A Century of /Science 7 



nebula of Orion, sodium and potassium, calcium 

 and iron, in the sun ; demonstrating the gaseous 

 character of nebulae ;* and revealing chemical ele- 

 ments hitherto unknown, such as helium, a mineral 

 first detected in the sun's atmosphere, and after- 

 ward found in Norway. A still more wonderful 

 result of spectrum analysis is our ability to mea- 

 sure the motion of a star through a slight shifting 

 in the wave-lengths of the light which it emits. 

 In this way we can measure, in the absence of all 

 parallax, the direct approach or recession of a star ; 

 and in somewhat similar wise has been discovered 

 the cause of the long-observed variations of bril- 

 liancy in Algol. That star, which is about the 

 size of our sun, has a dark companion not much 

 smaller, and the twain are moving around a third 

 body, also dark : the result is an irregular series 

 of eclipses of Algol, and the gravitative forces ex- 

 erted by the two invisible stars are estimated 

 through their effects upon the spectrum of the 

 bright star. In no department of science has a 

 region of inference been reached more remote than 

 this. From such a flight one may come back 

 gently to more familiar regions while remarking 

 upon the manifold results that have begun to be 

 attained from the application of a sensitive photo- 

 graph plate to the telescope in place of the human 



