A Century of Science 1 



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 



