A HALF-CEXTURY OF SCIENCE. 



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moreover, obvious that the powerful engine of investigation afforded 

 us by the spectroscope is by no means confined to the substances which 

 form part of our system. The incandescent body can thus be exam- 

 ined, no matter how great its distance, so long only as the light is 

 strong enough. That this method was theoretically applicable to the 

 light of the stars was indeed obvious, but the practical difficulties 

 were very great. Sirius, the brightest of all, is, in round numbers, a 

 hundred millions of millions of miles from us ; and, though as big as 

 sixty of our suns, his light when it reaches us, after a journey of six- 

 teen years, is at most one two-thousand-millionth part as bright. Nev- 

 ertheless, as long ago as 1815 Fraunhofer recognized the fixed lines in 

 the light of four of the stars, and in 1863 Miller and Huggins in our 

 own country, and Rutherfurd in America, succeeded in determining 

 the dark lines in the spectrum of some of the brighter stars, thus show- 

 ing that these beautiful and mysterious lights contain many of the 

 material substances w r ith which we are familiar. In Aldebaran, for 

 instance, we may infer the presence of hydrogen, sodium, magnesium, 

 iron, calcium, tellurium, antimony, bismuth, and mercury ; some of 

 which are not yet known to occur in the sun. As might have been 

 expected, the convposition of the stars is not uniform, and it would 

 appear that they may be arranged in a few well-marked classes, indi- 

 cating differences of temperature, or, in other words, of age. Some 

 recent photographic spectra of stars obtained by Huggins go very far 

 to justify this view. Thus we can make the stars teach us their own 

 composition with light which started from its source before we were 

 born light older than our Association itself. 



Until 1864, the true nature of the unresolved nebulas was a matter 

 of doubt. In that year, however, Huggins turned his spectroscope on 

 to a nebula, and made the unexpected discovery that the spectra of 

 some of these bodies are discontinuous that is to say, consist of bright 

 lines only, indicating that, " in place of an incandescent solid or liquid 

 body, we must probably regard these objects, or at least their photo- 

 surfaces, as enormous masses of luminous gas or vapor. For it is from 

 matter in a gaseous state only that such light as that of the nebulas is 

 known to be emitted." So far as observation has yet gone, nebulas 

 may be divided into two classes : some giving a continuous spectrum, 

 others one consisting of bright lines. These latter all appear to give 

 essentially the same spectrum, consisting of a few bright lines. Two 

 of them, in Mr. Huggins's opinion, indicate the presence of hydrogen : 

 one of them agrees in position with a line characteristic of nitrogen. 



But spectrum analysis has even more than this to tell us. The old 

 methods of observation could determine the movements of the stars 

 so far only as they were transverse to us ; they afforded no means of 

 measuring motion either directly toward or away from us. Now, 

 Doppler suggested in 1841 that the colors of the stars would assist us 

 in this respect, because they would be affected by their motion to and 



