284 IN STARRY REALMS. 



of a millimetre. Now if the star is coming towards us, 

 the line F will be displaced, with the rest of the spectrum, 

 towards the blue end, and we want to measure the amount 

 of that displacement. For this purpose we require a com- 

 parison spectrum which shall show where the line ought 

 to be, if the body were at rest, relatively to the earth. "We 

 obtain this by availing ourselves of the wonderful principle 

 discovered a quarter of a century ago, by which the 

 interpretation of the dark lines in the spectrum is 

 obtained. 



Suppose that the element hydrogen be heated to incan- 

 descence by the electric spark, or in any other way, it will 

 of course radiate light, and when that light is viewed 

 through the spectrum certain brilliant lines will be per- 

 ceived; these lines are characteristic of that particular 

 element ; where these lines are found hydrogen is present, 

 where they are absent, hydrogen is absent ; provided the 

 conditions as to incandescence have been attended to. But 

 these are bright lines on a dark background, whereas the 

 lines in the spectrum of a star are often black lines on a bril- 

 liant background ; how then are the two to be connected 

 together ? It is the unveiling of this connection that con- 

 stitutes the discovery to which I have referred. The 

 brilliant lines, it is to be observed, are produced by highly 

 heated hydrogen ; the dark lines in the spectrum of a star 

 are due, not to heated materials, but to comparatively cool 

 materials. In the upper regions of the star's atmosphere, 

 the actual glowing surface sheds forth in copious abund- 

 ance rays of every hue, but before these gain the outer 

 space they have to pierce through the comparatively cold 

 atmosphere with which each star may be surrounded. 



