290 IN STARRY REALMS. 



particular wave length of light corresponding to the 

 line F, but they also respond to the line C, and to several 

 other lines as well. As the solar light, therefore, streams 

 through the mighty encompassing atmosphere of the sun, 

 in which hydrogen is a very important ingredient, it has 

 filtered from it those rays which are of the same kind, as 

 incandescent hydrogen is itself competent to produce. 

 Accordingly, the dark lines belonging to hydrogen in the 

 solar spectrum occupy the same positions as do the series 

 of bright lines, which are generated when hydrogen is 

 rendered incandescent in a vacuum tube. 



I am aware that the illustrations I have given only 

 render a very imperfect account of a great scientific doc- 

 trine, but they may suffice, at all events, for my present ob- 

 ject, which is to show the use of the dark lines when we are 

 studying the movements of the body from which the light 

 radiates. Let us take a star, surrounded with hydrogen, 

 and therefore showing in its spectrum the F lines as well 

 as the other lines characteristic of this gas. Let us further 

 suppose that the star is hurrying towards us, then, as we 

 have already seen, the effect of this movement is to impart to 

 the vibrations of each ray from the star, as interpreted by 

 the eye, a somewhat greater rapidity than they actually 

 possess. The F line of hydrogen coming from the star will 

 pour more vibrations into the eye in a second than would 

 have been the case if the star had no relative movement. 



By the aid of the electric spark we are able to produce an 

 artificial spectrum of hydrogen, and by suitable appliances 

 we can introduce into the spectroscope the actual glowing 

 spectrum of hydrogen side by side with the dark line 

 spectrum of the same gas as it has come to us from the 



