322 THE AURORA AND THE SPECTROSCOPE. 



ment whose spectrum has a bright line where this one appeared. The 

 observation has been repeated over and over again, by Angstrom, by Otto 

 Struve, and recently by Mr. Plummer, always with the same result, — we 

 cannot tell what the substance may be to whose incandescence or luminosity 

 the aurora owes its brilliancy. But now a most remarkable discovery has 

 been effected. Angstrom has found that the mysterious line of the auroral 

 spectrum exists in the spectrum of another object which had been thought 

 to be wholly different in character. Ever since its discovery by Cassini, 

 the zodiacal light has been an object of interest to astronomers. Gradually 

 a theory had been formed respecting it, which had been sanctioned by the 

 authority of such men, as Humboldt and Sir John Herschel. It was held 

 that this appearance is due to the light reflected from a number of minute 

 cosmical bodies travelling around the sun within the orbit of our earth. 

 This theory had never been tested by spectroscopic analysis. Indeed, the 

 zodiacal light shines so faintly that it was hardly hoped its spectrum could 

 be rendered visible. But it was confidently anticipated that if the zodiacal 

 light ever were thus analysed, its spectrum would be that which the theory 

 required — that is, a very faint reproduction of the common solar spectrum. 

 Now, at length, we hear from Angstrom, that the spectrum of the zodiacal 

 light has been observed, and instead of being, as had been expected, a faint 

 rainbow-coloured streak, it presents but a single line. Tliat line is the 

 same that we see in the spectrum of the aurora ! In other words, the light 

 of the zodiacal gleam and that of the auroral streamers are due to the same 

 sort of electric discharge taking place in the same medium. Without pre- 

 tending to further interpret this startling result, we may indicate the 

 promise it affords of explaining a number of phenomena which have long 

 seemed most perplexing. When once we recognise the fact that electric 

 action is effective in producing any of the celestial lights, we have a resource 

 available to remove many difficulties. Astronomers were asking how com- 

 ets, for example, could exhibit the spectrum of the incandescent vapour of 

 carbon — that is, a spectrum indicative of the most intense heat, when, as 

 in the case of Winneck's comet (whose spectrum was of this nature), they 

 were farther from the sun than the earth is. The action of the sun in 

 exciting electrical discharges would be quite sufficient to account for this 

 and similar phenomena. Again, it has long been recognised that the 

 peculiarities of comets' tails seem only explicable as due to electrical action ; 

 but astronomers were unwilling to adopt such a theory without some posi- 

 tive evidence in its favour. We now have such evidence ; and it is most 

 probable that the first long-tailed comet which is submitted to spectro- 

 scopic analysis will establish the view which Euler put forth more than 

 half a century ago, that comets' tails have something in common with the 

 aurora and the zodiacal light. It would indeed be strange if three of the 

 most mysterious phenomena with which men of science are acquainted 

 should find their explanation simultaneously. 



