CH. xxxiv. SPECTRUM ANALYSIS. 345 



(No. 4), with the nebula spectrum (No. 6), you will see at 

 once that the nebula spectrum is that of a luminous gas, 

 and so the truth of Sir W. Herschel's idea was proved, and 

 there can be now no doubt that the light of some of the 

 nebulae comes from gaseous matter ; chiefly, so far as we 

 can learn, of nitrogen and hydrogen. 



Spectra of Falling Stars and of Comets. I have 

 said that it was difficult to examine the spectrum of the 

 stars and nebulae, but something which to an ordinary 

 observer seems still more wonderful has been lately done. 

 Mr. Alexander Herschel and others have actually caught 

 the light of falling stars in the spectroscope, and in this 

 way have discovered that some of them give a continuous 

 spectrum, showing that they are incandescent solid bodies, 

 while others give a gas spectrum, on which are the bright 

 lines of potassium, sulphur, and phosphorus, and sodium. 

 Even comets, those strange visitors to our solar system, 

 have now had their light analysed. In 1864 Donati 

 succeeded in obtaining the spectrum of a comet, and found 

 that, besides the continuous coloured band produced by 

 the sunlight reflected from its surface, there were three 

 bright bands, yellow, green, and blue, separated by wide 

 dark spaces. This showed that the comet was self-luminous 

 and formed, at any rate partly, of glowing gas. In 1868 

 Dr. Huggins discovered that in Winnecke's comet this gas 

 gave the same spectrum as our ' marsh or olefiant ' gas 

 when it is made luminous by electricity in a vacuum tube, 

 and since then this hydrocarbon spectrum has been found 

 in all comets examined. The gases of sodium and iron 

 have also been detected in the tails of comets which have 

 passed very close to the sun, and were therefore probably 

 in a great state of electrical excitement. Thus comets, 

 though very complex in their nature, and so attenuated 



