Photography of the Skies 



crossed by sharp thin lines of darkness. These 

 bright lines, when the gas has high pressure, 

 broaden out and become almost continuous, so 

 as to resemble those emitted by a glowing solid. 

 Hence an astronomer is told by one particular 

 spectrum that it comes from a star having a 

 highly condensed gaseous core, while another 

 spectrum betokens a true nebula a vast body 

 of gas aglow in extreme attenuation. A spectro- 

 scope, therefore, reveals not only what a heav- 

 enly body is made of, but also the physical con- 

 dition in which its substances exists, whether as 

 a solid, a liquid, or a gas. 



The lines in a stellar spectrum are liable not 

 only to be broadened out, but to be shifted from 

 their normal place, and this shifting has profound 

 significance, according to a principle first an- 

 nounced by Christian Doppler in 1841. If a 

 star is at rest, relatively to the earth the tints 

 and lines of the elements aglow on its surface 

 will have positions in its spectrum as changeless 

 as those due to the iron or the sulphur aflame 

 on the chemist's tray. But if the star is moving 

 towards the earth, or away from it, the spectral 

 lines will appear a little to the right or left of 

 their normal position, and in so doing disclose 

 the rate of approach or recession. To under- 

 stand this we have only to enter the field of 

 acoustics. Suppose that a listener takes up his 

 post midway between two railway stations some- 

 what distant from each other. As .a locomotive 

 approaches him let us imagine that its whistle is 

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