128 



NATURE 



\_Dec. 1 1, 1879 



aurora; the most diverse conclusions are quoted, trigono- 

 metrical measurements giving results varying from a few 

 thousand feet up to 1,000 miles, while there are several 

 ■well attested instances in which auroral rays have been 

 seen actually between the observer and terrestrial objects. 

 If these latter observations are correct it is evident that 

 aurorae may be produced near the earth's surface, and 

 consequently in air of considerable density. They are 

 supported by the fact that the lower trigonometrical 

 measurements are less liable to fallacy than the higher, 

 since in the latter it may always be objected that ob- 

 servers at different stations might have seen different 

 arches, or that the auroral arch in general is merely a 

 perspective illusion produced by the termination of vast 

 numbers of parallel rays at the same height. Additional 

 observations of aurora; seen between the observer and 

 mountain-tops or other elevated objects would be of great 

 scientific interest. 



Another very important line of inquiry noted by Mr. 

 Capron is that of the connection of clouds and aurora?, 

 some types of cirrus cloud so much resembling aurora; in 

 their forms and arrangement that it is very probable that 

 in some of the reported cases of daylight aurora; the 

 observers may merely have noted arches of cirrus. On 

 the other hand, it is by no means unlikely that some form 

 of cloud, especially that which consists of small particles 

 of ice, may be illuminated by electric discharges, and be 

 the actual material basis of the phenomenon. In this 

 connection the coincidence of aurora; with mock suns and 

 similar appearances is of interest, since these indicate 

 the presence of minute ice-crystals in the upper air. The 

 Whitby fishermen, on September 23 of this year, reported a 

 considerable aurora, and on the same night the moon, "prior 

 to being obscured by clouds, seemed to shed a radiant 

 glow straight up and down " (probably a rudimentary 

 paraselene). 1 If aurora really is ever visible by daylight, it 

 would seem almost incontestable that it must consist in 

 some form of mist capable of reflecting as well as of 

 emitting light, for the light of the brightest aurora is very 

 inferior in intensity to that of the moon's surface, and the 

 moon by daylight only appears like a faint white cloud. 

 An aurora is a very brilliant one which lights the earth 

 as brightly as the full moon, and yet it probably covers a 

 great part of the sky, while the moon's diameter is only 

 half a degree. 



On p. 47 Mr. Capron summarises a most interesting 

 investigation of Donati's on the time of appearance of the 

 great aurora of February 4, 1S72, in which he shows that 

 it did not appear everywhere really simultaneously, but 

 at ihf same local hour, as if it depended, like celestial 

 phenomena, on something fixed and external to the earth 

 and its rotation. If this were more than a mere coin- 

 cidence it would be of the utmost importance, as proving 

 the cosmical character of the aurora, and it is very 

 desirable that the investigation should be repeated as 

 soon as a sufficiently extended display presents the 

 opportunity. Probably there is already such information 

 stored in meteorological registers for whoever will take 

 the trouble to seek it out. 



In Chapter VII. some observations of the moon during 

 eclipse are described, and it is suggested that the curious 

 red lighting of the shadowed portion may be due to lunar 



1 Friends' Schools Nat. Hht. Journ., November 15. 



aurora. Spectral observations, however, seem to lend no 

 support to the theory. It is noted that the colours of 

 Jupiter's bands seem brightest during periods of auroral 

 frequency. 



The suggested connection between aurora; and zodiacal 

 light is dismissed as unfounded, the latter evidently 

 being some form of reflected sunlight, and having a 

 totally different spectrum. The relation of the aurora to 

 the solar corona seems almost equally shadowy, depending 

 solely on a supposed coincidence of one of the lines in the 

 coronal spectrum with a faint band of doubtful position in 

 that of the aurora. We entirely sympathise with the 

 author in his protest against the identification of spectra 

 by the mere coincidence of single lines. Such coinci- 

 dences within the limits of observation with instru- 

 ments of small dispersion are exceedingly'.' numerous, 

 and the only safe ground of identification is that of like- 

 ness of general features, or at least coincidence of many 

 lines. 



The latter half of the volume is mainly devoted to the 

 discussion of the auroral spectrum and its supposed coin- 

 cidences. A reference to the plate and catalogue of 

 auroral lines (p. 104), however, is sufficient to show that 

 it is little use as yet to compare these measures with the 

 accurate determinations of solar and spark lines, only one 

 line out of the nine or ten given being positioned with any 

 approach to accuracy or general agreement of the obser- 

 vers, even to the third figure. It is much to be hoped that 

 Mr. Capron's suggestion of photographing the spectrum 

 may prove practicable, and after his extraordinary suc- 

 cess with the lines of vacuum tubes, as evidenced in his 

 recent work on " Photographic Spectra," we can hardly 

 doubt it. Dry plates are now prepared of extraordinary 

 sensitiveness, and there is practically no limit to the time 

 of exposure which may be employed. 



We may briefly summarise, however, the results of 

 comparison, so far as it is possible to compare with such 

 defective measures. 



Perhaps the first supposed identification of the auroral 

 spectrum was that of Procter, who announced the corre- 

 spondence of the bright yellow-green line with a band in 

 a vacuum tube, which he supposed to be due to oxygen, 

 but afterwards ascribed to a hydrocarbon impurity. We 

 should not allude to this here, since the correspondence 

 broke down under high dispersion, the auroral line prov- 

 ing slightly more refrangible than that of the tube; but 

 that we wish to give a word of explanation as to the con- 

 stant recurrence of these carbon lines, which have proved 

 misleading to many experimenters. As is well known, 

 the glass tubes and apparatus employed in such re- 

 searches are made by the use of a blowpipe fed with coal- 

 gas. The imperfectly burnt products of combustion 

 inevitably pass into the comparatively cool glass tubes, 

 and some of them, such as naphthalene, being of high 

 density, they are condensed on the inner surfaces, and 

 obstinately retained. When, however, they are subjected 

 to the high vacuums of the Sprengel pump, they slowly 

 volatilise, and being good conductors of the electrical 

 discharge, become frequently so brilliant as completely to 

 mask the spectrum of the small residue of other gas in 

 the tube. By heating the tube strongly during exhaus- 

 tion and " washing out " many times with the pure gas of 

 which the spectrum is desired, these accidental spectra 



