatory Assistant, Dr. di Legge, likewise observed this spectrum 
 distinctly in various parts of the heavens. Respighi's observations corro- 
 borating Angstrom's in 1867, appeared to him to demonstrate the identity 
 of the Zodiacal Light with the Aurora, and to establish the identity of their 
 origin. 
 
 Pringle, in a letter to 'Nature' from South Canara, October 3, 1871, 
 
 K2 
 
 The Aurora 
 and the 
 Zodiacal 
 Light. 
 
 o 
 
 Angstroms 
 observation 
 on spectrum. 
 
 Eespighi's 
 at Cam- 
 pidoglio. 
 
68 
 
 THE AUKOBA AND ITS CHAEACTEES. 
 
 Pringle 
 thinks the 
 Aurora may 
 be consi- 
 dered as 
 allied to the 
 Zodiacal 
 Light. 
 
 Phosphores- 
 cence of sky 
 when Zodi- 
 acal Light 
 has been 
 seen bright. 
 
 Pringle 
 failed to 
 find bright 
 lines or 
 bands in the 
 Zodiacal 
 Light. 
 
 Prof. Piazzi 
 Smyth 
 confirms 
 this. 
 
 Colour of 
 the Zodiacal 
 Light. 
 
 Eev. Mr. 
 "Webb's ob- 
 servation, 
 February 2, 
 1862. 
 
 He found no 
 green line of 
 the Aurora. 
 
 alludes to the Aurora as being considered by many allied to the Zodiacal 
 Light, and does not think the evidence then hitherto adduced against the 
 theory at all conclusive. He says : " Assume the auroral light to consist of 
 solid particles of matter, planet dust, shining by reflected light, and it is not 
 difficult to imagine the Aurora playing amongst these tiny worlds, each of 
 which would have its own small magnetic system swayed like our own by the 
 monster magnet the sun." 
 
 He notices he has never found it to have a decided outline, nor traced it 
 east or west to 180 from the sun. He also refers to others having noticed that 
 when the Zodiacal Light has been seen unusually bright, a "phosphorescence " 
 of the sky was everywhere visible. 
 
 He does not seem at that time to have examined the matter spectro- 
 scopically ; and on June 23, 1872, he writes again, pointing out the peculiarity 
 in Respighi's observation that the green line was seen everywhere as bright 
 as in the Zodiacal Light, and suggesting that it was due to a concealed Aurora 
 
 O 
 
 present at the time of Angstrom's and Respighi's observations. He further 
 states he had examined the Zodiacal Light with a Browning 5-prism spectro- 
 scope (I presume a compound direct-vision form is meant) since the last 
 December, and, brilliant as the phenomenon had frequently been, failed to 
 detect the slightest appearance of bright lines or bands. A faint diffuse 
 spectrum about as intense as that of a bright portion of the Milky Way was 
 all he had obtained. 
 
 Professor Piazzi Smyth, in the clear sky of Italy, and with an instrument 
 specially designed for showing faint spectra, found no lines or bands, but only 
 a faint continuous spectrum extending from about midway between D and E 
 in the solar spectrum to nearly F (see Plate V. fig. 3, in which the continuous 
 spectrum is graphically shown, white on a black ground). 
 
 It may here be mentioned that the Zodiacal Light is usually described as, 
 in these latitudes, of a golden yellow or pale lemon tinge. 
 
 On one occasion, however, it has been described as not having this tinge, 
 but rather resembling the light of the Milky Way, but brighter. On another 
 occasion I saw the whole cone of a crimson hue without any mixture of yellow. 
 The Rev. Mr. Webb thought that a display seen at Hardwick Vicarage, 
 February 2nd, 1862, showed a ruddy tinge not unlike the commencement of 
 a crimson Aurora " it was certainly redder or yellower than the galaxy." He 
 examined it with a pocket spectroscope which would show distinctly the green 
 line of the Aurora (probably Browning's miniature), but nothing of the kind 
 was visible, nor could any thing be traced beyond a slight increase of general 
 
Till: AURORA IN CONNKXION WITH OTHER PHENOMENA. 
 
 69 
 
 light, which, on closing the slit, was extinguished long before the auroral band 
 would have become imperceptible. 
 
 A. W. Wright examined the Zodiacal Light with a Duboscq single-prism 
 spectroscope, the telescope and collimator having a clear aperture of 2'4 cen- 
 timetres, magnifying-power of telescope 9 diameters. Special precautions 
 were taken about the observations, and the conclusions arrived at were : 
 
 (1) The spectrum of the Zodiacal Light is continuous, and is sensibly the 
 same as that of faint sunlight or twilight. 
 
 (2) No bright line or band can be recognized as belonging to this spectrum. 
 
 (3) There is no evidence of any connexion between the Zodiacal Light and 
 the Polar Aurora. 
 
 The Polarization of the Zodiacal Light has been already referred to under 
 the head of "Polarization of the Aurora;" but it may be here noted that 
 Mr. Burton's observation of polarization of the light there mentioned has 
 been confirmed by Wright and Tacchini, and the presence of reflected sun- 
 light established. In this respect it differs from the Aurora, in which no 
 trace of polarization has hitherto been detected ; and looking at this, and at 
 the weight of evidence in the spectroscopic observations, the theory of a con- 
 nexion between the Aurora and the Zodiacal Light must, as the matter stands, 
 be given up. 
 
 A. \\'. 
 Wright's 
 observations 
 and conclu- 
 
 sions. 
 
 Polarization 
 
 of Zodiacal 
 
 Light. 
 
 Burton's 
 
 observation 
 
 confirmed 
 
 by Wright 
 
 and 
 
 Tacchini. 
 
70 
 
 THE AUKOKA AND ITS CHARACTERS. 
 
 CHAPTER VII. 
 
 Aurora-like 
 patches on 
 the partially- 
 eclipsed 
 moon, 
 Feb. 27, 
 1877. 
 
 Formerly it 
 was thought 
 the moon 
 was illumi- 
 nated by 
 auroral light. 
 
 Author's 
 notes of the 
 eclipse. 
 Colour-tints 
 Described. 
 
 A crimson- 
 scarlet tint 
 reminded 
 author of an 
 auroral 
 glow. 
 
 Eclipse,Aug. 
 23-24,1877. 
 
 Sky clear, 
 but eclipsed 
 moon misty 
 and indis- 
 tinct until 
 total obscu- 
 ration. 
 
 Succession 
 of colours. 
 
 AURORA-LIKE PATCHES ON THE PARTIALLY-ECLIPSED MOON. 
 
 1$ anticipation of the total eclipse of the Moon on the 27th February, 1877, 
 several articles appeared in the leading journals of the day describing, for the 
 public benefit, the appearances which might be expected during the occur- 
 rence of the phenomenon. 
 
 Among these was one by Mr. E. A. Proctor, in which the following passage 
 occurs : " That dull, or occasionally glowing red colour, shown by the moon 
 when she is fully and even deeply immersed in the shadow of the earth, is a 
 phenomenon whose explanation is not without interest. Formerly it was 
 thought that the moon possessed an inherent light, or perhaps was illuminated 
 by auroral light, which only became discernible at the time of total eclipse. 
 Indeed even Sir W. Herschel fell into the mistake of supposing this the only 
 available explanation, having miscalculated the efficiency of the true cause." 
 
 This passage was only pointed out to me by a friend after the eclipse had 
 actually taken place, and I had sent him some notes of what I then saw. My 
 notes on the occasion comprised, amongst others, the following remarks : 
 "The tints of colour also during partial eclipse, owing, no doubt, to the 
 moon's considerable altitude, were .singularly bright and well contrasted. 
 Silver-grey, dusky copper-red, and the same tint clearer and brighter were 
 ranged side by side with a lovely jewel effect. We noticed also at times a 
 crimson-scarlet tint, deeper and less mixed with yellow than the copper colour. 
 This last tint reminded me much of a crimson glow common to the Aurora, 
 and which I also once distinctly remarked (of course in a weaker degree) in 
 the zodiacal light" (ante, p. 68). 
 
 On the occasion of the eclipse of August 23-24, 1877, we were favoured 
 at Guildown, in common with many other places, by a singularly clear sky 
 during the progress of the moon's obscuration and subsequent clearing. In 
 the early part of the evening, however, the moon, from some cause (possibly 
 atmospheric vapour), seemed to have, as the earth's shadow advanced on its 
 disk, an unexpectedly misty and indistinct appearance, which lasted up to 
 and including total obscuration. Golden yellow, yellow copper, dull copper, 
 ruddy copper, and dull red were successively the principal colours observed 
 at different times and at various portions of the moon's surface. 
 
AUEOEA-LIKE PATCHES ON THE PARTIALLY-ECLIPSED MOON. 71 
 
 After referring to some spectroscopic appearances, my notes then ran on 
 thus : " As the shadow began to pass off, and the bright sharp crescent of the 
 illuminated portion of the moon to appear, the general aspect of the moon's 
 disk seemed to me to greatly change. The certain amount of indistinctness 
 noticeable during approach and continuance of totality, gave way to a con- 
 siderable sharpness of the moon's features as seen through the shadow. The 
 shadowed part glowed with a richer copper tint, on which were seen dark, 
 almost black, spots and patches." Then follows a description of these ; and 
 the notes continue : " Two features here struck me the one a continuation 
 of the upper limb of the illuminated crescent, so that it seemed to form a 
 bead of light just on the centre of the upper edge of the moon ; the other 
 two patches of crimson light, similar to those I described as having been seen 
 in the last total eclipse. One of these, quite a small one, was just under the 
 elongated bead before described ; the other, a much larger and more diffused 
 one, was seen towards the south-west limb of the moon, about midway be- 
 tween it and the centre. The spots or patches were of a decidedly crimson- 
 red, in contrast to the ordinary copper-red of the disk, and were noticed by 
 my friend as well as by myself." 
 
 These were eye observations. The patches were quite well seen (but not 
 so brightly as with the eye) with a double achromatic field-glass. With a 3|- 
 inch Cooke refractor and low power, they seemed lost in the general moon 
 tint ; but they were then diminishing in brightness. From a comparison of 
 my two sketches, the patches seem to have gradually deepened in tint, and 
 we considered them to have disappeared in a like gradual manner. 
 
 My first sketch was taken shortly after end of total phase; the second 
 about ten minutes later. I have reproduced the original sketches in prefer- 
 ence to any drawing prepared from them (Plate IV. figs. 2 and 3). 
 
 The patches did not last long, but were lost as the shadow swept off 
 the moon. I saw nothing of the sort during the approach of or pending 
 totality, nor until a small crescent of the moon began to appear behind the 
 shadow. 
 
 I have looked for other accounts of these patches, but cannot find any. 
 Most observers have described the deeper colour of the shadowed moon by 
 the word " copper." Some extend this colour to red ; but there is probably 
 much in the state of the atmosphere affecting this. 
 
 At Avignon, December 3, 1703, the moon appeared, pending eclipse, 
 " extraordinarily illuminated and of a very bright red," while other and 
 different features were seen at Montpelier. 
 
 As shadow 
 passed off 
 indistinct- 
 ness gave 
 way to a 
 sharpness of 
 the moon's 
 features as 
 seen through 
 shadow. 
 
 Two patches 
 of crimsoii 
 light de- 
 scribed. 
 
 Patches well 
 seen in field- 
 glass ; lost in 
 small re- 
 fractor. 
 They gra- 
 dually deep- 
 ened in tint. 
 
 Two 
 
 sketches 
 
 taken. 
 
 Dec. 3, 1703 
 moon's co- 
 lour de- 
 scribed. 
 
72 
 
 THE AUEOEA AND ITS CHAEACTEES. 
 
 Mr. Keye's 
 observation. 
 
 Prof. Pritch- 
 ard's. 
 
 M. Faye's. 
 
 Dr. Allnatt's 
 at Frant. 
 
 Observa- 
 tions as to 
 the patches. 
 
 Infraction 
 of sun's rays 
 not a satis- 
 factory ex- 
 planation. 
 
 On March 19, 1848, observers in England, Ireland, and Belgium described 
 the moon's disk as " intensely bright coppery red." On the occasion of 
 August 23-24, 1877, before mentioned, an article in one of the public papers 
 described the moon's disk, during totality, as of a " dull copper colour." 
 
 Mr. Henry Keye, in the Engadine, at a height of 4500 feet above sea-level, 
 and with the purest air, saw the partially covered moon (before totality) as a 
 " dull copper colour." 
 
 Prof. Pritchard, writing from the Oxford University Observatory, says that 
 at 12 h 10 m (about the time my sketches were taken) there was a good deal of 
 light on the moon's following limb, and the colour was " more red than copper," 
 and apparently redder than it had been at a similar distance of time before 
 totality. Mons. Faye reported to the French Academy of Sciences that " a 
 striking phenomenon not previously noticed was that the reddish tinge, resem- 
 bling that of a fine sunset, was deepest at the margin of the disk, a circum- 
 stance which he could not explain." Dr. Allnatt, writing from Frant, says : 
 " At totality the moon's disk presented a most extraordinary appearance : the 
 western limb was comparatively transparent, but the main body appeared as 
 though enveloped in a semi-opaque clot of coagulated blood, through which 
 the lunar features were dimly visible." 
 
 The observations of Prof. Pritchard and Mons. Faye point more imme- 
 diately to redness ; and this is the nearest approach I can find to the patches 
 I noticed. These patches do not seem to me easy of explanation. They could 
 not well be colours or details due to the actual surface of the moon itself. The 
 moon, we are aware, has only a certain portion of the visible disk slightly tinted. 
 The Mare Serenitatis is certainly of a slight green tinge ; and to the Palus 
 Somni and certain other districts is attributed a pale red or pink ; but these 
 tints could hardly have sufficed to produce the effect seen, as the patches were 
 conspicuous for a bright and decided colour. The positions, moreover, did 
 not correspond ; while the ease with which other details of the surface were 
 seen at the time would, if the tints had arisen from the surface itself, probably 
 have enabled the circumstance to be detected. 
 
 The refraction of the sun's rays by passage through the earth's atmosphere 
 is, too, not a satisfactory explanation. This, as judged by the appearance of 
 the covered moon immediately before and at totality, gives a disk of shadow 
 deeper in tone in the centre and lightening towards the edges, but in other- 
 respects fairly uniform, so that the whole disk seems to partake of the same 
 tint and its graduations ; and this is what might have been expected under 
 the circumstances. The patches, on the other hand, were quite local. 
 
AUEOEA-LIKE PATCHES OX THE PAETIALLY-ECLIPSED MOON. 73 
 
 The theory of the moon's possessing no atmosphere whatever is now 
 very generally, but perhaps too readily, received (mainly upon the evidence 
 of the spectroscopic observations of occulted stars*), as there still seems a 
 reasonable doubt whether our satellite may not possess an atmosphere, pos- 
 sibly rarefied, but yet sufficiently dense to permit of the formation of cloud or 
 vapour. 
 
 A curious case, in which a patch of vapour or cloud was supposed to be 
 detected on the moon's surface, is reported by the Rev. J. B. Emmett in a 
 communication to the ' Annals of Philosophy ' (New Series, vol. xii. p. 81). 
 It is dated "Great Ouseburn, near Boroughbridge, July 5, 1826," the obser- 
 vation being made with " the greatest care with a very fine telescope." 
 
 On the 12th April 8 h , while observing the part of the moon called Palus 
 Mceotis by Nevelius, with an excellent Newtonian reflector of 6 inches 
 aperture, at a particular part of the Palus, which he minutely describes, he 
 saw, with powers 70 and 130, "a very conspicuous spot wholly enveloped in 
 black nebulous matter, which, as if carried forward by a current of air, 
 extended itself in an easterly direction, inclining a little towards the south, 
 rather beyond the margin of Mceotis." April 13th 8 h to 9 h , the cloudy ap- 
 pearance was reduced both in extent and intensity, and the spot from which 
 it seemed to issue had become more distinctly visible. On April 17th 
 scarcely a trace of the nebulous matter remained ; but so long after as 
 June 10th 8 h " a little blackness " remained about the spot. Mr. Emmett 
 suggested " smoke of a volcano or cloudy matter." A copy of the drawing 
 annexed to the paper is given on Plate X. fig. 10 (black patch on moon). 
 If this observation was (as it certainly appears to be) critical and exact, there 
 must have been a disturbance of the moon's surface, indicating some sort of 
 cloud- or vapour-supporting atmosphere ; and probably, for the purposes of 
 Aurorse, an atmosphere of a very rarefied condition would suffice f- 
 
 * The proof from occulted stars merely goes to the fact that the moon possesses no atmo- 
 sphere appreciable in that way. It may still be a question whether there does not exist something 
 of the kind, lying low and close to the surface, and possibly of a rarefied character, which would 
 scarcely make itself visible by its effects in occultations. Cloud-vapour might form in an 
 atmosphere of inconsiderable density. 
 
 t This observation is not without a certain amount of confirmation by more recent ones, in 
 which certain lunar objects and regions have been suspected of mist or vapour. Mr. Birt 
 (' English Mechanic,' vol. xxviii. no. 725) mentions two the cloud-like appearance of the white 
 patch west of Picard, and the interior of Tycho, which at one time always misty and ill- 
 defined, is now become perfectly distinct and sharply defined. 
 
 December 4, 1878, 4" 45 m . I observed Klein's crater as a dull dark spot, larger than the true 
 
 L 
 
 Question of 
 lunar atmo- 
 sphere. 
 
 Instance of 
 patch of 
 vapour or 
 cloud on 
 moon's sur- 
 face. 
 
74 
 
 THE AUROKA AND ITS CHARACTERS. 
 
 Prof. Alex- 
 ander's evi- 
 dence in 
 favour of a 
 lunar atmo- 
 sphere. 
 
 Mars and 
 Jupiter. 
 
 Prof. 
 Dorna's 
 " Lunar 
 Aurora." 
 
 Spectro- 
 scopic ob- 
 servations 
 bearing on 
 the subject. 
 
 According to the ' New York Tribune,' at a recent semi-annual meeting of 
 the American Academy of Sciences, Professor Alexander " brought forward 
 a variety of evidence tending to indicate some envelope like an atmosphere 
 for the moon. The evidence was principally drawn from observations during 
 eclipses. The explanations usually offered for the bright baud seen around 
 the moon at such times was fully considered, and shown to be inadequate, 
 though good as far as they would apply. The ruddy band of light is much 
 too broad to be the sun's chromosphere. It was most apparent in those 
 instances where the moon was nearest the earth. It would best be accounted 
 for by supposing an atmosphere to the moon, a thin remnant of ancient 
 nebulosity, comparable to that which accompanies the earth and gives rise to 
 the appearance of the Aurora Borealis." Is it not, however, possible that 
 the appearance might have arisen from Aurora in action within the region 
 of the earth's own atmosphere during the passage of the sun's rays through 
 it at the time of the eclipse 1. The whole subject is difficult of explanation, 
 and should be one of the points for attention on the occasion of the next 
 total Lunar eclipse. It seemed to me appropriate for introduction into the 
 present history of the Aurora, whatever its solution may ultimately be. 
 
 In the case of Mars and Jupiter, whose atmospheres are sufficiently recog- 
 nized, red- and scarlet-tinted patches are frequently noticed. In Mars this is 
 generally attributed to the geological character of the surface of the planet 
 itself; but I have observed on Mars's surface during the recent opposition a 
 local rosy tint of a more diffused and indefinite character ; and in the case of 
 Jupiter the appearances seem almost always connected with the clouds' belts, 
 as distinguished from the regions lying nearer to the planet's surface. 
 
 Professor Dorna, of Turin, ascribed a flickering light seen on the reddened 
 disk of the moon during the Lunar eclipse of February 1877 to the action of 
 a Lunar Aurora, holding that the refraction of the sun's rays within the 
 cone of the earth's shadow was not an adequate explanation (' L'Opinione 
 Nazionale,' March 3, 1877). 
 
 The spectroscope might have afforded some information on the question ; 
 but my own telescopes (8|- and 3J in.) were not of sufficient aperture to 
 give a sensible spectrum of a portion of the moon's eclipsed surface, and my 
 observations were chiefly made on the entire disk with hand-spectroscopes 
 
 object ; and while definition was good and other objects were well defined, " the floor of Klein's 
 object, the oval spot near, and also Agrippa (especially), all had an odd misty look as if vapour 
 were in or about them" ('English Mechanic,' vol. xxviii. no. 727). The mystery of different 
 observers seeing and not seeing Klein's object on the same night is hardly to be accounted for 
 by the angle of illumination. 
 
\l ROEA-LIKK PATCHES OX THE PAETIALLT-ECLIPSED MOON. 75 
 
 without a slit. Mr. Christie, at the Royal Observatory, Greenwich, made a 
 set of observations during totality, and also during subsequent partial phase, 
 with a single-prism spectroscope. During totality a strong absorption band 
 was seen in the yellow, and the red and blue ends of the spectrum were 
 completely cut off, while the orange was greatly reduced in intensity. The 
 yellow and green were comparatively bright, and seemed to constitute the 
 whole visible spectrum. The absorption band became narrowed as the end 
 of the total phase approached, and during partial phase was reduced to a 
 mere line. The red end of the spectrum was cut off by a dark band com- 
 mencing about halfway from D to C, in which a black line was suspected. 
 The bands observed were characteristic of the spectrum of light which has 
 passed through a thick stratum of air. In the description of the spectrum 
 of the Aurora in Part II., it will be seen that the conspicuous red and green 
 lines of the Aurora are either coincident with, or very close to, some of these 
 atmospheric lines. It does not appear that Mr. Christie examined the 
 crimson patches specifically, nor that he saw bright lines on any part of the 
 moon's eclipsed disk. 
 
 Mr. Heniy Pratt has also kindly handed me for use his notes of the Lunar 
 eclipse of August 23, 1877, as seen at Brighton on a splendid night. They 
 were made as the phenomenon progressed, are 58 in number, and in many 
 instances only a few minutes, or even seconds, apart. A selection of them is 
 here given : 9 h 13 m 50 s , first contact of shadow. 9 h 30 m , shadow very dark ; 
 no details of disk easily seen. 9 h 40 m , first appearance of red. 9 h 50 m , red 
 all over disk, except margin bluish and S. part green tint. 10 h 2 m , a sudden 
 brightening of whole disk, in strong contrast to two minutes previously. 
 10 h 15 m , E. limb much darker. 10 h 35 m , south pole decidedly brightest. 10 h 44 m , 
 S.E. limb much brighter. 10 h 48 m , whole disk much darker. 10 h 51 m , S.E. 
 limb brightening again. ll h l m , N.E. limb brightening. ll h 3 m , N.E. limb 
 has darkened and brightened three times during last two minutes. ll h 20 m , 
 N. pole has darkened. ll h 21 m , N. pole has brightened. ll h 24 m 30', N. pole 
 darker red. ll h 35 m , N. pole bright. ll h 35 m 30 s , same dark and red. 
 ll h 42 m , N.E. limb especially bright for a few seconds, and then reddened 
 and shaded again. ll h 49 m , S. pole reddened. 12 h l m , S.W. limb reddest 
 part ; S. pole red ; N. pole paler red. 12 h 3 m 50", first appearance of E. limb 
 (my first sketch was made shortly after this, and my second about ten minutes 
 later). 12 h 21 m , a bright patch on N.N.W. separated from N. pole. 12 h 24 m , 
 S.W. region is reddest part of eclipse. 12 h 40 m , redness of shadow fading out. 
 
 With a small Browning star-spectroscope Mr. Pratt saw the red and blue 
 
 L2 
 
 Mr. Chris- 
 tic's obser- 
 vations at 
 Greenwich. 
 
 Mr. Pratt's 
 notes of 
 Lunar 
 Eclipse. 
 August '}. 
 1877. 
 
76 
 
 THE AUEOBA AND ITS CHAEACTEES. 
 
 Mr. Pratt's 
 observation 
 on the floor 
 of Plato. 
 
 Observation 
 by Mr. Hirst 
 of a dark 
 shade on the 
 
 ends of the spectrum cut off, but nothing else. Mr. Pratt adds that the 
 red colour was not an effect of contrast or an optical delusion in any way, as 
 was proved by using at times a limited field containing only the red portion 
 under examination. In reference to the curious brightening and darkening 
 of the disk, and the change from time to time of local colour, he says that 
 with much experience he has seen nothing of the same marked character on 
 other occasions, and that " the whole matter was at the time astonishing to 
 me, but none the less real." The local red patches seen by me seem also to 
 have been observed by Mr. Pratt. 
 
 As an addition to the instances of Tycho, Picard, &c., mentioned in the 
 note on p. 73, Mr. Pratt has also sent me his notes of some observations by 
 him, of " local obscuration of the floor of Plato." As somewhat condensed, 
 they are as follows : 1872, July 16. While in other parts of the floor spots 
 and streaks were well visible, " the N.W. portion was in such a hazy con- 
 dition that nothing could be defined upon it." 1873, Nov. 1. 27 light 
 streaks seen (7 new) : the brightness of the streaks was in excess of their 
 usual character, as compared with the fcraterlets ; " an obliteration or invisi- 
 bility of all the light streaks in the neighbourhood of craterlet no. 1 was very 
 noticeable ;" and also " a similar obliteration of the N. end of the streak 
 called the Sector, near craterlet 3." 1874, January 1. 18 light streaks seen, 
 including 3 new, " some of which outshone other longer known ones. This 
 was curious ; for had they been as bright within the last two years as on this 
 occasion I must have noticed them." Mr. Pratt points out, as worthy of 
 remark, that some months previous to November 1st, 1873, neither craterlets 
 nor streaks on the floor of Plato " had maintained their previous cha- 
 racteristic brightness," a fact which he thinks ought to be considered toge- 
 ther with the outbreak of brilliancy of both orders on that day, as well as 
 the apparently sudden existence of new ones. 
 
 The ' Observatory,' March 1, 1879, p. 375, contains an account, by Mr. 
 H. C. Russell, of some Astronomical Experiments made on the Blue Mountains, 
 near Sydney, N. S. W. Among these it is noticed that on 21st October, 1878, at 
 9 A.M., when looking at the moon, Mr. Hirst found that a large part of it was 
 covered with a dark shade, quite as dark as the shadow of the earth during 
 an eclipse of the moon. Its outline was generally circular, and fainter near 
 the edges. Conspicuous bright lunar objects could be seen through it ; but it 
 quite obliterated the view of about half the moon's terminator, while those 
 parts of the terminator not in the shadow were distinctly seen. 
 
 No change in the position of the shade could be detected after three hours' 
 
AURORA-LIKE PATCHES ON THE PARTIALLY-ECLIPSED MOON. 77 
 
 watching. The observation is made, " One could hardly resist the con- 
 viction that it was a shadow ; yet it could not be the shadow of any known 
 body. If produced by a comet, it must be one of more than ordinary 
 density, although dark bodies have been seen crossing the sun which were 
 doubtless comets." The diameter of the shadow from the part of it seen on 
 the moon was estimated at about three quarters that of the moon *. 
 
 * Some doubt has been cast on this observation, on the ground that nothing unusual was 
 seen, and that the appearances were only those ordinarily presented by the moon at its then 
 phase. I simply give the account as it appears in the scientific journal in which it was 
 published. 
 
78 
 
 THE ATJKOBA AND ITS CHAEACTEES. 
 
 Aurora and 
 the solar 
 corona. 
 Mr. Norman 
 Lockyer's 
 ' Solar 
 Physics.' 
 Extracts 
 from as to 
 Aurora's 
 connexion 
 with sun- 
 spots and 
 with solar 
 corona. 
 
 Evidence of 
 American 
 observers on 
 nature of the 
 corona con- 
 sidered. 
 
 Mr. Lock- 
 yer's conclu- 
 sion adverse 
 to the ques- 
 tion being 
 settled. 
 
 CHAPTEE VIII. 
 
 AUEOEA AND THE SOLAE COEOXA. 
 
 ME. NORMAN LOCKTEE, in his ' Solar Physics,' a work of 666 pages, gives but 
 little space- to the Aurora. The index comprises : " Aurora Borealis, con- 
 nexion with sun-spots, pp. 82-102." " Affirmed coincidence of spectrum 
 with that of the corona, pp. 244, 256." 
 
 Page 82. After referring to Gen. Sabine as having shown that there are 
 occasional disturbances in the magnetic state of the earth, and that these 
 disturbances have a periodical variation, coinciding in period and epoch with 
 the variation in frequency and magnitude of the solar spots as observed by 
 Schwabe, the author proceeds to state, " and the same philosopher has given 
 us reason to conclude that there is a similar coincidence between the outburst 
 of solar spots and of the Aurora Borealis." 
 
 Page 102. " We have also shown that sun-spots or solar disturbances 
 appear to be accompanied by disturbances of the earth's magnetism, and these 
 again by auroral displays." 
 
 Page 243. "What, then, is the evidence furnished by the American 
 observers on the nature of the corona (solar) \ It is bizarre and puzzling to 
 the last degree. The most definite statement on the subject is that it is 
 nothing more nor less than a permanent Solar Aurora ! the announcement 
 being founded on the fact that three bright lines remained visible after the 
 image of a prominence had been moved away from the slit, and that one 
 (if not all) of these lines is coincident with a line (or lines) noticed in the 
 spectrum of the Aurora Borealis by Professor Winlock." Mr. Lockyer then 
 adds, that amongst the lines he had observed up to that time, some forty in 
 number, this line was among those which he had most frequently recorded, 
 and was, in fact, the first iron line which made its appearance in the part of 
 the spectrum he generally studied, when the iron vapour is thrown into the 
 chromosphere. 
 
 Hence he thought he should always see it if the Aurora were a permanent 
 solar corona, and gave out this as its brightest line, and on this ground alone 
 should hesitate to regard the question as settled. 
 
 Page 256 is an extract from a communication by Prof. Young to ' Nature, 
 
AUEOEA AND THE SOLAE COEONA. 
 
 79 
 
 March 24, 1870, in which the Professor refers to the bright line 1474 as 
 being always visible with proper management. He also thinks it probable 
 that this line coincides with the Aurora line reported by Prof. Winlock 
 at 1550 of Dr. Muggins's scale, though he is by no means sure of it. He had 
 only himself seen it thrice, and then not long enough to complete a measure- 
 ment. He was only sure that its position lay between 1460 and 1490 of 
 Kirchhoff. 
 
 For this reason he did not abandon the hypothesis, which appeared to have 
 other elements of probability, in the general appearance of the corona, the 
 necessity of immense electrical disturbances in the solar atmosphere as the 
 result of the powerful vertical currents known to exist there, as well as 
 the curious responsiveness of our terrestrial magnets to solar storms ; yet he 
 did not feel in a position to urge it strongly, but rather awaited develop- 
 ments. Father Secchi was disposed to think the line hydrogen, while 
 Mr. Lockyer still believed it to be iron. 
 
 Dr. Schellen, in his ' Spectrum Analysis,' treats the matter more in detail. 
 Referring to the eclipse of 1869 as confirming the previous observations 
 that the coronal spectrum was free from dark lines, he points out that 
 Pickering, Harkness, Young, and others were agreed that with the extinction 
 of the last rays of the sun all the Fraunhofer lines disappeared at once 
 from the spectrum. He further says : 
 
 " The small instruments employed by Pickering and Harkness, with a large 
 field of view, exhibited a spectrum obtained at once from the corona, the pro- 
 minences, and the sky in the neighbourhood of the sun. These instruments 
 showed during totality a faint continuous spectrum free from dark lines, but 
 crossed by two or three bright lines. Young, with a spectroscope of five 
 prisms, observed the three bright lines in the spectrum of the corona, and 
 deduced the following positions according to Kirchhoff 's scale: 1250 + 20, 
 1350 + 20, and 1474. It had been already explained why the last and 
 brightest of these lines was thought to belong to the corona and not to that 
 of the prominences, and it seemed probable that the other two lines belonged 
 also to the light of the corona, from the fact that they were both wanting in 
 the spectrum of the prominences when observed without an eclipse. But 
 what invested these three lines with a peculiar interest was the circumstance 
 that they appeared to coincide exactly with the first three of the five bright 
 lines observed by Professor Winlock in the spectrum of the Aurora Borealis. 
 These lines of the Aurora were determined by Winlock according to Huggins's 
 scale ; and if these be reduced to KirchhofF's scale, the positions of the lines 
 
 Prof. 
 Young's 
 communica- 
 tion to 
 1 Nature.' 
 
 He does not 
 abandon his 
 hypothesis, 
 it having 
 other ele- 
 ments of 
 probability. 
 
 Dr. Schellen 
 reviews the 
 subject 
 in eclipse 
 of 1869. 
 
 Young ob- 
 served three 
 bright lines 
 in the spec- 
 trum of the 
 corona. 
 
 Coincidence 
 of these lines 
 with three 
 bright lines 
 observed bv 
 Winlock in 
 the Aurora. 
 
80 
 
 THE AUEOEA AND ITS CHAEACTEES. 
 
 Corona self- 
 luminous, 
 ^>and probably 
 of a gaseous 
 nature. 
 
 Corona sup- 
 posed to be 
 a permanent 
 polar light 
 existing in 
 the sun. 
 
 Polar light 
 in the sun 
 attributed to 
 electricity. 
 
 Dr. Schellen 
 thinks 
 
 nature of the 
 corona still 
 a problem. 
 
 would be 1247, 1351, and 1473, while the lines observed by Young were 1250, 
 1350, and 1474." Dr. Schellen then points out that if it be borne in mind 
 that Young found the positions of the two fainter lines more by estimation 
 than by measurement, the coincidence between the bright lines of the corona 
 and those of the Aurora would be found very remarkable. The brightest of 
 the lines, 1474, was the reversal of a strongly marked Fraunhofer line, 
 
 o 
 
 ascribed by Kirchhoff and Angstrom to the vapour of iron. Dr. Schellen 
 then details Professor Pickering's observations with the polariscope, showing 
 that the corona must be self-luminous, and that from the bright lines seen in 
 its spectrum it is probably of a gaseous nature, and forms a widely diffused 
 atmosphere round the sun ; and then adds, " It has been supposed, from the 
 coincidence of the three bright lines of the corona with those of the Aurora 
 Borealis, that the corona is a permanent polar light existing in the sun 
 analogous to that of our earth." Dr. Schellen here adds : " Lockyer, how- 
 ever, justly urges against this theory the fact that although the brightest of 
 these three lines, which is due to the vapour of iron, is very often present 
 among the great number of bright lines occasionally seen in the spectrum of 
 the prominences, it is by no means constantly visible, which ought to be the 
 case if the corona were a permanent polar light in the sun." (Professor 
 Young's answer to this, on the ground of line 1474 being always visible, 
 has been already given.) " A yet bolder theory is the ascription of such a 
 polar light in the sun to the influence of electricity, which has been proved, 
 it is well known, by the relation of the magnetic needle, and the disturbance 
 of the electric current in the telegraph wires, to play an important part in 
 the phenomenon of the Aurora Borealis ;" and Dr. Schellen then concludes with 
 an opinion that the nature of the corona was still a problem *. 
 
 On reference to the ' American Journal of Science,' vol. xlviii. pp. 123 and 
 404, it seems that the auroral observations before referred to were made on 
 15th April, 1869, by C. S. Pierce, with " an ordinary chemical spectroscope, 
 with the collimator pointed directly to the heavens," and were reported by 
 Winlock. The lines were 1280, 1400, and 1550 of Huggins's scale, and were 
 reduced to Kirchhoff's scale by Young. These lines have had all sorts of 
 
 * The question of a connexion between the waxing and waning of the solar corona and the 
 prevalence of sun-spots is now being mooted, and may have an important bearing on the subject 
 of the constitution of the corona. It would seem that when the corona has been examined about 
 the time of minimum of sun-spots, it has proved fainter though more extended, while the bright 
 lines of the spectrum have been absent, indicating a change or variance in the gaseous part of it 
 at those periods. 
 
AUEO1U AND THE SOLAE CORONA. 
 
 places of wave-length assigned to them by different writers. Proctor gives 
 5570, 5400, 5200; Pickering and Alvan Clarke, 5320 (assumed to be 5316, 
 coronal line); Barker, 5170, 5200, 5020; Backhouse, 5320, 4640, and 4310. 
 In my ' Aurora Spectrum,' Plate XII., I have assigned two, with a ?, to 5320 
 (Alvan Clarke) and 5020 (Barker). The third might perhaps be placed at 
 4640 (Backhouse and Winlock). 
 
 The coincidences relied on in the foregoing observations depend, of course, 
 upon (1) the accuracy of the observations themselves, and (2) the subsequent 
 reduction of the lines for comparison. Assuming the correctness of the latter, 
 what have we as to the former 1 Two of Professor Young's positions of 
 coronal lines, as stated, seem to have far too much of the element to make 
 them sufficiently accurate. Pierce's auroral observation does not state how 
 the lines were positioned. As they all end with a cypher, the suspicion 
 naturally arises that the measurements did not extend beyond the first three 
 places of the figures, and, if so, could not be used for accurate comparison. 
 The auroral lines, too, are generally rather wide and nebulous, and not easy 
 of comparison with sharper ones. 
 
 NIVERSITY 
 
 Variou* 
 places of 
 wave-length 
 assigned to 
 these lines. 
 
 Doubts 
 raised as to 
 closeness 
 of the ob- 
 servations 
 for the pur- 
 pose of 
 comparison. 
 
82 
 
 THE AUEOEA AND ITS CHAEACTEES. 
 
 CHAPTER IX. 
 
 Supposed 
 causes of the 
 Aurora. 
 Sulphurous 
 vapours. 
 
 Magnetic 
 effluvia. 
 
 Zodiacal 
 light. 
 
 Luminous 
 particles of 
 our atmo- 
 sphere. 
 
 Electric fluid 
 in vacua 
 resembles 
 Aurora. 
 
 Experiment 
 with electri- 
 cal machine 
 and ex- 
 hausted re- 
 ceiver. 
 
 Dr. Frank- 
 lin's theory. 
 
 SUPPOSED CAUSES OF THE AUEOEA. 
 
 AT first the Aurora was described to be sulphurous vapours issuing from the 
 earth ; and Musschenbroek pointed out that certain chemical mixtures sent 
 forth a phosphorescent vapour, in some respects resembling the Aurora. Dr. 
 Halley originally proposed a similar theory, but ultimately concluded that 
 the Aurora might be occasioned by the circulation of the magnetic effluvia of 
 the earth from one pole to another. 
 
 M. de Mairan, in 1721, in a treatise, ascribed the Aurora to the impulse 
 of the zodiacal light upon the atmosphere of the earth. 
 
 Euler combated this theory, and ascribed the Aurora to the luminous 
 particles of our atmosphere driven beyond its limits by the light of the sun, 
 and sometimes ascending to the height of several thousand miles. 
 
 Mr. Hawksbee very early showed that the electric fluid assumes, in vacuo 
 or in highly rarefied atmosphere, an appearance resembling the Aurora. Mr. 
 Canton contrived an imitation of the Aurora by means of electricity trans- 
 mitted through the Torricellian vacuum in a long glass tube, and showed 
 that such a tube would continue to display strong flashes of light for 24 
 hours and longer without fresh excitation. 
 
 In the 'Edinburgh Encyclopaedia,' date 1830, is mentioned an experiment 
 in which an electrical machine and air-pump are so disposed that strong sparks 
 pass from the machine to the receiver of the air-pump. 
 
 As the exhaustion proceeds the electricity forces itself through the receiver 
 in a visible stream, at first of a deep purple colour ; " but as the exhaustion 
 advances it changes to blue, and at length to an intense white, with which 
 the whole receiver becomes completely filled." [It will be noticed that this 
 
 o 
 
 experiment bears a close resemblance to Prof. Angstrom's exhausted flask 
 referred to later in treating of the spectrum of the Aurora.] 
 
 Dr. Franklin gave a different form to the electric theory of the Aurora, 
 supposing that the electricity which is concerned in the phenomenon passes 
 into the Polar regions from the immense quantities of vapour raised into the 
 atmosphere between the tropics (Exper. and Observ. 1769, p. 43). 
 
 Mr. Kirwan (Irish Trans. 1788) supposed that the light of the Aurora 
 
SUPPOSED CAUSES OF THE AUROEA. 
 
 83 
 
 Borealis and Australis was occasioned by the combustion of inflammable air 
 kindled by electricity. 
 
 Mons. Monge proposed the theory that the Aurorse were merely clouds 
 illuminated by the sun's light falling upon them after numerous reflections 
 from other clouds placed at different distances in the heavens (Lecons de 
 Physique par Prejoulz, 1805, p. 237). 
 
 Mons. Libes propounded a theory that the electric fluid, passing through a 
 mixture of azote and oxygen, produced nitric acid, nitrous acid or nitrous 
 gas, and that these substances, acted upon by the solar rays, would exhibit 
 those red and volatile vapours which form the Aurora Borealis (Traite de 
 Physique, ou Dictionnaire de Physique, par Libes; Rozier's Journal, June 
 1790, February 1791, and vol. xxxviii. p. 191). 
 
 Mr. Dalton considered the Aurora a magnetic phenomenon whose beams 
 were governed by the magnetism of the earth. He observed that the lumi- 
 nous arches were always perpendicular to the magnetic meridian (Dalton's 
 Meteorological Observations and Essays, 1793, pp. 54, 153). 
 
 The Abbe Bertholon ascribed the Aurora Borealis to a phosphorico-electric 
 light (Encyc. Method, art. Aurora). 
 
 Dr. Thompson (Annals of Philosophy, vol. iv. p. 429), from the observa- 
 tions of Mr. Cavendish and Mr. Dalton, concluded there was no doubt that 
 the arched appearance of the Aurora was merely an optical deception, and 
 that in reality it consisted of a great number of straight cylinders parallel 
 to each other and to the dipping-needle at the place where they were seen. 
 
 With many of us (at least it was so in my own case) our first viewed Aurora; 
 have been artificial ones, devised by electricians and having their locus at the 
 Royal Polytechnic in Regent Street or in some scientific lecture-room. The 
 effects in these cases are produced in tubes nearly exhausted by means of an 
 air-pump, and then illuminated by some form of electric or galvanic current. 
 
 In one instance the tube is usually of the form shown on Plate X. fig. 9, 
 supported on a base with a brass ball electrode at the lower end, and a pointed 
 wire at the upper. In another case the tube is of the form shown on same 
 Plate, fig. 8. After exhaustion it is permanently closed, the current passing 
 through it by means of the platinum-wire electrodes introduced into each 
 end of the tube. The first form of tube is usually excited by a frictional 
 plate machine ; the second by a galvanic current from a Grove or bichromate 
 battery, which, by the aid of a Ituhmkorff coil, has had its character changed 
 from quantity to intensity. In each instance, upon connexion with the source 
 supply of the electric current, a very similar effect is produced. 
 
 M2 
 
 Mr. Kir- 
 wan's theory. 
 
 Mons. 
 Monge's. 
 
 Mous.Libes'. 
 
 Mr. Dal- 
 ton's. 
 
 Abbe Ber- 
 tholon's. 
 
 Dr. Thomp- 
 son eon- 
 eluded the 
 arches to be 
 an optical 
 deception. 
 
 Artificial 
 Aurora pro- 
 duced in ex- 
 hausted 
 tubes. 
 
 Tubes de- 
 scribed. 
 
84 
 
 THE AUEOKA AND ITS CHAEACTEES. 
 
 Effects de- 
 scribed. 
 
 Tube with- 
 out elec- 
 trodes. 
 
 Tube excited 
 by friction. 
 
 Greissler's 
 
 mercury 
 
 tube. 
 
 Brilliant streams of rose-coloured light pass between the electrodes, some- 
 times as a single luminous misty band, sometimes in divided vibrating sprays 
 or streams, and sometimes in a flaky column of striae. 
 
 All this, before the spectroscope took its part in the investigation, we were 
 content to accept as a very fair and probable explanation of the Aurora accom- 
 panied by a mimic representation of the phenomenon. 
 
 These appearances may, of course, be produced at will in tubes having 
 electrodes ; but it is, moreover, possible to produce them, though with less 
 eifect, in certain other forms of tube having no such direct communication 
 with the external electric machine. 
 
 One electrode only may be connected with the coil or electrical machine. 
 The appearance is then a faint representation of what happens when the 
 current entirely passes (but see experiments with a single wire detailed in 
 Part III.). 
 
 In the case of an exhausted tube having no electrodes, the wires from the 
 coil may be made into a little helix and placed at each end of the tube, and 
 the induced currents within will show themselves in flashes and streams of 
 light, varying in colour and tint according to the gaseous or other contents of 
 the tube. 
 
 In some cases the ordinary forms of galvanic or electrical machine for sup- 
 plying the current of electricity may be dispensed with. A long straight tube 
 exhausted and closed at each end, and without electrodes, Plate X. fig. 6, being 
 slightly warmed and then excited by friction with the dry hand or a piece of 
 flannel, silk handkerchief, or the like, is soon filled with the most brilliant 
 flashes of light playing in the interior, and when once thoroughly charged 
 needs but little further excitation to keep up the effect. 
 
 Geissler has introduced a form of tube in which electricity in its form of 
 flashes and glow of light is produced by the friction of mercury. The outer 
 tube is strong, and contains within it a smaller tube of uranium glass with 
 balls blown upon it (Plate X. fig. 7). The tubes are exhausted and a small 
 quantity of mercury is introduced which has access to both surfaces of the 
 inner tube, as well as to the inner surface of the outer tube. Upon the tube 
 being reversed end for end or shaken, the mercury runs up or down the tube 
 and causes a very considerable display of whitish light. 
 
 The before-described tubes are also referred to, and their spectra described, 
 in the section " On the comparison of some tube and other Spectra with the 
 Aurora " (Part II.). 
 
 The aura or brush from the electrical machine has been considered as 
 
SUPPOSED CAUSES OP THE AURORA. 
 
 85 
 
 resembling the Aurora, while the hissing and crackling accompanying it has 
 been supposed to corroborate the reports of similar noises having been heard 
 during an auroral display. 
 
 Prof. Lemstrom, of the University of Helsingfors, has devised an instrument 
 for the purpose of demonstrating that Aurorae are produced by electrical 
 currents passing through the atmosphere. An illustration of this instrument 
 (for which I am indebted to the Editor of ' Nature ') is introduced (fig. 1). 
 
 The instrument was exhibited at the recent Scientific Loan Collection at 
 South Kensington, and a full description of it, together with an essay by 
 Prof. Lemstrom, " On the Theory of the Polar Light," will be found in the 
 third edition of the Official Catalogue, p. 386. no. 1751. The apparatus is 
 intended to show that an electric current passing from an insulated body does 
 not produce light in air of normal pressure ; but as it rises to the rarefied air 
 in the Geissler tubes a phenomenon very like the real Polar Light is produced. 
 
 Fig. 1. 
 
 A is an electrical machine, the negative pole being connected with a copper 
 sphere and the positive with the earth. 
 
 s s? is of ebonite as well as R R d, so that B is quite insulated as the earth 
 is in space. B is surrounded by the atmosphere, a' d a! a' a' a' are a series of 
 Geissler tubes with copper ends above and below. All the upper ends are 
 
 Prof. Lem- 
 strom's in- 
 strument to 
 demonstrate 
 the nature 
 of Aurora. 
 
86 
 
 THE ATJKOEA AND ITS CHAEACTEES. 
 
 M. de la 
 Eive's appa- 
 ratus de- 
 scribed. 
 
 De la Eive's 
 magnet in 
 an electric 
 
 Gassiot's 
 experiment 
 with 400 
 Grove cells 
 and ex- 
 hausted 
 receiver 
 between 
 poles of 
 magnet. 
 
 connected with a wire which goes to the earth ; consequently a current runs 
 in the direction of the arrows through the air, and the Geissler tubes become 
 luminous when the electrical machine is set into operation. 
 
 The Geissler tubes represent the upper part of the atmosphere which 
 becomes luminous when the Aurora Borealis is observed in the northern 
 hemisphere. The phenomena produced by the Lemstrom apparatus are 
 considered consistent with the theory advocated by Swedish observers that 
 electrical currents emanating from the earth and penetrating into the upper 
 regions produce Aurorse in both hemispheres. The experiment differs from 
 the apparatus of M. de la Rive, who placed his current in vacuo, and did not 
 show the property of ordinary atmospheric air, in allowing to pass unobserved, 
 at the pressure of 760 millims., a stream of electricity which illuminates a 
 rarefied atmosphere. 
 
 De la Rive's apparatus was also exhibited at the same time, and will be 
 found described at p. 385 of the Catalogue, No. 1749. A large sphere of 
 wood represented the earth, and iron cylinders the two extremities of the 
 terrestrial magnetic axis. These penetrated into two globes filled with 
 rarefied air, simulating the higher regions of the Polar atmosphere. The 
 electric discharge turned around a point situate in the prolongation of the 
 axis, in a different direction at either pole, when the two cylinders were 
 charged by means of a horseshoe electro-magnet, in accordance with obser- 
 vations on the rotation of the rays of the Aurora. 
 
 De la Rive placed an electro-magnet in an electric egg. As soon as the 
 magnet was set in action the discharge which had before filled the egg was 
 concentrated into a defined band of light, which rotated steadily round the 
 magnet. 
 
 Gassiot describes an experiment with his great Grove's battery of 400 cells, 
 in which an exhausted receiver was placed between the poles of the large 
 electro-magnet of the Royal Institution. 
 
 " On now exciting the magnet with a battery of 10 cells, effulgent strata 
 were drawn out from the positive pole, and passed along the under or upper 
 surface of the receiver according to the direction of the current. 
 
 " On making the circuit of the magnet and breaking it immediately, the 
 luminous strata rushed from the positive pole and then retreated, cloud fol- 
 lowing cloud with a deliberate motion, and appearing as if swallowed up by 
 the positive electrode." Mr. Marsh considered this bore a very considerable 
 resemblance to the conduct of the auroral arches, which almost invariably 
 drift slowly southward. 
 
SUPPOSED CAUSES OF THE AUROKA. 
 
 87 
 
 He considered it probable that the Aurora was essentially an electric 
 discharge between the magnetic poles of the earth, leaving the immediate 
 vicinity of the north magnetic pole in the form of clouds of electrified matter, 
 which floated southward, bright streams of electricity suddenly shooting forth 
 in magnetic curves corresponding to the points from which they originated, 
 and then bending southward and downward until they reached corresponding 
 points in the southern magnetic hemisphere, and forming pathways by which 
 ihc electric currents passed to their destination; and, further, that the mag- 
 netism of the earth caused these currents and electrified matter composing 
 the arch to revolve round the magnetic pole of the earth, giving them their 
 observed motion from east to west or from west to east. 
 
 Varley showed that when a glow-discharge in a vacuum tube is brought 
 within the field of a powerful magnet, the magnetic curves are illuminated 
 beyond the electrodes between which the discharge is taking place, as well 
 as in the path of the current, and also thought that this illumination was 
 caused by moving particles of matter, as it deflected a balanced plate of talc 
 on which it was caused to infringe. It has also been shown that in electrical 
 discharges in air at ordinary pressure, while the spark itself was unaffected by 
 the magnet, it was surrounded by a luminous cloud or aura which was driven 
 into the magnetic curve, and which might also be separated from the spark 
 by blowing upon it. 
 
 Most of the foregoing interesting results and experiments will be found 
 repeated and verified in Part III. 
 
 Mr. March 
 considered 
 the Aurora 
 an electric 
 discharge 
 between the 
 magnetic 
 poles of the 
 earth. 
 
 Varley's 
 observation 
 on a glow- 
 discharge 
 in vacua. 
 
 Spark sur- 
 rounded by 
 an aura 
 which could 
 be sepa- 
 rated. 
 
 Prof. Lemstroms Theory. 
 
 Prof. Lemstrom thinks that terrestrial magnetism plays only a comparatively 
 secondary part in the phenomena of the Polar Light, this part consisting 
 essentially in a direct action upon the rays. 
 
 That the experiments of M. de la Rive do not all furnish the proof that the 
 rays of the light are really united under this influence. 
 
 That the Polar Light considered as an electrical discharge gives the fol- 
 lowing results : 
 
 (1) An electric current arising from the discharge itself, which takes place 
 slowly. 
 
 (2) Eays of light consisting of an infinite number of sparks, each spark 
 giving rise to two induction currents going in opposite directions. 
 
 (3) A galvanic current going in an opposite direction to that of the discharge, 
 
 Prof. Lem- 
 
 strotn's 
 
 theory. 
 
 Character 
 of the Polar 
 Light, 
 
88 THE AUEOBA AND ITS CHAEACTEES. 
 
 and having its origin in the electromotive force discovered by M. Edlund in 
 the electric spark. That these currents require a closed circuit ; but this is 
 not necessary in the case of the Aurora, as the earth and rarefied air of the 
 upper regions are immense reservoirs of electricity producing the same effect 
 as if the circuit were closed. That permanent moisture in the air, a good 
 conductor of electricity, is the cause of a slow and continuous discharge 
 assuming the form of an Aurora, instead of suddenly producing lightning as 
 in equatorial regions and mean latitudes. 
 
 Polar Light He sums up, that the electric discharges which take place in the Polar 
 due to elec- re gj ons between the positive electricity of the atmosphere and the negative 
 charges electricity of the earth are the essential and unique cause of the formation of 
 
 the Polar Light, light the existence of which is independent of terrestrial mag- 
 netism, which contributes only to give to the Polar Light a certain direction, 
 and in some cases to give it motion. 
 
 This Prof. Lemstrom maintains contrary to those who believe they see in 
 terrestrial magnetism, or rather in the induction currents, what is capable of 
 developing the origin of the Polar Light. 
 
 Theories of MM. Becquerel and De la Rive. 
 
 Theories of M. Becquerel's theory is that solar spots are cavities by which hydrogen and 
 querel and other substances escape from the sun's protosphere. That the hydrogen takes 
 De la Eive. w ith it positive electricity which spreads into planetary space, even to the 
 earth's atmosphere and the earth itself, always diminishing in intensity because 
 of the bad conducting-power of the successive layers of air and of the earth's 
 crust. That would then only be negative, as being less positive than the air. 
 The diffusion of electricity through planetary space would be limited by the 
 diffusion of matter, since it cannot spread in a vacuum. That gaseous matter 
 extends further than the limits usually assigned to the earth's atmosphere, is 
 proved by the observation of Aurorse at heights of 100 and 200 kilometres, 
 where some gaseous matter must exist. M. de la Rive agrees with 
 M. Becquerel as to the electric origin of the Aurora, but considers the earth 
 is charged with negative electricity and is the source of the positive atmo- 
 spheric electricity, the atmosphere becoming charged by the aqueous vapour 
 rising in tropical seas. The action of the sun he considers is an indirect one, 
 varying with the state of the sun's surface, as shown by coincidences in the 
 periods of Aurora and sun-spots. 
 
SUPPOSED CAUSES OF THE AURORA. 
 
 M. Plante s Electric Experiments. 
 
 M. Plante has performed some experiments with a very considerable series 
 of secondary batteries. By inserting the positive electrode after the negative 
 in a vessel of salt water, luminous and other effects were observed which were 
 considered to have a strong resemblance to those of Aurorae. 
 
 M. Plante advocates the theory that the imperfect vacuum of the upper 
 regions, acting like a large conductor, plays the part of the negative electrode 
 in his experiments, while the positive electricity flows towards the planetary 
 spaces, and not towards the ground, through the mists and ice-clouds which 
 float above the Poles. 
 
 In an article in 'Nature,' March 14, 1878, a further account is given of 
 M. Plante" s experiments, under the head of " Polar Aurorae ;" and it is stated 
 that, in these experiments, the electric current, in presence of aqueous 
 vapour, yielded a series of results altogether analogous to the various 
 phases of Polar Aurorse. If the positive electrode of the secondary battery 
 was brought into contact with the sides of a vessel of salt water, there wa 
 observed, according to the distance of the film (electrode 1) from the liquid, 
 either a corona formed of luminous particles arranged in a circle round the 
 electrode (fig. 2, p. 90), an arc bordered with a fringe of brilliant rays (fig. 3), or 
 a sinuous line which rapidly folded and refolded on itself (fig. 4). This undu- 
 latory movement, in particular, formed a complete analogy with what had been 
 compared in Aurorse to the undulations of a serpent, or to those of drapery 
 agitated by the wind. The rustling noise accompanying the experiment was 
 analogous to that sometimes said to accompany Aurorae, and was caused by the 
 luminous electric discharge penetrating the moisture. As in Auroras, mag- 
 netic perturbations were produced by bringing a needle near the circuit, the 
 deviation increasing with the development of the arch. 
 
 The Auroras were produced by positive electricity, the negative electrode 
 producing nothing similar. 
 
 Illustrations of these miniature Aurora? are given in ' Nature,' and repro- 
 duced on p. 90. No mention of any spectroscopic observations is made. 
 
 In a communication to the Metropolitan Scientific Association (' Obser- 
 vatory,' March 1, 1879, p. 389), Mr. A. P. Holden, after supporting the theory 
 of a connexion between the waxing and waning of the solar corona and sun- 
 spots, adopts Mr. F. Pratt's hypothesis " that the Aurora is simply light filmy 
 cirrus cloud, first deposited at the base of a vast upper body of highly rarefied 
 vapour, and illuminated by the free electricity escaping in the condensation 
 through the very rarefied medium above, towards the north or south. The 
 
 M. Planted 
 experi- 
 ments. 
 Effects pro- 
 duced re- 
 sembling 
 Aurone. 
 
 M. PJaiite's 
 experiments 
 producing 
 a corona, 
 an arc, or a 
 sinuous line. 
 
 Mr. Hol- 
 den's views. 
 
90 
 
 THE AUEOEA AND ITS CHARACTERS. 
 
 Aurora would, according to this theory, have its origin in a vast electrical 
 storm, resulting from a violent condensation of vapour which causes a flow of 
 electricity from the pole to restore equilibrium." The Aurora would thus, in 
 Mr. Holden's opinion, " depend on storm phenomena of an intense character ; 
 and the frequency of Aurorse at the sun-spot maxima would indicate the con- 
 nexion of the latter with the weather." 
 
 Fig. 2. 
 
 Kg. 3. 
 
 The corona. 
 
 The arc and ravs. 
 
 The sinuous line. 
 
SPECTROSCOPE. MICROMETER, TUBES. 
 
 Fig:!. 
 SPECTROSCOPE FOR AURORA. 
 
 MICROMETER CARD. 
 
 Fig:*. 
 MICROMETER ENLARGED 
 
 Fig: 3. 
 DIAPHRAGM . MICROMETER. 
 
 Tig: 6. 
 EXHAUSTED TUBE. 
 
 Fig: 5. 
 DOUBLE SLIT PLATE. 
 
 Fig : 9. 
 AURORff. TUBE. 
 
 Fig : 7. 
 GEISSLER MERCURY TUBE. 
 
 Fig: 8. 
 AURORA TUBE. 
 
 Fig. 10. 
 
 BLACK PATCH ON MOON. 
 
PART II. 
 
 THE SPECTRUM OF THE AURORA. 
 
 CHAPTER X. 
 
 SPECTEOSCOPE ADAPTED FOB THE ATJEOEA. 
 
 ANT form of spectroscope of moderate dispersion will suffice for observations 
 of the spectrum of the Aurora ; but, for sake of convenience, a hand or 
 direct-vision spectroscope is to be preferred, and it is desirable also to have 
 some quick and ready mode of measuring the position of the lines while the 
 Aurora lasts. 
 
 Mr. John Browning arranged for me a form of instrument which I have 
 found very convenient for observations by hand of the Aurora-lines, and also, 
 when fixed on a stand, for tube and chemical investigations. A representation 
 of this instrument is given on Plate X. fig. 1. A brass tube carries a large 
 compound (5) direct-vision prism (shown dark in the drawing). An arrange- 
 ment is made so that a second prism can at will be slipped into the tube 
 (shown in outline in the drawing). With one prism and a fine slit the D 
 lines are widely separated, and the field of view extends at one glance from 
 near C to near G. When the second prism is inserted and used in combina- 
 tion, the nickel line can be seen between the two D lines, and the instrument 
 may be used for solar work. A photograph of the sun's spectrum, taken with 
 one prism only, shows a great number of the dark solar lines and many of the 
 bright ones, ascribed by Prof. Draper to oxygen and nitrogen. 
 
 o 
 
 Must be of 
 moderate 
 dispersion, 
 with ready 
 mode of 
 measuring 
 line-posi- 
 tions. 
 
 Mr. Brown- 
 ing's instru- 
 ment de- 
 scribed. 
 
92 
 
 THE SPECTEUM OF THE AURORA. 
 
 Diaphragm 
 micrometer 
 described. 
 
 Mode of use 
 of the 
 micrometer. 
 
 Advantage 
 of the 
 method. 
 
 The collimator and observing telescope are respectively 6 inches in length, 
 and carry achromatic lenses of one inch aperture. The telescope traverses 
 the field so that the extremities of the spectrum may be observed. The 
 dispersion of the instrument was ascertained by a set of observations of the 
 principal solar and some metallic lines, made with an excellent filar micro- 
 meter. For the Auroral observations, Dr. Vogel has described an instrument 
 (see Appendix E) in which the usual spider's-web wires are replaced by a 
 needle-point, as being easily seen upon a faint spectrum. Illuminated wires 
 may also be used ; but I was led ultimately to employ, in preference, a dia- 
 phragm micrometer which the spectrum itself illuminates, as being adapted 
 for speedy, yet fairly accurate, observations. It was made in this manner : 
 A card was first of all prepared (Plate X. fig. 2), and within a circle described 
 on this, a scale was drawn of moderately wide white spaces, with black 
 divisions between, short and long, so as to read off easily by eye. The upper 
 half of the circle was then entirely filled in with black ; and from the card as 
 thus prepared a reduced negative photograph was made. In this the spaces 
 and lower half of the circle were opaque, and the upper half of the circle 
 and the lines between the spaces were transparent (Plate X. fig. 4). This 
 photograph was about the size of a shilling (fig. 3, same Plate). It was 
 mounted carefully in Canada balsam, with a thin glass cover, and then 
 placed in the focus of the eyepiece. In use, the spectrum is brought upon 
 the scale so that the upper half shows above the scale without any inter- 
 ruption at all ; while the lower half illuminates the scale and renders the 
 divisions visible, showing the spectrum-lines falling either upon them or the 
 spaces between. The photographed scale was next enlarged to a considerable 
 size and printed upon faintly ruled paper; and the enlargement was so 
 arranged as to comprise five of the faint ruled lines between each division of 
 the scale. Each of these faint lines in turn represented a certain portion of 
 the spectrum as read off with the filar micrometer ; so that the scales as con- 
 structed with the filar micrometer and with the photographed micrometer 
 corresponded for all parts of the spectrum included in the field of the eye- 
 piece. 
 
 One of the photographed enlargements being laid on the table under the 
 spectroscope, the observed lines were marked off with ease and accuracy upon 
 it ; and as the photograph was an exact copy of the scale, any want of exacti- 
 tude in the divisions was of no moment. 
 
 One great advantage of this method was, that all the lines seen could be 
 recorded at one time and with all in view, and without the risk of slight 
 
SPECTROSCOPE ADAPTED FOE THE AURORA. 
 
 93 
 
 shift in the instrument, which frequently happens when lines are read off 
 seriatim. 
 
 I found this plan most effective for the rapid and correct recording of a 
 faint and evanescent spectrum, and it gave close results when compared with 
 traversing-micrometer measured spectra. The records, too, admitted of sub- 
 sequent examination at leisure. 
 
 Mr. Browning subsequently constructed for me a double-slit plate (lately in 
 the Scientific Loan Collection at South Kensington) for the same instrument 
 (Plate X. fig. 5). The lower half of the plate is fixed. The upper half 
 traverses the lower by the aid of a micrometer-screw. The slit is widened or 
 closed at pleasure by loosening the small screws by which the jaw-plates are 
 attached. A scale is engraved on the fixed lower half of the plate for an 
 approximate measurement, while the division of the micrometer-screw-head 
 completes it. 
 
 In use, one half of the spectrum slides along the other, and a bright line 
 in the upper spectrum is selected as an index. The distances between the 
 lines of the lower half of the spectrum are read off by means of the bright 
 line above. This form of micrometer was suggested by Mr. Procter (in 
 ' Nature ') many years ago as a substitute for a more complicated apparatus 
 by Zb'llner. Other instruments on a similar principle have been lately intro- 
 duced, but for Aurora purposes I prefer a fixed scale. 
 
 In ' Photographed Spectra ' I have pointed out that we shall probably 
 obtain no spectrum of the Aurora to be absolutely depended upon for com- 
 parison with other spectra until we succeed in a photographed one. From 
 experiments made with a special prism of the Rutherfurd form, constructed 
 for me by Mr. Browning (with which many gas-spectra have been already 
 photographed), I see no reason, should an unusually bright Aurora favour us 
 with a visit, why its spectrum may not be recorded in a permanent form, 
 and with lines sufficiently well marked to be compared with other spectra. 
 Rapid dry plates would be especially useful for such a purpose, and some 
 Aurora, if wanting in brilliancy, would doubtless compensate by their period 
 of endurance. 
 
 Mr. Adam Hilger has also made for me one of his " half-prism " spectro- 
 scopes, in which considerable dispersion is obtained with but very little loss 
 of light. This instrument has a simple and rapid micrometer arrangement, 
 with a bright line as an index. I have (for want of Aurorae) had no oppor- 
 tunity of trying it, but I doubt not it is well adapted for such a purpose. 
 
 Double-slit 
 plate ar- 
 rangement. 
 
 Photo- 
 graphed 
 spectrum 
 suggested. 
 
 Mr. Hilger's 
 half-prism 
 spectro- 
 scope. 
 
 02 
 
94 
 
 THE SPECTEUM OF THE ATJKOEA. 
 
 Lines or 
 bands and 
 continuous 
 spectrum. 
 
 Lines nine 
 in number. 
 
 Table from 
 Encyc. Brit. 
 
 Spectrum of the Aurora described. 
 
 The spectrum of the Aurora consists of a set of lines or bands upon a dark 
 ground at each extremity of the spectrum, but with more or less of faint con- 
 tinuous spectrum towards the centre. The extreme range of the spectrum, 
 as observed up to the present time, is from " " (between C and D) in the 
 red to " h " (hydrogen) in the violet. 
 
 The lines have been classified and arranged by Lemstrom and others as 
 nine in number, but I believe not more than seven have ever been seen 
 simultaneously. 
 
 The author of the article " Aurora Polaris," in the ' Encyclopaedia Britan- 
 nica,' classes the lines as nine, and gives a table with the following results 
 (to these I have added Herr Vogel's lines, for the purpose of identification 
 and comparison) : 
 
 No. of 
 line. 
 
 Number of 
 observations. 
 
 Mean 
 W.L. 
 
 Probable 
 error. 
 
 Vogel's 
 lines. 
 
 1. 
 
 5 
 
 6303 
 
 8-1 
 
 6297 
 
 2. 
 
 10 
 
 5569 
 
 2-9 
 
 5569 
 
 3. 
 
 4 
 
 5342 
 
 16 
 
 5390 
 
 4. 
 
 6 
 
 5214 
 
 5-4 
 
 5233 
 
 5. 
 
 4 
 
 5161 
 
 9-7 
 
 5189 
 
 6. 
 
 6 
 
 4984 
 
 11 
 
 5004 
 
 7. 
 
 4 
 
 4823 
 
 9-3 
 
 
 8. 
 
 8 
 
 4667 
 
 9-8 
 
 4663 
 
 9. 
 
 8 
 
 4299 
 
 9-3 
 
 
 Angstrom's 
 line. 
 
 Zb'Uner's 
 line in the 
 red. 
 
 The probable errors are large, and it is a question whether any thing is 
 gained by thus endeavouring to average the lines. 
 
 The principal and brightest line, in the yellow-green, is generally called 
 
 o 
 
 " Angstrom's," and his (probably the first) measurement of its position at 5567 
 adopted. This was in the winter of 1867-68, and he saw in addition, by 
 widening the slit, traces of three very feeble bands situated near to F. 
 Zollner is credited with the first observation of the line in the red. These 
 two lines are generally described as with similar characteristics, and in 
 about the same respective positions, by all observers, and have never been 
 
SPECTEUM OF THE AURORA DESCRIBED. 
 
 95 
 
 remarked to spread into bands. The other lines in the spectrum are difficult 
 to position, owing to the many discordant observations of them. They seem 
 also variable in intensity as well as in number (sometimes even in the same 
 Aurora), and are not unfrequently observed to have their places supplied by 
 bands. 
 
 The spectroscope was used in the second German expedition, but only the 
 one brightest line seen Dr. Borgen stating he had never seen a trace of the 
 weak lines in the blue and red, which were observed so distinctly with the 
 same spectroscope on 25th October, 1870, after the return of, the expedition. 
 Lieutenant Weyprecht used a small spectroscope during the Austro- 
 Hungarian Expedition, and saw only the well-known yellow-green line. 
 
 In the Swedish Expedition, 1868, Lemstrom mentions that in the Aurora 
 spectrum there are nine lines (he does not say he saw them simultaneously), 
 which he considers to agree with lines belonging to the air-gases. He also 
 thinks the Aurora could be referred to three distinct types, depending on the 
 character of the discharge. 
 
 At Tronsa an Aurora was seen October 21st, 1868, which commenced in 
 the north and became very brilliant. The spectroscope showed : 
 
 1. A yellow line at 74-9. 
 
 2. A very clear line in the blue at 65 - 90. 
 
 3. Two lines of hair's breadth, with very pronounced (horizontal 1) striae on 
 the side of the yellow, one at 125 and the other about 105. 
 
 [I presume the striae were really vertical, and that the explanation intended 
 to convey that these lines shaded off towards the yellow. From a comparison 
 of the figures they must have been in the red, and are the only instance 
 recorded of two auroral lines in that region. They are subsequently spoken 
 of as " shaded rays." J. R. C.] 
 
 M. Auguste Wijkander and Lieut. Parent, of the Swedish Expedition in 
 1872-73, under Professor Nordenskiold, used a direct-vision spectroscope, 
 with a micrometer-screw movement of the prisms, the reading being after- 
 wards reduced to wave-lengths upon Angstrom's line-values. 
 
 The following Table gives the results, with Dr. Vogel's lines added for the 
 sake of comparison : 
 
 " UBR/1 
 
 Other liues 
 of the 
 spectrum. 
 
 Second 
 German ex- 
 pedition ob- 
 servations. 
 
 Austro- 
 Hungarian. 
 
 Swedish ex- 
 pedition, 
 1868. 
 
 Lemstrom'g 
 observa- 
 tions. 
 
 Spectrum 
 of Aurora 
 seen at 
 Tronsa. 
 
 MM. Wij- 
 kander and 
 Parent's ob- 
 servations. 
 
96 
 
 THE SPECTKTJM OF THE ATJEOEA. 
 
 Spectrum 
 of Aurora 
 of October 
 24th, 1870. 
 
 T. F.'s ob- 
 servations. 
 
 W. B. 
 
 Gibbs's ob- 
 servation. 
 Elger's ob- 
 servation. 
 
 Lines. 
 
 Observations, "Wijkander. 
 
 Observations, Parent. 
 
 Mean of 
 both. 
 
 Vogel. 
 
 Number. 
 
 W.L. 
 
 Probable 
 error. 
 
 Number. 
 
 W.L. 
 
 Probable 
 error. 
 
 . . 
 
 
 . . 
 
 
 
 . . 
 
 
 
 
 6297 
 
 . , 
 
 . . 
 
 . , 
 
 . . 
 
 . . 
 
 . . 
 
 
 
 5569 
 
 (i) 
 
 5 
 
 5359 
 
 3 
 
 
 
 
 5359 
 
 5390 
 
 (2) 
 
 6 
 
 5289 
 
 5 
 
 3 
 
 5280 
 
 1 
 
 5286 
 
 
 (3) 
 
 6 
 
 5239 
 
 4 
 
 2 
 
 5207 
 
 11 
 
 5231 
 
 5233 
 
 . . 
 
 . . 
 
 
 
 
 . . 
 
 
 
 5189 
 
 (4) 
 
 5 
 
 4996 
 
 9 
 
 
 
 
 
 4996 
 
 5004 
 
 (5) 
 
 1 
 
 4871 
 
 
 1 
 
 4873 
 
 
 
 4872 
 
 
 (6) 
 
 8 
 
 4692 
 
 2 
 
 10 
 
 4708 
 
 5 
 
 4701 
 
 4663 
 
 (7) 
 
 1 
 
 4366 
 
 
 
 
 
 4366 
 
 
 (8) 
 
 4 
 
 4280 
 
 3 
 
 3 
 
 4286 
 
 +16 
 
 4284 
 
 
 
 The brightest line in all Aurora, 5567, was intentionally not included in 
 the Tables. The red line was not seen. Nos. 5 and 7 were only seen once, 
 and not in the same Aurora. 
 
 The Aurora of October 24th, 1870, came at a time when spectroscopes of 
 a direct-vision form were being introduced, and a number of observations were 
 communicated at the time to ' Nature.' 
 
 A correspondent, T. F., writing from Torquay, saw, with a direct-vision spec- 
 troscope, one strong red line near C, one strong pale yellow line near D, one 
 paler near F, and a still paler one beyond, with a faint continuous spectrum 
 from about D to beyond F. The C line was very conspicuous and the 
 brightest of the whole. It was intermediate in position and colour to the red 
 lines of the lithium and calcium spectra. Plainly there were two spectra 
 superposed, for while the red portions of the Aurora showed the four lines 
 with a faint continuous spectrum, the greenish portions showed only one line 
 near D on a faint ground. W. B. Gibbs saw, in London, only two bright 
 lines, one a greenish grey, situate about the middle of the spectrum, and the 
 other a red line very much like C (hydrogen). Thomas G. Elger, at Bedford, 
 on the 24th and 25th, saw : (1) a broad and well-defined red band near C ; 
 (2) a bright white band near D (same as Angstrom's W.L. 5567), on 25th 
 visible in every part of the sky ; (3) a faint and rather nebulous line, roughly 
 estimated to be near F ; (4) a very faint line about halfway between 2 and 3. 
 
SPECTRUM OF THE AURORA DESCRIBED. 
 
 97 
 
 The red band was absent from the spectrum of the white rays of the Aurora, 
 but the other lines were seen. 
 
 With a small Browning direct- vision spectroscope on the 24th, I found no 
 continuous spectrum, but two bright lines, one in the green (like that from 
 the nebulae, but more intense, and considerably flickering), the other in the 
 red (like the lithium line, but rather duskier : Plate V. fig. 6). The latter 
 was only well seen when the display was at its height ; it could, however, 
 be faintly traced wherever the rose tint of the Aurora extended. The line in 
 the green was well seen in all parts of the sky, but was specially bright in 
 the Auroral patches of white light. 
 
 Mr. Browning also saw the red line, but found comparison difficult. On 
 the evening of the 24th October, Mr. Alvan Clarke, jun., at Boston, used a 
 chemical spectroscope of the ordinary form, with one prism and a photo- 
 graphed scale illuminated with a lamp. Four Auroral lines were seen at 
 points of his scale numbered 61, 68, 80, and 98. These were reduced to 
 wave-lengths by Professor Pickering, with the following results : 
 
 Line. 
 
 Reading 
 on scale. 
 
 Wave- 
 lengths. 
 
 Assumed 
 line. 
 
 Comments. 
 
 Probable 
 error. 
 
 1. 
 
 61 
 
 5690 . 
 
 5570 
 
 Common Aurora-line. 
 
 -20 
 
 2. 
 
 68 
 
 5320 
 
 5316 
 
 Corona line. 
 
 + 1 
 
 3. 
 
 80 
 
 4850 
 
 4860 
 
 F, hydrogen. 
 
 - 3 
 
 4. 
 
 98 
 
 4350 
 
 4340 
 
 G, hydrogen. 
 
 + 6 
 
 [61 is evidently wrong, and was probably a mistake for 63.] 
 
 George F. Barker, observing at New Haven (U. S. A.), saw, on November 
 9th, a crimson and white Aurora, which he examined with a single glass- 
 prism spectroscope, by Duboscq, of Paris. The line positions were obtained 
 by an illuminated millimetre scale. In the white Aurora were four lines 
 (the red one being absent) ; in the red Aurora five. The wave-lengths of the 
 Aurora-lines were run out as follows : 
 
 (1.) Between C and D, 6230 (Zollner's 6270). 
 
 (2.) D and E, 5620 (Angstrom's 5570). 
 
 (3.) E and b, 5170 (Winlock's 5200). 
 
 (4.) b and F, 5020. 
 
 (5.) F and G, 4820 (Alvan Clarke's, jun., 4850). 
 
 J.R. 
 
 Capron's 
 observation. 
 
 Mr. Brown- 
 ing's obser- 
 vation. 
 Alvau 
 Clarke's, 
 jun., ob- 
 servations. 
 
 G. F. Bar- 
 ker's obser- 
 vations. 
 
98 
 
 THE SPECTETJM OF THE ATJEOEA. 
 
 Spectrum 
 of Aurora 
 of Feb. 4, 
 1872. 
 
 Prof. Piazzi 
 Smyth's ob- 
 servations. 
 
 Rev. T. W. 
 Webb's ob- 
 servations. 
 
 E. J. Fris- 
 well's ob- 
 servations. 
 
 The Eev. 
 8. J. Perry's 
 observa- 
 tions. 
 
 J. E. Ca- 
 
 pron's ob- 
 servations. 
 
 His cata- 
 logue of 
 lines up to 
 Nov. 9, 
 
 1872. 
 
 Mr. Procter's Aurora-lines will be found noticed in connexion with the 
 spectrum of oxygen; and Lord Lindsay's lines, with a comparison scale 
 drawing, are separately described further on in this Chapter. The Aurora of 
 February 4th, 1872, had many observers; some of whom communicated at 
 the time spectroscopic notes. Professor Piazzi Smyth minutely describes the 
 
 o 
 
 display as seen in Edinburgh, and saw " Angstrom's green Aurora-line per- 
 petually over citron acetylene * at W.L. 5579, and the red Aurora-line 
 between lithium a and sodium a, but nearer to the latter, say at W.L. 6370." 
 Extremely faint greenish and bluish lines also appeared at W.L. 5300, 5100, 
 and 4900 nearly. 
 
 The Eev. T. W. Webb, with a very fine slit, saw the green Auroral line even 
 in the light reflected from white paper. With a wider slit he saw a crimson 
 band in the brighter patches of that hue, and beyond an extent of greenish 
 or bluish light, which he suspected to be composed of contiguous bands. 
 
 E. J. Friswell, coming up the Channel at 9.40, with a Hoffman's direct- 
 vision spectroscope (the observing telescope removed), saw the green line, a 
 crimson line near C, and faint traces of structure in the blue and violet. 
 
 The Eev. S. J. Perry observed at Stonyhurst four lines, and, on examining 
 one of the curved streamers, found the red line even more strongly marked 
 than the green. A magnetic storm was observed to be at its height from 4 
 to 9 P.M. of the same day. 
 
 With a Browning 7-prism direct-vision spectroscope I saw the green line 
 in all parts of the Aurora, attended with a peculiar flickering movement. I 
 did not see the other lines. 
 
 In a letter to ' Nature,' dated November 9th, 1872, I catalogued the lines 
 observed up to that date as follows : 
 
 O 
 
 1. A line in the red between C and D. W.L., Angstrom, 6279. 
 
 2. A line (the principal one of the Aurora) in the yellow-green, between 
 D and E. W.L., Angstrom, 5567. 
 
 3. A line in the green, near E (corona line ?). W.L., Alvan Clarke, jun., 
 and Backhouse, 5320. 
 
 4. A faint line in the green, at or near b. W.L., Barker, 5170. 
 
 5. A faint line or band in the green, between b and F. W.L., Barker, 
 5020 (chromospheric V). 
 
 * There seems to be some confusion as to the W.L. here given ; 5567 is usually accepted as 
 Angstrom's line, while Prof. Smyth refers to it as 5579. The position, too, when examined with 
 a spectroscope of greater dispersion, is not exactly over the citron-line of acetylene, both the above 
 referred to lines lying somewhat more towards the violet end of the spectrum (see Plate V. fig. 7). 
 
SPECTRUM OF THE AURORA DESCRIBED. 
 
 99 
 
 6. A lino in the green-blue, at or near F. W.L., Alvan Clarke, juh., 
 4850. 
 
 7. A line in the indigo, at or near G. W.L., Alvan Clarke, jun., 4350. 
 s. The continuous spectrum from about D to beyond F. 
 
 Dr. H. C. Vogel, formerly of the Bothkamp Observatory, near Kiel, and 
 since of the Astrophysical Observatory, Potsdam, made several observations 
 of the Auroral lines, October 25th, 1870. Besides the bright line between 
 I) and E, he found several other fainter lines stretching towards the blue 
 end of the spectrum on a dimly-lighted ground. February llth, 1871, he 
 observed the same set of lines, and an average of six readings gave 5572 as 
 
 O 
 
 the W.L. of the Angstrom line. February 12th gave 5576 as Dr. Vogel's 
 reading, and 5569 as Dr. Lohse's. April 9th gave 5569, and April 14th 
 V>r,9. The Aurora of April 9th, 1871, was exceedingly brilliant, so that 
 micrometer measurements of the lines were taken. The spectrum consisted 
 of one line in the red, five in the green, and a somewhat indistinct broad line 
 or band in the blue. The lines are thus described : 
 
 Table of Dr. Vogel's lines. Aurora, April 9th, 1871. 
 
 W.L. 
 
 Probable error. 
 
 Remarks. 
 
 6297 
 
 14 
 
 Very bright stripe. 
 
 
 
 
 
 
 p 
 
 5569 
 
 2 
 
 Brightest line of the spectrum, became 
 
 
 
 
 
 noticeably fainter at appearance of 
 
 CR 5* 
 -' 
 
 
 
 the red line. 
 
 >|| 
 
 5390 
 
 .. 
 
 Extremely faint line ; unreliable observa- 
 
 cr5' 
 
 
 
 tion. 
 
 ET 
 
 
 
 
 t 
 
 5233 
 
 4 
 
 Moderately bright. 
 
 5189 
 
 . 9 
 
 This line was very bright when the red line ap- 
 
 
 
 peared at the same time ; otherwise equal in 
 
 
 
 brilliancy with the preceding one. 
 
 5004 
 
 8 
 
 Very bright line. 
 
 46941 
 
 
 f Broad band of light, somewfiat less brilliant in 
 
 46631 
 
 3 
 
 < the middle ; very faint in those parts of the 
 
 4629 J 
 
 
 Aurora in which the red line appeared. 
 
 A translation of Dr. Vogel's interesting paper will be found printed in ex- 
 tenso in Appendix E, and his lithographed drawings of the spectrum in the 
 and red portions of the Aurora respectively on Plate VI. figs. 2 and 3. 
 
 Or. II. ( 
 Vogel's ob- 
 
 servations 
 of Auroral 
 lines. 
 
 Spectrum 
 described. 
 
 Table of 
 lines. 
 
100 
 
 THE SPECTRUM OF THE AURORA. 
 
 Mr. Back- 
 house's 
 catalogue of 
 lines. 
 
 Subsequent 
 full cata- 
 logue of Au- 
 roral lines. 
 
 The observations of April 9th by Dr. Vogel are probably, up to the present 
 time, the most exact of" any one Aurora, and I have therefore in most cases 
 used them for comparison. 
 
 Mr. Backhouse, in a letter to ' Nature,' commenting upon my catalogue 
 of lines, gave the following as the latest determinations from his own 
 
 observations : 
 
 No. 1. Wave-length 6060 
 
 5660 
 
 2. 
 3. 
 
 4. 
 6. 
 
 7. 
 
 5165 
 5015 
 
 4625 
 4305 
 
 (6060 must be a mistake for 6260, and 5660 for 5560. J. E. C.) Mr. Back- 
 house never saw a line at 5320 again. He found the continuous spectrum to 
 reach from No. 2 to No. 7, being brightest from a little beyond No. 2 to 
 No. 6. This part of the spectrum did not give him so much the idea of 
 a true " continuous spectrum " as of a series of bright bands too close to be 
 distinguished. 
 
 I have subsequently, in another section of this Chapter, added a full cata- 
 logue of the Auroral lines, prepared by myself from the foregoing and other 
 sources and observations ; and I also append to it a Plate [Plate XII.], in 
 which these lines are positioned and the wave-lengths and names of observers 
 are given. The numbers of the lines on the Plate correspond with those in 
 the catalogue. The solar spectrum and the spectrum of the blue base of a 
 candle-flame are added for purposes of comparison. [The telluric bands in 
 the solar spectrum are shown more distinctly than they actually appear, and 
 do not profess to give details.] 
 
 Flickering of 
 the green 
 line. 
 
 Herschel's 
 observation. 
 
 J. R. Ca- 
 
 pron's ob- 
 servation. 
 
 Flickering of the Green Line. 
 
 A. S. Herschel noticed this, April 9th (1871 1). He says : "A remarkable 
 circumstance connected with the appearance of the single line observed on 
 this occasion was the flickering and frequent changes with which it rose and 
 fell in brightness ; apparently even more rapidly than the swiftly travelling 
 waves, or pulsations of light, that repeatedly passed over the streamers, near 
 the northern horizon, towards which the spectroscope was directed." In the 
 spectrum of the Aurora of 20th October, 1870, I saw and noted the green 
 line as "considerably flickering; " and in the Aurora of 4th February, 1872, 
 
GRAPHICAL SPECTRA OF THE AUEOEA. 
 
 101 
 
 I again saw and noted "the peculiar nickering " I had remarked in 1870. I 
 have not seen the peculiarity noted by other observers. 
 
 Mr. Backhouses graphical Spectra of four Aurora. 
 
 Mr. Backhouse has been good enough to supply me with some details of 
 four several Aurora? seen by him at Sunderland, accompanied by drawings, 
 showing in a graphical way the spectrum of each display as seen with a spectro- 
 scope with rather a wide slit and as drawn by eye. I have reduced the four 
 drawings to the same scale, and in this way they are extremely interesting for 
 
 O 
 
 comparison (Plate V. fig. 4). The line on the left in each spectrum is Ang- 
 strom's bright Auroral line, and is supposed to be considerably prolonged. 
 The height of the lines denotes intensity. 
 
 April 18th, 1873, was a bright Aurora. No. 3 is a faint band, which 
 Mr. Backhouse had not perceived before. No. 5 had not been visible lately, 
 and Mr. Backhouse thought it must belong to Aurorse of a different type 
 from those which had appeared latterly. 
 
 February 4th, 1874. In the spectrum of this Aurora Mr. Backhouse saw 
 seven lines, all that he had ever seen. (The red line, not shown in the diagram, 
 makes the seventh.) 
 
 The spectrum is represented as seen between 6.50 and 7.5 P.M. Mr. Back- 
 house had only once before seen No. 4, and it became quite invisible between 
 7.45 and 7.55, though the other lines were as bright as before and the red 
 line had appeared. 
 
 October 3rd, 1874. This spectrum was examined, and diagram made be- 
 tween 10 and 10.25 P.M. Five lines only are indicated. 
 
 It is mainly distinguished from the two preceding spectra by the brightness 
 of the continuous spectrum on which the lines 2, 3, and 4 lie, and by the 
 weakness of No. 5. 
 
 October 4th, 1874. Taken between 11.10 and 11.20 P.M. ; distinguished, 
 like the last, by a considerable amount of continuous spectrum and by a faint 
 line (No. 3), not seen in the last spectrum, while No. 3 in the last is missing 
 in this spectrum. 
 
 Mr. Backhouse, as to both these last spectra, remarks that the lines were 
 very variable in intensity, and sometimes some were visible and sometimes 
 others. They varied also in relative brightness in different parts of the sky 
 at the same time. Mr. Backhouse, in a communication to ' Nature,' referring 
 to a statement of Mr. Procter's, that the bands of the Auroral spectrum are 
 
 P2 
 
 Mr. Back- 
 house's 
 graphical 
 spectra of 
 Auroras. 
 
 April 18, 
 1873. 
 
 Feb. 4, 
 1874. 
 
 Oct. 3, 1874. 
 
 Oct. 4, 1874. 
 
 Mr. Back- 
 house's re- 
 marks as to 
 comparative 
 frequency of 
 some of the 
 Auroral 
 lines. 
 
102 
 
 THE SPECTEUM OF THE AUEOEA. 
 
 Lord Lind- 
 say's Aurora 
 of 21st Oct., 
 1870. 
 
 Spectrum 
 described. 
 
 seldom visible, except the bright line at 5570, says that he always found two 
 bands, " doubtless Winlock's 4640 and 4310," to be invariably visible when 
 the Aurora was bright enough to show them. Of thirty-four Auroras ex- 
 amined by Mr. Backhouse, fourteen showed the lines 4640 and 4310, and 
 three others at least one of these, while eight showed the red line. (Ang- 
 strom only once saw this line.) In five Aurorse, all more or less red, he saw 
 a faint band, the wave-length of which he placed at 5000 or 5100. He never 
 saw the line 5320 (also Winlock's coronal line), unless it were once, probably 
 from want of instrumental power. With regard to these observations, 1 may 
 say that with a Browning's miniature spectroscope I saw only two lines (the 
 red and the green) in the grand display of the 24th October, 1870 ; and with 
 an instrument of larger aperture the green line only on the 4th February, 
 1872; while I saw the green line and three others towards the violet with 
 the same instrument during the Aurora of 4th February, 1874. (See descrip- 
 tion of this Aurora, ante p. 21, and drawing of spectrum, Plate VI. fig. 1 a.) 
 
 Lord Lindsay s Aurora-Spectrum, 21s October, 1870. 
 
 Lord Lindsay observed a fine Aurora at the Observatory at Dun Echt on 
 the night of the 21st October, 1870. It commenced about 9.30, reached its 
 maximum about 11, and faded -away suddenly about 11.30 P.M. 
 
 A spectrum obtained in the north-west gave five bright lines with* a Brown- 
 ing's direct-vision Spectroscope two strong, one medium, two very faint. A 
 tallow candle was used to obtain a comparison spectrum of sodium and car- 
 buretted hydrogen. 
 
 A drawing of the spectrum obtained is given on Plate XI. fig. 2. No. 1 
 is a sharp well-formed line visible with a narrow slit. 
 
 No. 2, a line very slightly more refrangible than F. The side towards D 
 is sharp and well defined, while on the other side it is nebulous. 
 
 No. -3, slightly less refrangible than G, is a broad ill-defined band, seen only 
 with a wide slit. 
 
 No. 4, a line near E, woolly at the edges, but rather sharp in the centre. 
 This, says Lord Lindsay, should be at or near the position of the line 1474 of 
 the solar corona. 
 
 No. 5, a faint band, coincident with b, extending equally on both sides 
 of it. 
 
 The lines are numbered in order of intensity. It is questionable, from 
 observations with instruments carrying a scale, whether the line-positions are 
 exact ; but the description of their characters is valuable. 
 
Pla,t,e XI. 
 
 AURORA SPECTRA. CANDLE SPECTRUM. 
 
 COMPARISON OF AURORA AND VIOLET POLE. 
 
 AURORA 
 
 VOG EL 
 
 I' VIOLET POLE. 
 
 CAPRON 
 
 fig: 2. 
 LORD LINDSAY. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 1 
 
 
 1 
 
 
 
 
 
 
 D E 
 
 
 F 
 
 
 
 G 
 
 
 
 
 
 II 
 
 
 I I 
 1! 
 
 
 I 
 
 
 
 
 
 SUN 
 
 CANDLE 
 
 AURORA 
 
 Firf.S. 
 
 AURORA AUSTRALIS. CAP 1 ? MACLEAR. 
 A A A. A 
 
 1 
 
 
 1 1 
 
 
 T) 
 
 1, 
 
 F G 
 
 1 
 
 Car 
 
 1 
 
 Car. 
 
 loir 
 
 A 
 
 A A A? 
 
 AURORA BOREALIS. VOGEL. 
 
 1) 
 
 I 
 
 OXYGEN 
 
 AURORA 
 
 HYDROGEN 
 
 Fig:*. 
 PROCTER 
 
 Tfa Vu 
 
 HjJ lly 
 
 D '<<> 
 
 100 CARBON WATTS. 
 
 130 
 
 Mmtrnflro lith 
 
SPECTRUM OF THE AUKOKA AUSTEAL1S. 
 
 103 
 
 As a candle blue-base spectrum is at times a ready and handy mode of 
 reference in Auroral observations (as was found in this instance), I have, on 
 Plate XI. fig. 5, given a representation of it as seen with my Auroral spectro- 
 scope. Dr..\Vatts's corresponding carbon-spectrum is added on the lower 
 
 Candl.- 
 spectr urn. 
 
 margin. 
 
 The numbers on the upper margin refer to my scale. 
 
 Spectrum of the Aurora Australia. 
 
 Captain Maclear, on examining the streamer seen by him Feb. 9th, 1874 
 (ante, p. 27), with the spectroscope, found three prominent lines in the yellow- 
 gri'en, green, and blue or purple, but not the red line. In the Aurora of 
 March 3rd, 1874 (p. 27), he could trace four lines, three bright and one rather 
 faint. They must have been exceedingly bright to show so plainly in full- 
 moon light. 
 
 The spectroscope used was a Grubb single-prism with long collimator. 
 A needle-point in the eyepiece marked the position of the lines ; and a cor- 
 responding needle-point, carried on a frame by a screw movement in concord 
 with the point of the eyepiece, scratched the lines on a plate of blackened 
 glass. Two plates were taken. On the first were scratched the auroral 
 lines and the solar lines as seen in the moonlight; on the second plate were 
 scratched the auroral lines, the Solar lines from the moon, and the carbon 
 lines in a spirit-lamp. 
 
 The next morning the solar lines were verified in sunlight. I subjoin 
 (Plate XI. fig. 3) copies of the two spectra as printed in ' Nature,' the auroral 
 lines being marked A, the solar lines by the usual designating letters, and 
 the carbon by Car. To these spectra I have added for comparison Dr. Vogel's 
 spectrum of the Aurora Borealis. Captain Maclear could not account for the 
 different positions of the auroral lines in the two plates ; for the prism, as far 
 as he was aware, was not moved during the observations. As the solar lines 
 are indicated in the same place in both spectra, the case would seem one of 
 actual change of position of the auroral lines during observation. A compa- 
 rison of the two spectra gives the impression that the lower one is the same 
 as the upper, except that the dispersion is greater, the lines remaining rela- 
 tively in position. One does not, however, see how the dispersion could have 
 so varied in a single-prism instrument, and the position of the solar lines is 
 adverse to such an explanation. 
 
 There is a suspicion that Aurora? are not always identical in position of 
 some of the lines ; but the line in the green (considerably out of place in the 
 lower Australis spectrum) has always, within small limits, the same position. 
 
 Captain 
 Maclean's 
 spectra of 
 Aurora Aus- 
 tralis. 
 
 Instrument 
 used, and 
 mod^ of 
 registering 
 lines. 
 
 Copies of 
 the two 
 spectra ob- 
 tained. 
 
 Discrepancy 
 in the spec- 
 tra. 
 
 Eemarks on 
 the spectra. 
 
104 
 
 THE SPECTEUM OF THE AUEOEA. 
 
 Comparison 
 of the lines. 
 
 It will be noticed how much further the Australia spectrum runs into the 
 violet than Vogel's Borealis, the latter having no lines much beyond F. 
 
 The faint line (No, 2) mentioned by Captain Maclear possibly corresponds 
 with Dr. Vogel's band. The absence of the four lines of the Aurora Borealis 
 in the green part of the spectrum of the Australis is peculiar ; and in this 
 respect, too, the two Australis spectra agree. 
 
 The nearest approaches to Captain Maclear 's lines (of the upper spectrum) 
 which I can find are : 
 
 Line Corresponding line. 
 
 o 
 
 1. 5567, Angstrom. 
 
 2. (The faint line.) Vogel's band, 4694-4629. 
 
 3. I find no approximately corresponding line. 
 
 4. 4350, Alvan Clarke. 
 
 But the comparisons are not by any means close. 
 Australis spectrum are very desirable. 
 
 Further observations of the 
 
 Prof. Pia/./.i 
 Smyth's 
 chemical 
 and auroral 
 spectra. 
 
 Prof. Piazzi Smyth's Aurora-Spectra. 
 
 Prof. Piazzi Smyth, in volume xiv. of the ' Edinburgh Astronomical Obser- 
 vations,' 1870-77, has compared simultaneously the Aurora-spectrum with 
 the sets of bright lines seen in the blue base of flame the lines of potassium, 
 lithium, sodium, thallium, and indium being also introduced for comparison. 
 The spectra are drawn as seen under small dispersion, and will prove most 
 useful in cases where an Aurora is not bright enough to admit of the lines 
 being measured by micrometer, and the eye and comparison spectrum are 
 obliged to be resorted to. ^ 
 
 Author's Catalogue of the Auroral Lines. 
 (See Plate XII.) 
 
 1. W.L. 6297, Vogel. Very bright stripe; first noticed by Zollner. Seen 
 only in red Aurora ; stands out on a dark ground, without other lines near it. 
 Character of line sharp and well defined ; varies in colour from dusky red to 
 bright crimson. Intensity, Herschel, to 4 or 8. According to same, position 
 coincident with atmospheric absorption-group " a" in solar spectrum (between 
 C and D). I confirm this position according to my scale of solar lines, and a 
 
o 
 
 UJ 
 
 a. 
 en 
 
 UJ 
 
 _i 
 
 Q 
 
 Z 
 
 Q 
 
 Z 
 
 K 
 O 
 
 UJ 
 
 a. 
 (0 
 
 a: 
 
 o 
 
 (O 
 
 o: 
 
 K 
 O 
 uJ 
 
 CL 
 (f) 
 
 cr 
 o 
 cc 
 
 D 
 
 cr 
 O 
 
 IE 
 
 D 
 
 UJ 
 
 I 
 H 
 
 U. 
 
 O 
 
 D 
 a: 
 
 H 
 O 
 ul 
 a. 
 
CATALOGUE OF AURORAL LINES. 105 
 
 drawing of the coincidence (in which, and in Plate XII., the absorption-lines 
 are drawn too dark) is given on Plate XIII. fig. 2. Herschel says this line 
 coincides with a red band in the negative glow-discharge, but its identity is 
 doubtful. Its isolation and want of adjacent lines seem to separate it from 
 the air-spectrum and gas-spectra in general. At the appearance of this line, 
 5569 (No. 2) becomes noticeably fainter. When this line is bright, 5189 
 (No. 5) is bright also (Vogel). 
 
 If we propose to assign to this line, as well as to 5569, a phosphorescent 
 origin, it would be strongly confirmatory of such a theory (in connexion with 
 the phosphoretted-hydrogen spectrum) to find it brighten at low temperatures. 
 
 Note. Sir John Franklin says, in his ' Polar Expeditions,' that a low state of 
 temperature is favourable for the production of brilliant coruscations. It was 
 seldom witnessed that the Aurorae were much agitated, or that the prismatic 
 tints were very apparent, when the temperature was above zero. 
 
 2. Line in the yellow-green. Brightest of all lines in the Aurora-spectrum. 
 
 O 
 
 W.L. 5567, Angstrom ; 5569, Vogel. Intensity 25, Herschel. To me more 
 pale green than yellow, sometimes nickering and changing in brightness 
 (Herschel and Capron). Seen in all Aurora? usually sharp and bright, but 
 Procter has once recorded it nebulous. Its character as to width, sharpness, 
 and intensity, if carefully observed, might indicate height and structure of 
 Aurora. Becomes noticeably fainter at appearance of red line (Vogel). 
 Found by me to correspond in position with a faint atmospheric absorption- 
 band (see Plate XIII. fig. 2). According to Angstrom and Herschel, arising 
 from a phosphorescent and fluorescent light, emitted when air is subjected to 
 the action of electrical discharge. 
 
 3. Line in green near last. W.L. 5390. An extremely faint and unreliable 
 observation (Vogel). Seen only by him, unless Alvan Clarke's 5320 (coronal I) 
 be the same. 
 
 4. Line in green-blue. W.L. 5233, moderately bright (Vogel) ; 5200, Win- 
 lock. Intensity, 2 or 1 to 6, Herschel. Coincides with line in the negative 
 glow according to same. Frequently observed. 
 
 5. Line in green-blue. W.L. 5189. This line is very bright when the red 
 line appears at the same time ; otherwise equal in brilliancy with No. 3 (Vogel); 
 Winlock, 5200. Not so frequently observed as No. 3. Barker gives a band 
 extending from 5330 to 5200. Intensity of 5189, to 8, Herschel, who con- 
 siders it coincident with a constant strong line in the spark-discharge. 
 
 6. Line in blue. W.L. 5004. Very bright line, Vogel; 5020, Barker 
 (coronal 1). Intensity 2 or 1 to 8, Herschel. Coincides with line of nitrogen 
 
106 THE SPECTRUM OF THE AURORA. 
 
 in the nebulae according to same. Barker gives a band extending from 5050 
 to 4990. 
 
 7. Line in the blue not found by Vogel in Aurora, April 9th, 1871. "W.L. 
 4850, Alvan Clarke ; 4820, Backhouse and Barker. Intensity, Herschel, of 
 4820-4870, to 4"? Herschel suspects this and No. 4 to be seen only in 
 Auroral streamers of low elevation. Barker gives a band extending from 
 4930 to 4850. 
 
 8. 4694, 4663, 4629. Broad band of light, somewhat less bright in the 
 middle ; very faint in those parts of the Aurora in which the red line appears 
 (Vogel). Intensity 3-6 (Herschel). A double band, consisting of two lines, 
 the first rather more frequently noted than the second in Auroral spectra, 
 agrees well in position with the principal band in the negative glow-spectrum 
 (same). Barker gives a band extending from 4740 to 4670 ; Backhouse and 
 Winlock give a line at 4640, situate within the same. 
 
 9. There seems a good deal of confusion about a fairly bright line (intensity 
 0-6, Herschel) seen in most Aurorae (not, however, by Vogel, April 9th, 1871), 
 and situate somewhere near G in the solar spectrum. Alvan Clarke places it 
 at 4350, on the less refrangible side of G ; Backhouse and Barker at or very 
 near to G ; while Lemstrom and others position it on the more refrangible 
 side of G. Accurate observations, for which a quartz spectroscope might be 
 useful, are much wanted. Herschel makes this line, at 4285, correspond with 
 a strong band in the violet in the negative glow-spectrum. 
 
 Herschel also refers to an apparently additional line near the hydrogen- 
 line, or between G and H : , in the solar spectrum, as mentioned once by Lem- 
 strom at Helsingfors. I am not aware of any other observation of this line, 
 which must be considerably beyond that at or near G, and would probably 
 be difficult to detect, except in instruments specially adapted for examination 
 of the violet end of the spectrum. 
 
 Theories in relation to the Aurora and its Spectrum. 
 
 Lemstrom's. Lemstrom (1) : That the Polar light is caused by an electric current passing 
 from the upper rarefied layers of the air to the earth, producing light-phe- 
 nomena that do not arise in the denser layers of the air. (2) That there are 
 nine rays (lines or bands) in the Aurora-spectrum, which in all probability 
 agree with lines which belong to the gases of the air. (3) That the Aurora- 
 spectrum can be referred to three distinct types, which depend on the 
 character of the discharge. 
 
THEOEIES IN RELATION TO THE AURORA. 
 
 107 
 
 Dr. Vogel : (1) That the Aurora are electric discharges in rarefied-air strata 
 of very considerable thickness. (2) That the Aurora-spectrum is a modifica- 
 tion of the air-spectrum, involving the question of alteration of the spectrum 
 by conditions of temperature and pressure. 
 
 Angstrom : (1) seems to adopt the hypothesis that the Aurora has its final 
 cause in electrical discharges in the upper strata of the atmosphere, and that 
 these, whether disruptional or continuous, take place sometimes on the outer 
 boundary of the atmosphere, and sometimes near the surface of the earth. 
 
 (2) That the Aurora has two different spectra. 
 
 (3) That the green line is due to fluorescence or phosphorescence, and that 
 there is no need to resort to Dr. Vogel's variability of gas-spectra according 
 to circumstances of pressure and temperature. 
 
 (4) That an agreement exists between the lines of the Aurora (except the 
 red and green before mentioned) and the lines or bands of the violet light 
 which proceed from the negative pole in dry air. 
 
 Zollner has pointed out that the temperature of the incandescent gas of the 
 Aurora must be exceedingly low, comparatively, and concludes that the 
 spectrum does not correspond with any known spectrum of the atmospheric 
 gases only because, though a spectrum of our atmosphere, it is one of 
 another order, and one which we cannot produce artificially. 
 
 Vogel's. 
 
 Angstro 
 
 roin s. 
 
 Zolliier's 
 remark as to 
 temperature 
 of Aurora 
 and charac- 
 ter of spec- 
 trum. 
 
108 
 
 THE SPECTBUM OF THE AUEOKA. 
 
 Testing 
 
 Angstrom's 
 
 Aurora 
 
 theory. 
 
 Battery and 
 spectro- 
 scope de- 
 scribed. 
 
 Vogel's spec- 
 trum se- 
 lected for 
 comparison. 
 
 Central part 
 only of spec- 
 trum 
 mapped. 
 
 Hydrogen- 
 tube. 
 
 CHAPTER XI. 
 
 THE COMPAEISON OF SOME TUBE AND OTHEE SPECTRA WITH THE 
 SPECTEUM OF THE AUEOEA. 
 
 [In part from an Article in the ' Philosophical Magazine ' for April 1875.] 
 
 IN order to test Professor Angstrom's theory of the Aurora, referred to in the 
 last Chapter, in an experimental way, I examined, in the winter of 1874, some 
 tube and other spectra, not only for line-positions, but also for general re- 
 semblance to an Aurora-spectrum. It did not seem desirable to use powerful 
 currents. A J-inch-spark coil, worked by a quart bichromate-cell, was found 
 sufficient to illuminate the tubes steadily. The spectroscope used was one 
 made for me by Mr. Browning specifically for Auroral purposes, and of the 
 direct-vision form, being the same instrument as is described ante, p. 91, and 
 figured in Plate X. fig. 1. The micrometer was the diaphragm one, also before 
 described and figured on same Plate, figs. 2, 3, and 4. I selected Dr. Vogel's 
 spectrum for comparison, it being, so far as I am aware, the most accurately 
 mapped, with regard to wave-length, at one observation, of any Auroral spec- 
 trum. It seemed an unsafe plan to attempt to obtain an average Aurora by 
 comparison of different observations made at various times by different ob- 
 servers with all sorts of instruments the difficulty, too, being increased by 
 the suspicion that the spectrum itself at times varies in number and position 
 as well as intensity of its lines. 
 
 In most cases the central part of the spectrum only (corresponding to the 
 central lines of the Aurora) was mapped, the red line in the Aurora not being 
 found to correspond with any prominent line in the gas-spectra examined, 
 and the Auroral line near solar G being so indefinitely fixed as to render 
 comparison almost valueless. (See Plate XIII. fig. 1.) 
 
 Dr. Vogel's spectrum does not comprise the line near G ; but I have added 
 this (in an approximate place only) in order to complete the set of lines. For 
 drawing of Dr. Vogel's spectrum, with its scales attached, see Plate XIII. 
 
 Hydrogen-tube. 
 
 This tube was one of Geissler's and of rather small calibre. On illumi- 
 nation the wide ends were easily lighted with a silver-grey glow, having a 
 
o 
 It 
 
 3 
 
 o 
 
 U) 
 
 oe 
 
 H 
 
 00 
 
 *- 
 
 H 
 
 111 O 
 
 go* 
 e z < 
 
 n 
 
 I 
 
 CO 
 
 111 
 
 U] 
 
 a: 
 
 1C 
 Id 
 
 c/i 
 
 O 
 in 
 
 o: 
 O 
 o: 
 
 3 
 
 3 ONV a QNV 
 
 QKIV 3 N33MX38 
 
COMPARISON OP SOME TUBE AND OTHER SPECTEA. 
 
 109 
 
 considerable amount of stratification. The capillary part glowed brilliantly 
 with silver-white, bright green, and crimson light, according to the intensity of 
 the current. With the commutator slowly working, white running into green 
 and bright green were the main features of the thread of light; on the 
 current passing more rapidly, the capillary thread became of an intense 
 crimson, at the same time apparently increasing in diameter (an effect pro- 
 bably due to irradiation). 
 
 The spectrum was very brilliant, consisting of the three bright lines 
 usually distinguished as Ha, H/3, and Hy, and a number of shaded bands 
 and fainter lines between these, with a bright continuous spectrum as a back- 
 ground to the whole. 
 
 The lines Hu, H/3, and Hy were found to vary in intensity with the 
 current, and in accordance with the colour of the light as seen by the eye a 
 fact, as I think, not without bearing on the question of the Aurora, the vary- 
 ing tints of which are so well known. The fainter lines or bands were mostly 
 stripes of pretty equal intensity throughout, and all about the width of the 
 H/3 line. I did not trace any marked degrading on either side of the lines, 
 though the edges were not uniformly so sharp as Ha and H/3. Some of the 
 lines were found coincident in position with lines of the air-spectrum. 
 
 It is a question whether these subsidiary lines are hydrogen, or are due to 
 some tube impurity. A photograph I have taken of this tube-spectrum 
 shows 17 lines in the part of the spectrum between F and H 2 , some of which 
 are repeated in the hydrocarbon-tube spectra. 
 
 No principal line, and one subsidiary line only, actually coincide with the 
 Aurora-spectrum, this last being that to which Dr. Vogel assigns an identical 
 wave-length, viz. 5189. Other of the subsidiary lines, however, fall some- 
 what near the Aurora-lines 5569, 5390, 5233, and 5004, two faint lines also 
 falling within the band 4694 to 4629. 
 
 The lines (adopting Dr. Vogel's wave-lengths for the H lines) were, when 
 
 compared, as under : 
 
 (4694) 
 
 Aurora .... 5569 5390 5233 5189 5004 { to \ band. 
 
 I 4629 J 
 
 Hydrogen . . . 5555 5422 .... 5189 5008 4632 
 
 I remarked that a line (5596) described by Dr. Vogel as " very bright " 
 in his H spectrum does not appear in my tube, though in most other respects 
 our H spectra agree. 
 
 I thought this tube afforded a good opportunity for testing the effect of 
 
 Q2 
 
 Colour of 
 glow varied 
 with inten- 
 sity of cur- 
 rent. 
 
 Spectrum 
 described. 
 
 Lines a, j3, 
 and y varied 
 in intensity 
 with colour 
 as seen by 
 eye. 
 
 Fainter lines 
 or bands de- 
 scribed. 
 
 Purity of 
 subsidiary 
 lines ques- 
 tioned. 
 
 Coincidence 
 of lines with 
 Aurora- 
 spectrum. 
 
 Comparison 
 of the lines. 
 
110 
 
 THE SPECTEUM OF THE AUEOEA. 
 
 Efiect of dis- 
 tance on the 
 spectrum. 
 
 distance upon the spectrum. The slit was made rather fine. At 6 inches 
 distance from it the line a in the blue-green (F solar) was very bright. The 
 lines marked /3, 7, S, e, and also survived, but were faint. At 12 inches 
 from the slit a and y were alone seen, and at 24 inches a stood by itself 
 upon a dark ground. I also noticed that the red and yellow parts of .the 
 spectrum first lost their light on the tube being withdrawn from the slit ; 
 and this appeared to account for /3 disappearing while -y survived. For 
 drawing of the hydrogen- tube spectrum see Plate XIV. spectrum 1. 
 
 The question of eifect of distance upon the spectroscopic appearance of a 
 glowing light, as tested for this and other tubes, seems an important one. It 
 may possibly account for the generally faint aspect of the lines in the more 
 refrangible part of the Auroral spectrum. 
 
 Carbon- and 
 oxygen- 
 tubes. 
 
 Tubes de- 
 scribed. 
 
 Carbon- 
 tubes lighted 
 up. 
 
 Spectra of 
 the carbon- 
 tubes de- 
 scribed. 
 
 Carbon- and Oxygen-tubes. 
 
 The following tube-observations were taken together, because my friend 
 Mr. Henry E. Procter (to whom I am indebted for many profitable hints and 
 suggestions in Auroral work) was disposed to regard the spectra found in the 
 carbon-tubes and in those marked " O " as identical, suggesting that pure O, 
 with the ordinary non-intensified discharge, gives only a continuous spectrum ; 
 and that the O tubes are in fact generally lighted up by a carbon-spectrum, 
 as the result of impurity from accidental causes. The tubes examined for the 
 purpose of comparison were as follows: A coal-gas tube, a tube marked 
 " C.A.," three O tubes (two of, I believe, London make, and the third from 
 Geissler), and an OH 2 tube, also from Geissler. The carbon-tubes were both 
 brilliantly and steadily lighted by the current. * The C.A. tube glowed with' 
 a peculiar silvery -grey green light in the capillary part, and with a grey glow, 
 considerably stratified, in the bulbs. The coal-gas tube illumination was 
 whiter and still more brilliant than the C.A., and with even finer stratification 
 in the bulbs. The spectra of both tubes were conspicuous for the same 
 three well-known principal bright lines or bands in the yellow, green, and 
 blue (with one fainter in the violet), all shading off towards the violet, and 
 in both cases with fainter intervening bands or lines. These last bands or 
 lines only partially coincided when the two tubes were compared. 
 
 The spectra in both cases were rich and glowing, with a certain amount 
 of continuous spectrum between the lines ; and the three principal bands 
 or lines showed well and distinctly their respective place-colours. 
 
UJ 
 
 O 
 
 >- 
 X 
 O 
 
 CO 
 
 z 
 
 O 
 
 QQ 
 OL 
 < 
 O 
 O 
 
 tr 
 
 Q 
 
 ce 
 o 
 
 Of. 
 
 j 
 
COMPAKISON OF SOME TUBE AND OTHER SPECTEA. 
 
 Ill 
 
 Tubes tested for distance. In the case of the C.A. tube, at 18 inches from 
 the slit the continuous spectrum and fainter lines disappeared, while the 
 four principal lines still shone out, that in the green being the strongest. At 
 24 inches the same lines were still visible, though somewhat faintly. 
 
 In the case of the coal-gas tube, at 24 inches the whole spectrum was 
 quite brilliant, the four principal lines being very bright and even pre- 
 serving their distinctive colours. The H line, near the line or band in the 
 blue, was also plainly seen. 
 
 The O tubes, when treated by the same current as the carbon-tubes, were 
 found to be all three identical in general features. The discharge lighted up 
 each of the tubes feebly and somewhat intermittently. Grey in the bulbs 
 and a faint but decidedly pinkish white in the capillary part were the distin- 
 guishing light colours ; while nothing could be more marked than the differ- 
 ence in brilliancy between these and the preceding carbon-tubes. 
 
 The OH 2 tube presented very much the same character ; but the discharge 
 occasionally varied from a pinkish white to a yellow colour, somewhat like 
 that which artists call brown-pink, and reminding one of the " golden rays " 
 in certain Auroras. These O spectra presented, in common with the carbon- 
 tubes, three principal bright lines or bands in the yellow, green, and blue, 
 with a fainter one in the violet, all shading off towards the violet. The bands, 
 however, showed but very little trace of local colour, and the whole spectrum 
 had a faint and washed-out look, very different from the carbon-spectra. (I 
 certainly, by a little management, subsequently succeeded in getting the 
 same look to the C.A. spectrum ; but it was only by removing the tube to 
 some distance from the slit, and thus depriving the spectrum of very much of 
 its brightness.) The hydrogen line (solar F) was bright, more so than any of 
 the O lines. 
 
 The intensity of the three principal lines seemed to me to run in the 
 following order : 
 
 Yellow. Green. Blue. 
 
 Coal-gas /3 a y 
 
 Oxygen y a /3 
 
 Between the lines y and a in the Geissler O tube I found a rather bright 
 line, which I shall have occasion to refer to hereafter. 
 
 At 12 inches distance from the slit the O spectrum lost nearly all its light ; 
 the H line, and the three lines y, a, and /3, alone faintly remaining, a being 
 decidedly the brightest. At 24 inches no spectrum at all was to be seen. 
 
 Tubes tested 
 
 O tubes 
 lighted up. 
 
 OH a tubes 
 lighted up. 
 
 O tubes 
 spectra de- 
 scribed. 
 
112 
 
 THE SPECTKIJM OF THE ATTROEA. 
 
 Comparison 
 of spectra 
 of coal-gas 
 and O tubes. 
 
 Dr. Vogel's 
 O spectrum 
 reduced and 
 compared. 
 
 Tube- and 
 flame-spec- 
 tra of car- 
 bon do not 
 correspond. 
 
 I carefully compared together the three principal lines of the two spectra 
 of coal-gas and O by means of : 
 
 1st, the photographed micrometer before described ; 
 
 2nd, a comparison-prism on the slit plate ; 
 
 3rd, a piece of very fine brass foil cut as a pointer and fixed in the focus 
 of a positive eyepiece. 
 
 The lines or bands in both tubes were found to be slightly nebulous 
 towards the less-refrangible end (where they were measured), and the O tube 
 was not bright under a moderately high power (positive eyepiece). Subject 
 to these remarks, the three principal lines in both tubes were found to cor- 
 respond in position within the limits of my instrument. The spectra did not, 
 however, I am bound to say, look alike. 
 
 Puzzled by these observations, it then occurred to me to reduce Dr. Vogel's 
 spectrum of O, given in his memoir, to the same scale with my own. This 
 I did independently, and I then compared the result with my own spectrum 
 as mapped out. From the comparison, I judge that if my O tubes, one and 
 all, showed a carbon-spectrum, the learned Doctor's tube must have been 
 subject to a similar infirmity, as the tubes all agreed in main features. 
 
 There is, however, one point to which I desire to draw attention, which is 
 this, that common to both the Doctor's and my own Geissler spectrum 
 I found the before-mentioned rather bright line between y and a. This 
 line I found no equivalent for in either of the carbon-tubes. For spectra 
 of coal-gas and oxygen-tubes, see Plate XIV. spectra 2, 3, & 4. 
 
 In comparing the spectra, it should be remembered that the tube- and 
 flame-spectra of carbon do not correspond. Compare, for instance, the 
 spectrum of coal-gas or CO 2 in tube, and the well-known lines or bands in 
 the blue base of a candle-flame. The sharper edge of the yellow line or 
 band of the carbon-tubes will be found about midway between the two bright 
 yellow candle-lines or bands. The first of the very beautiful group of lines 
 or bands in the green in the candle-flame falls considerably behind the 
 sharper edge of the green line or band in the tube, while the third bright 
 band in the tube, alone of the three, corresponds with a very faint band in 
 the candle-flame. A line or band in the violet in the tube-spectrum finds no 
 equivalent in the candle-spectrum. For comparison of the carbon-tube and 
 flame spectra (the principal lines of the tube being alone shown), see Plate 
 XVI. spectra 6 & 7. 
 
 Note. Prof. Piazzi Smyth has been good enough, at my instance, to 
 measure the components of the citron band of the carbo-hydrogen spectrum 
 
COMPARISON OF SOME TUBE AND OTHER SPECTRA. 
 
 113 
 
 Prof. Piazzi 
 Smyth's 
 measure- 
 ments of 
 the com- 
 ponents of 
 the citron - 
 band in a 
 
 (near Angstrom's Aurora-line), as seen in a coal-gas blowpipe-flame urged 
 with common air. 
 
 The spectroscope used had prisms giving 22 of dispersion between A and H, 
 and the observing telescope magnified 10 times. The following is a table 
 of the results communicated to me by the Professor : 
 
 ., Reading of 
 Intensity. Micron f eter . flame. 
 
 Reference line, lithium /3 4 16-55 
 
 sodium, a 1 .... 10 18'45 
 
 a 2 . . . . 10 18-51 
 
 Citron band. Carbo-hydrogen. 
 
 Line 1, exquisitely clear 6 21-28 
 
 2, 5 21-88 
 
 3, 3 22-44 
 
 4, faint but clear 2 22-95 
 
 5, faint 1 23-38 
 
 6, faint and hazy 1 23-70 
 
 7, doubtful ? 23-92 
 
 Reference line, thallium a .... 10 25-08 
 
 From Dr. Watts's ' Index of Spectra ' I have extracted the three principal Comparison 
 carbon-tube bands or lines ; and they compare with Dr. Vogel's oxygen- yp g o 
 tube as under : lines and 
 
 Yellow. Green. Blue. Dr. Watts'* 
 
 carbon- 
 Dr. Vogel's oxygen-lines 5603 5189 4829 lines. 
 
 Dr. Watts's carbon-tube bands or lines 5602 5195 4834 
 
 Now these wave-length differences are so small that they raise a pre- 
 sumption of the possibility of the spectra being identical. On the other hand, 
 assuming the spectra are not identical, the comparison tells the other way, 
 viz. that the differences are so minute as to escape detection in instruments 
 of moderate dispersion. With my own instrument I found the O spectrum 
 too faint to increase the dispersive power with advantage. Considering the 
 extremely different character of the two discharges, the identity of all the 
 O tubes, and the presence of the line found between y and a in the 
 O spectrum, I think the two spectra are independent, though I admit there 
 is room for doubt. 
 
 Note. Since this examination I have photographed both spectra side by O aud CO 3 
 side (see 'Photographed Spectra,' plate xxxi., text, pp. 69, 70). The 
 
114 
 
 THE SPECTKUM OF THE AUEOEA. 
 
 Mercury- 
 and baro- 
 meter-tubes 
 examined. 
 
 Mercury- 
 tube de- 
 scribed. 
 
 Barometer- 
 tube. 
 
 Spectrum 
 of both 
 these tubes 
 described. 
 
 pictures include, of course, only the blue and violet parts of the spectrum ; 
 but they are widely different in aspect, and show that, photographically at 
 least, in this part of the spectrum there is a complete want of identity. 
 Subsequent investigations, however, by Schuster and others (detailed later in 
 this Chapter), go to establish that the principal lines shown in mine and 
 Dr. Vogel's tubes were due to (probably hydrocarbon) impurity. The excep- 
 tion is the single line common to mine and Dr. Vogel's tubes, but absent 
 from the coal-gas spectrum. This line proves to be oxygen. Compare 
 oxygen-tube spectra (Plate XIV. spectra 3 and 4) with Schuster's oxygen-tube 
 spectrum (Plate XVIII. fig. 15). The line in question is found identical in 
 the three tubes. 
 
 The tube OH 2 was found to give the principal lines of the O and H spectra 
 combined on a faint continuous spectrum. 
 
 Geissler Mercury-tube (Plate X. fig. 7) and Barometer 
 Mercurial vacuum, 
 
 I next examined two vacuum-tubes of an entirely different character. The 
 one was a tube from Geissler of stout glass, some fifteen inches long, 
 without electrodes, and an inch across. Within this tube was a second of 
 uranium glass, with bulbs blown in it. In contact with both tubes a quantity 
 of fluid mercury ran loose (Plate X. fig. 7). Upon shaking this tube with 
 the hand brilliant flashes of blue-white light, like summer lightning, flashed 
 out. These were discernible (though faintly) even in daylight. The fine 
 terminal wires of the coil being wrapped round each end of this tube, when 
 the current passed, a bright and white induced discharge, with a considerable 
 amount of stratification, was seen in the tube. The other tube was that of a 
 mercurial siphon-barometer. This being placed in a stand, one terminal 
 wire was placed in the mercury in the short leg of the siphon, while the other 
 terminal was made into a little coil and placed on the upper closed extremity 
 of the barometer-tube. On passing the current, the entire short space above 
 the mercury was filled with a grey-white light, not stratified, but showing a 
 conspicuous bright ring just above the level of the mercury. 
 
 Both these tubes, when examined with the spectroscope, showed four 
 bright rather uniform bands (the central one being the brightest), which I 
 assigned to the carbon-spectra (see Plate XIV. spectra 5 and 6). 
 
 The Geissler tube was probably filled designedly with coal-gas. In the 
 case of the barometer-tube the spectrum must be assumed to be the result of 
 some carbon impurity. 
 
X 
 
 CO 
 Ul 
 QQ 
 
 I- 
 <* 
 < 
 
 Q 
 
 Z 
 < 
 
 o 
 
 cc 
 
COMPARISON OF SOME TUBE AND OTHEE SPECTEA. 
 
 115 
 
 No lines of mercury could be detected in either case. 
 
 An effort was made to examine the light of the Geissler mercury-tube as 
 excited by motion only, but the spectrum could not be kept in the field ; 
 the four lines were, however, seen to flash out as the light passed before 
 the slit. 
 
 Air-tubes. 
 
 The first tube I examined was an ordinary Geissler tube charged with 
 rarefied air. The bulbs, on passing the discharge, were filled with the well- 
 known rose-tinged light like to the Aurora-streams. This in the capillary 
 part was condensed into a brighter and whiter thread, while the platinum 
 wire of the negative pole was surrounded by its characteristic mauve or 
 violet glow. 
 
 The spectrum, even with a weak current, was quite bright, and consisted 
 mainly of the nitrogen-lines and bands, with the lines Ha, H/3, and Hy, and 
 some of the intermediate lines of the H tube. 
 
 The double line a was undoubtedly the brightest in the spectrum when 
 taken in the capillary part of the tube. After this followed /3, and then y (H), 
 B, and e. I was, however, uncertain as to the relative brightness of the last 
 three, and marked their intensities with hesitation. I tested them several 
 times independently with differing results, and suspected them of variability 
 with the current. 
 
 The rest of the lines were very much of the same intensity. (For drawing 
 of spectrum of air-tube in capillary part see Plate XV. spectrum 1.) 
 
 Violet [negative] Pole, same tube. 
 
 I next turned my attention to the violet or negative-pole glow ; and here a 
 remarkable change took place in the spectrum, not only in the position of 
 the principal bands or lines, but in their relative intensity (see Plate XV. 
 spectrum 2). 
 
 The double line a hi the capillary part was replaced in the violet glow by 
 a shaded band of second intensity /3, the sharp edge of which was extended 
 towards the red, and formed (except for some faint indications) the limit of 
 the spectrum in that direction. The somewhat faint line next a in the 
 capillary tube had its faint representative in the violet pole ; but the next two 
 lines (capillary) were represented by the bright band j in the violet pole 
 lying in a position between them. Next y in the violet pole came three 
 faint lines, representing /3, y, and $ in the capillary spectrum ; and then the 
 
 Air-tube 
 illuminated. 
 
 Spectrum 
 described. 
 
 Violet 
 (negative) 
 pole : spec- 
 trum de- 
 scribed. 
 
116 
 
 THE SPECTKIJM OF THE AUEOEA. 
 
 Ked (posi- 
 tive) pole : 
 spectrum 
 described. 
 
 Aurora- 
 tube : 
 discharge 
 described. 
 
 Spectrum 
 described. 
 
 Phospho- 
 rescent tube 
 described. 
 
 Dis charge 
 described. 
 
 bright band a, which was the brightest of the violet-pole group, and repre- 
 sented a medium-intensity band in the capillary spectrum. After this was a 
 faint band near a, representing two rather bright ones in the capillary 
 spectrum, this last being succeeded by other bands in the violet, a, j3, and -y 
 in the violet pole were examined carefully for relative brightness, and were, 
 I believe, correctly marked. 
 
 Eed [positive] Pole. 
 
 The red [positive] pole was next examined, but presented no peculiar 
 features. It appeared as a fainter representation of the capillary air-spectrum, 
 with some few lines or bands absent, and (as will be seen after) was also a fair 
 representation of a diffused air-spectrum (see Plate XV. spectrum 3). 
 
 Examined for comparative intensity, at 24 inches from the slit, the whole 
 capillary air-spectrum showed faintly. The marked lines in the centre of the 
 spectrum generally retained their prominence ; but after a I judged e next in 
 brightness. On examining the violet pole at 12 inches from the slit, the 
 whole spectrum was faint and the bands a and /3 were alone distinctly seen. 
 
 Aurora (air)-tube. (Plate XV. spectrum 4.) 
 
 Next to the Geissler air-tube I examined an " aurora "-tube, about 
 15 inches long and IJ inch across, with platinum terminals, and of the same 
 diameter throughout (Plate X. fig. 8). The discharge was of a rosy-red 
 colour, and the long flickering stream from pole to pole certainly much 
 reminded one optically of an auroral streamer. .Spectroscopically examined, 
 the discharge presented a faint banded air-spectrum similar to that of the 
 positive pole (see Plate XV. spectrum 4) ; but the relative intensity of the 
 lines was somewhat altered, while a very bright line in the green (seen also in 
 the tube next described) was characteristic of the spectrum, and in this respect 
 distinguished it from the ordinary air-spectrum. 
 
 Phosphorescent tube. 
 
 Following this last tube I examined one purchased as " phosphorescent." 
 It was rather short (6^ inches), of equal calibre, and about the size of the 
 bulb of a Geissler tube. It was filled with a white powder (probably one of 
 the Becquerel compounds). On passing the current between the electrodes, 
 a bright rose-coloured stream appeared ; and wherever this was in contact with 
 the powder, the tube glowed with a brilliant green light. On stopping the 
 current, the tube still continued to shine, but with a fainter green glow, which 
 gave only a continuous spectrum. When examined in full glow, the tube- 
 
UNIVERSITY] 
 
to r- 
 
 
 
 Z 
 
 o 
 
 Q 
 
 uJ 
 
 Ld 
 
 a 
 o 
 
 i 
 a. 
 <f> 
 o 
 
 x. 
 CL 
 
 a: 
 o 
 ce 
 
COMPARISON OF SOME TTBE AND OTHER SPECTRA. 
 
 117 
 
 spectrum was also in the main continuous and of a green tinge ; but upon it 
 were bright lines in the blue and violet portions of the spectrum, while in 
 the red, yellow, and green a faint but distinct air-spectrum was seen ; and 
 with this was also found the same bright line in the green which distin- 
 guished the ' ; aurora "-tube. [Five out of six of the lines in the blue and 
 violet will be also found in Schuster's oxygen-tube, violet pole (Plate XVIII. 
 fig. 15). The air-spectrum probably arose from impurity.] 
 
 Spark in Air. 
 
 I next took a ^-inch spark in air between platinum terminals (see Plate XV. 
 spectrum 6). The principal lines in this spectrum were the line o (by far the 
 brightest), corresponding to y in the violet pole ; next was /3, a line in the 
 yellow, not appearing in the tube-spectrum, and then other lines of less 
 intensity. In the "aurora" and " phosphorescent" tubes was found, as before 
 mentioned, a line in the green prominent for its brightness, and, indeed, in 
 the " aurora "-tube the only one which survived when it was moved away 
 from the slit. This line also appeared in the spark-spectrum, but there only 
 of an average brightness. I examined it carefully for position in the respective 
 tubes ; and on comparing them by means of a pointer in the eyepiece, found 
 it coincident with the ridge or centre of the wedge-like bright-green broad 
 band which is so conspicuous in the air-tube spectrum. 
 
 I think this edge-like centre has actually a line coincident with the line I 
 refer to ; but if so, its intensity little exceeds that of the band itself. 
 
 Spark over Water. 
 
 To complete the set of air-experiments, I examined the same spark taken 
 from the surface of a small meniscus of water, placed in a glass cup upon the 
 lower platinum wire. In this case the air-spectrum was plainly, but not 
 brightly, seen at the violet end of the spectrum the red, yeDow, green, and 
 blue being filled with a continuous spectrum, through which some of the 
 air-lines faintly showed (see Plate XV. spectrum 7). 
 
 Phosphoretted- Hydrogen Flame. 
 
 This was obtained from a hydrogen-bottle fitted with glass tubing, two or 
 three minute pieces of phosphorus being placed with the zinc. The flame was 
 of a bright yellow colour, with a cone of vivid green light in its centre. 
 
 The spectrum was found to consist mainly of three bright bands in the 
 yellow, green, and green-blue respectively (see Plate XVI. spectrum 3). 
 
 The central band was very striking in its emerald-green colour, while all 
 the bands were remarkable as being very broad in proportion to the slit 
 
 B2 
 
 Spectrum 
 described. 
 
 Spark in air: 
 
 spectrum 
 
 described. 
 
 Spark over 
 water: 
 
 spectrum 
 described. 
 
 Phospbo- 
 retted- 
 
 hydrogea 
 flame. 
 
 Spectrum 
 described. 
 
118 
 
 THE SPECTEUM OF THE AUEOEA. 
 
 Mons .Lecoq 
 de Boisbau- 
 dran's re- 
 marks 
 on the 
 spectrum 
 increasing 
 in brilliancy 
 when the 
 flame is 
 cooled. 
 
 Iron- 
 spectrum. 
 
 How 
 
 obtained. 
 
 Spectrum 
 described. 
 
 Mous.Lecoq 
 de Boisbau- 
 dran's 
 spectra also 
 given. 
 
 Comparison 
 of iron- and 
 Aurora- 
 spectrum. 
 
 (which, however, was not fine). The yellow band had a rich glow of colour. 
 My spectrum was mapped out at ordinary temperature, and I found the bands 
 sufficiently bright ; but Mons. Lecoq de Boisbaudran, in his ' Spectres 
 Lumineux' (texte, p. 188), has described how the brilliancy of these bands is 
 increased when the name is artificially cooled (refraidie). 
 
 The idea of cooling the flame was due to M. Salet, who effected it either 
 by a jet of water or by an air-blast. 
 
 The less refrangible bands seem the most susceptible to increase of brilliancy. 
 
 Mons. Boisbaudran also makes the important remark that the relative 
 intensities of the bands are in such case altered, adding : " La plus importante 
 de ces modifications consiste en un renforcement tres-considerable de la bande 
 rouge 8 97'03 (W.L. 5994) qui devient vive de presque invisible qu'elle etait 
 en 1'absence du refroidissement artificiel de la flamme." 
 
 Full details of the changes are given by M. de Boisbaudran. 
 
 The bearing of these observations as connected with the variable character 
 of the red line in the Aurora-spectrum seems to me in the highest degree 
 
 noteworthy. 
 
 Iron-Spectrum. 
 
 A comparison of this spectrum suggested itself, partly from the suspected 
 relations between the Aurora and solar corona, and partly from a consideration 
 of the views expressed by M. Gronemann and others in favour of the Aurora 
 having its origin in the fall of an incandescent meteoric powder. 
 
 The spectrum was obtained from a spark taken over a solution of perchloride 
 of iron in a small glass cup, and was remarkable for its brightness in and 
 about the green region. The lines varied considerably in intensity, and with 
 a fine slit the principal ones were sharp, distinct, and clear. A group of three 
 lines (a) stood out boldly in the green as the most marked, and next to these 
 a group of three others more towards the violet end of the spectrum (see 
 Plate XVI. spectrum 4). By the side of my phosphoretted-hydrogen and 
 iron spectra I have placed the principal lines of Mons. Lecoq de Boisbaudran's 
 same spectra (reduced to my scale), and with figures of wave-lengths for com- 
 parison with the Aurora-spectrum (see Plate XVI. spectra 1 and 2). 
 
 A difficulty in comparing the iron-spectrum with that of the Aurora arises 
 from the large number of fine lines found in the former spectrum. In a pho- 
 tograph (taken with the same prism as before described) of a small piece of 
 meteoric iron fused in an electric arc by the aid of 40 Grove cells, about 
 154 lines are easily counted in the blue and violet parts of the spectrum. 
 Double this number at least would be seen with a spectroscope of moderate 
 dispersion in the region comprising the entire set of auroral lines. 
 
COMPARISON OF SOME TUBE AND OTHEE SPECTRA. 
 
 119 
 
 Spectrum of Mercury. 
 
 This spectrum is given as useful for comparison with the bright and Mercury- 
 principal Aurora-line. It is easy to obtain with a small coil, the metal Hov 
 being used as one electrode. The yellow lines are distinct and steady; but obtained, 
 the green, which is very bright, is apt to nicker as the spark moves on the 
 surface of the metal (see Plate XVI. spectrum 5). 
 
 The following Table was compiled for the purpose of comparing the 
 foregoing results with the Aurora-spectrum. 
 
 TABLE showing comparative position of Aurora-lines with the principal lines Table of co- 
 in the examined spectra. C. means coincident within the limits of my 
 instrument and scale, N. near, and VN. very near. 
 
 
 6297 
 fr 
 
 5569 
 a. 
 
 5390 
 5. 
 
 5233 
 
 S. 
 
 5189 
 
 I. 
 
 5004 
 
 r- 
 
 4694 to 4629 
 
 6. 
 
 4350? 
 
 6. 
 
 
 
 X 
 
 o 
 
 N. 
 
 
 
 N. 
 
 C., 
 same 
 W.L. 
 
 
 
 Band 
 includes 
 2 lines. 
 
 I 
 
 a 
 
 5' 
 I 
 
 S 1 
 | 
 
 I 
 
 1 
 H 
 
 
 Coal-gas tube 
 
 
 
 N. 
 
 VN. 
 
 
 Band 
 includes 
 1 line. 
 
 
 
 
 
 Oxygen-tube 
 
 
 
 
 VN. 
 
 
 
 
 
 
 | 
 
 f 
 
 5- 
 
 r- 
 
 r 
 
 I 
 
 00 
 
 1 
 9 
 
 =r 
 
 c 
 
 rt- 
 
 hj 
 1 
 H 
 
 14 
 P 
 CD 
 V 
 
 ta 
 
 
 
 
 
 Band 
 includes 
 
 Band 
 includes 
 
 
 
 N. 
 
 
 
 Band 
 includes 
 2 lines. 
 
 
 
 
 
 
 Band 
 includes 
 
 c. 
 
 Band 
 includes 
 1 line. 
 
 
 
 
 
 Air, red-pole 
 
 See Air, capillary. 
 
 
 Aurora-tube and phospho- \ 
 rcscent tube j 
 
 See Air, capillary ; 
 
 and note bright line. 
 
 
 
 N. 
 
 
 
 N. 
 
 c. 
 
 Band 
 includes 
 2 lines. 
 
 Band 1 
 includes 
 2 lines. 
 
 
 
 
 
 Continuous spectrum and faint air-lines. 
 
 
 Phosphoretted hydrogen ... 
 
 N. 
 
 Faint 
 band. 
 
 Band 
 
 includes 
 
 
 
 
 
 
 
 
 VS. 
 
 N. 
 
 VN. 
 
 VN. 
 
 
 
 
 
 
120 
 
 THE SPECTEUM OF THE ATJEOEA. 
 
 Additional 
 Table of 
 compared 
 spectra. 
 
 Tested by coincidence, or close proximity of lines to those of the Aurora, 
 we arrange the spectra in the following order : (1) iron, (2) air-spark, 
 (3) hydrogen, (4) air-tube, (5) phosphoretted hydrogen, (6) carbon and oxygen. 
 
 The air-tube spectrum might perhaps stand higher in the scale but for 
 its broad bands, which make comparison doubtful. Lines of oxygen possibly 
 escape detection in the Aurora from the faint character of its spectrum. 
 
 The phosphorus and iron spectra are especially interesting in connexion 
 with Professor Nordenskiold's " metallic and magnetic cosmic dust in the 
 Polar regions " (see Phil. Mag. ser. 4, vol. xlviii. p. 546). 
 
 As an addendum to the foregoing, on Plate IX. fig. 1 will be found a 
 Table I have prepared, in which a type Aurora and also Vogel's and 
 Barker's Aurorse are compared with eight other spectra, viz. : 
 
 S. Solar spectrum. 
 
 N. Nitrogen (air) : Watts. 
 
 0. Oxygen (air) : Watts. 
 
 C.H. Carburetted-hydrogen vacuum-tube : Watts. 
 C.I. Carburetted-hydrogen flame : Watts. 
 C.C. Blue base of candle-flame : Capron. 
 O.P. Oxygen vacuum-tube : Procter. 
 
 1. Iron: Watts. 
 
 The divisions and vertical lines will guide the eye in making comparison of 
 the spectra. 
 
NOTES ON PKOFESSOB ANGSTEOM'S THEOEY. 
 
 121 
 
 CHAPTER XII. 
 
 SOME NOTES ON PEOFESSOE ANGSTEOM'S THEOEY OF THE 
 AFBOBA-SPECTEUM. 
 
 [The substance of these appeared in the ' Philosophical Magazine' for April 1875, in conjunc- 
 tion with the " Comparison of the Tube and other Spectra" (Chapter XI.), but they are now, 
 for the sake of convenience, made a separate article.] 
 
 o 
 
 IN a contribution by the late Professor Angstrom to a solution of the pro- 
 blem of the Aurora-spectrum (an abstract of which appeared in ' Nature ' of 
 July 16, 1874), the Professor is stated, amongst other things, to have laid 
 down certain propositions in substance as follows : 
 
 1st. That the Aurora has two different spectra the one comprising the 
 one bright line in the yellow-green only, and the other the remaining fainter 
 lines. 
 
 2ndly. That the bright line falls within a group of hydrocarbon lines, but 
 does not actually coincide with any prominent line of such group, and that 
 Dr. Vogel's finding this line to coincide with a not well-marked band in the 
 air-spectrum must be regarded as a case of accidental coincidence. 
 
 3rdly. That moisture in the region of the Aurora must be regarded as 
 nil, and that oxygen and hydrogen must alone there act as conductors of 
 electricity. 
 
 O 
 
 Professor Angstrom then details the examination of an exhausted dry air- 
 flask filled with a discharge analogous to the glow of the negative pole of a 
 vacuum air-tube. 
 
 The experiment is described as follows : " Into a flask, the bottom of 
 which is covered with a layer of phosphoric anhydride, the platinum wires 
 are introduced, and the air is pumped out to a tension of only a few milli- 
 metres. If the inductive current of a Ruhmkorff coil be sent through the 
 flask, the whole flask will be filled, as it were, with a violet light, which other- 
 wise only proceeds from the negative pole, and from both electrodes a spectrum 
 is obtained composed chiefly of shaded violet bands." The comparison of 
 the spectrum of this violet glow with that of the Aurora gives, according to 
 Angstrom, the following results : 
 
 Aurora-lines, wave-lengths 4286 4703 5226 
 
 Violet light, wave-lengths 4272 4707 5227 
 
 Professor 
 
 o 
 
 Angstrom s 
 proposi- 
 tions. 
 
 That the 
 Aurora has 
 two spectra. 
 
 That bright 
 line does not 
 coincide 
 with HC . 
 
 That mois- 
 ture is nil 
 in Aurora 
 region. 
 
 o 
 
 Angstroms 
 flask-expe- 
 riment de- 
 scribed. 
 
 Flask-spec- 
 trum com- 
 pared with 
 Aurora- 
 spectrum. 
 
122 
 
 THE SPECTKUM OF THE ATJKOKA. 
 
 Bright line 
 is due to 
 fluorescence 
 or phospho- 
 rescence. 
 
 No need of 
 Dr. Vogel's 
 theory of 
 variability. 
 
 Professor 
 
 Angstrom's 
 conclusions 
 tested. 
 
 Kfisult of 
 examination 
 of the Pro- 
 fessor's pro- 
 positions. 
 
 Moisture 
 probably not 
 nil in the 
 Aurora re- 
 gion. 
 
 Two weak light bands, found by Dr. Vogel at 4663 and 4629, are also 
 compared with other lines in the violet light 4654 and 4601 ; and the Pro- 
 fessor then concludes that it may be in general assumed that the feeble 
 bands of the Aurora-spectrum belong to the spectrum of the negative pole, 
 possibly changed more or less by additions from the banded or the line air- 
 spectrum. 
 
 4thly. That the only probable explanation of the bright line is, that it 
 owes its origin to fluorescence or phosphorescence. The Professor remarks on 
 this point that " an electric discharge may easily be imagined which, though 
 in itself of feeble light, may be rich in ultra-violet light, and therefore in a 
 condition to cause a sufficiently strong fluorescence." He notes also that 
 oxygen and some of its compounds are fluorescent. 
 
 5thly. That there is no need, in order to account for the spectrum of the 
 Aurora, to have recourse to the " very great variability of gas-spectra accord- 
 ing to the varying circumstances of pressure and temperature " (Dr. Vogel's 
 theory). Professor Angstrom does not admit such variability, and does not 
 admit that the way a gas may be brought to glow or burn can alter the nature 
 of the spectrum. 
 
 In order to test some of the Professor's conclusions in an experimental way, 
 I examined some tube and other spectra not only for line-positions, but also 
 for general resemblance to an Aurora-spectrum. 
 
 These experiments are detailed in the last Chapter, and the results are 
 comprised in Plates XIV., XV., and XVI., in which the spectra obtained are 
 represented in black for white. 
 
 The result of the examination of Professor Angstrom's principal propositions 
 seems to be this : 
 
 1st. Two Auroral spectra. I agree in this, but question whether the fainter 
 lines may not possibly comprise more than one spectrum. 
 
 2nd. I agree also that the bright yellow-green line falls, as Professor Ang- 
 strom describes, just behind the second line in the hydrocarbon yellow group 
 (see Plate V. fig. 7). And I find, in common with the Professor, no well- 
 marked or prominent line in the air-spectrum with which it accords. 
 
 3rd. This may be conveniently divided into two parts, viz. : 
 
 A. The proposition that " moisture in the region of the Aurora must be 
 regarded as nil." 
 
 Here I see reason to differ, since (to quote a letter of Mr. Procter's) " the 
 vapour-density of OH 2 is only 9 against 14 for N and 16 for O;" and again, 
 " electrical or heat-repulsion may carry water-dust up to enormous heights." 
 
NOTES ON PEOFESSOE ANGSTEOM'S THEOEY. 
 
 There are, too, I think, circumstances connected with the Aurora itself which 
 make the assumption of moisture being nil in the Auroral regions untenable. 
 The first of these is the fact that the white arc, streamers, and floating 
 patches of light, found in some Aurora?, have frequently the peculiarly dense 
 and solid look of vapour-clouds a circumstance with which I have been 
 frequently struck. Mr. Procter and others have also remarked that the 
 Aurora is generally formed in a sort of " mist or imperfect vapour." The 
 second, that Aurora, or portions of them, are frequently near to the earth's 
 surface. Instances of this are given in the section on the Height of the 
 Aurora, notably the experiences of Sir W. Grove and Mr. W. Ladd. 
 
 On this point, too, note the peculiarities of the red line, which (and, as I 
 find, the green line also) are coincident with, or very close to, telluric bands or 
 groups of lines in the solar spectrum usually attributed to moisture. (See 
 Plate XIII. fig. 2.) 
 
 I think we may also claim the continuous spectrum in the Aurora in 
 further proof of water- vapour (see Plate XV. spectrum 7). The continuous 
 spectrum of the Aurora is also, to my observation, more local and dense in 
 the spectroscope than the glow generally seen between the lines or bands 
 in gas-spectra. 
 
 B. The question of the violet-pole spectrum. Here I make the remark 
 that in comparing other spectra with that of the Aurora, it is, I think, too 
 much the practice to trust to the coincidence (more or less perfect) of one 
 or perhaps two lines out of many ; whereas we know by experience that 
 most spectra have so well-marked a general as well as special character 
 that, when once seen, they are recognized afterwards with the greatest ease 
 and without measurements. An experience and proof of this is found in a set 
 of " Photographed Spectra" which the Autotype Company have reproduced 
 for me. 
 
 Of course no two given spectra can be considered identical unless their 
 principal lines coincide ; but, on the other hand, the coincidence of one or 
 two lines out of many, without other features, cannot be satisfactorily or con- 
 clusively held to establish identity. 
 
 In Professor Herschel's letter (Phil. Mag. ser. 4, vol. xlix. p. 71), Professor 
 Angstrom's representation of the " spectrum of the glow discharge round the 
 negative pole of air-vacuum tubes " is given, in comparison with the Aurora- 
 lines and those of olefiant gas. This illustration is here introduced. 
 
 Beasous for 
 this given. 
 
 Aurora in 
 vapour or 
 mist. 
 
 Frequently 
 near to 
 earth's sur- 
 face. 
 
 Coincidence 
 of Auroral 
 lines with 
 telluric solar 
 lines. 
 
 Continuous 
 spectrum. 
 
 Violet-pole 
 
 spectrum 
 
 discussed. 
 
 Most spec- 
 tra have a 
 general as 
 well as spe- 
 cial charac- 
 ter. 
 
 Coincidence 
 of one or 
 two lines not 
 sufficient to 
 establish 
 identity. 
 
 o 
 
 Angstrom's 
 
 compared 
 
 spectra. 
 
124 
 
 THE SPECTEUM OF THE AUKOBA. 
 
 Angstrom's representation of the Spectrum of the glow discharge round the negative pole of Air- 
 vacuum tubes, and its comparison with the Spectrum of the Aurora. 
 
 Spectra of (1) olefiant gas, (2) Aurora, (3) negative pole in air. 
 D E F G 
 
 H,H 2 
 
 (1) 
 
 
 tjfck 
 
 
 V 
 
 , 
 
 
 
 
 (2) 
 
 
 
 
 
 
 
 
 1 
 
 
 1 
 
 
 
 (3) 
 
 1 i <T 
 
 A 
 
 Arv X-N _ 
 
 
 
 Ks 
 
 h 
 
 Jb 
 
 k 
 
 
 
 l i 
 
 1 67 1 65 
 
 63 
 
 1 
 
 61 
 
 59 57 1 
 
 55 53 
 
 1 
 51 
 
 1 '1 
 49 1 47 
 
 1 A 
 
 i i 
 
 43 1 1 39 1 
 
 Table of 
 
 compared 
 
 Aurora. 
 
 Angstrom's 
 
 drawing 
 
 discussed. 
 
 Wave-lengths, in hundred-thousandths of a millimetre. 
 
 It is unfortunate that in this illustration and in Professor Herschel's paper 
 the wave-lengths of the Aurora-lines are not given in figures, but must be 
 
 Q 
 
 roughly calculated from the scale. Professor Herschel speaks of Angstrom's 
 drawing as representing a normal spectrum, and as derived from authentic 
 sources, such as Vogel, Barker, and others; but beyond this we are not 
 certain as to its origin. 
 
 In illustration of the difficulty of constructing any thing like a general 
 typical Aurora-spectrum I append a Table of eight Auroral spectra taken at 
 hazard : 
 
 Auroral lines and bands. 
 
 Observers. 
 
 Bed. 
 
 Yellow. 
 
 Green. 
 
 Blue. 
 
 Indigo. 
 
 Violet. 
 
 Vogel, April 9, 1871 
 
 6297 
 6230 
 6300 
 
 6060 
 * 
 * 
 
 5569 
 5620 
 5550 
 
 5690 
 5660 
 5570 
 
 * 
 * 
 
 5390 
 
 5233 
 
 5189 
 5170 
 
 5004 
 
 5020 
 f 5050 
 
 1 to 
 [4990 
 
 4820 
 
 4930 
 to 
 4850 
 
 4850 
 4830 
 
 f4694] 
 1 to \ 
 I.4629J 
 
 47401 
 to \ 
 4670 J 
 
 4625 
 4640 
 
 * 
 
 4310 
 
 4350 
 4305 
 4320 
 
 * 
 * 
 
 Barker Nov. 9, 1871 
 
 Barker Oct. 14, 1873 
 
 
 f 53301 
 
 i to r 
 
 [5200J 
 5320 
 
 A. Clarke, June., Oct. 24, 1870 
 Backhouse, 1873 
 
 5165 
 5180 
 
 5015 
 4980 
 
 Backhouse, Feb. 4, 1874 . , 
 
 
 
 H. K. Procter, 1870 . 
 
 
 
 Lord Lindsay, 1870 
 
 * 
 
 * 
 
 * 
 
 
 
 
 
 
 
 
 * Mr. Procter's and Lord Lindsay's lines had no wave-lengths. 
 
 O 
 
 On examining Angstrom's diagram it certainly seems to me that, upon 
 the showing of the drawing itself, the coincidences are not very exact. All 
 three of the violet-pole bands appear to be less refrangible than the Aurora- 
 
NOTES ON PROFESSOR ANGSTROM'S THEORY. 
 
 125 
 
 lines with which they are compared the middle one (at 47) considerably 
 so, the one near E (at about 52-30) appreciably so, and the third (at 43) 
 slightly so. 
 
 As it seemed desirable to adopt a specific Aurora-spectrum for comparison, 
 
 O 
 
 and to show such comparison on a somewhat larger scale than Angstrom's 
 drawing, I prepared the diagram shown on Plate XI. fig. 1. The upper 
 spectrum is Vogel's, already described and figured on Plate XIII. The lower 
 spectrum is that of " Air, violet pole," Plate XV. spectrum 2, graphically 
 shown. I can only find one absolute coincidence in the two compared spectra 
 in this diagram. 
 
 It should, too, I think, be borne in mind that there is a great difference 
 in the character of the compared spectra, whether as shown in Angstrom's 
 drawing or mine the bands of the violet-pole spectrum mostly degrading 
 towards the violet, while the lines or bands of the Aurora in no way possess 
 that character*. 
 
 To assist in the foregoing violet-pole comparison I add the following Table 
 derived from Dr. Vogel's memoir : 
 
 W.L. 
 
 6100, 
 5945, 
 5459, 
 5289, 
 5224, 
 5147, 
 5004, 
 4912, 
 4808, 
 4704, 
 4646, 
 4569, 
 4486, 
 4417, 
 4346, 
 4275, 
 
 Violet-pole lines. 
 
 Aurora-lines. 
 
 W.L. 
 
 [ broad, moderately bright stripe 6297, very bright stripe. 
 
 }f 5569, brightest line of spectrum, 
 broad, moderately bright stripe \ . r 
 
 [ 5390, extremely faint line. 
 
 very bright line 5233, moderately bright. 
 
 faint line 5189, moderately bright. 
 
 bright line 5004, very bright line. 
 
 fainter than last. 
 
 very faint line. c 6694, -\ 
 
 very intense line \ 4663, > band less brilliant in the middle. 
 
 very faint line. 1 4629, J 
 
 moderately bright. 
 
 moderately bright. 
 
 quite faint line. 
 
 moderately bright line. 
 
 very bright line. 
 
 * Angstrom's drawing, in giving this character to the two Aurora-bands which are said to 
 correspond with violet-pole bands about 47 and 43, is incorrect, and calculated to mislead by 
 giving the Aurora-bands a feature corresponding to the violet-pole bands which they do not 
 possess. I am not aware of any Aurora-line or band which is described as distinguished by 
 degrading towards the violet. 
 
 s2 
 
 Diagram of 
 Vogel's 
 Aurora and 
 violet-pole 
 spectrum. 
 
 Dr. Vogel s 
 violet-pole 
 and Aurora- 
 lines. 
 
126 
 
 THE SPECTRUM OF THE AURORA. 
 
 Conclusions 
 arrived at 
 adverse to 
 the violet^ 
 pole theory. 
 
 Phospho- 
 rescence or 
 fluorescence 
 of the yel- 
 low-green 
 line. 
 
 External 
 features of 
 Auroras con- 
 firmatory of 
 this. 
 
 Other con- 
 firmatory 
 circum- 
 stances. 
 
 On examination of these figures it will be seen that 5224 and 5233 are 
 fairly close, and that 5004 is coincident. Beyond these there is little to 
 identify the spectra. 
 
 As the general result of my observations and a comparison of the fore- 
 going spectra and tables, I see no reason for giving to the violet-pole glow 
 any special or distinguished place in a comparison with the Aurora, and cer- 
 tainly not for assigning to it the nearly absolute monopoly of the spectrum. 
 It is true that the line y in the violet-pole glow (Plate XV. spectrum 2), 
 which, by the way, degrades towards the red, is in close coincidence with one 
 of the Aurora-lines ; but how are the brighter bands a and (3 accounted for ? 
 These, as I have before pointed out, alone survive when the tube is placed at 
 a distance from the slit. It is true they are thus reduced to shaded-off lines 
 in lieu of bands ; but the difficulty still remains, that they are conspicuous 
 for their absence in the Aurora-spectrum. On the whole, I cannot but 
 conclude that Professor Angstrom's theory fails. At all events, if the violet- 
 pole glow-spectrum is to represent the Aurora-spectrum, it must be under 
 conditions different from those by which it obtains in dry-air vacuum-tubes 
 or flasks at ordinary temperature. 
 
 4th. I feel more in accord with Professor Angstrom's memoir upon the 
 subject of the phosphorescence or fluorescence of the bright yellow-green 
 Aurora-line. 
 
 I do not notice that the Professor touches upon the external features of the 
 Aurora in respect of this question. 
 
 October 20, 1870. I noted the grand Auroral display of that evening, in- 
 cluding " streamers of opaque-white phosphorescent cloud very different from 
 the more common transparent Auroral diverging streams of light." 
 
 February 4, 1872. A fine display. The first signs were (in dull daylight) 
 " a lurid tinge upon the clouds, which suggested the reflection of a distant 
 fire, while, scattered among these, torn and broken masses of white vapour, 
 having a phosphorescent appearance, reminded me of a similar appearance 
 in October 1870." (Other instances of this effect will be found in the section 
 Aurora and Phosphorescence.) Day Aurora, too, we might suppose could 
 hardly be seen without the presence of some phosphorescent glow. 
 
 Having regard to the near proximity of the phosphoretted-hydrogen band 
 to the bright Aurora-line, to the circumstance of this band brightening by 
 reduction of temperature (a phenomenon probably connected with ozone), to 
 the peculiar brightening of one line in the green in the "Aurora " and " phos- 
 phorescent" tubes (the phosphorescent tubes probably containing O), and 
 
NOTES ON PROFESSOR ANGSTROM'S THEORY. 
 
 127 
 
 to the observed circumstance that the electric discharge has a phosphorescent 
 or fluorescent afterglow (isolated, I believe, by Faraday), I feel there is strong 
 evidence in favour of such an origin to the principal Aurora-line, if not to the 
 red line as well. 
 
 O 
 
 5th. Professor Angstrom opens a wide door to discussion in his proposition 
 of the invariability of gas-spectra, and I do not now attempt to follow in 
 detail this interesting part of the present subject. Suffice it to say, that if the 
 Professor lays down this proposition in its strictest sense (I can hardly sup- 
 pose he so meant it), there is, so far as I am aware, no one spectrum that can 
 at all claim comparison with the Aurora-spectrum. Giving greater latitude 
 to the Professor's words, I reply, upon competent authority, that lines vary 
 in number and brilliancy with temperature, and in breadth with pressure. 
 Kirchhoff, too, in speaking of vapour-films as increasing the intensity of lines, 
 states " it may happen that the spectrum appears to be totally changed when 
 the mass of vapour is altered." We may, too, now add magnetism as capable 
 of effecting a change in certain spectra, not only as to brilliancy, but even as 
 to position of lines. (Chautard's Eesearches, ' Philosophical Magazine,' 4th 
 series, vol. 1. p. 77, and experiments detailed in Chap. III. of this work.) 
 
 Conclusion 
 in favour of 
 the theory. 
 
 Invariabi- 
 lity of gas- 
 spectra 
 questioned. 
 
128 
 
 THE SPECTRUM OF THE AURORA. 
 
 CHAPTER XIII. 
 
 Procter's 
 oxygen- 
 spectrum. 
 
 How ob- 
 tained. 
 
 Compared 
 spectra de- 
 scribed. 
 
 Mr. Proc- 
 ter's subse- 
 quent views. 
 Yellow- 
 green line 
 traced to 
 some form 
 of hydro- 
 carbon. 
 
 THE OXYGEN-SPECTRUM IN RELATION TO THE AURORA 
 (PROCTER AND SCHUSTER). 
 
 IN a communication to ' Nature,' Mr. H. R. Procter has pointed out an 
 apparent coincidence in position of several of the Auroral lines with those of 
 a spectrum occasionally obtained from air at low pressure with a feeble 
 discharge. It is, he says, sometimes exhibited in lumiere (phosphorescent 1 ?) 
 tubes, and he believed it, in part at least, to be the spectrum described by 
 Wiillner (Philosophical Magazine, June 1869) as a new spectrum of oxygen. 
 
 He had obtained it very vividly in pure electrolyzed oxygen with a 
 feeble discharge, but some perplexing observations made him doubtful of 
 its origin. 
 
 Plate XI. fig. 4 gives a representation of this spectrum as shown by 
 Mr. Procter, except that my drawing is in black for white. 
 
 The upper spectrum is that above mentioned, the centre one that of the 
 Aurora, the lower one the lines of Na and H for comparison. The Auroral 
 yellow-green line, in January 1870, was found by Mr. Procter coincident with 
 a bright line or band in the tube (with a spectroscope of a 60 bisulphide 
 prism, and magnifying-power about six). The third and fifth lines in the 
 Aurora seemed also to correspond with tube-lines. As to these Mr. Procter 
 says they were not bright enough to be compared with the same accuracy as 
 the yellow-green line, but that the positions could not be far wrong. 
 
 Mr. Procter subsequently (' Edinburgh Encyclopaedia,' art. " Aurora ") 
 considered he traced the yellow-green tube-line to some form of hydrocarbon. 
 On examination with instruments of greater dispersion, it was found that, 
 though more refrangible than the first band of citron acetylene (candle-flame), 
 it was less so than the Aurora-line. The tube-band, too, was shaded towards 
 the violet, which was not the case with the Aurora-line. 
 
 The question as between hydrocarbon and oxygen I did not then consider 
 as disposed of. With the lumiere tubes the question might be open, but I did 
 not see how it could be in the case of the electrolyzed oxygen-spectrum. 
 
 From a comparison of the tube-spectra, I have shown that although the 
 spectra of the carbon and oxygen tubes are proved to be, photographically, 
 
OXYGEN-SPECTEUM IN RELATION TO THE AUEOEA. 
 
 129 
 
 as a whole, distinct, they have, as to position of some of the principal lines 
 in the central part of the spectrum, a very close resemblance. 
 
 That oxygen may in some form play a part in the Aurora seems highly 
 probable ; how far it is spectroscopically detected seems a different question. 
 
 Angstrom and Herschel suggest its presence in the Aurora in connexion 
 with phosphorescence or fluorescence. With a spark-discharge in air at ordi- 
 nary pressure, a mixed spectrum of bright lines of N and O is found ; while 
 in the case of Geissler vacuum-tubes (representing a glow-discharge in a 
 much more rarefied atmosphere) the N lines appear mainly to usurp the 
 spectrum. 
 
 It must, however, be borne in mind that a Geissler tube, as to temperature 
 at least, in no way represents the conditions of the Aurora ; and when we 
 remember the association of oxygen and ozone, and the way in which the 
 latter is affected by heat, it may well be that temperature plays an important 
 part in the matter. In proof of this conduct of oxygen, it may be cited that, 
 in the case of a H 2 O tube, the H lines come out sharp and brilliant in the 
 spectrum, while what is seen of the O lines is comparatively weak, misty, and 
 ill-defined. Vogel, it will be remembered, makes 5189 of the Aurora coinci- 
 dent with an O line. 
 
 Professor Herschel has pointed out, and I have confirmed, that the residual 
 phosphorescence in Geissler tubes, after the spark has passed, is probably 
 associated with oxygen. He also alludes to the fact that when one of the 
 globes of a "Garland" tube was heated, it did not shine after the spark had 
 passed, apparently because of the destruction of the ozone by heat. 
 
 [Some experiments with a tube of this description will be found detailed 
 in Part III. Oxygen was not, I think, the gas it was filled with.] 
 
 Subsequently to my examination and comparison of the O and CO 2 spectra 
 before detailed, Dr. Arthur Schuster was good enough to send me three 
 vacuum-tubes of his own preparation, showing an oxygen-spectrum. 
 
 One, with large disk-shaped brass electrodes, was unfortunately broken in 
 transit. Dr. Schuster informed me it showed the carbonic-oxide spectrum as 
 well as that of oxygen. The other two tubes had aluminium electrodes. They 
 were similar in shape to ordinary Geissler tubes, but had attached to each a 
 supplemental bulb containing dry oxide of manganese. Illuminated by the 
 larger coil, one of these tubes (which had a slight crack in the manganese 
 bulb) lighted up faintly ; the other was fairly bright, and the glow had a 
 somewhat reddish tint. 
 
 Plate XVIII. fig. 15 represents as the upper spectrum Vogel's Aurora, 
 
 Probability 
 that O may 
 play a part 
 in the 
 Aurora- 
 spectrum. 
 Difference 
 between air- 
 spark and 
 tube- 
 spectra. 
 
 HO tube 
 referred to. 
 
 Eesidual 
 phospho- 
 rescence in 
 Geissler 
 tubes. 
 
 Garland 
 tube. 
 
 Dr. Schus- 
 ter's tubes 
 described. 
 
130 
 
 THE SPECTRUM OF THE AUEOEA. 
 
 Spectra de- 
 scribed. 
 
 Capillary. 
 
 Violet-pole. 
 
 Dr. Schu- 
 ster's re- 
 marks on 
 the spectra. 
 
 with W.L. numbers, as the middle spectrum the capillary part of Dr. 
 Schuster's O tube, and as the lower spectrum the negative (violet) pole of the 
 same tube. 
 
 The tube-spectra were mapped out with the aid of the diaphragm micro- 
 meter before described. 
 
 The capillary spectrum was mainly distinguished by four bright sharp 
 lines one in the red, between the red Aurora-line and D, two in the green, 
 but considerably more refrangible than the yellow-green Aurora-line, while 
 the fourth was found to be hydrogen F. The other lines in the spectrum 
 were considerably fainter, and misty and band-like. The red line, though 
 not brilliant, was fairly bright and sharp. 
 
 The place of the less refrangible of the two bright bands in the violet- 
 pole spectrum was occupied in the capillary spectrum by a faint glow only. 
 
 The violet-pole spectrum was recognized by two very bright broad bands 
 of light in the green, each including within its limits one of the Aurora-lines. 
 The bright red line in the capillary had a faint representative in the violet- 
 pole spectrum, as also had the two bright lines in the green. Other fainter 
 lines appeared in the blue, and three fairly bright ones towards the violet. 
 
 Dr. Schuster remarks that one of these O bright bands is closely coincident 
 with a band in the CO spectrum, but that the CO band is bright towards 
 one edge and fades off gradually thence, while the O band is of pretty 
 uniform strength throughout. Dr. Schuster finds the wave-lengths of the 
 violet-pole O bands to be as follows: 
 
 5205-0) 
 
 Brightest part 5255. 
 
 Brightest part 5586. 
 
 He also gives as weak bands 5840-5900 and 5969-6010. Dr. Schuster 
 comes to the conclusion that the green line of the Aurora is not due to 
 oxygen, as, under considerable dispersion and with good definition, the 
 oxygen-bands can be broken up into a series of lines, when the brightest part 
 is found to lie at 5586, which is too much towards the red to compare with 
 the Aurora-line. He notices that the more refrangible of the O bands 
 corresponds with a line sometimes seen in the Aurora (Vogel's 5233). The 
 same remark will, however, apply to this last as to the other coincidence, 
 viz., that a broad band can hardly represent a line at least, the line can 
 only be said to coincide in a loose and indefinite way. It is evident that 
 
OXYGEN-SPECTEUM IN RELATION TO THE AUEOEA. 
 
 131 
 
 Dr. Schuster's tubes were free from what must now be considered an impurity 
 in those examined by me and by Dr. Vogel, and that Mr. Procter's suspicions 
 of carbon impurities in these, and the ordinary oxygen-tubes, are thereby quite 
 confirmed. 
 
 In some experiments which we made (after receiving Dr. Schuster's tubes) 
 with an open Geissler tube, so arranged as to connect with an air-pump and 
 gas-receiver, and thus from time to time to wash out the tube and vary its 
 contents, we found the same impure spectrum as in the case of the sealed O 
 tubes ; and it seems to require a very large amount of precaution to avoid 
 these impurities. 
 
 Dr. Schuster was kind enough to examine the spectra I mapped out, and 
 which are shown in Plate XVIII. fig. 15, with the following results : The 
 lines Oa, O/3, Oy are those he has referred to under that designation in his 
 communications to ' Nature,' and undoubtedly belong to oxygen. The bands 
 A, B, and C are the bands characteristic of the negative pole. He finds A 
 divided into two parts by a dark space. The spectrum of the negative pole, 
 under good exhaustion, stretches into the capillary part ; hence B appears in 
 the capillary as a faint band. A similar thing happens with nitrogen. L, II., 
 III., and possibly 8 and 9, he thinks, are due to the spark-spectrum of oxygen, 
 obtained when the jar and a break are interposed, the brighter lines of the 
 line-spectrum being always present at the negative pole. These last-men- 
 tioned lines I have already referred to, as having been found by me in a tube 
 showing phosphorescence after the spark has passed. (Compare Plate XVIII. 
 fig. 15, O violet pole, with Plate XV. spectrum 5.) Nos. 1 and 2, he thinks, 
 are due to some foreign matter, as they are not in all his tubes. 
 
 Dr. Schuster often finds that a spectrum due to the aluminium electrodes 
 is seen in tubes under great exhaustion ; and this he considers is the spectrum 
 of aluminium oxide. A drawing of this spectrum is found in Watts's ' Index 
 of Spectra,', plate iii., "Aluminium first Spectrum." To this, he thinks, are 
 also due the bands, or sets of lines in my aluminium-arc spectrum (' Photo- 
 graphed Spectra,' plate ii.), and he believes lines 3, 4, 5, 6, and 7 in the 
 mapped-out spectra are due to it. It would thus appear that the lines due 
 to O are few in number, and do not well compare with the Aurora-spectrum. 
 
 His tubes 
 free from 
 impurity. 
 
 Experi- 
 ments with 
 an open 
 Geissler 
 tube. 
 
 Spectra of 
 Dr. Schus- 
 ter's O tube 
 examined. 
 
PART III. 
 
 MAGNETO-ELECTRIC EXPERIMENTS IN 
 CONNEXION WITH THE AURORA. 
 
 INTRODUCTION. 
 
 THE set of experiments detailed in Chapters XIV. to XIX. was mainly con- 
 ducted for the purpose of testing, in connexion with the Aurora, the action of 
 a magnet upon the electric glow in vacuo and on the spark at ordinary pressure. 
 It also includes some observations on the glow from the violet pole with and 
 without the magnet, and on the glow obtained from one wire only. The 
 apparatus employed was a Ladd's electro-magnet, with poles 10 inches high 
 by 2 inches across, each pole being surrounded by a movable helix, com- 
 posed of two sets of stout copper wire wound together, so that they could 
 be used either in one length or as independent coils excited at the same time. 
 The latter form of arrangement was employed by us. In most of the experi- 
 ments conical armatures were employed for the purpose of bringing the action 
 of the poles to bear upon the subjects examined. A contact-maker was 
 added to the magnet, so that it could be put rapidly in or out of action 
 without disturbing the wires. The battery used to excite the magnet was of 
 the form known as that of Dr. Huggins, and consisted of four vulcanite cells 
 in a frame, each holding seven pints of bichromate solution, and containing 
 two carbon and one zinc plate, each 13^ by 6 inches. 
 
 A winch and pulley enabled the whole set of plates to be lowered into the 
 liquid and withdrawn at pleasure, and the large quantity of solution gave the 
 battery a considerable amount of constancy. We found it could be used 
 
 T2 
 
 Object of 
 experi- 
 ments. 
 
 Description 
 of apparatus 
 employed. 
 Electro- 
 magnet. 
 
 Battery. 
 
134 
 
 MAGNETO-ELECTEIC EXPERIMENTS. 
 
 Small coils. 
 
 Larger coil. 
 
 Magnetic 
 curves ob- 
 tained. 
 
 Chautard's 
 investiga- 
 tions kept 
 in view. 
 
 Evidence 
 obtained of 
 change in 
 colour, form, 
 &c. of 
 Aurora. 
 
 Angstrom's 
 flask-ex- 
 periment 
 tried. 
 
 General 
 results of 
 experiments. 
 
 for two evenings' work, of four hours each, without any material dropping in 
 power. For obtaining the glow in the Geissler and other small tubes, a 
 Ruhmkorff coil, giving a -inch spark, excited by one plate of a ^-gallon 
 bichromate (bottle form), was used. For the glow in the larger tubes and 
 the spark in air a larger coil, giving a two- to three-inch spark, and worked by 
 two ^-gallon double-plate bichromates, was employed. Notes were taken 
 of the experiments, and drawings of the effects at the time ; and these 
 are reproduced almost literally in the text and Plates comprised in 
 this Part. To ascertain the direction and extent of the magnetic curves, 
 we covered large sheets of cardboard, placed over the poles, with iron 
 filings ; excited the magnet so as to obtain the curves, and then obtained 
 permanent prints from the filings by spraying the cardboard with tannin 
 solution. The magnetic effects were thus found to extend to a radius of 
 at least ten inches (see diagram, Plate XVII. fig. 1, showing magnet-poles 
 and curves on a ^ scale). 
 
 In the vacuum-tube experiments we held Mons. J. Chautard's investigations 
 (on the action of magnets on rarefied gases in capillary tubes rendered 
 luminous by the induced current, Phil. Mag. 4th series, vol. 1. p. 77) in view. 
 We obtained in our experiments plenty of evidence of a change of colour and 
 form in the discharge under the magnetic influence ; and both simple and 
 compound spectra were found to be much varied by the exaltation or sup- 
 pression of some parts of the spectrum, so that apparently new lines sprang 
 up ; but we failed to trace actual change of position or wave-length in any 
 given line, though we carefully looked for it. A portion of our researches 
 
 O 
 
 was directed to the subject of Angstrom's experiment of filling a dry flask 
 with a violet glow, analogous to that from the negative pole. We entirely 
 failed in obtaining the same result while two wires and an uninterrupted 
 circuit were employed. When, however, we attached a negative wire only 
 (the other wire being left free) to an exhausted globular receiver, we obtained 
 an effect very similar to that referred to in Prof. Angstrom's memoir. 
 
 The general result of the experiments was to prove, assuming the Aurora 
 to be an electric discharge, the great influence the magnetic forces may 
 exercise on the colours, form, motions, and probably the spectrum also 
 of that phenomenon. It is easy to conceive that the variation in number, and 
 intensity of the lines which has been remarked in Auroral spectra may have 
 its origin in such a cause. The influence of the magnet on the capillary 
 stream was mainly in colour and intensity ; but in the bulbs the effects were 
 still more marked and striking, and, in a greater or less degree, different in 
 
Plate XVI I. 
 
 ELECTRO- MAGNET AND TUBE. '/ 
 
 
 ! 
 
INTRODUCTION. 
 
 135 
 
 the case of each gas which we examined. A careful and extended study of 
 these effects, conjointly with the changes in the spectrum, might possibly 
 form a new and valuable mode of analysis of compound gases. This is well 
 illustrated in the case of the iodine and sulphur tubes which we examined. 
 
 Bulb effects 
 noticed as a 
 mode of 
 analysis of 
 
 -:l> > 
 
 TVERSITY 
 
136 
 
 MAGNETO-ELECTRIC EXPERIMENTS. 
 
 CHAPTER XIV. 
 
 EXAMINATION OF GEISSLEE TUBES UNDER ACTION OF THE MAGNET. 
 
 Nitrogen- 
 tube No. 1. 
 
 Discharge 
 described. 
 
 Spectrum 
 described. 
 
 Capillary 
 stream. 
 
 Positive 
 bulb. 
 
 Violet-pole 
 glow. 
 
 Nitrogen- 
 tube No. 2. 
 
 Glow de- 
 scribed. 
 
 Difference 
 of spectra of 
 capillary 
 stream and 
 violet-pole 
 glow. 
 
 Nitrogen-tubes. 
 
 (1) A SMALL Geissler tube (No. 1) was lighted up by the small coil. The capil- 
 lary part showed a very bright, slightly rosy-tinted stream. Negative bulb was 
 filled with rosy-purple light, the violet-pole glow being confined to the extent of 
 the electrode. Positive bulb of the same rosy-purple colour, but stream slightly 
 contracted in volume. Glow throughout quiescent, and no stratification in 
 the tube. A compound-prism spectroscope, taking in the whole of the spec- 
 trum, showed in the capillary stream, from yellow to red, a fairly bright wedge, 
 having a dark band in the centre, and six bright columns, with dark lines at 
 intervals, shading off on either side. On the more refrangible side of the 
 yellow, the spectrum was composed of a set of bright bands and lines in the 
 green, blue, and purple, one line only (in the green) standing out very bright. 
 In the yellow and red no bright line stood out alone. The positive bulb gave 
 a fainter spectrum of the same character, mainly confined to the centre, the 
 violet, yellow, and red not being well seen. When the violet-pole glow was 
 examined, the general character of the spectrum was quite changed : a bril- 
 liant broad band in the violet, a bright narrower one in the blue, and two 
 bright lines in the green, with intermediate fainter lines throughout, were the 
 main features. The yellow and red part of the spectrum was also changed. 
 The yellow was fairly and evenly distinct up to the dark band ; then came a 
 somewhat brighter orange band, and after that the red, but rather obscure 
 and cut off. No absolutely bright line could be traced in the red. 
 
 (2) To compare the capillary stream and the violet-glow, a second nitrogen- 
 tube (No. 2) was used. This tube was larger in bulk and bore than No. 1. 
 The glow in the bulbs was considerably fainter and more salmon-coloured ; 
 and there was much stratification in both, extending to the capillary bore. 
 (This stratification was considered due to H, as the three principal lines of 
 that gas came out very brightly in the spectrum.) The difference of the 
 spectra of the capillary stream and of the violet-pole glow was extremely well 
 marked the former consisting of a set of bright lines and bands of fairly 
 uniform intensity, while the latter was split up into a few bright bands with 
 
EXAMINATION OF MAGNETIZED GEISSLER TUBES. 
 
 137 
 
 fainter lines between. The yellow and red of the violet-glow were very weak 
 as compared with the same region of the capillary spectrum. No bright line 
 appeared in the red. The tube being properly adjusted for the purpose, the 
 junction of the violet-pole glow and the capillary red-glow was easily ob- 
 served. The bright bands of the violet-pole were seen to run into the 
 capillary line-spectrum, and then, gradually getting finer and more pointed, 
 to fade out. 
 
 (3) The capillary part of tube No. 1 was arranged between the poles of 
 Ladd's electro-magnet, the conical ends of the armatures almost touching the 
 tube (Plate XVII. fig. 1). With the magnet not excited, the capillary stream 
 was bright and of a slightly rosy-yellow tinge. It varied a little in apparent 
 diameter with the current. As soon as the magnet was excited the capillary 
 stream, as also (in a less degree) that in the bulbs, were seen to contract, and 
 to change from a rosy tint to a distinctly blue-violet. The polished armatures, 
 acting as reflectors, showed this change of tint in a most marked manner each 
 time the magnet was excited. At the same time the capillary stream was 
 seen to run into the negative bulb, as if overflowing, and with an effect resem- 
 bling the " tailing-over " of a gas-flame. This effect took place each time the 
 magnet was excited, and was not found at the positive-bulb end. 
 
 Occasionally, when the magnet was excited, flashes of light were discharged 
 in the negative bulb from the capillary towards the violet-pole. The 
 spectrum was then carefully examined. No change was seen in the actual 
 position of any of the lines or bands when the tube was influenced by the 
 magnet, but those towards the violet end of the spectrum were conspicuously 
 brightened. 
 
 (4) The extremity of the negative bulb was now placed between the poles 
 of the magnet. A bright violet-coloured arc, following the magnetic curve, 
 was at once formed, as in the case of the large Pliicker tubes ; and at the 
 same time a straight stream of not very bright light ran along the bulb. The 
 positive bulb was next placed within the action of the magnet ; and imme- 
 diately a brilliant spiral of flickering light appeared in the bulb, lighting it 
 up, and reminding one in shape of the spiral which water forms on being 
 poured from a lipped jug (see Plate XVII. fig. 9). 
 
 This was repeated each time the magnet was excited. The spiral, though 
 flickering in character, was permanent in form, and inclined to the side of the 
 tube which was in contact with the N pole of the magnet. 
 
 Junction ot 
 the violet- 
 pole glow 
 and capil- 
 lary stream. 
 
 Tube No. 1 
 between the 
 poles of the 
 magnet. 
 
 Change of 
 colour iu 
 capillary 
 stream. 
 
 " Tailing- 
 over " of 
 capillary 
 stream. 
 
 Spectrum 
 examined. 
 
 Negative 
 bulb be- 
 tween poles 
 of the mag- 
 net. 
 
 Positive 
 bulb within 
 action of 
 the magnet. 
 
 Spiral 
 formed. 
 
138 
 
 MAGNETO-ELECTRIC EXPERIMENTS. 
 
 O tube No. 
 1; spectrum 
 described. 
 
 O tube No. 
 '2; spectrum 
 described. 
 
 Tube-glow 
 described. 
 
 Effect upon 
 glow when 
 magnet 
 excited. 
 
 Bulbs be- 
 tween poles 
 of the mag- 
 net. 
 
 Effect of 
 magnet on 
 spectrum. 
 
 Oxygen-tubes. 
 
 A tube (No. 1) was lighted up and examined with the spectroscope, and 
 found to give the spectrum shown on Plate XIV. spectrum 3, but with a 
 strong set of H lines in addition. 
 
 A second tube (No. 2) was then lighted up. The spectrum was a bright 
 one, similar to the foregoing, the principal H lines being present, but not 
 strong. 
 
 The red region was indistinct, and showed no prominently bright line. The 
 bulbs were mainly of a slightly blue-grey tint, with a steady glow. Capillary 
 stream quite pale white, with a veiy slight tinge of red. Violet-glow small 
 and confined to the electrode. Upon the magnet being excited, the capillary 
 stream became intensely brighter, and the glow in both bulbs contracted into 
 a single bright stream, which curved towards the sides of the bulbs at right 
 angles to the magnetic poles, and changed from side to side with the current. 
 This effect was very marked, and was more apparent in the positive than the 
 negative pole. A faint stratification was seen in both bulbs. Upon either 
 bulb being placed between the armatures, the glow left the electrode point 
 and condensed into one bright stream, running along the side of the tube and 
 curving at each end (Plate XVII. fig. 10). No trace whatever of tendency 
 to form a spiral was seen. The spectrum with the magnet on was very con- 
 spicuously brightened up throughout. A set of fluted bands with a bright 
 line among them appeared in the red, and several lines or bands appeared 
 in the violet which could not be seen before. The bright red line, upon 
 measurement, proved to be the hydrogen-line C. It thus seemed brighter in 
 proportion than the F line, although, with the magnet off, the latter was well 
 seen, while the C was not. No actual change in position of the spectrum-lines 
 could be detected. 
 
 [It is to be noticed that the O tubes employed were those used by me in 
 former experiments, and had the bright lines now attributed to hydrocarbon 
 impurity. Their bulb-effects differed, however, entirely from those of the CO 2 
 tube. (Compare figs. 10 and 11, Plate XVII.)] 
 
 Hydroyen-tubes. 
 
 H tube, No. A sma11 H Geissler tube (No. 1) was selected, and lighted up by the small 
 
 1 ; glow de- co iL The capillary was a bright white-pink stream, with a tendency to redden 
 
 at times. The bulbs were both of a faint blue-grey tint, with coarse lenticular 
 
EXAMINATION OF MAGNETIZED GEISSLER TUBES. 
 
 139 
 
 When the 
 magnet was 
 excited, 
 whole cha- 
 racter of 
 tube 
 changed. 
 
 TJnexcited 
 
 spectrum 
 
 described. 
 
 Effect when 
 magnet was 
 excited. 
 
 No. 2H 
 tube; effects 
 described. 
 
 stratification. The violet-pole glow was pale and white as compared with 
 that of N. 
 
 When the magnet was excited, the whole character of the tube changed. 
 The capillary stream diminished in brightness and in apparent volume, and 
 changed to a deep amber-yellow. The bulbs lost some of their light, and their 
 coarse stratification ; being, in lieu, filled with a vertical condensed stream of 
 moderate light, in which a fine stratification only was seen. The stream in 
 the positive bulb had a tendency to the spiral form. The capillary, each time 
 the magnet was excited, " tailed over " into the negative bulb, as in the case 
 of N, looking as if it were squeezed out of the capillary bore. The unexcited 
 spectrum was found to consist of the usual principal lines of H on a continuous 
 glow, with the intermediate bands and finer lines, which are usually suspected 
 to be due to impurity. The sodium-line was also seen. When the magnet 
 was excited, the spectrum grew much fainter the continuous glow in the red 
 and blue, and the red and blue lines, nearly disappearing, and the line in the 
 green alone shining out conspicuously. No change of place in the lines could 
 be noticed. 
 
 A longer H tube (No. 2) was then tried, with similar effects, except that 
 the diminution in brightness was not so conspicuous. When the negative bulb 
 of the tube No. 1 was placed between the poles of the magnet, a stream of 
 light was formed, and the stratification became finer. The same effect took 
 place with the positive bulb, with a tendency to the spiral form. 
 
 Water-Gas (H 2 O) tube. 
 
 A faint purple glow was seen in each bulb, the tube not lighting-up brightly. 
 The capillary showed a slightly rosy-tinted, grey stream of brighter light. 
 ^ ith the magnet on, the glow in the bulbs was condensed into a single bright 
 stream. The capillary brightened up, and assumed a yellow tint this effect 
 being principally confined to that portion which was between the conical ends 
 of the armatures, and gradually diminishing as the distance increased from 
 these. Without the magnet, the principal H lines showed brightly in the 
 spectrum, the O lines being misty and indistinct. With the magnet on, the 
 O lines and spectrum generally brightened up. 
 
 Ammonia-tube. 
 
 This tube was difficult to light up. Hardly any light was seen in the bulbs, Ammonia- 
 except a very faint purple glow at the electrodes. In the capillary part a 
 fairly bright stream of purple-white light appeared. The spectrum was a scribed. 
 
 U 
 
 Water-gas 
 (H..O) tube ; 
 effects pro- 
 duced de- 
 scribed. 
 
140 
 
 MAGNETO-ELECTEIC EXPEKIMENTS. 
 
 Spectrum 
 described. 
 
 faintly shown one of N and H. The effect of the magnet was to reduce the 
 brightness of the glow in the capillary, but with little marked action on the 
 bulbs, except to condense the faint glow into a slightly bright stream running 
 along the side of the tube. 
 
 On a subsequent examination the tube and spectrum both brightened up 
 under the influence of the magnet. The N lines, which were faint without 
 the magnet, shone out under its influence distinctly the red and yellow 
 parts of the spectrum specially showing this effect. The H lines also 
 brightened up, but hardly so much in proportion as the N. 
 
 Tube 
 marked 
 C A ; light- 
 ing-up de- 
 scribed. 
 
 Effects 
 when mag- 
 net was 
 excited. 
 
 Chlorine- 
 tube No. 1 
 lighted-up. 
 
 Action of 
 magnet 
 upon the 
 tube and 
 spectrum. 
 
 Carbonic-Acid tube. 
 
 A Geissler tube marked C A was examined. Capillary stream a brilliant 
 bluish white ; bulbs grey-blue, with a slight tint of green ; slight stratification 
 in positive bulb ; stream diffuse, not quite filling the bulbs, and changing in 
 volume as the coil-break was touched ; glow round the violet-pole consider- 
 able, but markedly white in tint, rather than violet ; stratification strong in 
 capillary. With magnet excited, the capillary stream diminished in volume, 
 but greatly increased in brightness. It " tailed over " into the negative bulb, 
 and the stream through both bulbs curved towards the sides. A slight 
 pattering noise was heard in the tube. In the positive bulb bright, imper- 
 fectly formed, saddle-shaped rings of light, with a tendency to spiral formation, 
 were seen, somewhat similar to the effects in the Pliicker tube after described 
 (see Plate XVII. fig. 11). 
 
 The whole spectrum, under influence of the magnet, became much brightened 
 up. Faint bands in the red came out bright, as also did some in the violet. 
 The violet-glow was examined (without the magnet), and the light was found 
 condensed into four prominent shaded bands, one red, one yellow-green, one 
 green, and one blue, with fainter bands seen between. 
 
 Chlorine-tubes. 
 
 A chlorine-tube (No. 1) was lighted-up with the small-coil. Capillary stream 
 of a pale green tint. Bulbs with very little glow in them ; spectrum pale, and 
 not very distinct. Under action of the magnet this tube brightened up 
 throughout, and the glow became more condensed, and ran to the sides of the 
 tube. The spectrum also brightened, the faint lines becoming stronger, but 
 the general character was preserved. 
 
 A second chlorine-tube (No. 2) was then tried. Both bulbs were completely 
 
EXAMINATION OF MAGNETIZED GEISSLEE TUBES. 
 
 141 
 
 filled with a dense white (very slightly rosy-tinted) opaque light, and capil- 
 lary the same, but brighter. A very slight violet tinge was seen at the nega- 
 tive pole. When the magnet was put on, both bulbs were at once filled with 
 flickering bright streams of light, running towards the side of the tube, 
 according to the direction of the current. 
 
 The capillary stream at the same time changed from white to an intense 
 bright green. The spectrum without the magnet consisted of sets of lines, 
 with two well-marked absorption-spaces between, all seen somewhat faintly, 
 as if through a mist. 
 
 When the magnet was put on, the marked character of the absorption 
 spaces was lost. The sets of lines in the yellow-green and green started up 
 intensely bright, while those in the blue only slightly brightened. 
 
 The misty appearance was altogether lost, and the bright lines all shone up 
 upon a perfectly dark background, with a strikingly metallic look ; we could 
 not, however, trace change of position or actually new lines. It seemed as if 
 lines which had been faint in the yellow-green and green region suddenly 
 increased in intensity, the other parts of the spectrum not being similarly 
 influenced. They quite flashed up when sudden contact was made with the 
 magnet commutator. 
 
 Iodine-tubes. 
 
 This tube (Xo. 1) had been used for photographic purposes, and the bulbs 
 were partly obscured by a white deposit. 
 
 On lighting it up, both bulbs were filled with a violet-grey diffused light, 
 with much coarse well-marked lenticular stratification. This stratification was 
 mainly lost on changing the direction of the current, but made its reappear- 
 ance when one conducting-wire was touched with a finger. This effect was 
 still more marked when one finger of each hand was applied to the wire. 
 The capillary stream was of a pale lemon-yellow. On putting on the magnet 
 the light in the whole tube was nearly extinguished, a faint thin stream of 
 condensed light running through the centre of the tube alone remaining. 
 
 On placing the bulbs between the magnet-poles, effects were produced 
 similar to those in the case of the tube, after described (p. 144, and marked 
 Si F1 6 ), but in a less marked degree. 
 
 The iodine-tube was subsequently again tested, and it lighted-up better 
 than on the last occasion, showing nearly the same effects in bulbs and 
 capillary, the former having somewhat of a rosy tint and the latter an amber. 
 
 On exciting the magnet, the capillary part of the tube changed from amber 
 
 u2 
 
 Chlorine- 
 tube No. '2 
 lighted-up. 
 Effect on 
 glow when 
 magnet was 
 put on. 
 
 Changes in 
 spectrum 
 when mag- 
 net was 
 excited. 
 
 Iodine-tube 
 No. 1. 
 
 Lighting-up 
 described. 
 
 Effect of 
 touching 
 one wire 
 with the 
 finger. 
 
 Effect of the 
 magnet. 
 
 Tube again 
 tested. 
 
 Magnet 
 effects. 
 
142 
 
 MAGNETO-ELECTKIC EXPEEIMENTS. 
 
 The spec- 
 trum de- 
 scribed. 
 
 Change 
 when mag- 
 net was 
 excited. 
 
 No change 
 in line- 
 position. 
 
 Comparison 
 of iodine- 
 tubes No. 1 
 and No. 2. 
 
 Comparison 
 of the 
 spectra. 
 
 The two 
 tubes exa- 
 mined in 
 detail. 
 No. 2. 
 
 to a decided light green. The spectrum, without the magnet, gave one very 
 bright line, and several less bright ones near, in the blue-green. The rest of 
 the spectrum, with the exception of the absorption-spaces, was misty and con- 
 tinuous, with lines showing faintly through. The red and yellow portions of 
 the spectrum were quite bright. When the magnet was excited, the 
 spectrum entirely changed. The red and yellow portions of the spectrum, 
 and the misty continuous light, all quite disappeared ; while a set of 
 sharp lines on the yellow-green and green flashed up bright and clear, and 
 stood out alone upon a dark background, in which the absorption-spaces 
 were lost. The effect was very strongly marked, and gave a totally different 
 character to the appearance of the spectrum. The change seemed to arise 
 from the suppression of one part of the spectrum, and the increase in intensity 
 of the lines in the other part. 
 
 The principal lines could not be traced to change in actual position. 
 
 This tube differing somewhat from a second one we examined (No. 2) in 
 tint of glow and spectrum, it suggested itself to us that there might be a 
 partial mixture of N or H (or both) with the iodine vapour, giving rise to 
 some of the brighter parts of the spectrum which were extinguished under 
 the action of the magnet. 
 
 We therefore compared these two tubes, viz. the old one (No. 1) and the 
 new one (No. 2), and also their spectra, by means of a comparison-prism on 
 the slit of the spectroscope. To the eye, the tubes differed much in appear- 
 ance. No. 1 had a distinct transparent rosy tint throughout, with consider- 
 able coarse flickering stratification ; and this contrasted strongly with the 
 dense whitish light of tube No. 2, which showed neither movement nor 
 stratification. The spectra were also found different in general look. That 
 of tube No. 1 was strongly tinged in the red and yellow, and showed a bright 
 continuous spectrum, crossed by many sharp lines, with little trace of absorp- 
 tion-spaces. The spectrum of No. 2 was much whiter in tint, showed very 
 little of the red and yellow, and the absorption-spaces were very dark. A 
 few bright lines, mainly in the yellow-green and green, were faintly seen. 
 
 The two tubes were then examined separately in detail. No. 2, excited by 
 the magnet, showed curious effects. The glow was rendered weak and inter- 
 mittent, and the rosy tint almost disappeared. The capillary changed to a 
 decided green colour, and the positive electrode was surrounded by a yellow 
 glow. The changes in the spectrum were no less decided. Without the 
 magnet, the spectrum was found to be a bright continuous one of H (with a full 
 set of principal and intermediate lines) and N the N spectrum being rather 
 
EXAMINATION OF MAGNETIZED GEISSLEE TUBES. 
 
 143 
 
 faint and misty, with very slight, if any, traces of the iodine-spectrum. On 
 the magnet being excited, the spectrum changed as if by magic ; the H and N 
 spectra disappeared (except hydrogen F, which still faintly remained), and the 
 iodine lines, mostly in the yellow-green and green, shone out wonderfully 
 sharp and bright on quite a dark ground. No. 1, upon examination, showed 
 between the magnet-poles only the same changes as on last occasion. The 
 spectrum seemed to be one of iodine, with the addition of slight traces of the 
 H spectrum. 
 
 On excitation of the magnet, the misty continuous part of the spectrum 
 nearly disappeared, and the bright lines shone up sharply upon the dark 
 background as before. The effects in the case of both tubes were strongly 
 marked. The impression as to tube No. 2 was that, without the magnet, the 
 slight iodine-spectrum was overpowered and masked by the N and H spectra ; 
 while under the influence of the magnet the N and H spectra were almost 
 altogether suppressed, the iodine-spectrum being at the same time intensified. 
 The disappearance of the continuous spectrum under the action of the magnet 
 in No. 1 (with the supposition it was mainly H) would be accounted for in the 
 same way. 
 
 Bromine-tubes. 
 
 This tube (No. 1) had been previously worked for photographic purposes. 
 Excited by the small coil, the whole tube was filled with a faint flickering 
 light. The positive bulb contained a faint purple glow, with a yellow-green 
 tinge at the electrode, a curious flickering stream of light flashing from the 
 electrode to the side of the tube. The negative pole showed pretty much 
 the same effect as the positive. The capillary stream expanded at the open- 
 ing into the positive bulb, but ran in a condensed stream into the negative 
 bulb. In colour it was of a rather bright lilac. Upon putting the magnet 
 on, the light-glow in the tube was at once and permanently extinguished, 
 the coil still working as if the current passed. The same effect happened 
 repeatedly ; but now and then the tube lighted-up for a second, showing 
 spiral arrangement in the bulb. We tried another bromine-tube (No. 2) : 
 it lighted-up easily ; both bulbs were filled with a purple stream of light ; 
 capillary stream bright grey. The glass of the tube was strongly fluorescent 
 and of a yellow tinge. When the magnet was excited the stream of light was 
 somewhat condensed in the bulbs, and flew to the side of the tube ; while 
 the capillary stream at the same time brightened. The spectrum without 
 the magnet was fairly bright ; it increased in brightness under the influence 
 of the magnet, and additional lines appeared ; but we considered them to be 
 
 Effects 
 discussed. 
 
 Bromine- 
 tube No. 1. 
 
 Lighting-up 
 described. 
 
 Effect of the 
 magnet. 
 
 Bromine- 
 tube No. 2. 
 
 Effect of 
 magnet. 
 
144 
 
 MAGNETO-ELECTEIC EXPEEIMENTS. 
 
 Si Fl e tube. 
 
 Lighting-up 
 described. 
 
 Effect of 
 magnet. 
 
 Comparison 
 of Si F 4 and 
 Si FK tubes. 
 
 only faint existing ones brightened up. No change in the position of the 
 principal lines was traced. 
 
 Silicic-Fluoride tubes. 
 
 (1) A tube marked SiFl 6 had been worked for photographic purposes; it 
 lighted-up easily. Both bulbs were filled with a brown-pink diffused light, 
 inclined to condense into a stream in the positive bulb. The violet glow was 
 very bright, and nearly filled the space round the electrode. The capillary 
 stream was of a bright violet tint. The effect of the magnet was to decrease 
 the intensity of the light throughout the whole tube. 
 
 In the positive bulb the stream broke up into a number of vibrating 
 streamlets, with little bright threads of light intermixed, which flew towards 
 the side of the tube at right angles to the magnetic poles. There was an 
 inclination to spiral arrangement in the streamlets. This stream changed from 
 side to side of the tube coincidently with change in the magnetic poles. At the 
 negative pole the violet glow formed an arc in the direction of the magnetic 
 curves, while a spiral of fainter (positive]) light was formed in the upper 
 part of the bulb. A slight ringing sound was heard in the tube. 
 
 (2) We compared two tubes (SiF 4 and the one marked SiFl 6 ). The SiFl 6 
 tube in general effect, and in its spectrum, when lighted-up, resembled Si F 4 . 
 We compared the one tube under the influence of the magnet with the other 
 not so, by means of a comparison-prism on the slit. As the spectroscope and 
 second tube were necessarily removed some distance from the magnet, the 
 spectrum of the tube between the poles was not bright. We could not trace 
 a change of position in any of the principal lines. The tube between the 
 poles was brightened up when the magnet was in action*. 
 
 SO, tube 
 No. 1 ; 
 lighting-up 
 described. 
 
 Effect of the 
 magnet. 
 
 Changes in 
 the spec- 
 trum. 
 
 Sulphuric-Acid (SO 3 ) tubes. 
 
 (1) Excited by the small coil, both bulbs of this tube (No. 1) lighted-up 
 brightly, with a misty light-blue tinted stream of oqaque light, a yellow glow 
 appearing at the negative pole. The capillary stream partook of the same blue 
 tint, but was whiter and brighter. Under the magnet's influence, the glow 
 in the bulbs flew to the side of the tube in flickering streams of light, the 
 capillary at the same time changing to a distinctly green tint. The spectrum 
 without the magnet consisted of four fairly bright bands of light in the yellow, 
 
 * The tubes generally seem marked Si Fl instead of the ordinary notation Si F. Si F1 6 is pro- 
 bably, in fact, SiF 4 . 
 
EXA.AQNATION OF .MAGNETIZED GEISSLEE TUBES. 
 
 145 
 
 green, blue, and violet, connected by a faint misty continuous spectrum (O or 
 possibly the hydro-carbon spectrum found in O tubes by way of impurity). 
 
 When the magnet was excited, this spectrum entirely disappeared ; and a set 
 of bright metallic-looking lines upon a dark background (line-spectrum of S) 
 took its place. This effect was produced whenever the magnet was excited, 
 and we tried it several times, to make sure of the complete change. After a 
 time, when the magnet, battery, and the coil-power were all weaker, with the 
 magnet on, we obtained a compound of both spectra, the bright lines being 
 seen upon the continuous spectrum in which the bands appeared. When the 
 magnet was taken off, the bright lines disappeared, and the O spectrum alone 
 remained. 
 
 (2) We also tried another SO 3 tube (No. 2) which had been worked for 
 photographic purposes, and was suspected of a carbon impurity. Without 
 the magnet, the spectrum was very like that of the first tube ; but when the 
 magnet was excited, the spectrum only brightened, and no bright metallic- 
 looking lines appeared. 
 
 Sulphur-tube. 
 
 (1) A small bent vacuum-tube containing some solid sulphur, excited by 
 the smaller coil, and without being heated, gave a narrow stream of bright 
 blue-green light running straightly through it. With the magnet on, this 
 stream was deflected in the bulbs, and the capillary changed from a blue-green 
 to a distinct rosy tint. 
 
 Without the magnet, the spectrum consisted of four bright bands, with a 
 continuous spectrum between, resembling that of SO 3 tube No. 1. With the 
 magnet on, the spectrum brightened, especially in the yellow and red, which 
 were dull before ; and a set of lines appeared upon it (a line or band in the 
 yellow especially showing) which were not seen before. The lines were dis- 
 tinct, but not very bright. The action on the capillary was noticed to be 
 strongest just between the conical points of the armatures ; and, in accord- 
 ance with this, the central part of the spectrum-band in the red and yellow 
 showed an increased brightness. 
 
 (2) One of the bulbs of the tube was then gradually heated with a small 
 gas-flame. The single stream in the heated bulb became somewhat deflected 
 and broken up into a number of smaller streams ; and these, when placed 
 under the magnetic influence, had small spark-like threads of light running 
 among them. The capillary, as the tube was heated, and the sulphur rose 
 in it, changed somewhat in tint, and, under the magnetic influence, became of 
 
 SO 3 tube 
 No. 2 ex- 
 amined. 
 
 Sulphur- 
 tube. 
 
 Lighting-up 
 (without 
 heating) de- 
 scribed. 
 
 Effect of 
 magnet. 
 Changes in 
 the spec- 
 trum. 
 
 Effects when 
 one of the 
 bulbs of the 
 tube was 
 heated. 
 
146 
 
 MAGNETO-ELECTRIC EXPERIMENTS. 
 
 Changes in 
 the spec- 
 trum under 
 influence of 
 magnet. 
 
 yellow-rose hue. As the heat was applied to the bulb the bands of sulphur 
 gradually appeared in the field of the spectroscope, until at last the band- 
 spectrum of sulphur entirely took the place of the spectrum seen in the cool 
 tube. The magnet being excited, the spectrum changed at once, a set of 
 bright sharp lines (line-spectrum of S) appearing upon a faint and dull image 
 of the band-spectrum. 
 
 This effect was constantly repeated upon the magnet being excited. The 
 magnet being taken off, the band-spectrum alone was to be seen. 
 
KITECT OF MAGNET ON A CAPILLAEY GLASS TUBE. 
 
 147 
 
 CHAPTER XV. 
 
 EFFECT OF MAGNET ON A CAPILLAET GLASS TUBE. 
 
 THE capillary portion of a Geissler tube was cut away from the bulbs, 
 cleaned, and connected by a small vulcanite tube with the gas-pipe in the 
 room conveying coal-gas at ordinary pressure. The flame was small and oval 
 in shape, 8 millims. high, by 4 millims. wide, and burnt quite steadily. (Plate 
 XVII. fig. 13.) 
 
 (1) The capillary tube was placed between the poles of the excited 
 magnet, almost, but not quite, touching them ; no effect at all was produced 
 on the flame. 
 
 (2) The tube was placed so that the conical ends of the armatures were 
 allowed to compress the centre of it between them ; still no effect was pro- 
 duced on the flame." 
 
 (3) The tube was placed so that the straight sides of the armatures com- 
 pressed it between them ; still no effect took place on the flame. 
 
 (4) The flame itself was placed between the poles of the magnet It was 
 slightly drawn towards one pole with an inclination to form the magnetic 
 curve. 
 
 (5) A piece of quill glass tubing was selected, 5 millims. in diameter and 
 1 millim. thick, and drawn out to a point, the end of which was snapped off 
 and the tubing connected as before. The flame was 20 millims. high, and 
 5 millims. across, and somewhat lambent. On being placed (1) between the 
 conical ends and (2) between the flat ends of the armatures, no effect could 
 be seen on the flame. 
 
 (6) A small taper-flame was placed between the poles of the magnet : no 
 effect was produced, except that the flame gave a slight " jump " each time 
 the magnet was excited. A spirit-lamp flame was tried with a similar result. 
 
 Action of Magnet on a bar of heavy glass. 
 
 A piece of heavy yellow-tinted glass was selected, being a bar 10 centi- 
 metres in length, and 8 millimetres square. This was mounted in a frame 
 with a Nicol prism at one end, and a double-image prism (next the eye) at 
 the other. 
 
 Capillary 
 portion of a 
 Geissler 
 tube tested 
 in three 
 ways. 
 
 No effect on 
 flame. 
 
 Flame be- 
 tween poles 
 of magnet. 
 
 Quill glass 
 
 tubing 
 
 tested. 
 
 No effect on 
 the flame. 
 
 Effect on 
 taper and 
 spirit-lamp 
 flames. 
 
 Heavy glass 
 bar and 
 mounting 
 described. 
 
148 
 
 MAGNETO-ELECTEIC EXPEEIMENTS. 
 
 Placed along 
 poles of 
 magnet. 
 
 Effect of 
 magnet on 
 candle- 
 images. 
 
 Effect on 
 using a tour- 
 maline as 
 analyzer. 
 
 Bar placed 
 at right 
 angles to the 
 poles : no 
 effect pro- 
 duced. 
 
 Slight effect 
 on second 
 experiment. 
 
 Effects pro- 
 duced when 
 a biquartz 
 was intro- 
 duced. 
 
 Change in 
 colour of the 
 halves. 
 
 (1) The glass bar and mounting were placed upon and along the poles of 
 the magnet (in the direction of the magnetic curves), and the double-image 
 prism and Nicol were so adjusted that two images of a candle were seen 
 the one below bright and normal, the one above, by rotation of the prism, as 
 nearly as possible extinguished (Plate XVII. fig. 4). On exciting the magnet 
 the faint image at once conspicuonsly brightened, at the same time assuming 
 a slightly green tinge. To get full effect of brightening, it seemed necessary 
 to have good pressure-contact between the battery-wires and the binding- 
 screws. 
 
 (2) Using a tourmaline as analyzer in lieu of the double-image prism, the 
 candle-flame was seen alternately brightened and darkened, as the tourmaline 
 was rotated ; and when the image was obscured by rotation, excitation of the 
 magnet caused it to brighten strongly. This effect was accompanied by the 
 apparent removal of a dusky red patch or spot, which occupied the centre of 
 the field when the flame was obscured. 
 
 (3) The bar of glass and double-image prism being placed between the 
 conical ends of the armatures, but at right angles to, instead of along, the 
 poles, upon excitation of the magnet no effect at all was produced. 
 
 (4) The bar and prism being placed in the same position between the flat 
 ends of the armatures, no effect at all was produced. 
 
 (4 a) Experiment No. 4 was repeated. It was thought that on excitation 
 of the magnet the secondary image slightly brightened ; but there was a 
 doubt about it, and the effect (if any) was slight. 
 
 (5) The apparatus was now changed for one of the following arrangement: 
 1, a rotating Nicol prism next the eye ; 2, the glass bar ; 3, a biquartz 
 with the halves horizontal ; 4, another Nicol prism. The neutral-passage 
 tint of the biquartz was found to be rather green (from mixture with the 
 yellow of the glass). 
 
 (i.) Placed along the poles of the magnet and the magnet excited, a change 
 of tint was seen in both halves of the biquartz, the slightly purple-reddish 
 tint of the upper half passing into a full purple. Effect not so marked as 
 with the double-image prism. 
 
 (ii.) Placed across flat ends of the armatures (as in experiment No. 4) no 
 effect was seen. 
 
EFFECT OF MAGNET ON WIDE AIE (AUROEA)-TUBE. 
 
 149 
 
 CHAPTER XVI. 
 
 EFFECT OF MAGNET ON WIDE AIE (AUEOEA) TUBE. 
 
 A LARGE, wide air-tube was tried ; it was 14^ inches long by 1 inch in dia- 
 meter, of the same bore throughout, and with straight platinum electrodes. 
 
 (1) To excite it the larger coil was used. The tube was filled with bright, 
 steady, rosy light, and beautiful stratification, which, as it flickered, seemed 
 to incline to a continuous spiral (Plate X. fig. 8). This stratification was 
 very close and fine, and .extended nearly throughout the tube. On excitation 
 of the magnet (the tube having been placed vertically between the conical 
 armatures), the glow was condensed into a bright solid line or stream of light 
 at the point which lay directly between the poles. This line or stream ex- 
 panded into an elongated funnel-shape as it retreated from this centre towards 
 the extremities of the tube, the stratification showing itself more distinctly 
 as the glow of light became less dense (Plate XVIII. fig. 3). The stream of 
 light was driven away at right angles to the poles, and changed from side to 
 side of the tube with the direction of the current. 
 
 [With the small coil this tube showed only a flickering stream of light, 
 with very slight indications of stratification.] 
 
 (2) The tube was placed horizontally between the conical ends of the 
 armatures. The condensed stratified stream of light flew upwards and down- 
 wards (according to direction of current) instead of to the respective sides of 
 the tube. 
 
 (3) The tube was placed along the poles of the magnet. In the interval 
 between these the stream was driven upwards, but at either end sideways, 
 right or left according to whether the pole was N. or S. (Plate XVIII. 
 fig. 4). The result gave a complete spiral of stratified condensed light within 
 the tube. 
 
 Note on Stratification. 
 
 The current from the large coil was sent through a set of five small French 
 vacuum-tubes, of equal calibre, containing salts of strontium and calcium, 
 and showing phosphorescent effects. These tubes were arranged in single 
 series ; and, from the colour of the glow-discharge, were presumed to contain 
 rarefied air in contact with the salts. 
 
 x2 
 
 Wide air- 
 tube de- 
 scribed. 
 
 Magnet 
 effect when 
 tube placed 
 vertically 
 between 
 conical ar- 
 matures. 
 
 Effect when 
 tube placed 
 horizontally 
 between the 
 armatures. 
 
 Tube placed 
 along the 
 poles of the 
 magnet. 
 
 Stratifica- 
 tion in 
 small tubes 
 arranged in 
 series. 
 
150 
 
 MAGNETO-ELECTKIC EXPEKIMENTS. 
 
 A strong coarse stratification was seen in the central (No. 3) tube. Tubes 
 Nos. 2 and 4 also showed stratification, but in a less degree ; while the out- 
 side tubes, Nos. 1 and 5, showed no stratification at all. The current was 
 steady, and these effects did not fluctuate. 
 
 Pliicker 
 air-tube. 
 
 Lighting-up 
 described. 
 
 Effect of 
 magnet on 
 the positive- 
 pole stream. 
 
 Effect on 
 negative 
 violet glow. 
 
 Arc of light 
 followed the 
 magnetic 
 curves. 
 
 Direction of 
 the current 
 changed. 
 Effects on 
 glow de- 
 scribed. 
 
 Effect of Magnet on Pliicker (Air-) Tube. 
 
 (1) A Pliicker air-tube was selected of the form shown on Plate V. fig. 1, 
 and was excited by the small coil. The ring was used for the positive pole, 
 the straight electrode for the negative. When lighted up, the tube glowed 
 with a perfectly steady and quiescent light. The negative electrode was sur- 
 rounded by the usual bright violet glow, extending itself and being gradually 
 lost at a short distance from the wire, while the ring let fall a faint, tubular, 
 salmon-coloured, diffused stream of light, which met the violet glow as it 
 approached the negative pole. The tube was then placed vertically between 
 the poles of the electro-magnet, the armatures being almost in contact with 
 the sides of the tube around the negative pole. On excitation of the magnet, 
 an instantaneous change took place. The stream of light from the positive 
 pole contracted itself, so that it became of a long funnel-shape (the ring 
 forming the mouth of the funnel), while it tapered almost to a point where 
 it met the violet glow. 
 
 The stream also became very brilliant (the sides of the tube being left pro- 
 portionately free from light), and crossing it were a set of bands, or striae, 
 ' having a waving or vibratory motion. The whole of the negative violet glow 
 was simultaneously gathered into a brilliant narrow arc, which stretched 
 across between the poles of the magnet. These effects are shown on Plate V. 
 fig. 1. The edges of the arc were remarkably sharp and well defined, and 
 with no surrounding aura or shading off. 
 
 By moving the tube between the armatures it was seen that the arc of 
 light followed the magnetic curves. If the tube was moved upwards, the arc 
 curved towards the zenith, if downwards, contrariwise ; and a middle position 
 could be selected, in which the edges of the arc were nearly parallel. Moving 
 the tube a short distance from the pole had the effect of rendering the arc 
 more diffuse, but not of otherwise altering its character. 
 
 (2) The direction of the current in the tube was then changed ; and, with- 
 out the magnet, the ring electrode was surrounded by a diffused violet glow ; 
 while the straight wire gave forth a faint salmon-coloured stream of light, 
 spreading up to the ring. 
 
EFFECT OF MAGNET ON PLUCKER (AIR-) TUBE. 
 
 151 
 
 On excitation by the magnet (the positive pole being now placed between 
 the armatures), the violet glow of the negative pole contracted into a cojnpact 
 mass round the ring electrode. At the same time from the positive pole 
 sprang a set of bright saddle-shaped rings, which increased in size as they 
 advanced ; and spreading upwards with a rapid but smooth motion towards 
 the negative pole, closely approached to, but never actually came in contact 
 with, the violet glow. The positive end of the tube was otherwise but 
 slightly lighted, and the sudden appearance of this brilliant stream of rings 
 of light was very striking. A single bright ray was also seen running from 
 the positive wire, in a somewhat transverse course, along one side of the tube. 
 AVhen wire-contact with the magnet ceased, so that it was not excited, the 
 rings ran back in succession to the positive pole and disappeared, and by 
 making and breaking contact they were caused to advance and retire at will. 
 They were accompanied by a waving or vibratory motion, and were evidently 
 of the same character as the smaller stria3 or bands mentioned as seen when 
 the ring formed the positive pole. The general appearance was that of a 
 hollow cone of light (the base towards the negative pole), composed of bril- 
 liant rings with dark spaces between, which appeared and expanded under 
 the magnetic influence, and contracted and disappeared on its removal. The 
 rings did not appear to be flat disks, but were somewhat curved or saddle- 
 shaped. They reminded one much of the diatom Campylodiscus spiralis; 
 that is to say, they were apparently flat if looked at from above, but like a 
 figure of 8 when viewed sideways, the peak of the saddle forming a kind of 
 brilliant point or apex. 
 
 All this is difficult to describe ; but an illustration from a sketch made of 
 the tube is given on Plate XVII. fig. 2. 
 
 (3) The negative pole (straight electrode) was then placed vertically on 
 one of the poles of the electro-magnet. On excitation, the violet glow was 
 contracted into a small upright brush or column of bright light, with a slight 
 inclination to curvature. 
 
 (4) The same Pliicker tube was laid horizontally across the poles of the 
 electro-magnet (without armatures), the respective electrodes being above 
 each pole. 
 
 From the negative (straight electrode) pole sprang a dense and compact 
 arc of violet light, in the direction of the magnetic curves, which terminated 
 at the upper circumference of the tube, but which, if prolonged, would have 
 followed the curves to the opposite pole. The stream from the positive pole 
 was very considerably brightened, as in the other experiments, but did not 
 
 Magnet 
 effects de- 
 scribed. 
 On negative 
 pole. 
 
 Kings from 
 positive 
 pole de- 
 scribed. 
 
 Effects on 
 rings of 
 making 
 and break- 
 ing contact 
 with 
 magnet. 
 
 Shape of 
 rings de- 
 scribed. 
 
 Negative 
 pole placed 
 vertically 
 on the 
 magnet. 
 
 Tube laid 
 horizontally 
 across poles 
 of magnet. 
 
 Effects pro- 
 duced. 
 
152 
 
 MAGNETO-ELECTRIC EXPERIMENTS. 
 
 Effects like 
 those ob- 
 tained by 
 Gassiot. 
 
 Pliicker 
 tube (tin 
 chloride). 
 Lighting-up 
 described. 
 
 Effects of 
 magnet 
 upon the 
 stream. 
 
 Peculiar 
 noise within 
 the tube. 
 
 Spectrum] 
 described. 
 Without 
 magnet. 
 
 appear in the form of rings or waves. It assumed that of a bright steady 
 continuous glow, which formed round the tube a not perfectly continuous, 
 but distinct and well-marked, spiral. This form of discharge seems connected 
 with the peculiar contour of the rings mentioned in experiment 2. One 
 might, indeed, conjecture the spiral-shaped glow to be a ring of light extended 
 or drawn out towards the negative pole. 
 
 Experiment No. 2 seems in result very like that of Gassiot's with his grand 
 battery and the Royal Institution magnet, the effects (though of course upon 
 a smaller scale) being similar to those obtained by him. 
 
 Effect of Magnet on Pliicker Tube (Tin Chloride). 
 
 A large Pliicker tube was examined, which had a bulb attached at each 
 end, communicating with the central portion by a narrow neck or constric- 
 tion. On connexion with the small coil, a narrow stream of pale diffused 
 cobalt-blue light ran along the whole tube, from point to point of the elec- 
 trodes, the positive wire at the same time glowing with an aura of amber- 
 yellow light. (See Plate XVII. fig. 3, where the narrow stream of light is 
 shown by dotted lines.) At the two necks or constrictions the stream of 
 light was perceptibly brightened. 
 
 When the magnet was connected, the stream in the positive bulb was not 
 much changed, but only slightly bent. In the central partition of the tube 
 and in the negative bulb, the stream of light was broken and split into a 
 number of smaller streams, and at the same time bent or forced against the 
 sides of the tube. (See Plate XVII. fig. 3.) 
 
 In the central partition, the blue streamlets were accompanied by a number 
 of spark-like threads of golden light, which shone out among them as the 
 whole vibrated against the side of the tube ; at the same time a peculiar 
 pattering, as of a miniature hail-storm within the tube, made it ring with a 
 slightly metallic tinkle. 
 
 The direction of the bending or deflection of the stream was at right angles 
 to the axis of the poles of the magnet, and changed from side to side of the 
 tube as the direction of the current from the coil was varied. 
 
 In the positive bulb the stream, instead of joining the point of the electrode, 
 left this and ran along one side of the whole length of the wire. (See effect, 
 Plate XVII. fig. 3.) The spectroscope was applied to the neck of one of the 
 bulbs where the stream was bright. Without the magnet a faint continuous 
 spectrum, mainly of the blue and green, with very slight traces of the yellow 
 and red, was seen. Upon this, five or six faint but sharp and metallic-looking 
 
EFFECT OF MAGNET ON TIN-CHLOEIDE TUBE. 
 
 153 
 
 lines were seen. On the magnet being excited, the continuous spectrum was 
 not changed ; but the sharp lines shone out brighter and clearer, one in the 
 blue being especially conspicuous. These lines were measured with a mi- 
 crometer ; and their places being compared with Lecoq de Boisbaudran's 
 " Spectres lumineux," they were easily recognized to be those of tin. On 
 each excitation of the magnet the same brightening of the lines took place. 
 
 Effect of Magnet on Tin-Chloride Geissler Tube. 
 
 We then examined a Geissler tube, marked Sn C1 4 . When first excited 
 by the small coil, the spark passed freely. The glow in the bulbs was of a 
 diffused, light purple tint ; the positive electrode had a bright yellow glow 
 around it. The capillary stream was of a sharp green-yellow, at times 
 brightening up to a metallic-looking green. When the magnet was first 
 employed, the tube distinctly and permanently brightened up throughout. 
 
 The negative bulb was not much changed in appearance; but in the 
 positive bulb a curious permanent and steady cloud-like spiral, of a purple 
 colour, made its appearance, and lasted while the tube was under the magnetic 
 influence. (See Plate XVII. fig 12.) After a short time the tube seemed to 
 lose a great deal of its conducting-power, and to light up in a feeble and 
 intermittent manner, brightening only when the coil was made to work its 
 best. While in this condition, the magnet (which had been previously dis- 
 connected) was excited, and at once what moderate glow was still shining in 
 the tube was totally extinguished. At first it was thought some accident 
 might have happened to the conducting-coil wires ; but repeated trials satisfied 
 us that the effect was due to the magnetic influence alone. Efforts were 
 made, by looking to the coil and battery, to brighten up the tube as at first, 
 but they quite failed ; and it was evident some change had taken place in its 
 conducting qualities. This tube was accidentally broken, so that we had no 
 opportunity to renew the experiments. 
 
 We subsequently tried another tin-chloride tube, purchased of Mr. Brown- 
 ing. This lighted up like the former tube, but brighter. There was an 
 amber glow at the junction of the negative bulb which adjoined the capillary 
 part. This was lost on putting on the magnet. At the same time a percep- 
 tible pattering ringing noise was heard in the tube, and metallic-looking 
 threads of light ran through the bulbs. 
 
 Without the magnet, the spectrum was a continuous faint misty one, with 
 bright lines of tin occasionally flashing up. With the magnet, the tin lines 
 at once shone out bright, strong, and clear upon a black background, the 
 change in effect being very marked. 
 
 With the 
 
 magnet 
 
 excited. 
 
 Geissler 
 tube, Sn C1 4 , 
 examined. 
 Glow de- 
 scribed. 
 
 Effect of 
 the magnet. 
 
 Spiral 
 formed in 
 positive 
 bulb. 
 
 Glow in 
 tube extin- 
 guished. 
 
 Another tin- 
 chloride 
 tube tried. 
 Glow de- 
 scribed. 
 
 Spectrum 
 described. 
 
154 
 
 MAGNETO-ELECTEIC EXPEEIMENTS. 
 
 Large phos- 
 phorescent 
 bulbed tube. 
 Lighting-up 
 described. 
 
 Spectrum 
 described. 
 
 Glow when 
 discharge 
 stopped, de- 
 scribed. 
 
 Comparison 
 with SO 3 
 Geissler 
 tube. 
 
 Effects in 
 bulbs on 
 lighting-up 
 the tube 
 described. 
 
 Effects of 
 reversal of 
 the current. 
 
 After- glow 
 restored by 
 passing of 
 current. 
 
 Effect of 
 reversal of 
 current on 
 positive- 
 pole glow. 
 
 CHAPTER XVII. 
 
 EFFECT OF MAGNET ON BULBED PHOSPHOEESCENT TUBE. 
 
 Mr. JOHN BROWNING kindly lent me a large phosphorescent tube with five 
 bulbs, said to be filled with anhydrous sulphurous-acid gas (SO 2 ). (See 
 Plate XVIII. fig. 1.) This tube lighted up beautifully with the large coil. 
 The connecting tubular parts of it were filled with a bright, beaded, transparent, 
 rosy light ; while the bulbs glowed with a more opaque blue-tinted effect. The 
 spectrum of the tubular part was found to agree exactly with the principal 
 bright band seen in a SO 3 Geissler tube. The spectrum of the bulb-glow 
 was a faint green-blue continuous one, with bright bands or lines faintly flash- 
 ing up at times. When the discharge was stopped, the tube still glowed with 
 a moderately bright, opaque, grey-green light. This glow gradually faded out, 
 always commencing with the bulb forming the negative or violet pole, and so 
 dying out, bulb by bulb, towards the positive pole. The negative-pole bulb 
 at times was, on suddenly stopping the current, hardly lighted at all, the other 
 bulbs being luminous. 
 
 (1) We compared the large tube with a SO 3 Geissler tube, by means of a 
 comparison-prism on the slit, with the result before detailed. The Geissler 
 tube, however, showed no after-glow. 
 
 (2) We lighted up the Browning tube with the large coil. The negative 
 bulb was always the least filled with the blue opaque vapour, and the other 
 bulbs increased in vapour-density in the order they approached towards the 
 positive bulb. When the current was reversed, so that the negative and posi- 
 tive glow changed places, the negative bulb still remained transparent, 
 although the positive opaque glow had (presumably) been thrown into it. 
 When the after-glow had quite disappeared in the bulbs, it was again 
 strongly restored, by the passing of the current for a few seconds only^through 
 the tube. 
 
 (3) The tube was well excited, and the four bulbs (other than the negative 
 one), upon stopping the current, glowed strongly. The current was then sent 
 through reversed, so as to throw the negative glow for a few seconds into the 
 positive bulb. The after-glow in the positive bulb was at once extinguished. 
 On once more reversing the current, it was only restored after a certain 
 amount of continuance of the positive stream. 
 
XVIII. 
 
 POLE.. 
 
 PHOSPHORESCENT TUBE. 
 
 WIDE 
 AIR TUBE 
 
 LARGE TUBE. + 
 
 GLOBULAR RECEIVER. 
 
 BELL GLASS 
 RECEIVER. 
 
 1 - 
 
 VIBRATING i STREAM. 
 
 BELL 
 RECEIVER 
 
 BRASS PUMP PLATE. 
 
 ROTATING 
 Fi A. 13. \ STREAM 
 
 IRON DISCS 
 LET IN. 
 
 AURORA VOGEL . 
 
 SCHUSTERS OXYGEN TUBE. 
 
 0. VIOLET POLE. 
 
 Oa U. 
 
 Op. <>v 
 
 i. n. in. 
 
UNIVERSITY) 
 
EFFECT OF MAGNET ON BULBED PHOSPHOEESCENT TUBE. 155 
 
 The time during which the negative glow was thrown into the positive bulb 
 did not appear sufficient to have heated it. After rapidly changing the 
 direction of the current several times and then stopping it, the three central 
 bulbs alone had an after-glow, the two extreme ones having none, being both 
 equally transparent. 
 
 (4) A moderate heat from a spirit-lainp was applied to the centre bulb (a) 
 while the current was on ; and also (b] when this was stopped, and the bulb 
 glowed. In the first case the bulb was found to get more transparent ; and in 
 the second case the after-glow disappeared in a proportionately shorter time 
 in the heated bulb than in the others. To test the result of cooling the bulbs, 
 the negative-pole bulb and also the central one were each subjected to the 
 action of ether-spray, and also of ether and water-spray mixed. This was 
 done, (a) when the current was passing, and (b) when it was stopped and the 
 glow only was in the bulb. The bulbs were cooled until a marked cold effect 
 to the touch was produced. We did not notice any difference in the behaviour 
 of the bulbs so treated as compared with the others, either when the current 
 was passing or in the case of the after-glow. 
 
 (5) We placed the negative-pole bulb between the conical points of the 
 armatures, and excited the magnet. The negative glow contracted itself into 
 a condensed violet-tinted crescent, in accord with the magnetic curves. The 
 positive glow of the same bulb lost its beaded (stratified) character, and was 
 condensed into a bright stream of light, which latter protruded from the small 
 inner tube and formed a spreading spiral set of cloud-rings within the bulb 
 (see Plate XVIII. fig. 2). The action of the magnet seemed to be exercised 
 in subduing the stratification, condensing the glow into a bright stream of 
 light, and forcing the latter to " tail over " at each extremity of the tubular 
 joints into the bulbs this effect extending even so far as the second bulb. 
 
 When the positive bulb was placed between the poles of the magnet, the 
 glow was simply condensed into a bright stratified stream, which flew to either 
 side of the bulb. 
 
 (6) a. Bulb No. 4 (see Plate XVIII. fig. 1) was placed between the poles 
 of the excited magnet, and the current was passed and then stopped. The 
 glow in that bulb faded away out of its order, and earlier than in ordinary 
 cases (nearly as soon as No. 2). 
 
 b. The same and other bulbs were tested in a similar manner. In all cases 
 the bulb influenced by the magnet, when the current was stopped, was found 
 perceptibly fainter in after-glow. 
 
 c. The tube was arranged with one of the bulbs between the poles of the 
 
 Effect of 
 change of 
 current on 
 the three 
 central 
 bulbs. 
 
 Effect of 
 heat on the 
 bulbs. 
 
 Effect of 
 cooling by 
 eth/er-spray. 
 
 Negative- 
 pole bulb 
 between the 
 armatures 
 of magnet. 
 Effects on 
 negative 
 and positive 
 glow. 
 
 Effect of 
 magnet on 
 glow in bulb 
 No. 4. 
 
 Other bulbs 
 tested in 
 similar 
 manner. 
 
156 
 
 MAGNETOELECTKIC EXPEKIMENTS. 
 
 Effect of 
 magnet 
 upon the 
 after-glow 
 itself. 
 
 Mr. Thomp- 
 son's experi- 
 ments on 
 action of 
 magnets 
 upon liquid 
 rings. 
 
 Mr. Ladd's 
 explanation 
 of some of 
 the pheno- 
 mena ob- 
 served. 
 
 Tubes con- 
 taining 
 phospho- 
 rescent 
 powders 
 described. 
 
 Lighting-up 
 of the tubes 
 described. 
 
 unexcited magnet ; the current was passed and stopped, and the after-glow 
 obtained. The magnet being then quickly excited, the after-glow in the bulb, 
 under its influence, faded out ; and the bulb became transparent, perceptibly 
 sooner than under ordinary circumstances. We tried this several times, with 
 the same result in each case. 
 
 Note. In relation to these experiments, it may be mentioned that Mr. S. 
 P. Thompson, of Bristol, is reported to have studied the action of magnetism 
 upon rings of coloured liquid projected through water, and to have observed 
 their retardation and partial destruction in passing through a powerful mag- 
 netic field. 
 
 Mr. Ladd has suggested to me that some of the phenomena produced indi- 
 cate a driving of the gas in the direction from the negative to the positive 
 pole a theory which is supported by the action of the magnet on the bulbs, 
 if this be considered a repulsive one as regards the gas influenced. 
 
 Effect of Magnet on small Phosphorescent (powder) Tubes. 
 
 We examined six vacuum-tubes containing phosphorescent powders, which, 
 upon exposure to sunlight and removal to the dark, or after passing of the 
 electric current over them, continued to glow in the tubes after the exci- 
 ting cause had ceased. They were of thin glass, and of equal calibre 
 throughout. 
 
 One was 6-| inches long and -f inch in diameter, and had no label ; 
 the other five were 7J inches long and \ inch in diameter, and were labelled 
 respectively : 
 
 Strontium vert, 
 
 jaune, 
 Calcium violet, 
 
 orange, 
 
 vert-bleuatre. 
 
 The powders in tubes of this description are said to contain either sulphide 
 of strontium, or calcium, or sulphate of quinine. The first-mentioned tube 
 shone with a white and bright light, and probably contained the latter sub- 
 stance. The general effect of the current on the tubes was similar in all 
 cases. Under a sufficiently strong current, they lighted up with a brilliant, 
 slightly green-white glow ; in which, however, by looking sideways, it was 
 possible to detect a delicate rosy tint. Any colours beyond these in the tubes 
 seemed to depend on the powders enclosed in them. When the current was 
 
EFFECT OF MAGNET ON PHOSPIIOEESCENT TUBES. 
 
 157 
 
 stopped, the powders alone glowed in accordance with the colours mentioned 
 on the labels, the rarefied gas or air in the tubes not giving any after-glow, as 
 in the case of the sulphurous-acid tube. When the f-diameter tube was 
 excited by the small coil, the effect of the magnet was to entirely suppress 
 and extinguish the glow. When this and the other tubes were worked with 
 the larger coil, the spectrum, without the magnet, was bright and continuous, 
 either showing no lines or else very faint traces of them, and, extending through 
 the whole range of colours was brightest in and about the green. 
 
 With the magnet excited, a bright line of pink light was condensed against 
 the upper side of the tube ; while the glow in the tube generally became very 
 decidedly fainter, except at the electrodes, which still preserved a certain 
 amount of brilliancy. The spectrum also was much changed. The bright 
 continuous glow became much fainter, and many sharp and fairly bright lines 
 were seen upon it. These lines were, as to character, not easy to recognize. 
 Hydrogen (F) was, however, plainly distinguished ; and other lines, which we 
 considered to be N, were common to all the tubes. Some lines were also 
 remarked as being, without the magnet, not so constant. 
 
 Calcium orange and calcium violet, compared for spectra, were identical ; 
 the two strontium tubes hardly so, but with strontium vert a bright continuous 
 spectrum mainly hid the lines. 
 
 Strontium jaune and calcium orange were not alike; strontium vert and 
 calcium violet differed. Calcium orange and calcium vert-bleuatre were 
 considered alike ; but the comparison was not easy, as the calcium vert 
 was bright, and the lines were only seen faintly upon the continuous 
 spectrum. 
 
 In order not to shift the powders, the tubes were laid horizontally, and two 
 spectra simultaneously examined across the tubes. 
 
 Effect of 
 
 magnet on 
 
 f-diameter 
 
 tube. 
 
 Spectrum 
 
 without 
 
 magnet. 
 
 Magnet 
 effect on 
 glow. 
 
 Same on 
 spectrum. 
 
 Tubes exa- 
 mined and 
 compared 
 for spectra. 
 
 Lighting-up Tubes with One Wire only (Marquis of Salisbury's 
 
 Observations). 
 
 The vacuum-tubes employed were examined in the usual way, but one wire 
 only was connected with an electrode. The other wire was attached to the 
 end of a glass rod, and circuit was from time to time completed while the 
 tube was before the spectroscope. 
 
 The large coil was used. In all cases, with the one wire, the glow was very 
 faint as compared with that of the closed circuit. 
 
 (1) Ether Vapour. With both wires, in company with the usual bright 
 
 T2 
 
 One wire 
 only con- 
 nected with 
 an elec- 
 trode. 
 
158 
 
 MAGNETO-ELECTEIC EXPEEIMENTS. 
 
 Ether va- 
 pour. 
 
 Coal-gas. 
 Nitrogen. 
 
 Hydrogen. 
 
 Oxygen, N 
 andH. 
 
 Water-gas. 
 
 Turpentine 
 vapour. 
 
 bands of the carbon spectrum, shading-off towards the violet, the H lines were 
 very sharp and brilliant. With the one wire only, the carbon bands were left 
 faintly shining, with both sides nebulous alike, and with no shading-off towards 
 the violet. (We were not quite sure whether this was not the effect of the 
 reduction of the light.) The H lines, though originally stronger than the 
 carbon bauds, quite disappeared from the spectrum. 
 
 (2) Coal-gas. The same effects were produced ; but we thought we could 
 detect very faint traces of the H lines. 
 
 (3) Nitrogen. The N lines, as well as those of H (also seen in the 
 tube), were much fainter with one wire, but the H lines more so in pro- 
 portion. 
 
 (4) Hydrogen. Only a marked reduction in brilliancy of the whole 
 spectrum. 
 
 (5) Oxygen. AM impure tube, showing O (some of the lines hydrocarbon "?), 
 N, and H spectra simultaneously. With one wire the O lines still remained 
 fairly bright, the N and H being only faintly seen. 
 
 (6) Water-gas. Same effect. 
 
 (7) Turpentine Vapour. Same effect as ether, but the H lines could be 
 faintly seen. 
 
ACTION OF THE MAGNET ON THE ELECTRIC SPAKK. 
 
 159 
 
 CHAPTER XVIII. 
 
 ACTION OF THE MAGNET ON THE ELECTRIC SPARK. 
 
 THE magnet was excited with two plates of the large battery, and the larger 
 coil with the other two plates, the action in both cases being strong. 
 
 1. A spark from the coil was passed between two platinum wire electrodes, 
 about three centimetres apart. 
 
 It consisted centrally of a thin stream of bluish-white light, vividly bright, 
 around which was seen a narrow, uniform, diffuse, yellow-tinted aura, which 
 accompanied the spark in all its movements. The spark always struck 
 across from the extreme points of the electrodes (see Plate XVII. fig. 5). 
 
 2. On being placed between the conical poles of the excited magnet the 
 bright thread of the spark did not change ; but instead of the inconsiderable 
 yellow-tinted aura which accompanied the unmagnetized spark, there now 
 struck out, at right angles to the magnet-poles, a thin rosy-tinted half-disk 
 of aura-like flame. This extended aura ran considerably along each electrode, 
 though the spark proper still struck from the points. 
 
 The aura was somewhat larger in extent upon one electrode than on the 
 other. In the first case, it sprang from a considerable number of minute 
 illuminated points ; on the other electrode, these illuminated points were 
 fewer in number, and the flame was more purple in tint. Reversing the 
 current these effects were reversed. The aura was uniformly thin and disk- 
 like, and the curved edge remarkably true in shape (see Plate XVII. fig. 6). 
 
 The lateral direction of the aura was changed when the current was 
 reversed. 
 
 3. The aura was found not proportionate to the length of the spark. 
 When the electrodes were approached, so as to very much shorten the spark, 
 the aura still sprang out to a distance and extent quite out of proportion to 
 the length of the spark. Even when the electrodes were approached so 
 close that the spark was very short indeed, still, under the magnetic influence, 
 a very considerable aura made its appearance. 
 
 4. Upon working the coil-break, it was found that in proportion as the 
 contact screw was drawn apart from the break, so the aura gradually 
 
 Apparatus 
 employed. 
 
 Spark 
 and aura 
 described. 
 
 Effect of 
 magnet 
 upon the 
 aura. 
 
 Extended 
 aura de- 
 scribed. 
 
 Aura not 
 propor- 
 tionate to 
 length of 
 spark. 
 
160 
 
 MAGNETO-ELECTEIC EXPERIMENTS. 
 
 Effect of 
 working coil- 
 break upon 
 the aura. 
 
 Spark taken 
 in glass 
 bulb. 
 
 Aura could 
 be blown 
 away from 
 the spark. 
 
 Effect of 
 withdrawing 
 spark from 
 central posi- 
 tion between 
 the poles. 
 
 Magnet had 
 no effect 
 upon con- 
 densed 
 spark. 
 
 diminished in extent, until at last, by continuing to increase the distance 
 between the screw and the break, a point was reached when thin bright 
 sparks, without any aura, passed. Upon the screw being worked up closer, 
 thicker sparks passed, and the aura again made its appearance. As the 
 aura diminished in size it gradually changed in tint from yellowish rose- 
 pink to purple. 
 
 5. The spark was taken in a glass bulb, the tube in which it was blown 
 being open at both ends, with the same effect as in the open air. 
 
 6. A plate of glass was laid on the poles of the magnet, and the spark was 
 passed along the poles (in the same direction as the heavy glass was laid in 
 the Faraday experiment). No aura was formed. The points were then 
 moved round, so as to carry the spark at right angles to the poles, and the 
 aura was formed as before. 
 
 7. The aura, it was found, could be blown away at right angles to the 
 spark. When strongly urged, it assumed the shape of a flickering tongued 
 curtain of flame, flying away in the contrary direction to that from which the 
 current of air proceeded, and again returning to its original shape as the 
 impulse was removed. The spark proper was not influenced (see Plate XVII. 
 fig. 8). 
 
 8. As the spark was withdrawn from its central position between the 
 poles of the magnet, the convex edge of the aura became gradually less 
 perfect, and assumed a ragged and broken-up appearance, the inequality at 
 times amounting almost to jets or flickering sprays of light. The spark was 
 also slightly curved away from the electrodes (see Plate XVII. fig. 7). 
 
 9. A condenser of four coated plates was introduced into the circuit, causing 
 a sharp brilliant blue-white spark, apparently divided into streams and with 
 no aura. The magnet had no effect whatever upon this form of spark. 
 
TIIK msril \KiiK IN VACUO IN LAEGEB VESSELS. 
 
 1 01 
 
 CIIAl'TKK \1.\. 
 
 TIIK Dl.scilAKiiK IN VA<T<> IN I, A.BGEB VESSELS, AND MAGNETIC 
 
 KI'I'T.rTS TIIKK'KON 
 
 \ TVTK'S iiir-|iuiiip wan used, :nnl !!>. .spark from tho larger coil. The 
 
 exhaustion Cdllld Mdl. In- earned \er) I'lir. 
 
 (1) A globular receiver was used, having brass caps fur exhaust ion, and 
 I'hiinimii wires- passing through the opposite sides for electrodes (see Plate 
 X VIII. (ig. <i). With partial e\lians|inn, from tho po.sitivo electrode pro- 
 ceeded long, sharp, bright, rosy sparks, striking in y.ig/.ags across the receiver. 
 I'Yom the negative lenniiial sprang a laiv.ei nuinlier of bluer and nioie diffuse 
 streams of liyjit, lik.- pul. i \vehs; and the.se were en\eloped, for a short 
 .lr.la.nee lioin (.lie terminal, in a slight misty aura. Both UOts pla> ed round 
 the sides of the j'.la.ss us \\ell MS across. 
 
 (2) A boll-shaped receiver, with terminals inserted at the sides and one 
 also at the top, wan next used (see 1'lilto XVIII. %. U). When the : id.- 
 terminals uere employed, the ell'ect was much the same as in the last case. 
 \\ hen tin- lop I. i nun. 1 1 \\as ir-ed for one wire (the ot her wi ro hein^' connected 
 \\illi tin- pump-plate) a single si ream of bright rosy li^hl ran from the upper 
 ii i m ina I to I lie plate. First striking the central part of the plate, I he itroain 
 
 then glided louanls o| the lateral terminals, and so to the ed^e of I In 
 
 notirer. Ml^i paill\ dischar-'jn:; itself by contact, \\ilh the terminal, the 
 itroaill as la|Mill\ retivaled to Ihe eenlie ot' I he plale a^aill this ell'ect. bcill^ 
 from lime to time repealed \\hile the cnrretil \\as passing. Tin- < in lent 
 liein;; reversed, a number of bright, lint \\eakcr and more ditfnsed, sin am: o| 
 
 I ! Ill had the appeal a nee of :,l n n ,| i n : . I i the npp. i elect i -ode, and of sinking 
 
 Upon the plale helou . \\ilh a tendency to ll) oil Imm \\heie lhe\ Mini k. Ill 
 
 a similar mannei i.> the -,in-de in-ain liel'ore described. \\ hen- eai h 
 touched the plate a brilliant poinl of light appealed, and a strong paltering 
 noise was heard in the receiver. 
 
 (II) Another hell-shaped rereivci ol i in i la I sha |ie \\ .1 n ed. This had llo 
 
 elecliodes forming a din-el c mnnicalion \\ith tin- interior; bnl, in lieu of 
 
 l\\o ',, I thin sheet, brass wen- cemented, one inside and one oiil- 
 
 (lluliulivr 
 
 n-i-i , 
 
 niciivi<r 
 
 I|MM| 
 
 (Itmcrilx'il. 
 
 i:. n II.-IJK .I 
 
 \\llllnlll 
 
162 
 
 MAGNETO-ELECTKIC EXPEE1MENTS. 
 
 Induction 
 discharge 
 described. 
 
 Long large 
 tube ex- 
 hausted and 
 illuminated. 
 
 Spiral form 
 of discharge. 
 
 Globular 
 receiver 
 again used. 
 
 Phosphores- 
 cent after- 
 glow suc- 
 ceeding the 
 spark. 
 
 Positive 
 wire only 
 attached. 
 
 Negative 
 wire only 
 attached. 
 
 Effect of 
 gradual 
 exhaustion 
 on the 
 discharge. 
 
 side the glass, opposite to one another. On connexion being made with the 
 outside wafer, the effects produced by induction were similar to, and very 
 nearly as strong as, those in the cases where direct communication with the 
 interior of the receiver was made. 
 
 (4) A large tube, 24 inches long and 2 inches in diameter, with ball and 
 point electrodes respectively, was exhausted, and the current passed through 
 it. The effects were similar in most respects to those produced in the 
 globular and bell receivers, but the streams of light assumed a distinctly 
 spiral form in their passage (see Plate XVIII. fig. 5). This tube when placed 
 between the poles of the magnet showed no effect, except a slight conden- 
 sation of the streams of light towards the sides of the glass. 
 
 (5) The globular receiver first described was again used (the Tate pump 
 having been cleaned and working easier). 
 
 () When exhaustion was as good as it could be got, the spark struck 
 across in a single, slightly expanded, stream of rosy light, having a tendency 
 to curve upwards (see Plate XVIII. fig. 6). The electrodes had but little 
 glow round them, only just enough to distinguish the poles apart. When 
 the flow of the stream was interrupted by breaking contact with one terminal, 
 so that sparks passed in succession, we thought we detected a faint blue 
 phosphorescent after-glow succeeding eacn spark. 
 
 (1} The positive wire only was attached to one electrode, the negative 
 being unconnected. A set of faint whity-blue cobweb-looking streams of 
 light spread from the electrode all over the receiver, having a vibratory 
 motion. The spaces between these were dark, and there was no aura the 
 effect being similar, but not quite so bright and pronounced, as when both 
 wires were attached (see Plate XVIII. fig. 7). 
 
 (c) The negative wire only was attached. The cobweb streams were 
 absent, or only shot out very occasionally. The main effect was a straight 
 nebulous stream of violet light, which commenced at the electrode and spread 
 out in a fan-shape towards the lower brass cap of the receiver ; while, at the 
 same time, an aura or glow of similar light, but fainter in quality, spread 
 from the electrode over at least one half of the receiver. This aura would 
 no doubt have filled a small flask (see Plate XVIII. fig. 8). 
 
 (d) When exhaustion was first commenced, both electrodes of the receiver 
 (both wires being connected) threw out spider-web-like streams, as in Experi- 
 ment 1, pale blue from the one pole and somewhat rosy from the other. 
 
 As the exhaustion progressed the pale-blue streams disappeared, while the 
 rosy flickering ones diminished in quantity and extent until ultimately a 
 
THE DISCHAEGE IX VACUO IN LAEGEE VESSELS. 
 
 163 
 
 single rosy stream of light crossed the receiver as in Experiment 5 a. Upon 
 admitting the air, these effects took place in an inverse order the single 
 stream being gradually broken up, and the spider-webs taking its place. 
 
 (e) The exhausted globular receiver was placed upon the poles of the 
 excited magnet, with the stream at right angles to them. Looking across the 
 S. pole of the magnet, the negative electrode was on the left hand, and the 
 positive on the right. The effect of the magnet on the stream was apparently 
 to split it up into several ; but this appearance must have been due to vibra- 
 tion only, as a revolving mirror showed the stream as single. When the 
 current was reversed, the stream which, without the magnet; was somewhat 
 flickering and vibrating, slightly straightened at the positive pole, and the 
 whole stream became steadier. 
 
 (f) Single wires were successively attached to the negative and positive 
 poles, and the cobweb streamers and glow before described obtained. The 
 magnet was found to have no decided effect on either of these. 
 
 (6) The Plvicker air-tube (Plate XVII. fig. 2) was placed between the 
 poles of the magnet, and the negative wire only was connected with the 
 straight electrode. A pale violet glow was seen round this electrode, and 
 another, but rather fainter, glow of a similar description at the ring electrode, 
 the intermediate space being filled with a salmon-coloured light. This violet 
 glow was condensed into an arc by the action of the magnet. Eeversing the 
 current, the violet glow still remained at each electrode, and that between 
 the poles of the magnet was still influenced into an arc. 
 
 (7) An air Geissler tube was substituted for ( the Pliicker tube, with very 
 much the same result. Whichever wire was attached, a violet glow appeared 
 at the connected electrode, and a fainter one of the same character at the 
 other; and the magnet influenced both. The connecting salmon-coloured 
 glow was faint. 
 
 (8 a) The globular receiver had some phosphoric anhydride shaken into 
 it ; and it was then exhausted. The cobweb streamers and violet glow each 
 appeared according to which wire was connected. There was no marked 
 difference between the receiver with the anhydride and without ; except that in 
 the former case the streamers and glow were reduced in extent and strength, 
 and were comparatively faint. 
 
 (b) The anhydride having been washed out, first with plain and afterwards 
 with distilled water, some drops of the latter were allowed to remain in the 
 receiver. On exhaustion a vapour-cloud was formed, and the discharge passed 
 (both terminals being connected with the coil) through this. The rosy stream 
 
 z 
 
 Globular 
 receiver 
 placed on 
 poles of the 
 magnet. 
 
 Magnet 
 effect. 
 
 Single wires 
 attached. 
 
 Pliicker tube 
 placed be- 
 tween poles 
 of magnet 
 with nega- 
 tive wire 
 only at- 
 tached. 
 
 Geissler 
 tube substi- 
 tuted, with 
 similar 
 results. 
 
 Globular 
 receiver 
 treated with 
 phosphoric 
 anhydride. 
 
 Discharge 
 in water- 
 vapour 
 described. 
 
164 
 
 MAGNETO-ELECTRIC EXPERIMENTS. 
 
 Large bell- 
 receiver and 
 plate 
 described. 
 
 Receiver 
 exhausted 
 and stream 
 of light 
 formed. 
 
 Effect on 
 same when 
 magnet 
 excited. 
 
 Experi- 
 ments with 
 the conical 
 armature 
 removed. 
 Vibrating 
 stream. 
 
 of light was formed as usual, but was more flickering and unsteady. As the 
 exhaustion was lessened, the rosy stream disappeared, and the cobweb streams 
 began to fill the receiver. These were, however, not so bright and sharp as 
 in a dry receiver, but were faint and broad ; while some diffused and nebulous 
 streams of light, running (slightly bent) from pole to pole, and from J to f of 
 an inch broad, were intermixed with them. When one wire only was con- 
 nected, the glow and streamers from the electrode were very faint. 
 
 (9) A large bell-shaped receiver, 11x8 inches, was next used. It was 
 open at the bottom, which was ground as usual ; and had a small opening at 
 the top, also carrying a ground edge. A solid brass plate was prepared, 
 ground only round the edge (in order to take the receiver), and in the centre 
 of this brass plate were inserted two disks of soft iron, corresponding in 
 position and size with the poles of the Ladd electro-magnet (see Plate XVIII. 
 fig. 11). When this plate^was placed on the magnet, the poles and disks were 
 in contact ; and the disks became N. and S. poles within the receiver. A 
 small brass plate carrying a tap and exhaust-tube, and a binding-screw for 
 attaching an electrode within, closed the receiver at top. The receiver and 
 plate being placed on the magnet-poles, the former was exhausted until the 
 discharge became a rosy slightly-diffused stream of light ; with a small un- 
 illuminated space between it and the negative pole, where the usual violet 
 glow appeared round the wire. 
 
 This stream of light was used for the experiments after detailed. In some 
 cases the conical armatures were placed within the receiver, in others the 
 disks alone were used as the, magnetic poles. 
 
 (a) With the apparatus arranged as shown on Plate XVIII. fig. 10, and 
 the magnet excited, a violet glow appeared round the end of the wire 
 which was negative. A small unilluminated space then intervening, the 
 stream ran in a curve between the wire and the armature, which latter was 
 positive. The stream was not steady and had a tendency to rotate; but as 
 this was better observed with the disks only, it is described further on. 
 
 (b) The conical armature within the receiver was removed, and the stream 
 allowed to connect with the centre of the pump-plate. When the magnet 
 was excited, the stream was violently projected at right angles to the poles, 
 with a vibrating movement to either side according to the direction of the 
 current. When the wire was positive the movement was towards the left, 
 with a slight inclination towards the N. pole. When the wire was negative 
 the movement was to the right, but in a rather strong curve towards the 
 N. pole. The vibrating motion was very distinct, and gave the appearance 
 
THE DISCHAEGE IN VACUO IN LARGER VESSELS. 
 
 1G5 
 
 of six or seven streams running off at regular intervals (see Plate XVIII. 
 fig. 12). 
 
 (c) The wire was next placed over the centre of the disk forming the S. 
 pole. With the wire negative and the pole positive, rotation of the stream 
 was decidedly, but not very strongly, from right to left from the centre of the 
 plate (as the hands of a watch). With the wire positive and the pole 
 negative, rotation was strongly left to right, with a disposition to spiral twist 
 in the stream (see Plate XVIII. fig. 13). 
 
 (d) The wire was placed over the disk forming the N. pole. With the 
 wire negative and the pole positive, rotation of the stream was left to right. 
 With the wire positive and the pole negative, rotation was right to left (see 
 Plate XVIII. fig. 14). 
 
 (e) The stream was thrown across the receiver from the lateral binding- 
 screws above, and at right angles to, the disks, and afterwards in the opposite 
 direction, i. e. along them. In neither case was there any marked change 
 when the magnet was excited. 
 
 (/) The conical armatures were placed with the pointed ends upon the 
 disks in the receiver, and the stream thrown above and along them. It 
 diverged one part running straight across between the electrodes, whilst 
 another stream and some cobwebs ran from each electrode to its nearest pole. 
 The streams and cobwebs flickered a good deal. There was no marked 
 change when the magnet was excited. 
 
 Rotating 
 wire over 
 S. pole. 
 
 Same over 
 N. pole. 
 
 Stream 
 thrown 
 across the 
 receiver 
 above the 
 magnet- 
 poles. 
 
 Some of Baron Beichenbach's Magnetic Eesearches tested. 
 
 In 1846 Dr. W. Gregory published an abstract of Baron Eeichenbach's 
 ' Researches on Magnetism and on certain allied subjects, including a supposed 
 new Imponderable.' 
 
 From a paragraph in this work, it would seem that the Baron considered 
 his observations as tending to an explanation of the Aurora Borealis ; and, 
 since it was generally admitted that these phenomena occur within our 
 atmosphere, that there appeared a great probability of Aurora? being visible 
 magnetic lights. The Baron, in the original work, fully describes the Aurora 
 Borealis ; and concludes it must be similar in its nature to the flames of light 
 seen streaming from the magnet-poles by Mdlle. Keichel and other sensitive 
 patients of the Baron's. It is unfortunate that these flames were only seen 
 by certain " sensitive " persons. The drawings given of them, too, show no 
 analogy to the magnetic curves. 
 
 z2 
 
 Baron Rei- 
 
 chenbach's 
 
 researches. 
 
 Aurorse con- 
 sidered to be 
 magnetic 
 lights. 
 
 Elames 
 seen by 
 " sensitive ' 
 persons. 
 
166 
 
 MAGNETO-ELECTRIC EXPERIMENTS. 
 
 Magnet 
 tested for 
 such flames. 
 
 Mr.Brooks's 
 experiments 
 on action of 
 the magnet 
 on a sensi- 
 tive photo- 
 graphic 
 plate. 
 
 Having the opportunity of a powerful magnet in that used during our 
 tube-experiments, we made an attempt to detect the Baron's magnetic flames, 
 on or around the poles of our magnet, in a perfectly dark room. Arrange- 
 ments were made to silently connect and disconnect the battery with the 
 magnet, without the knowledge of any one except the operator. The experi- 
 ment proved a complete failure ; no flames or discharges of light of any kind 
 were to be seen. The observers were five in number, two gentlemen and 
 three ladies, but not one of the party proved " sensitive." 
 
 Some experiments made by Mr. W. Brooks, and detailed in a paper read 
 by him before the South London Photographic Society, seem to corroborate 
 (to a certain extent) the statements made by the Baron in regard to the 
 influence of the magnet on a sensitive photographic plate. 
 
 Remembering, however, how it has been demonstrated that light may be 
 " bottled up " as an actinic source for a considerable period of time, it seems 
 a question whether the images obtained were not due to some such source 
 rather than to any magnetic aura. 
 
SUMMARY OF THE EXPERIMENTS. 1G7 
 
 SUMMARY OF THE FOREGOING EXPERIMENTS AND THEIR RESULTS. 
 
 Chapter XIV. Action of magnet on glow and spectrum of Geissler gas Summary of 
 vacuum-tubes demonstrated. 
 
 Chapter XV. Action of magnet on glass capillary tube negatived. 
 Faraday's experiment with heavy-glass bar repeated. 
 
 Chapter XVI. Action of magnet on glow in wide air-tube demonstrated. 
 Note on stratification. In Pliicker tube, action of magnet on negative pole 
 (arc formed) and positive pole (Gassiot's rings produced) demonstrated. 
 Effects of magnet upon glow and spectrum of tin-chloride vacuum-tubes 
 demonstrated. 
 
 Chapter XVII. Effect of magnet upon after-glow in a bulbed phospho- 
 rescent tube demonstrated Effect of magnet upon glow in small phospho- 
 rescent (powder) tubes examined. Marquis of Salisbury's experiments 
 (lighting-up with one wire only) tested, and confirmatory results arrived at. 
 
 Chapter XVIII. Action of magnet on aura of electric spark demonstrated. 
 
 Chapter XIX. Effects of magnet on discharges in vacuo in larger vessels 
 demonstrated. Angstrom's flask experiment tested ; same results not obtained 
 unless one wire only was connected. Experiments demonstrating the action 
 of a magnet on an electric stream, viz. vibration between, and rotation round, 
 poles.' Baron Reichenbach's magnetic flames tested without result. 
 
168 CONCLUDING EEMAKKS. 
 
 CHAPTER XX. 
 
 SOME CONCLUDING EEMAEKS. 
 
 IT is usual, in concluding a work on a special subject, to sum up its contents, 
 and to examine the general results arrived at. This, however, it is not easy 
 to do in the present case. The contents of our volume comprise a short 
 history of the Aurora, its qualities and spectrum ; and a statement has been 
 given of the several conclusions at which various observers have arrived as to 
 its character and causes. In the present state of our knowledge of the subject, 
 to add an opinion to these might seem to savour of presumption ; and the 
 questions involved may perhaps be better treated as still sub judice, and as 
 requiring further and fuller evidence before arriving at a verdict. The 
 following observations must therefore be taken rather as further notes and 
 memoranda, than as conclusions. Apart from the spectroscopic questions 
 involved, the oldest and most received theory of the Aurora that of its being 
 some form of electric discharge in the more rarefied regions of the atmosphere, 
 seems to hold its own : and if, as is probable, some form of phospho- 
 rescence is involved in the discharge, M. Lecoq de Boisbaudran's observations 
 on the brightening of the red line under the influence of cold, and the falling 
 of the yellow-green line within a band of phosphoretted hydrogen, come into 
 play ; and a connexion, though slight and imperfect, may be in this respect 
 traced between the discharge and its spectrum. The experiments detailed in 
 
 \ Part II. seem to have an important bearing, as showing the very marked 
 effect of the magnet on the rarefied glow, as well as on the spark in air at 
 ordinary pressure. The well-defined arc formed by the aura of the spark, the 
 flickering jets which replace the even edge of the arc when partially with- 
 drawn from the magnetic influence, and the streamers formed when the aura 
 is blown away from the spark (Plate XVII. figs. 6, 7, and 8), are certainly 
 
 \ highly suggestive of frequent forms of Auroral discharge ; and, but for trial 
 and failure, might lead one to expect results from a comparison of the line 
 air-spectrum with that of the Aurora. The experiments with a wire attached 
 to one electrode only, show how the glow may be affected and varied in colour 
 and character when the discharge is interrupted and incomplete. Differences 
 in electric tension may also considerably vary the character of the discharge. 
 
CONCLUDING KEMAKKS. 169 
 
 The influence of the magnet in exciting and brightening the glow and 
 spectrum of one gas, while it depresses and extinguishes the glow and spec- 
 trum of another gas in the same tube, suggests an explanation of the ob- 
 served variation in intensity, and difference in number, of the Aurora-lines. 
 Intensity of lines depending on temperature, and this again on resistance, 
 and it appearing that resistance is influenced by the magnetic action, the 
 same effects of brightening or depressing of the spectrum are probably pro- 
 duced in the Aurora, as in the vacuum-tubes placed between the magnet- 
 poles. 
 
 In the Marquis of Salisbury's observations, paraffin-vapour gave C and H 
 lines when connected with both poles of the battery, but C lines only when 
 connected with one pole ; and in that case the lines were equally sharp on 
 both sides. These observations (repeated in our experiments) may afford an 
 explanation why the hydrogen-lines are not seen in the Aurora-spectrum; 
 although there can be hardly any doubt that the phenomenon usually takes 
 place in air more or less moist. Professor Angstrom's researches on the violet- 
 pole glow are not entirely corroborated by our experiments ; and it seems 
 doubtful whether his results in the exhausted flask were not obtained from 
 the negative pole only. One great difficulty in the comparison of the Aurora- 
 spectrum with the violet pole of air-tubes and some other spectra (including 
 oxygen), arises from the presence in the latter of broad bands ; and it is diffi- 
 cult to understand how these bands can be aptly compared with the definite, 
 though faint, lines observed by Dr. Vogel and others in the Aurora-spectrum. 
 It must, too, be borne in mind that the conditions under which we may con- 
 sider the Aurora to obtain, are such as can be only very imperfectly imitated 
 in the laboratory. Aurora? also no doubt differ in density and thickness of 
 layer ; and KirchhofTs observation must be remembered : " That if thick- 
 ness of a film of vapour be increased, the lines are increased in intensity, the 
 bright lines more slowly than the fainter ; and it may happen that the spec- 
 trum appears to be totally changed when the mass of the vapour is altered." 
 Were it possible to test with the spectroscope a cloud or film of gaseous 
 vapour corresponding in some degree in density and thickness with an 
 Auroral discharge, we might perhaps get nearer the truth. Mr. Procter also 
 remarks (as we proved in our magnet experiments) : " That frequently very 
 small traces appropriate to themselves the whole of electrical discharges at 
 low pressures, and completely mask the spectra of any other gases present." 
 The oxygen-spectrum, with its possible variation by the conversion of that 
 gas into the allotropic condition termed ozone, seemed at first to afford a 
 
170 CONCLUDING EEMAEKS. 
 
 prospect of close relation to the Aurora-spectrum ; which, however, dis- 
 appeared on closer examination. If nitrogen could be modified in some such 
 way as oxygen is converted into ozone, it might perhaps afford another oppor- 
 tunity for investigation ; but we have no evidence at present of such a change. 
 The spectrum of nitrogen is usually found singularly distinct and persistent ; 
 and, except as varied from band to line by intensity of the discharge, not 
 liable to alteration *. 
 
 Colours of lines are functions of wave-length, subject, however, to the 
 observation that in a weak spectrum the colours lose their intensity. The 
 red line in the Aurora has sometimes been found brighter than the green. 
 It has been suggested that the red and green may be independent spectra; 
 but the variations of tint observed in the capillary of hydrogen and other 
 tubes according to resistance of the current, demonstrate that the varying 
 colours of the Aurora may be connected with the lighting-up of particular 
 parts of the spectrum, and do not necessarily indicate that different gases and 
 spectra are excited. 
 
 Absorption may also play an important part in the nature of the Aurora- 
 spectrum (Zollner's theory that the lines are really spaces between absorption 
 bands). Most gases will give a continuous spectrum under certain circum- 
 stances, even at a low pressure. 
 
 The question of cosmic dust is inviting, but the facts collated hardly 
 warrant at present its probable connexion with the Aurora. 
 
 If Auroras were composed of incandescent glowing meteors, it would be 
 reasonable to expect to find in the spectrum the lines of iron, a metal consti- 
 tuting so prominently the composition of meteorites. No connexion between 
 the iron and the Aurora-spectra is, however, proved ; though it may be 
 suspected. The iron-spectrum, as remarked elsewhere, contains so many lines 
 that some may, as a mere accidental circumstance, closely agree with the 
 Aurora-lines. 
 
 The iron-lines are, it may be remarked, as a rule, sharper and finer than 
 the Auroral lines, though it is possible that these characteristics might vary 
 if the spectrum were obtained in a rarefied medium. Tubes with iron ter- 
 minals are said to evolve a compound gas of H and Fe. I have not had an 
 opportunity to verify this. 
 
 * Dr. Schuster has found that while the line-spectrum of lightning is attributable to N", it has 
 also a band-spectrum, which he considers due to and a slight trace of C0 2 (Phil. Mag. 5th ser. 
 vol. vii. p. 321). 
 
CONCLUDING REMARKS. 171 
 
 It may be added that the comparative faintness of the more refrangible 
 lines of the Aurora-spectrum suggests a feeble resistance to the exciting cur- 
 rent, and a low temperature inconsistent with a meteoric theory ; and this is 
 not contradicted by the brightness of the red and green lines, if these are due 
 to a phosphorescent origin. Expansion of a line is recognized to be dependent 
 on pressure, and consequently the breadth of the green or red lines might 
 indicate the height of the Aurora ; while their brightness or otherwise might 
 also give some idea as to its density. No observations in this direction have, 
 as far as I am aware, been recorded. 
 
 As the general result of spectrum work on the Aurora up to the present 
 time, we seem to have quite failed in finding any spectrum which, as to posi- 
 tion, intensity, and general character of lines, well coincides with that of the 
 Aurora. Indeed, we may say we do not find any spectrum so nearly allied to 
 portions even of the Aurora-spectrum, as to lead us to conclude that we have 
 discovered the true nature of one spectrum of the Aurora (supposing it to 
 comprise, as some consider, two or more). The whole subject may be charac- 
 terized as still a scientific mystery which, however, we may hope some future 
 observers, armed with spectroscopes of large aperture and low dispersion, but 
 with sufficient means of measurement of line positions, and possibly aided by 
 photography, may help to solve. The singular absence of Aurorse has, for 
 some time past, given no opportunity in that direction. May some of my 
 readers be more fortunate in obtaining opportunities of viewing the glorious 
 sky-fires, and assist to unravel so interesting a paradox ! 
 
 2A 
 
APPENDICES. 
 
 APPENDIX A, 
 
 REFERENCES TO SOME WORKS AND ESSAYS ON THE 
 
 AURORA. 
 
 (Most of these are cited in the 'Edinburgh Encyclopaedia' and the 'Encyclopaedia Britannica.') 
 
 Musschenbroeck, Instit. Phys. c. 41. 
 
 ' Trai. Phys. et Hist, de 1'Aurore Boreale,' par M. de Mairan. 
 
 Beccaria, ' Dell' Elettricismo Artif. e Nat.' p. 221. 
 
 Smith's ' Optics,' p. 69. 
 
 D'Alembert's ' Opuscules Mathematiques,' vol. vi. p. 334. 
 
 ' Philosophical Transactions ' as under : 
 
 Paris, 1754. 
 
 Vol. 
 
 1716 406 
 
 1717 584,586 
 
 1719 1099, 1101, 1104, 1107 
 
 1720 21 
 
 1721 180, 186 
 
 1723 300 
 
 1724 175 
 
 1726 128,132,150 
 
 1727. 
 1728. 
 1729. 
 1730. 
 1731. 
 1734. 
 1736. 
 
 245, 301 
 
 453 
 
 137 
 
 279 
 
 53-55 
 
 243,291 
 
 241 
 
 Vol. 
 
 1740 368 
 
 1741 744, 839, 840,843 
 
 1750 319,345,346,499 
 
 1751 
 
 1752 
 
 1753 
 
 1762 
 
 1764 
 
 1767 
 
 1769 
 
 1770 
 
 1774 
 
 1781 
 
 1790.. 
 
 39, 126 
 
 169 
 
 85 
 
 474,479 
 326, 332 
 
 108 
 86,307 
 
 532 
 
 128 
 
 228 
 32, 47, 10 J 
 
 2A2 
 
174 APPENDIX A. 
 
 ' Miscell. Berolinens.' 1710, vol. i. p. 131. 
 
 ' Comment. Petrop.' torn. i. p. 351, torn. iv. p. 121. 
 
 ' Acta Petrop.' 1780, vol. iv. p. 1. 
 
 ' Mem. Acad. Paris,' 1747, pp. 363, 423 ; 1731 ; 1751. 
 
 ' Mem. Acad. Berl.' 1710, vol. i. p. 131 ; 1747, p. 117. 
 
 Schwed. ' Abhandlungen,' 1752, p. 169 ; 1753, p. 85; 1764, pp. 200, 251. 
 
 Bergman, ' Opusc.' vol. v. p. 272. 
 
 ' Americ. Trans.' vol. i. p. 404. 
 
 ' Mem. de Mathemat. et Phys.' torn. viii. p. 180. 
 
 Rozier, vol. xiii. p. 409 ; vol. xv. p. 128 ; vol. xxxiii. p. 153. 
 
 Franklin's Works, vol. ii. 
 
 Weidler, ' De Aurora Boreale.' 4to. 
 
 Nocetus, 'De Iride et Aurora Boreale, cum Notis Boscovisch.' Rome, 1747. 
 
 Chiminello, ' Mem. Soc. Ital.' vol. vii. p. 153. 
 
 Gilbert's ' Journal,' vol. xv. p. 206 ; and (particularly) Dr. T. Young's ' Nat. 
 
 Phil.' vol. i. pp. 687, 716, and vol. ii. p. 488. 
 Wiedeburg, 'Ueber die Nordlichter.' Jena, 1771. 
 Hu'psch, ' Untersuchung des Nordlichts.' Cologne, 1778. 
 Van Swinden, ' Recueil de Memoires.' Hague, 1784. 
 Wilke, 'Von den neuesten Erklarungen des Nordlichts,' Schwed. Mus. 
 
 Wismar, 1783. 
 
 Dalton's ' Meteor. Observ.' 1793, pp. 54, 153. 
 Loomis, ' Sill. Journal,' 2nd series, xxxii. p. 324 ; xxxiv. p. 34. The same, 
 
 3rd series, v. p. 245 ; B. V. Marsh, 3rd series, xxxi. p. 311. 
 Oettingen and Vogel, Pogg. Ann. cxlvi. pp. 284, 569. 
 Galle and Sirks, ibid, cxlvi. p. 133 ; cxlix. p. 112. 
 Silbermann, ' Comptes Rendus,' Ixviii. pp. 1049, 1120, 1140, 1164. 
 Prof. Fritz, "Geog. Distrib.," Petermann's Mitth., Oct. 1874. 
 Zehfuss, ' Physikalische Theorie.' Adelman, Frankfort. 
 ' Nature,' iii. pp. 6, 7, 28, 104, 126, 346, 348, 510 ; iv. pp. 209, 213, 345, 497, 
 
 505 ; x. 211 (Angstrom). 
 
 'Edinburgh Astronomical Observations,' vol. xiv. 1870-1877. 
 ' English Mechanic,' No. 461 (January 23, 1874), pp. 445-447 ; and No. 462, 
 
 pp. 475, 476. 
 
APPENDIX B. 
 
 EXTRACTS FROM THE MANUAL AND INSTRUCTIONS FOR THE 
 (ENGLISH) ARCTIC EXPEDITION, 1875. 
 
 Note on Auroral Observations. By Prof. STOKES, Sec. E.S. 
 
 THE frequency of the Aurora in Arctic regions affords peculiar facilities for 
 the study of the general features of the phenomenon, as in case the observer 
 thinks he has perceived any law he will probably soon, and repeatedly, have 
 opportunities of confronting it with observation. The following points are 
 worthy of attention : 
 
 Streamers. It is well known that, at least as a rule, the streamers are 
 parallel to the dipping-needle, as is inferred from the observation that they 
 form arcs of great circles passing through the magnetic zenith. It has been 
 stated, however, that they have sometimes been seen curved. Should any thing 
 of this kind be noticed, the observer ought to note the circumstances most 
 carefully. He should notice particularly whether it is one and the same 
 streamer that is curved, or whether the curvature is apparent only, and arises 
 from the circumstance that a number of short, straight streamers start from 
 bases so arranged that the luminosity as a whole presents the form of a curved 
 band. 
 
 Have the streamers any lateral motion ] and if so, is it from right to left or 
 left to right, or sometimes one and sometimes the other, according to the 
 quarter of the heavens in which the streamer is seen, or other circumstances I 
 Again, if there be lateral motion, is it that the individual streamers move 
 sideways, or that fresh streamers arise to one side of the former, or partly the 
 one and partly the other 1 Do streamers, or does some portion of a system of 
 streamers, appear to have any uniform relation to clouds, as if they sprang 
 from them ? Can stars be seen immediately under the base of streamers I Do 
 
176 APPENDIX B. 
 
 streamers appear to have any definite relation to mountains 1 Are they ever 
 seen between the observer and a mountain, so as to appear to be projected on 
 it ? This or any other indication of a low origin ought to be most carefully 
 described. 
 
 When streamers form a corona, the character of it should be described. 
 
 Auroral Arches. Are arches always perpendicular to the magnetic meri- 
 dian ? If incomplete, do they grow laterally I and if so, in what manner, and 
 towards which side 1 Do they always move from north (magnetic) to south I 
 and if so, is it by a southerly motion of the individual streamers, or by new 
 streamers springing up to the south of the old ones ? What (by estimation, 
 or by reference to known stars) may be the breadth of the arch in different 
 positions in its progress I Do arches appear to be nothing but congeries of 
 streamers, or to have an independent existence 1 What relations, if any, have 
 they to clouds 1 and if related, to what kind of clouds are they related ? 
 
 Pulsations. Do pulsations travel in any invariable direction ? What time 
 do they take to get from one part of the heavens to another ? Are they run- 
 ning sheets of continuous light, or fixed patches which become luminous, or 
 more luminous, in rapid succession ? and if patches, do these appear to be 
 foreshortened streamers'? Are the same patches luminous in successive 
 pulsations ? 
 
 Sounds (1). As some have suspected the Aurora to be accompanied by 
 sound, the observer's attention should be directed to this question when an 
 Aurora is seen during a calm. If sound be suspected, the observer should 
 endeavour, by changing his position, brushing off spicules of ice from the 
 neighbourhood of the ears, his whiskers, &c., to ascertain whether it can be 
 referred to the action of such wind as there is on some part of his dress or 
 person. If it should clearly appear that it is not referable to the wind, then 
 the circumstance of its occurrence, its character, its relation (if any) to bursts 
 of light, should be most carefully noted. 
 
 These questions are prepared merely to lead the observer to direct his 
 attention to various features of the phenomenon. Answers are not demanded, 
 except in such ca'ses as definite answers can be given ; and the observer should 
 keep his attention alive to observe and regard any other features which may 
 appear to be of interest. It is desirable that drawings should be made of 
 remarkable displays. 
 
 Observations with Sir William Thomson's electrometer would be very 
 interesting in connexion with the Aurora, especially a comparison of the 
 readings before, during, and after a passage of the Aurora across the zenith. 
 
INSTRUCTIONS FOR THE ARCTIC EXPEDITION. 177 
 
 Spectroscope Observations. Ey Prof. G. G. STOKES, Sec. B.S. 
 
 Spectrum of tlw Aurora. 
 
 The spectrum of the Aurora contains a well-known conspicuous bright line 
 in the yellowish green, which has been accurately observed. There are also 
 other bright lines of greater refrangibility, the determination of the positions 
 of which is more difficult on account of their faintness, and there are also one 
 or more lines in the red, in red auroras. 
 
 Advantage should be taken of an unusually bright display to determine the 
 positions of the fainter lines. That of the brightest lines, though well known, 
 should be measured at the same time to control the observations. The cha- 
 racter of the lines (i. e. whether they are strictly lines, showing images of the 
 apparent breadth of the slit, or narrow bands, sharply defined or shaded-off) 
 should also be stated. 
 
 Sometimes a faint gleam of light is seen at night in the sky, the origin of 
 which (supposed from the presence of clouds) is doubtful. A spectroscope of 
 the roughest description may in such cases be usefully employed to deter- 
 mine whether the light is auroral or not, as in the former case the auroral 
 origin is detected by the chief bright line. The observer may thus be led 
 to be on the look-out for a display which otherwise might have been 
 missed. 
 
 It has been said, however, that the auroral light does not in all cases exhibit 
 bright lines, but sometimes, at least in the eastern and western arch of the 
 Aurora, shows a continuous spectrum. This statement should be confronted 
 with observation, special care being taken that the auroral light be not con- 
 founded with light which, though seen in the same direction, is of a different 
 origin, such, for example, as light from a bank of haze illuminated by the 
 moon. 
 
 Sir Edward Sabine once observed an auroral arch to one side (say north) of 
 the ship, which was in darkness. Presently the arch could no longer be seen, 
 but there was a general diffuse light, so that a man at the mast-head could be 
 seen. Later still, the ship was again in darkness, and an auroral arch was 
 seen to the south. 
 
 Should any thing of the kind be observed, the whole of the circumstances 
 ought to be carefully noted, and the spectroscope applied to the diffuse 
 light. 
 
178 APPENDIX B. 
 
 Polarization of Light. By W. SPOTTISWOODE, M.A., LL.D., Treas. B.S. 
 
 It has been suggested that the Aurora, inasmuch as it presents a structural 
 character, may afford traces of polarization. Having reference to the fact that 
 the striae of the electric discharge in vacuum-tubes present no such feature, the 
 probability of the suggestion may be doubted. But it will still be worth while 
 to put the question to an experimental test. 
 
 If traces of polarization be detected, it must not at once be concluded that 
 the light of the Aurora is polarized ; for the Aurora may be seen on the back- 
 ground of a sky illuminated by the moon, or by the sun, if not too far below 
 the horizon, and the light from either of these sources is, in general, more or 
 less polarized ; therefore, if the light of the Aurora is suspected to be polarized, 
 the polariscope should be directed to an adjacent portion of clear sky, free 
 from Aurora, but illuminated by the moon or sun as nearly as possible 
 similar, and similarly situated to the former portion ; and the observer must 
 then judge whether the polarization first observed be merely due to the illu- 
 mination of the sky. 
 
 The presence of polarization is to be determined : 
 
 (1) With a Nicol's prism, by observing the light through it by turning the 
 prism round on its axis, and by examining whether the light appears brightest 
 in some positions and least bright in others. If such be the case, the posi- 
 tions will be found to be at right angles to one another. The direction of 
 " the plane of polarization " will be determined by that of the Nicol at either 
 of these critical positions. The plane of polarization of the light transmitted 
 by a Nicol, is parallel to the longer diagonal of the face ; and, accordingly, the 
 plane of polarization, or partial polarization, of the observed light is parallel 
 to the longer diameter of the Nicol when the transmitted light is at its greatest 
 intensity, or to the shorter when it is at its least. 
 
 (2) The observation with a double-image prism is similar to that with a 
 Nicol. This instrument, as its name implies, gives the images which would 
 be seen through the Nicol in two rectangular positions, both at once, so that 
 they can be directly compared ; and when in observing polarized light the 
 instrument is turned so that one image is at a maximum, the other is 
 simultaneously at a minimum. Both these methods of observation, (1) and 
 (2), are especially suitable for faint light ; because in such a case the eye is 
 better able to appreciate differences of intensity than differences of colour. 
 
 (3) The observation with a biquartz differs from (1) only by holding a bi- 
 quartz (a right-handed and a left-handed quartz cemented side by side) at a 
 
INSTRUCTIONS FOE THE ARCTIC EXPEDITION. 179 
 
 convenient distance beyond the Nicol, and by observing whether colour is or 
 is not produced. If the Nicol be so turned that the two parts of the biquartz 
 give the same colour (choose the neutral tint, tcint de passage, rather than the 
 yellow), we can detect a change in the position of the plane of polarization by 
 a change in colour, one half verging towards red, the other towards blue. This 
 observation is obviously applicable to a change in the plane, either at different 
 parts of the phenomenon at the same time, or at the same parts at different 
 times. 
 
 (4) We may use a Savart's polariscope, which shows a series of coloured 
 bands in the field of view. For two positions at right angles to one another 
 corresponding to the two critical positions of a Nicol, these bands are most 
 strongly developed ; for two positions midway between the former the bands 
 vanish. In the instruments here furnished, the plane of polarization of the 
 observed light will be parallel to the bands when the central one is light, per- 
 pendicular to them when the central band is dark. 
 
 Instructions in the use of the Spectroscopes supplied to the Arctic Expedition. 
 
 By J. NORMAN LOCKYEB, F.E.S. 
 
 Spectroscopic Work. 
 
 Scales prepared on Mr. Capron's plan, together with forms for recording 
 positions, also accompany the instrument. 
 
 A. In using these, carefully insert the principal solar lines in their places 
 on the forms, as taken from a fine slit, and keep copies of this scale for use. 
 If the slit opens only on one side, note on scale in which direction the lines 
 widen out, whether towards red or violet. Also fill up some of these forms 
 with gas and other spectra, as taken at leisure with the same instrument and 
 scale. 
 
 When observing, close the slit (after first wide opening it) as much as 
 light will permit, and then with pen or pencil record the lines as seen upon 
 the micrometer-scale on the corresponding part of the form, and note at once 
 relative intensities with Greek letters, a, ft, &c. (or numbers). 
 
 Reduce at leisure line-places on scale to wave-lengths, and note as to each 
 line the probable limits of instrumental error. B. 
 
 In case the auroral spectrum is so faint that the needle-point or micro- 
 meter-scale is invisible, half of the field of view may be covered with tinfoil, 
 with a perfectly straight smooth edge running along the diameter of the field, 
 in perfect focus, and parallel to the lines of the spectra. The reading-screw 
 being set to 10, the bending-screw should then be adjusted so that the green 
 
 2B 
 
180 APPENDIX B. 
 
 line of the Aurora is just eclipsed behind the blackened edge of the tinfoil. A 
 similar eclipse of other lines will give their positions. 
 
 In this instrument the reference-prism is brought into action by turning the 
 slipping piece to which is fixed the two terminals. Care should be taken that 
 the prism itself is adjusted before commencing observations, as it may be shaken 
 out of position on the voyage. The tubes provided for the reference-spectra 
 may be either fastened to the terminals or arranged in some other manner. 
 The air-spectrum may also be used as a reference-spectrum. To get this, 
 two wires should be screwed into the insulators, their ends being at such 
 a distance apart and in such a position that the spectrum is well seen. 
 
 General Observations regarding the Spectrum of the Aurora *. 
 
 C. Note appearance, colour, &c. of arc, streamers, corona,&n.d patches of light. 
 
 Get compass positions of principal features, and note any change of mag- 
 netic intensity. If corona forms, take its position and apparent height. 
 
 Look out for phosphorescence of Aurora and adjacent clouds. Listen for 
 reported sounds. Note any peculiarity of cloud scenery, prior to or pending 
 the Aurora. 
 
 Sketch principal features of the display, and indicate on this sketch the 
 parts spectroscopically examined. 
 
 Examine line in red specially in reference to its assumed connexion with 
 telluric lines (little a group), and note as to its brightening in sympathy with 
 any of the other lines. 
 
 Examine line in yellow-green (Angstrom's) as to brightness, width, and 
 sharpness (or nebulosity) at the edges. Notice as to a peculiar flickering in this 
 line sometimes seen ; note also whether this line is brighter (or the reverse) 
 with a fall of temperature. Note ozone papers at the time of Aurora. 
 
 Note whether the Aurorse can by their spectra be classed into distinct types 
 or forms, and examine for different spectra as under : 
 
 . The auroral glow, pure and simple. 
 j3. The white arc. 
 y. The streamers and corona. 
 
 S. Any phosphorescent or other patches of light, or light cloud in or 
 near the Auroras. D. 
 
 * In these obsemations some suggestions made by Mr. Capron have been incorporated. 
 
 [This was Mr. Lockyer's note. In point of fact, the Author was responsible for the verbatim 
 paragraphs comprised between the letters A and B, and C and D, in the instructions as now 
 reprinted.] 
 
INSTRUCTIONS FOE THE ARCTIC EXPEDITION. 181 
 
 The information collected together in the ' Manual ' should be carefully 
 consulted, and the line of observations suggested by Angstrom's later work 
 followed out. To do this, not only record the positions of any features you 
 may observe in the spectrum, but endeavour to determine, if any, and if so 
 which, of the features vary together. Compare, for instance, the two spectra 
 of nitrogen in the Geissler tube supplied, by observing first the narrow and 
 then the wider parts of the tube. It will be seen that the difference in colour 
 and spectrum results simply from an addition to the spectrum in the shape of 
 a series of channelled spaces in the more refrangible end in the case of the 
 spectrum of the narrow portion. 
 
 Try to determine whether the difference between red and green Auroras 
 may arise from such a cause as this, and which class has the simpler 
 spectrum. 
 
 See whether indications of great auroral activity are associated with the 
 widening or increased brilliancy of any of the auroral lines. 
 
 Remember that if auroral displays are due to gaseous particles thrown into 
 vibration of electric disturbance, increased electric tension may either (1) dis- 
 sociate those particles and thus give rise to a new spectrum, the one previously 
 observed becoming dimmer ; or (2) throw the particles into more intense 
 vibration without dissociation, and thus give rise to new lines, those previously 
 observed becoming brighter. 
 
 Careful records of auroral phenomena from both ships may enable the 
 height of some, observed from both, to be determined. It will be very impor- 
 tant that those the heights of which are determined by such means should be 
 carefully observed by the spectroscope, in order to observe whether certain 
 characteristics of the spectrum can be associated with the height of the 
 Aurora. 
 
 2n2 
 
APPENDIX C. 
 
 EXTEACTS FROM PARLIAMENTARY BLUE BOOK, CONTAINING 
 THE "RESULTS DERIVED FROM THE ARCTIC EXPEDITION 
 
 1875-76." (Eyre and Spottiswoode, 1878.) 
 
 Auroras observed 1875-1876, at Floebery Beach and Discovery Bay. 
 By Lieutenant A. C. PARR, E.N. 
 
 THOUGH the auroral glow was often present, and served in some degree 
 to lighten the darkness of the sky during the long winter, when the moon 
 was absent, the actual appearances of the Aurora itself were few, and the 
 nimbus worthy of any particular remark extremely small. Those which 
 were stationary assumed the form of low arches, with streamers flashing 
 up to them from the horizon, and usually to the eastward. But the more 
 common form was for an arch to appear low down in some part of the sky 
 where the glow was brightest ; at first it was very faint and narrow, but as it 
 rose gradually in the heavens it would increase both in size and intensity, till 
 on arriving near the zenith, with its ends extending nearly to the horizon, it 
 would be about the breadth of three or four rainbows, and its colour that of 
 white fleecy clouds lit up by the rays of the full moon. On reaching this 
 point, however, its course was nearly run ; for after appearing to remain 
 stationary, as little white gaps would suddenly rend the arch asunder, the 
 portions thus detached seemed to roll together and concentrate all their 
 brightness in the smaller space, and then gradually fade away and become 
 extinct. Sometimes a very pale green would show itself in the more luminous 
 patches, and once or twice there was a slight suspicion of red ; but never was 
 the whole sky illuminated by streams running in all directions, and forming 
 coronae, while these colours varied every moment. 
 
 When instead of the arch rising up from the horizon a streamer appeared, 
 its origin was in the north. From the northern horizon it would stretch out 
 towards the zenith, passing nearly overhead, and reaching to within a few 
 
AUEOEAS OBSERVED DURING THE AECTIC EXPEDITION. 183 
 
 degrees of the land to the south. In appearance they would be the same as 
 the arches, but sometimes a second would grow out of the first, and on one 
 occasion three were visible at the same time. They had lateral motion either 
 from east to west, or west to east, but there was no flashing to brighten them, 
 and they gradually faded away. 
 
 The time at which Auroras usually occurred was between 9 P.M. and mid- 
 night, the last display being on February 19th, commencing at 11 P.M. It 
 was a beautifully clear night, without mist or haze of any description, and 
 small stars visible close down to the horizon. At the above-named hour two 
 arches made their appearance, and remained stationary ; the lower one was 
 the brighter, being of a pale green colour, its centre bearing E.S.E. (true), 
 and having an altitude of about 5, with a breadth of about twice that of a 
 rainbow. The second arch was concentric with the first, and about 7 above 
 it, but rather broader and fainter. These arches maintained their altitude, 
 the upper one at about the same intensity, but that of the lower one varied 
 considerably. It would gradually lighten up, then send flashes to the upper 
 one, then break up and fade away ; before, however, it had quite disappeared, 
 flashes would come up to it from the horizon which seemed to endue it with 
 new life, for the arch would be reformed, brighten up, and the same per- 
 formance would be again repeated. This occurred three or four times in the 
 course of three quarters of an hour ; but the flashes from the horizon never 
 extended beyond the lower arch, and those from the lower never went beyond 
 the upper. During this display the citron-line was obtained very clearly 
 with the spectroscope, but no other lines were visible. 
 
 On six or seven occasions Auroras were visible at the same time on board 
 both the ' Alert ' and ' Discovery ; ' but the absence of characteristic features 
 makes it impossible to determine whether they were the same display, or 
 merely two distinct ones which happened to occur at the same time. But as 
 by far the larger number of those recorded in the one ship were not visible 
 at the other, it was certainly only under exceptional conditions that they 
 could be simultaneously observed at both stations, if, indeed, they ever were. 
 Auroras seemed to appear indifferently both when there was wind and when 
 it was calm, with either a high or low barometer, and seemed quite uncon- 
 nected with the temperature, although on an occasion the thermometer was 
 observed to fall 3 during the display, and to rise 2 almost immediately after- 
 wards. But it was never seen illuminating the edges of clouds, as we saw it 
 on the passage home, nor playing about the outline of the land, and never 
 was there the slightest suspicion of sound being produced by it. 
 
184 APPENDIX C. 
 
 The opportunities for observing the spectrum of the Aurora in this position 
 have been most unsatisfactory, as the displays were small in number and 
 deficient in brilliancy. 
 
 The form they generally assumed was to rise like an arch from a portion 
 of the horizon where there was a luminous glow, at first very faint, but gra- 
 dually increasing in brilliancy till near the zenith, where it would remain 
 stationary for a short time and then break up and disappear. Sometimes 
 they would rise up as streamers, but only occasionally was more than one 
 visible at a time, and they lasted for such a short time, that even if they had 
 been bright it would have been very difficult to make satisfactory obser- 
 vations. 
 
 Very few showed any signs of colour, and those only the slightest tinge. 
 Nearly all that were observed gave the citron-line with the small pocket 
 spectroscope with more or less distinctness, though no signs of any other 
 lines were ever seen ; but on only two occasions was it bright enough to get 
 the line with Nury's spectroscope, and then only for such a short time that 
 a satisfactory measure could not be obtained. 
 
 Then follows a descriptive list of the Aurora seen, from which I have 
 selected three of the finest, viz. January 2nd, February 14th, and February 
 19th, 1876. 
 
 January 2nd, 1876. Lieut. Parr. Floelerg Beach. 9 P.M. Streams of 
 Aurora. Stars shining brightly. 
 
 Register. Discovery Say. 9 P.M. Observed an Aurora like a pale band 
 of light in the form of an arch whose centre was on the true meridian and 
 15 from the zenith. It shortly afterwards broke up into feathered edges, 
 their direction being a little to the eastward of the zenith. The arch grew 
 fainter, and shifted to the eastward of the meridian four points ; the left 
 extremity of the arch faded away, and the right assumed the shape of the 
 folds of a curtain doubled over. The weather was clear and calm. The dis- 
 play lasted upwards of 30 minutes. 
 
 A spectroscope, one of Browning's 8-in. direct-vision, was directed towards 
 the Aurora, but the light was not sufficient to give any spectrum. 
 
 The temperature was 39. Barometer 29'56 inches. No wind. Clouds 
 stratus 2. Eight meteors were observed during the time the Aurora was 
 visible. 
 
 February 14th. Register. Discovery Say. At 2 A.M. a faint Aurora 
 passing across the heavens from S.E. to S.W. was observed, like an arch of 
 
AURORAS OBSERVED DURING THE ARCTIC EXPEDITION. 185 
 
 a pale colour. It lasted only a short time, and was very indistinct. Tem- 
 perature 47. Barometer 30'44 inches. No wind or clouds. 
 
 Lieut. Aldrich. Floeberg Beach. 2 A.M. A faint Aurora towards the 
 S.W. Weather calm. Cumulus-stratus clouds 3. Temperature 46. 
 8 P.M. Faint flashes of Aurora in the E. and S.W. 
 
 Lieut. Aldrich and Lieut. Parr. Floeberg Beach. 11.50 P.M. A mo- 
 derately bright arch of Aurora extended from due N. to about S.S.W., where 
 it terminated close down to the horizon in a crook turned to the eastward. 
 In a few moments a streamer flashed from the end of the crook parallel to 
 the first and right across the heavens, its edges being quite sharp and parallel 
 to each other. A third streamer shot up a minute afterwards, but did not 
 extend more than 80 upwards. The streamers were visible for a very short 
 time, the first remaining longest. The second-named arch gradually faded 
 away till within a few degrees of the S.S.W. horizon, and (still being a con- 
 tinuation of the crook) bent round to the eastward, and towards the horizon, 
 going on to what was left of the stump of the third arc. A lateral motion 
 to the eastward now began, the whole body gradually turning round until it 
 disappeared about due south. Stars were visible through it at its brightest, 
 but not very distinctly. This is the most intense and variegated Aurora we 
 have experienced, but scarcely any colours were to be seen. Temperature 
 51. Barometer 30'43 inches, stationary. Calm weather. Clouds cumu- 
 lus 1. Preceded and followed by calm weather. 
 
 Meteorological Register. Discovery Bay. 9.15 P.M. An Aurora was 
 observed to the southward, spreading out like a fan in separate ways. It 
 was faint. A few cirro-stratus clouds were visible, apparently between the 
 observer and the Aurora. It lasted about 40 minutes, and then gradually 
 faded away. Temperature 47. Barometer 30*51 inches, stationary. No 
 wind. Clouds cirro-stratus 4. 
 
 February 19th. Meteorological Report. Discovery Bay. 9.45 P.M. An 
 Aurora like a fluted arch, with rays flashing towards the Pole, was observed 
 spanning the hills from the south to the east. The direction of the lines of 
 light from all parts of the arch was towards the zenith. Above the arch a 
 pale band of colour appeared, like a secondary arch above the other. It 
 appeared very much as if it was caused by the reflected light of the Aurora. 
 The Aurora was bright for a few seconds, and then gradually died away. It 
 lasted altogether about 30 minutes. The centre of the arch bore S.E., having 
 an altitude of about 30. The secondary arch was about 15 above the 
 former. Both arches were of a pale light colour, the upper one very faint. 
 
186 APPENDIX C. 
 
 Temperature 34. Barometer 29'87 inches, rising rapidly. Weather calm. 
 Misty. No clouds. 
 
 Lieut. Parr. Floeberg Beach. An Aurora appeared shortly after 11 P.M., 
 consisting of bright arch, whose centre bore about E.S.E., and had an alti- 
 tude of about 5, with a second broader and fainter arch about 7 above the 
 first. These arches maintained their altitudes, the upper one at about the 
 same intensity, but that of the lower one varied considerably. It would 
 gradually brighten up, then send streamers up to the second, then break up 
 into light patches, and gradually fade away. This happened three or four 
 times during the 40 minutes that the display lasted. At times streamers 
 would come up from the horizon to the lower arch, for it was a splendidly 
 clear night, and seemed to brighten it up, but none of them extended beyond 
 it. Neither did the streamers from the lower arch extend beyond the upper 
 one. It was slightly green in colour when brightest, and the citron-line was 
 well defined, but no others were visible. Temperature 46. Barometer 
 29-95 inches, steady. Weather calm. Cumulus clouds 4. Misty. 
 
 Auroras and Magnetic Disturbances. 
 
 The appearances of Auroras and the synchronous movements of the declino- 
 meter-magnet were subjects of special observation during the stay of the 
 'Alert' and 'Discovery' at their winter-quarters. The Table on page 187 
 gives the dates and hours when Auroras were visible. On all occasions they 
 were observed to be faint, with none of those brilliant manifestations which 
 are described by our own officers as seen at Point Barrow, and by the Austro- 
 Hungarian Expedition at Franz-Josef Land, where the magnetical instruments 
 were so sensibly disturbed. 
 
 These phenomena were not observed either in the 'Alert' or the 'Dis- 
 covery,' especially no connexion between magnetical disturbances and the 
 appearances of Auroras could be traced. 
 
 This is quite in accordance with the remarks of previous observers within 
 the region comprehended between the meridians of 60 and 90 west, and 
 north of the parallel of 73 north. For example : 
 
 In the Phil. Trans. 1826, Part IV. p. 76, Capt. Parry and Lieut. Foster 
 remark, in the discussion of their magnetical observations at Port Bowen : 
 " As far, however, as our own observations extended, we have reason to believe 
 that on no occasion were the needles in the slightest degree affected by 
 Aurora, meteors, or any other perceptible atmospheric phenomenon." 
 
 Again, in the Smithsonian Contributions, vol. x., 1858, Mr. A. Schott, in 
 
AUEOEAS OBSERVED DURING THE ARCTIC EXPEDITION. 
 
 187 
 
 TABLE of DATES when AURORAS were observed by the AKCTIC EXPEDITION, 1875-76*. 
 
 Date. 
 
 75, October 25 
 
 November 
 
 December 
 
 1876, January 
 
 February 
 
 30 
 1 
 2 
 
 21 
 22 
 25 
 26 
 26 
 87 
 28 
 29 
 30 
 30 
 2 
 :! 
 
 3 
 16 
 19 
 
 22 
 23 
 24 
 26 
 29 
 31 
 1 
 2 
 17 
 18 
 
 19 
 20 
 23 
 24 
 24 
 27 
 27 
 28 
 30 
 31 
 3 
 11 
 13 
 14 
 
 19 
 20 
 22 
 24 
 26 
 
 H.M.S. 'Alert,' Floeberg Beach. 
 
 11.45P.M. Faint. 
 
 10 P.M. Very faint. 
 
 Sky obscured. Faint. 
 
 Ditto. Faint, but well marked. 
 
 9 to 10 P.M. Arches and streamers. 
 Ditto. Bright streamer. 
 
 2 P.M. and 8 P.M. Slight, red. 
 9.30 A.M. Character not recorded. 
 
 10 A.M. Stream of light. j 
 Cloudy to 10 P.M., bright afterwards. \ 
 Midnight. Slight. 
 
 1 A.M. Bright streak. 
 
 Cloudy, brighter at 11 A.M. Faint glow. 
 
 A few clouds. Very faint. 
 
 P.M., 8 P.M., and 10 P.M. Flashes. 
 Evening. Streamers. 
 
 1 A.M. Flashes. 
 
 Bright sky. Faint Aurora. 
 
 10 P.M. Slight ; showed citron-line. 
 
 3 P.M. to 5 P.M., faint ; and 9 to 10 P.M., 
 moderately bright arc. 
 
 10 P.M. Slight. 
 
 6 P.M. Ditto. 
 
 Misty, a few stars visible. Arch. 
 
 Very bright sky. Faint. 
 
 Ditto. Very faint. 
 
 4 P.M. Same. 
 
 5 P.M. and 11 P.M. Slight. 
 
 9 P.M. Described and figured. 
 
 Very bright sky. Very faint streamers. 
 9.45 P.M. and 10.5 P.M. Character not 
 
 recorded. 
 Very bright sky. Faint. 
 
 2 A.M. Slight. 
 
 7.55 A.M. and 2 P.M. Slight. 
 Bright sky. Slight flash. 
 
 5 P.M. and 11.15 P.M. Faint Aurora. 
 2 A.M. to 3.45 A.M. Faint. 
 
 Very bright sky. Faint double arch. 
 
 6 P.M. and 7 to 9 P.M. Faint flashes. 
 8 P.M. Streak. 
 
 8.30 A.M. and 7.30 P.M. Very faint. 
 
 10 P.M. Slight flash. 
 
 Sky obscured. Very faint. 
 
 11 P.M. Flashes. 
 
 2 A.M., 9.15 to 10 P.M. Described and 
 
 figured. 
 9.45 P.M. 
 
 '2 A.M. Very faint. 
 2 A.M. Character not recorded. 
 Bright sky. Very faint. 
 10 P.M. and 11 P.M. Faint flashes. 
 
 H.M.S. ' Discovery,' 
 Discovery Bay. 
 
 Cloudy. 
 
 10 P.M. 
 
 Ditto. 
 
 Ditto. 
 
 A few clouds. 
 
 9 to 10 P.M. and 10 to 11 P.M. 
 Clear sky. 
 
 Ditto. 
 
 A few clouds. 
 
 10 P.M. 
 11.40P.M. 
 Clear sky. 
 9.30 A.M". 
 4.30 A.M. 
 
 5 P.M. 
 Clear sky. 
 Ditto. 
 2.30 P.M. 
 
 11 P.M. 
 
 Very clear sky. 
 
 Ditto. 
 Ditto. 
 
 9A.M. 
 
 6 P.M. 
 6.15 P.M. 
 Sky obscured. 
 A few clouds. 
 9 P.M. 
 
 9.25 A.M. 
 10.15 P.M. 
 
 9.45 P.M. 
 2.30 A.M. 
 8.45 P.M. 
 
 2 A.M. 
 
 Very clear sky. 
 
 1 A.M. to 4 A.M. 
 8.30 P.M. 
 
 7.20 P.M. 
 7.50 to 9 P.M. 
 8.25 A.M., 5.30 P.M. 
 Very clear sky. 
 
 11 P.M. 
 
 Clear sky. 
 
 2 A.M. and 11.50 P.M. 
 
 11 P.M. 
 2.30 A.M. 
 Very clear sky. 
 Midnight. 
 Sky obscured. 
 
 * I have added to the above Table the character of the Aurora in each instance as taken from the fuller 
 descriptions given. J. R. 0. 
 
 2 c 
 
188 APPENDIX C. 
 
 his discussion of Dr Kane's observations at Van Rensselaer Harbour, in 
 1854, remarks "In conformity with the supposed periodicity of this pheno- 
 menon as recognized by Professor Olmstead, no brilliant and complete 
 Auroras have been seen ; with an exception of very few, they may all be 
 placed in his fourth class, to which the most simple forms of appearances 
 have been referred." The following statement is given in the same page 
 as a footnote : " The processes have no apparent connexion with the magnetic 
 dip, and in no case did the needle of our unifilar indicate disturbance." 
 
 The following description of the Aurora observed on 21st November, 1875, 
 is given by Commander Markham and Lieut. Gifiard, in their abstract of 
 observations at Floeberg Beach : 
 
 " Between 10 and 11 P.M. bright broad streamers of the Aurora appeared 
 10 or 15 above the north horizon, stretching through the zenith, and ter- 
 minating in an irregular curve about 25 above the south horizon, bearing 
 S.S.W. During the Aurora's greatest brilliancy the magnet was observed 
 during five minutes to be undisturbed." 
 
 [Note. I appEed for a loan of the lithographic stones to enable me to give copies of the three 
 diagrams of Aurorae referred to in the Arctic " Results ;" but the Lords Commissioners of H.M. 
 Treasury refused this, except on the terms of my paying one third of the original cost of production 
 of such diagrams. I did not think it worth while to accept these conditions. Only one of the 
 drawings has any special interest ; and this is a " curtain " Aurora, similar to that figured on 
 Plate H. of this work. J. E. C.] 
 
APPENDIX D. 
 
 THE AURORA AND OZONE. 
 
 WHILE Part I. was in the press, Dr. Allnatt, formerly of Frant, and for many 
 years the well-known meteorological contributor to ' The Times ' newspaper, 
 kindly placed at my disposal his large series of notes. Upon an examination 
 of these we came to the following conclusions : 
 
 1. That Auroral periods are also periods of comparative abundance of 
 ozone. 
 
 2. That instances are by no means wanting in which an abnormal de- 
 velopment of ozone appears to be coincident with the manifestation of an 
 Aurora. 
 
 In reference to the first point, it is found, as the result of an examination 
 of Dr. Allnatt's notes, that particular years and months are notable at once 
 for Aurorae and for ozone in abundance. 1870 was one of these years, and 
 was specially recorded by Dr. Allnatt, in his ' Summary for the Year,' as 
 remarkable for sun-spots, Aurorae, and ozone. 
 
 The month of February in that year was marked by intense cold and 
 brilliant Aurora. Atmospheric electricity was feeble, but ozone was, 
 throughout the month, well developed ; and there was no tangible period of 
 antozone. 
 
 In the month of April of the same year, eight days consecutively (19th to 
 26th) were marked for ozone 10, the maximum of Dr. Allnatt's scale. 
 
 In May of the same year there were magnificent Aurorae, and atmospheric 
 electricity was intense. Ozone was scanty ; but this was accounted for by 
 the wind being generally E.N.E., ozone being mostly developed with a W. 
 or S. W. wind, and a moist state of the atmosphere. 
 
 2c2 
 
 Aurora and 
 
 ozone. Dr. 
 
 Allnatt'g 
 
 notes and 
 
 conclusions 
 
 deduced 
 
 therefrom. 
 
 Year 1870 
 remarkable 
 for gun- 
 
 aurora, and 
 ozone. 
 Particulars 
 of some of 
 the monthly 
 records. 
 
190 
 
 APPENDIX D. 
 
 Year 1874. 
 
 Instances 
 showing a 
 connexion 
 between a 
 specific 
 Aurora and 
 an ozone 
 maximum. 
 
 In August 1870 the unusually large number of 22 days were recorded for 
 a maximum of ozone. 
 
 September 1870 was hardly less remarkable, with 19 days of maximum. 
 It was recorded that there were splendid Auroras during this month, and the 
 solar spots were very large. 
 
 October 1870 had 20 days of maximum ozone, and November had several 
 fine Aurora and maxima of ozone noted. In fact, nearly every month in that 
 year was referred to by Dr. Allnatt for displays of Aurora (of both Arctic and 
 Antarctic forms) and for a development of ozone very considerably above the 
 average. 
 
 The year 1871 had more or less of the same character. In the month of 
 October of that year, fine Aurorse were prevalent, and ozone was registered as 
 at its maximum during 22 days. 
 
 There seems reason to conclude that if a systematic comparison of annual 
 or other periods of Aurora and ozone development were made, it would result 
 in disclosing a connexion (probably an intimate one) between the two phe- 
 nomena. 
 
 With reference to the second point, the following (among other) instances 
 may be quoted, for the purpose of showing a connexion between a specific 
 Aurora and an ozone maximum. 
 
 The Aurora of 24th September, 1870, was splendid and universal, being 
 seen in Europe, Asia, Africa, America, and Australia. Ozone reached, on the 
 morning of the 24th, 8 of the scale (the scale running from 1 to 10), and, on 
 the morning of the 25th, 10, the maximum. 
 
 In October 1870 there were grand displays on the 14th, 20th, 22nd, 24th, 
 and 25th, and ozone was correspondingly abundant, as is seen by the follow- 
 ing Table : 
 
 Date. 
 
 Aurora. 
 
 Ozone. 
 
 
 1870, October 14th . 
 
 Aurora. 
 
 8 
 
 The display of the 24th 
 
 20th . 
 
 Aurora. 
 
 10 
 
 was accompanied by 
 the formation of a 
 
 21st . 
 
 None seen. 
 
 5 
 
 corona, and that of 
 
 22nd. 
 
 Aurora. 
 
 10 
 
 the 25th was splen- 
 didly seen in Edin- 
 
 23rd . 
 
 None seen. 
 
 8 
 
 burgh. 
 
 24th . 
 
 Aurora. 
 
 10 
 
 
 25th . 
 
 Aurora. 
 
 8 
 
 
THE AURORA AND OZONE. 
 
 191 
 
 The foregoing figures somewhat point to the conclusion that ozone quantity 
 rises and falls coincidently with the Aurora displays. 
 
 The following seems, however, a case still more strongly in point. 
 
 Date. 
 
 Wind. 
 
 Aurora. 
 
 Ozone. 
 
 1871, January 25th . 
 
 E.S.E. 
 
 None seen. 
 
 
 
 2Gth . 
 
 N.N.W. 
 
 None seen. 
 
 2 
 
 -'7th . 
 
 E.S.E. 
 
 Aurora at night in N. 
 and S. horizons. 
 
 10 
 
 28th . 
 
 E. 
 
 None seen. 
 
 8 
 
 29th . 
 
 S.E. 
 
 None seen. 
 
 2 
 
 It is curious, in examining the above Table, to note how the ozone rose, 
 notwithstanding an east wind, from on the 25th, and 2 on the 26th, to 10 
 on the 27th, when the Aurora appeared, and 8 on the 28th, when it might 
 have lingered ; and how it again descended to 2 on the 29th. 
 
 The case of the Aurora of 6th of October, 1869, when a broad belt of 
 Aurora was in the north, is also an illustrative one, as will be seen by the 
 following data : 
 
 Date. 
 
 Wind. 
 
 Ozone. 
 
 Aurora. 
 
 1869, October 5th.. 
 6th.. 
 7th.. 
 
 8th . . 
 9th.. 
 
 s.s.w. 
 
 S.S.E. 
 S.S.W. 
 
 s. 
 
 S.E. 
 
 1 
 5 
 10 
 10 
 5 
 
 
 Aurora. 
 
 
 
 
 The Aurora of the night of the 6th was here represented by the ozone- 
 paper of the morning of the 7th with a maximum of 10, which lasted till 
 the 8th. 
 
 [It should be borne in mind, in examining these Tables, that the 
 Aurora is of the night of the given date, while the ozone-papers are taken 
 and recorded in the morning of the date quoted.] 
 
 We will now take instances where the ozone has not reached its maximum ; 
 but even in these cases a certain amount of rise and fall of the ozone develop- 
 ment towards and from the Aurora is traceable. 
 
 Other 
 instances. 
 
192 
 
 APPENDIX D. 
 
 Date. 
 
 Wind. 
 
 Ozone. 
 
 Aurora. 
 
 1871, April 8th 
 9th 
 10th 
 
 S.8.E. 
 S.8.E. 
 S.E. 
 
 5 
 
 8 
 5 
 
 Aurora on 9th, but 
 wind E. and unfa- 
 vourable to ozone. 
 
 1871, November 9th 
 10th 
 llth 
 
 N. 
 N.W. 
 
 N. 
 
 5 
 
 8 
 5 
 
 Aurora on all three 
 nights. 
 
 1872, February 3rd 
 4th 
 5th 
 6th 
 
 8.W. 
 
 S.S.W. 
 S.W. 
 
 s.w. 
 
 4 
 5 
 
 8 
 5 
 
 Aurora on night of the 
 4th represented by 
 ozone-paper of morn- 
 ing of the 5th. 
 
 Other cases are, we are bound to say, found, in which ozone was either not 
 remarkable for quantity, or positively fell during the Aurora, as, for instance, 
 this : 
 
 Date. 
 
 Wind. 
 
 Ozone. 
 
 Aurora. 
 
 1874, March 16th.. 
 
 W.N.W. 
 
 6 
 
 Aurora on the 18th repre- 
 
 17th.. 
 
 S.W. 
 
 6 
 
 sented by test-paper of 
 the 19th with only two 
 
 18th.. 
 
 W. 
 
 5 
 
 degrees of discoloration. 
 
 19th.. 
 
 s.s.w. 
 
 2 
 
 
 It is, however, possible that such instances may be accounted for, either 
 by some reaction on the test-papers after they have been coloured, or by 
 some accidental antagonistic circumstance affecting the tests. The following 
 is a case well illustrating this : 
 
 Date. 
 
 Wind. 
 
 Ozone. 
 
 Aurora. 
 
 1874, January 31st 
 February 1st 
 2nd 
 3rd 
 
 N.N.W. 
 N.W. 
 N.W. 
 
 N.N.W. 
 
 6 
 
 8 
 2 
 4 
 
 There was an Aurqra on 
 the night of the 2nd re- 
 presented by the ozone- 
 paper (4 only) on the 
 morning of the 3rd. 
 
 4th 
 
 E.N.E. 
 
 8 
 
 
 This instance would seem strongly opposed to the theory of a connexion 
 between Aurora and ozone but for the fact that on the 2nd, when the Aurora 
 
:THE AURORA AND OZONE. 193 
 
 was seen at night, and on other days in the same month, Dr. Allnatt has 
 recorded a strong wave of antozone to have swept over the whole of England, 
 and blanched the ozone-papers, however deep their coloration might have 
 previously been. Indeed, it is easy to understand that some antozonic 
 influence may, at times, disturb the evidence of the test-papers, even in so 
 elevated and apparently pure an atmosphere as that of Frant. 
 
 It may not be considered that the foregoing instances are enough to esta- 
 blish a case of ozone = Aurora; but there seems, at least, sufficient to base a 
 requisition for further inquiry upon. 
 
 It would, too, be interesting to investigate whether Auroras and ozone 
 development are respectively localized. Mr. Ingall's fine Aurora, seen at 
 Champion Hill, S.E., July 18th, 1874 (antt, pp. 22 and 23), was not observed 
 at Frant, and the ozonoscopes there were described as blanched by antozone. 
 
APPENDIX E. 
 
 INQUIRIES INTO THE SPECTRUM OF THE AURORA. 
 
 BY H. C. VOGEL.* 
 
 THE frequent appearance of the Aurora in the past winter, as well as this 
 spring, has given me opportunity to institute exact inquiries into the spectrum 
 of the Aurora. It is known that the nature of Aurora? is as yet but little 
 explored. It has been considered necessary to abandon the former view 
 that they are discharges of the electricity collected at the poles because it 
 has been hitherto found impossible to bring the chief lines of the Aurora- 
 spectrum into coincidence with the spectra of the atmospheric gases. Theo- 
 retical considerations, based on the great alterations to which the spectrum 
 of the same gas is subject under varying conditions of temperature and 
 density, have very recently led Zollner to the opinion that probably the 
 spectrum of the Aurora does not coincide with any known spectrum of 
 the atmospheric gases, only because it is a spectrum of another form of our 
 atmosphere hitherto incapable of artificial demonstration f . 
 
 The following article will show how far I have succeeded, in conjunction 
 with Dr. Lohse, in supporting this view by exact observations of the Aurora- 
 spectrum itself, as well as by comparison with the spectra of the gases 
 constituting the air. 
 
 The star-spectrum apparatus belonging to the 11-inch equatorial of the 
 Bothkamp Observatory was used for these observations. It consists of a 
 set of prisms a vision directe, five prisms with refracting angle 90, slit, 
 collimator, and observing telescope. The lowest eyepiece (magnifying four 
 times) of this telescope was employed. The telescope is capable of being 
 
 * Communicated by the author to the Eoyal Saxon Academy of Science, 1871. 
 t Beports of the Eoyal Saxoii Academy of Science, Oct. 31, 1871. 
 
INQUIRIES INTO THE SPECTEUM OF THE AURORA. 195 
 
 moved in such a way, by the aid of a micrometer-screw, that different 
 portions of the spectrum can be brought into the centre of the field of 
 vision. As fractions of the rotation of this screw are marked, the distances 
 of the spectral lines can be readily found. 
 
 Repeated measurements of 100 lines of the solar spectrum have enabled 
 
 O 
 
 me, upon the basis of Angstrom's Atlas ('Spectre normal de Soleil'), to express 
 the indications of the screw directly in wave-lengths. 
 
 In place of the cross wires originally introduced into the focus of the 
 observing telescope, I have inserted a tiny polished steel cone, the very fine 
 point of which reaches to the centre of the field of vision. The axis of this 
 cone stands perpendicular to the length of the spectrum, therefore parallel 
 with the spectral lines, and the setting of the point of the cone on the latter 
 is accomplished with great sharpness. If the spectrum is very faint, or 
 consists only of bright lines, the cone is lighted by a small lamp. For this 
 purpose, opposite to the point of the cone, there is an opening in the 
 telescope, through which, regulated by a blind, light can be thrown on the 
 point. As the latter is polished, a fine line of light thus appears, which 
 extends to the centre of the field of vision, and the brilliancy of which can 
 be altered by withdrawing the lamp to a greater distance or lowering the 
 blind, so that even the faintest lines of a spectrum can be brought with 
 facility and certainty into coincidence with this line of light. 
 
 The head of the micrometer-screw is divided into 100 parts, and each part, 
 in the neighbourhood of the Fraunhofer line F, answers to about -00016 
 wave-length. The probable error of position on one of the well-marked lines 
 in the sun's spectrum amounts to about O'OOS of a turn of the screw with the 
 lowest eyepiece of the telescope. 'I have subjected the screw itself to a 
 thorough examination with reference to such range, as well as to periodical 
 inequalities in the single worms of the screw, but could discover no error 
 exceeding O'Ol of a turn of the screw. I have to mention, further, that after 
 each observation in the position in which the instrument was used, readings 
 followed on the sodium-lines, or on some of the hydrogen-lines, in order to 
 eliminate errors which might arise in the unavoidable disturbance of any 
 particular part of the spectral apparatus. 
 
 1. Observations of the Aurora. 
 
 1870, Oct. 25th. A very bright Aurora. In the brightest parts, besides 
 a very bright line between D and E, several other fainter lines were to be 
 discerned, situated further towards the blue end of the spectrum. They 
 
 2 D 
 
196 APPENDIX E. 
 
 appeared on a dimly-lighted ground, and stretched out over the Fraunhofer 
 lines E and b to about midway between b and F. Towards the red end the 
 spectrum was terminated by the bright line first mentioned. No measure- 
 ments could be taken, as the apparatus had not yet undergone the above- 
 mentioned alterations, and even the brightest line of the spectrum did not 
 diffuse sufficient light to be able to perceive the fine cross wires. The red 
 rays of the Aurora were not examined. 
 
 1871, Feb. llth. Towards ten o'clock appeared in the north-west a very 
 bright light-bow of greenish colour as the edge of a dark segment. Even 
 with a very narrow slit, the line between D and E could be well recognized 
 and measured. The average of six readings gave 7'11 turns, equal to 5572 
 wave-length. In a small spectroscope of low dispersion which is arranged 
 on Browning's plan, a few more lines placed further towards the blue could 
 be recognized (as in October). Towards the red end of the spectrum no lines 
 were observable. The greatest development of the Aurora was about 
 midnight. Magnificent rays rose to about 60 elevation ; they had the 
 same greenish colouring as the bow of light, and the appearance of the 
 spectrum also was exactly the same. I again obtained two sets of measure- 
 ments : the average of six readings in the first set gave 7'10 turns, 5572 
 wave-length; in another part of the heavens at the same time 7'10 was the 
 result of four readings. 
 
 On Feb. 12, towards eight o'clock, the intensity of the Aurora was already 
 great enough to allow measurements of the brightest line. The average of 
 six readings gave 7'09 turns, or 5576 wave-length. Dr. Lohse took observa- 
 tions later, with the same apparatus, and found from six readings 7'12 turns, 
 or 5569 wave-length. 
 
 Yet the appearance of the spectrum in the spectroscope of low dispersion 
 was essentially distinct from that of February llth. The green continuous 
 spectrum was present ; it extended from the bright Aurora-line to the lines 
 b of the solar spectrum, and was traversed by some bright lines. Between 
 band b and F, was another line standing alone, out beyond F, in the blue part 
 of the spectrum, a clear bright stripe ; and just before G a very faint broad 
 band of light was perceived. 
 
 Amongst the rays which, later on, shot upwards, and were coloured red at 
 their ends, another very intense red line appeared in the spectrum between 
 C and D, yet placed nearer to C *. 
 
 April 9th. An exceedingly brilliant Aurora, of which the greater develop- 
 * This red line was first noticed by Zollner. 
 
INQUIRIES INTO THE SPECTRUM OF THE AUEOEA. 
 
 197 
 
 ment took place in the early morning hours. Magnificent red sheaths rose 
 up to the zenith. The spectrum was like that observed on February 12th, 
 only much more intense, so that the lines could be seen and measured with 
 the larger spectral apparatus. In the brightest part of the Aurora was 
 the dark segment ; the spectrum consisted of five lines in the green, and a 
 somewhat indistinct broad line or band in the blue. 
 
 The red rays, on the other hand, allowed us to recognize seven lines, whilst 
 the bright line again appeared in the red part of the spectrum. I could not 
 again perceive the faint stripe observed on February 12th, in the vicinity of 
 line G. The mean measurements of four readings on an average, for each 
 line, gave : 
 
 Turns of 
 screw. 
 
 Probable 
 errors. 
 
 Wave- 
 length. 
 
 Probable 
 errors. 
 
 Remarks. 
 
 4-62 
 
 0037 
 
 6297 
 
 00014 
 
 Very bright stripe. "^ jjf- Q 
 
 7-12 
 
 9 
 
 5569 
 
 2 
 
 Brightest line of the spectrum ; becomes a 
 
 
 
 
 
 noticeably fainter at appearance of the I (L* 8 
 
 
 
 
 
 red line. M^ g! 
 
 7-92 
 
 .... 
 
 5390 
 
 
 Extremely faint line; unreliable observa- | g_ 
 
 
 
 
 
 tion. J (i.*** 
 
 8-71 
 
 21 
 
 5233 
 
 4 
 
 Moderately bright. 
 
 8-95 
 
 49 
 
 5189 
 
 9 
 
 This line is very bright when the red line appears 
 
 
 
 
 
 at the same time, otherwise equal in brilliancy 
 
 
 
 
 
 with the preceding one. 
 
 10-06 
 
 20 
 
 5004 
 
 3 
 
 Very bright line. 
 
 12-33 
 
 .... 
 
 4694 
 
 .... 
 
 1 Broad band of light, somewhat less brilliant in the 
 
 12-59 
 
 22 
 
 4663 
 
 .... 
 
 J middle. 
 
 12-88 
 
 .... 
 
 4629 
 
 3 
 
 Very faint in those parts of the Aurora in which 
 
 
 
 
 
 the red line appears. 
 
 April 14th. Faint Aurora ; only the bright line in the green could 
 be recognized in its spectrum. The mean of two readings gave 7'12 turns, or 
 5569 wave-length. 
 
 I append a table of the wave-lengths of the brightest line, as exactly 
 measured on four evenings : 
 
 1871, February 11 5573 
 
 12 5573 
 
 April 9 5569 
 
 14 5569 
 
 Therefore the average result (if only half-weight is allowed to the last 
 
 2D 2 
 
198 APPENDIX E. 
 
 observation, because it only depends upon two readings) gives for the wave- 
 length of the brightest line 5571'3, with a probable error of -000-92. 
 According to Angstrom *, the wave-length of this line is 5567 ; according to 
 Winlock f , on the other hand, 5570. 
 
 2. On the Spectra of some Gases in Geissler's Tubes, as well as on the 
 Spectrum of the Atmospheric Air. 
 
 Numerous experiments have been made in order to find out some admitted 
 connexion between the spectrum of the Aurora and the spectra of the 
 principal gases composing the atmosphere. I limit myself to noticing some 
 of the often-repeated observations in Plucker's tubes, which contained oxygen, 
 hydrogen, and nitrogen, as well as the observations of the spectrum of the 
 air under different conditions. The experiments were made with a small 
 inductive apparatus, in which the length of the spark between platinum 
 points in ordinary air was 15 millims. at the most. As Zollner (in the 
 pamphlet mentioned) comes to the conclusion, that if the development of the 
 light in the Aurora, according to the analogy of gases brought to glow in 
 rarefied spaces, is of an electric nature, it must belong to very low tem- 
 perature in order to bring the gases enclosed in the tubes to glow at the 
 lowest possible temperature, I have always employed such weak currents that 
 the gas was only just steadily alight. 
 
 The following observations have been repeated often and at various times. 
 The figures are averages of the indications of the micrometer-screw, so that 
 the uncertainty of the figures will, in the rarest cases, amount to no more than 
 0-015 turn of the screw, and must be reckoned somewhat more highly only 
 in the case of completely faint misty lines. The spectrum apparatus was that 
 described above, and the slit was nearly the same in every experiment, and so 
 narrow that the sodium-lines could be seen separated. The measurements, 
 for the most part, extend only to the Fraunhofer line G, as I feared lest, 
 through further turning the telescope by means of the micrometer-screw, too 
 great a pressure might be exercised on the worms of the latter. 
 
 i 
 
 * Eecherches sur le Spectre Solaire, p. 42. 
 t American Journal of Science, Ixviii. 123. 
 
INQUIRIES INTO THE SPECTRUM OF THE AURORA. 
 
 199 
 
 I. Oxygen, 
 a. In the narrow part of the Pliicker tube. 
 
 Screw. 
 
 Wave-length. 
 
 Remarks. 
 
 3-97 
 
 8662 
 
 Moderately bright. 
 
 5-04 
 
 6146 
 
 Very bright. 
 
 6-98 
 
 5603 
 
 Very bright, misty towards the violet. 
 
 8-19 
 
 5332 
 
 Faint. 
 
 8-95 
 
 5189 
 
 Moderately bright. 
 
 10-97 
 
 4870 
 
 i> 
 
 11-02 
 
 4863 
 
 Faint. 
 
 11-26 
 
 4829 
 
 Bright ; misty towards the red end of the spectrum. 
 
 13-30 
 
 4583 
 
 Very faint. 
 
 14-05 
 
 4506 
 
 Moderately bright. 
 
 15-55 
 
 4372 
 
 
 
 b. In the wide part of the Pliicker tube. 
 
 Screw. 
 
 Wave-length. 
 
 Remarks. 
 
 6-98 
 
 5603 
 
 Very faint. 
 
 
 8-95 
 
 5189 
 
 Very bright. 
 
 
 11-26 
 
 4829 
 
 Moderately bright. 
 
 
 The lines near 3'97 and 11-02 belong to hydrogen. Probably traces of 
 aqueous vapour were present in the tube, which were decomposed by the 
 galvanic current. These two lines are not to be found in a lower temperature 
 in the broad part of the tube. It is striking that the red nitrogen-line near 
 5-04 is also missing there. In the narrow part of the tube the lines stand out 
 in the green on a very dimly-lighted ground, whilst in the wider part they 
 appear on a perfectly dark ground. 
 
200 
 
 APPENDIX E. 
 
 II. Hydrogen, 
 a. In the narrow part of the tube. 
 
 Screw. 
 
 Wave-length. 
 
 Remarks. 
 
 3-98 
 
 6558 
 
 Very bright. 
 
 6-16 
 
 5813 
 
 Moderately bright, on both sides very faint lines. 
 
 7-01 
 7-18 
 
 5596 
 5555 
 
 Moderately bright. 1 On a dimly lighted ground, 
 Moderately bright. > which becomes fainter to- 
 
 7-77 
 
 5422 
 
 Faint, wards the violet. 
 
 8-95 
 
 5189 
 
 Moderately bright. ) n ,,., ,., i, 1.4. 
 
 10-03 
 
 5008 
 
 71 * i 1 vJQ. fli I ;i 1 1 ) i St/GZldlly DriTnT 
 
 1 i M 1 1 1 . Y -i 
 
 10-55 
 
 4929 
 
 Moderately bright.] 
 
 11-04 
 12-86 
 
 4861 
 4632 
 
 Very brieht 1 From U ' 5 to 12 ' 9 a bright 
 
 Moderately bright. \ 3*% which tOTvar , ds g 6 
 J J violet becomes very bright. 
 
 13-32 
 
 4581 
 
 Very faint. 
 
 14-05 
 
 4506 
 
 L On a dull ground. 
 
 15-90 
 
 4342 
 
 Very bright. 
 
 b. In the broad part of the tube. 
 
 Screw. 
 
 Wave-length. 
 
 Remarks. 
 
 5-30 
 
 6063 
 
 Faint. 
 
 
 7-00 
 
 5598 
 
 Bright. 
 
 
 8-96 
 
 5187 
 
 Very bright. 
 
 
 11-28 
 
 4828 
 
 
 
 14-04 
 
 4507 
 
 Moderately bright. 
 
 
 The lines appeared on a perfectly dark ground. 
 
 The tube shows in the narrow part the hydrogen-spectrum of the first order ; 
 the lines in the green do not coincide with the lines of the nitrogen, though 
 some lines belonging to nitrogen are found. Here, too, most probably small 
 particles of aqueous vapour have been enclosed in the tube and are decom- 
 posed. Very striking is the spectrum in the broad part of the tube ; nothing 
 is to be seen of the bright shining lines Ha 3'98, H/3 11*04, Hy 15*90 ; on 
 the other hand, four very bright lines and one quite faint one are in the red 
 end of the spectrum, which appear, in opposition to the spectrum of the 
 narrow part, not on a partially lighted, but on an entirely dark ground. The 
 appearance is very striking if we bring the tube in front of the slit ; and so, 
 by degrees, at first the light in the narrow part, then the light at the con- 
 necting-point of the narrow and wide parts, and, finally, the light in the 
 latter fall upon the slit. At the connecting-point of the wide ends of the 
 
INQUIRIES INTO THE SPECTRUM OF THE AUEOEA. 
 
 201 
 
 tube the three well-known hydrogen-lines decrease in intensity, the con- 
 tinuous ground of some parts of the spectrum disappears, and a new line 
 appears near 11-28, which has about the same brilliancy as H/3. 
 
 A comparison with the spectrum of oxygen shows the bright lines which 
 are in the spectrum in the wide end of the tube as belonging to that element. 
 The heat evolved by the current appears insufficient to bring the hydrogen to 
 glow, whilst by it the oxygen, which is of a more rarefied character, becomes 
 incandescent. An alteration of the direction of the current has no influence 
 on the appearance. 
 
 III. Nitrogen, 
 a. In the narrow part of the tube. 
 
 Screw. 
 
 Wave-length. 
 
 Remarks. 
 
 3-84 
 
 6620 
 
 1 Several faint, broad, close lines, increasing in 
 
 4-85 
 
 6213 
 
 j brilliancy as they approach the violet end. 
 
 5-30 
 
 6063 
 
 1 
 
 
 5-51 
 
 6000 
 
 Broad bright lines, so close together that the 
 
 5-69 
 
 5948 
 
 intervening spaces appear like fine dark lines. 
 
 5-87 
 
 5896 
 
 > This part of the spectrum is very bright, but 
 
 6-04 
 
 5846 
 
 not uniform, being brighter towards the violet 
 
 6-20 
 
 5802 
 
 end. 
 
 
 6-43 
 
 5741 
 
 
 
 "13 
 
 5607 
 5567 
 
 Group of faint but at the same time very broad 
 
 7-28 
 
 *J*J\J 1 
 
 5532 
 
 lines. The last is the brightest. 
 
 7-55 
 
 5470 
 
 The dark intervening spaces are somewhat broader, 
 
 7-74 
 
 5428 
 
 the bright lines somewhat more intense than 
 
 7-92 
 
 5389 
 
 in the preceding group, and all of almost equal 
 
 8-09 
 
 5353 
 
 brilliancy. 
 
 
 8-32 
 
 5306 
 
 
 
 8-50 
 
 5272 
 
 Very faint fine lines. 
 
 
 8-69 
 
 5237 
 
 
 
 9-01 
 
 5178 
 
 Very bright broad misty line. 
 
 9-67 
 
 5066 
 
 Very bright line. 
 
 
 10-25 
 
 4975 
 
 
 
 10-66 
 
 4913 
 
 is 
 
 The bright lines are 
 
 11-03 
 
 4862 
 
 Very faint line. 
 
 sharply defined to- 
 
 11-41 
 
 4811 
 
 Bright line. 
 
 wards the red end of 
 
 12-11 
 
 4721 
 
 >9 5 
 
 > the spectrum, fading 
 
 12-57 
 
 4666 
 
 Faint line. 
 
 away towards the 
 
 12-57 
 
 4644 
 
 Bright, broad, misty line. 
 
 other end of the 
 
 13-42 
 
 4570 
 
 Very bright line. 
 
 spectrum. 
 
 14-24 
 
 4487 
 
 5 59 
 
 
 15-02 
 
 4417 
 
 Bright line. 
 
 
 15-66 
 15-72 
 
 4363 
 4357 
 
 } Bright lines. \*L 
 
 lines sharply defined 
 Is red end, indistinct 
 
 15-87 
 
 4345 
 
 Bright line. towards other end of spec- 
 
 16-72 
 
 4273 
 
 J trum. 
 
 
 Here follow several lines. 
 
202 
 
 APPENDIX E. 
 
 b. In the wide part of the tube. 
 
 Screw. 
 
 Wave-length. 
 
 Eemarks. 
 
 6-20 
 
 5802 
 
 Faint, indistinct, broad line. 
 
 7-72 
 
 5433 
 
 Dull stripe. 
 
 8-20 
 
 5330 
 
 Faint line. 
 
 8-94 
 
 5191 
 
 Very faint line. 
 
 9-03 
 
 5175 
 
 Broad band of light. 
 
 9-90 
 
 5029 
 
 Dull band of light. 
 
 10-68 
 
 4911 
 
 Moderately bright line. 
 
 11-42 
 
 4809 
 
 Faint line. 
 
 12-59 
 
 4663 
 
 Bright line. 
 
 13-43 
 
 4569 
 
 55 
 
 14-07 
 
 4504 
 
 Moderately bright line. 
 
 14-25 
 
 4486 
 
 Very bright line. 
 
 15-85 
 
 4347 
 
 ii 
 
 16-76 
 
 4273 
 
 Moderately bright line. 
 
 c. At the aura of the negative pole. 
 
 Screw. 
 
 Wave-length. 
 
 Remarks. 
 
 5-18 
 
 6100 
 
 ! Broad, moderately bright stripe of light, indistinct 
 
 5-70 
 
 5945 
 
 at the edges. 
 
 7-60 
 8-41 
 
 5159 
 5289 
 
 Broad, moderately bright stripe. 
 
 8-76 
 
 5224 
 
 f Very bright line, somewhat indistinct towards the 
 \ violet. 
 
 9-19 
 
 5147 
 
 Faint line. 
 
 10-0 
 
 5004 
 
 Bright line, indistinct towards the red. 
 
 10-67 
 
 4912 
 
 f Somewhat fainter than the last, indistinct towards 
 \ the red. 
 
 11-43 
 
 4808 
 
 Very faint line. 
 
 12-25 
 
 4704 
 
 Very intense, broad, indistinct towards the violet. 
 
 12-73 
 
 4646 
 
 Very faint line. 
 
 13-43 
 
 4569 
 
 Moderately bright, indistinct towards the violet. 
 
 14-25 
 
 4486 
 
 Like the last. 
 
 15-03 
 
 4417 
 
 Quite a faint line. 
 
 15-86 
 
 4346 
 
 Moderately bright line. 
 
 16-76 
 
 4275 
 
 Very bright line. 
 
 Here follow several other lines. 
 
INQUIRIES INTO THE SPECTEUM OF THE ATJEOEA. 
 
 203 
 
 The observations in the different parts of the tube show plainly the 
 dependence of the spectrum on the temperature. The aura of the negative 
 pole gives the line near 10-07 so characteristic of the air-spectrum. This is 
 the same line which is met with in the spectra of most of the nebulae. The 
 very striking groups of lines in the red and yellow in the spectrum of the 
 narrow part of the tube disappear entirely in the wide part. If we compare 
 the spectra with those above quoted, of oxygen and hydrogen, we find line 
 H/3 very faint in the spectrum of the narrow part of the tube near 11'03 ; 
 on the other hand, oxygen-lines appear in the broad part near 8'20, 8' 94, and 
 14-07. Thence I would conjecture that the tube was not filled with pure 
 nitrogen, the appearance of which is precise, but with dry rarefied air, since 
 Wiillner's researches have proved that dry air yields the same spectrum as 
 nitrogen gas. Perhaps the air in the tube examined by me had not been 
 thoroughly dried, and thus the appearance of some lines of the elements 
 before named is to be explained. 
 
 I must further mention that the electrodes of the tubes consisted of 
 aluminium ; yet a comparison of the spectra observed and the aluminium 
 spectrum has shown no connexion between them. 
 
 IV. Atmospheric Air. 
 
 Screw. 
 
 Wave-length. 
 
 Remarks. 
 
 5-88 
 
 6892 
 
 Very bright double line (Na). 
 
 6-67 
 
 5680 
 
 Very bright line. 
 
 7-20 
 
 5550 
 
 Faint line. 
 
 9-00 
 
 5180 
 
 Very bright line. 
 
 9-79 
 
 5047 
 
 Fine faint line. 
 
 10-03 
 
 10-07 
 
 500S 
 5002 
 
 I Very bright double line. 
 
 11-43 
 
 4803 
 
 Faint confused line. 
 
 12-69 
 
 4651 
 
 i 
 
 12-84 
 
 4633 
 
 } Faint line not sharply defined. 
 
 13-04 
 
 4612 
 
 
 From 14-61 
 
 44.-,:! 
 
 ) Confused band of light, which ends with a broad 
 
 to 15-88 
 
 4444 
 
 J washy line. 
 
 Here follow several other lines. 
 
 Jr. 
 
204 
 
 APPENDIX E. 
 
 Rarefied air saturated with aqueous vapour. 
 
 Screw. 
 
 Wave-length. 
 
 Remarks. 
 
 3-97 
 
 6562 
 
 Moderately bright line. 
 
 (5-88) 
 
 5892 
 
 Bright double line (Na). 
 
 (6-25) 
 
 5789 
 
 Bright line (H). 
 
 From 7-03 
 
 5591 
 
 } Broad dull band of light ; near 7'03 a somewhat 
 
 to 7-55 
 
 5470 
 
 brighter line. 
 
 7-59 
 
 5461 
 
 Bright line (H). 
 
 8-72 
 
 5231 
 
 Dull stripe. 
 
 8-96 
 10-07 
 
 5187 
 5002 
 
 -p , , * [ On a dull steady ground. 
 
 11-05 
 
 4859 
 
 Very bright line. 
 
 12-21 
 
 4709 
 
 Moderately bright line. ~\ Qn ^ , ^ hted CTO und 
 
 12-75 
 13-28 
 
 4644 
 5585 
 
 \j V uocoini us? imiiiGr to 
 preceding. . , 
 ir *.! /-TT\ wards the violet. 
 Very faint line (H). J 
 
 (15-71) 
 
 4358 
 
 Very bright line (H). 
 
 15-90 
 
 4341 
 
 U 11 
 
 Here follow several more lines. 
 
 In the first observations, the electric spark, about 1 centim. in length, was 
 allowed to pass between platinum points in ordinary air. 
 
 The sodium-line near 5-88 appeared continually. The bright double line 
 at 10-03 and 10-07, with a weaker current or longer spark, was no longer to 
 be recognized as a double line, but appeared as a broad somewhat confused 
 line, of which the brightest part was near 10 - 05. No lines belonging to the 
 platinum spectrum appeared. Ordinary rarefied air, under a pressure of 
 25 to 30 millims., and which was enclosed by mercury in a tube 8 millims. 
 wide, showed exactly the same lines as Pliicker's nitrogen-tube (#), except 
 that some lines belonging to the spectrum of mercury also appeared. 
 
 This observation may be regarded as a confirmation of the conjecture above 
 expressed as to the condition of Pliicker's tube III. (nitrogen). In the obser- 
 vations described under b, the air saturated with aqueous vapour was under 
 a pressure of 22 millims. Besides the sodium-lines,, lines of the mercury- 
 spectrum appeared at 6'25, 7'59, and 15-71. The spectrum of rarefied air 
 under similar pressure was found to accord completely with the spectrum of 
 the light in the broad part of Pliicker's tube. 
 
 III. (Nitrogen b.) A comparison of the spectrum of rarefied air saturated 
 with aqueous vapour with the former shows the striking alterations in the 
 spectrum which are brought about by the presence of the aqueous vapour. 
 
INQUIRIES INTO THE SPECTRUM OF THE AURORA. 205 
 
 3. Comparison of the Aurora-Spectrum with the Spectra of Atmospheric 
 Gases and of Inorganic Substances. 
 
 In the next place, I turn to the comparison of the observed spectra of 
 different gases and of the air with the spectrum of the Aurora. The first 
 band of light in the red part of the Aurora-spectrum most probably coincides 
 with the first system of lines in the spectrum of nitrogen (a). Probably only 
 the bright part of this group of lines is perceptible, on account of the 
 extreme faintness of the Aurora ; and as in nitrogen the increase of the 
 brilliancy of the spectrum takes place towards the violet end, the absence of 
 the intermediate spectrum towards this direction would find its explanation. 
 The most intense line of the Aurora-spectrum at 7'12 is to be also found in 
 the spectrum of nitrogen (a) as a very faint line, however. That this line 
 appears in the Aurora by itself, and with intensity relatively great, need not 
 appear strange, considering the great alteration of the gas-spectra under 
 different conditions of pressure and temperature. The third line of the 
 Aurora-spectrum, very vaguely defined on account of its great faintness, 
 coincides in the same way with a nitrogen-line. 
 
 The line at 8*71 is met with in the nitrogen-spectrum (c), as well as in the 
 air-spectrum (b). The third line of the oxygen-spectrum at 8'95, which 
 seems to appear under very different conditions, is found again, as the fifth 
 line in the spectrum of the Aurora. Moreover, the sixth line in the Aurora 
 at 10'06 coincides very exactly with the known nitrogen-line appearing in the 
 spectra of some of the nebulae. Lastly, as to the broad band of light in the 
 Aurora-spectrum from 12'33 to 12-88, several lines are found in this place in 
 the spectrum of nitrogen as well as the air-spectrum (a, b) ; so that here, too, 
 a coincidence between the spectra may be regarded as probable. 
 
 The observations show with some certainty that at least one line at 10-06 
 of the Aurora-spectrum coincides with the maximum brilliancy of the air- 
 spectrum, and that the other lines appear with great probability in the 
 spectra of atmospheric gases. 
 
 In the very great difference of the gas-spectra under varying conditions of 
 pressure and temperature, it would indeed be difficult to succeed in pro- 
 ducing artificially a spectrum which should resemble that of the Aurora in all 
 parts. Moreover, it must be admitted, under the hypothesis that the Auroras 
 are electric discharges in rarefied air-strata, that these strata, qualified for the 
 transmitting of electricity, will have a very considerable thickness. 
 
 2 E 2 
 
206 
 
 APPENDIX E. 
 
 In this case the conditions of pressure on these air-strata are themselves so 
 different that, within certain limits, each will yield its own peculiar spectrum ; 
 but we shall see the sum of collective spectra, so to speak, spread out behind 
 each other ; and therefore the impossibility of attaining a perfect agreement 
 between the Aurora-spectrum and the artificially exhibited spectra of mixed 
 gases is evident. 
 
 A comparison of the Aurora-spectrum with the spectra of inorganic substances 
 may be easily worked out by the help of the above-quoted wave-lengths of the 
 single lines of the former, with due regard to probable errors, and with the 
 aid of Angstrom's Atlas of the Solar Spectrum. Here the perfect harmony of 
 the brightest Aurora-line (which was fixed with an exactitude of about one 
 seventh of the separation of the sodium-lines) with the lines of the iron- 
 spectrum is especially striking. The wave-lengths in the above-cited ob- 
 servations of the bright Aurora-line vary between 556'9 and 557'3, whilst, 
 
 o 
 
 according to Angstrom, two lines of the iron-spectrum are situated at 556-85 
 and 557-17. 
 
 Iron-lines corresponding to the other Aurora-lines, within certain limits of 
 accuracy, are also to be found, as will be seen from the following com- 
 parison : 
 
 Aurora-lines. 
 
 Lines of the iron- 
 spectrum. 
 
 Remarks. 
 
 629-7 
 
 f 630-08 1 
 { 629-85 / 
 f 539-60 1 
 j 539-92 1 
 1 539-05 f 
 ( 538-85 J 
 f 523-43 
 J 523-21 
 [ 522-90 
 T519-79 
 | 519-40 
 <( 519-16 
 1 519-06 
 1 518-51 
 f 500-65^1 
 500-52 | 
 < 500-49 > 
 | 500-30 | 
 1^500 -20 J 
 
 f , [ 3 stronger and < 
 y j 
 
 Moderately bright. 
 
 Mostly very faint. 
 
 Very faint. 
 Moderately bright. 
 Very faint. 
 > 
 * 
 Moderately bright. 
 
 
 Very faint. 
 
 Very faint. 
 t very faint iron-lines. 
 
 539-0 
 
 523-3 
 
 518-9 
 
 500-4 
 
 From 469 
 to 462 
 
 Yet this agreement, though remarkable, can only be considered as complete 
 
INQUIRIES INTO THE SPECTRUM OF THE AURORA. 207 
 
 proof of the presence of iron-vapour in the atmosphere when we shall have 
 succeeded in showing by observation analogous modifications of the relative 
 conditions of brilliancy in the iron-spectrum by alterations of temperature 
 and density ; and in this way explain the appearance of relatively very faint 
 iron-lines in the Aurora-spectrum, or, on the other hand, the absence of the 
 most intense lines. 
 
 It will meanwhile remain far more in accordance with probability to 
 regard the Aurora-spectrum as a modification of the air-spectrum ; since we 
 are already aware, in the case of gases, of the alteration of the spectra by 
 conditions of temperature and pressure ; and an agreement, at any rate, quite 
 as certain between the spectrum in question and the spectra of atmospheric 
 gases has been proved above. 
 
 [I am indebted to Miss Annie Ludlam for a translation from the German 
 of the above Memoir J. R. C.] 
 
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