CHAPTERS ON THE STARS. 511 



Now, we may consider the waves which form light when they strike 

 our apparatus as beats in the ethereal medium which follow each other 

 with extraordinary rapidity, millions of millions in a second, moving for- 

 ward with a definite velocity of more than 186,000 miles a second. 

 Each spectral line produced by a chemical element shows that that 

 element, when incandescent, beats the ether a certain number of times 

 in a second. These beats are transmitted as waves. Since the velocity 

 is the same whether the number of beats per second is less or greater, it 

 follows that, if the body is in motion in the direction in which it emits 

 the light, the beats will be closer together than if it is at rest; if moving 

 away they will be further apart. The fundamental fact on which this 

 result depends is that the velocity of the light-beat through the ether 

 is independent of the motion of the body causing the beat. To show 



A B X 



® 



® 



the result, let A be a luminous body at rest; let the seven dots to the 

 right of A be the crests of seven waves or beats, the first of which, at the 

 end of a certain time, has reached X. The wave-length will then be one 

 seventh the distance A X. Now, suppose A in motion toward X with 

 such speed that, when the first beat has reached X, A has reached the 

 point B. Then the seven beats made by A while the first beat is trav- 

 eling from A to X, and A traveling from A to B, will be crowded into 

 the space B X, so that each wave will be one seventh shorter than before. 

 In other words, the wave-lengths of the light emitted by any substance 

 will be less or greater than their normal length, according to the motion 

 of the substance in the direction in which its light is transmitted, or in 

 the opposite direction. 



The position of a ray in the spectrum depends solely on the wave- 

 length of the light. It follows that the rays produced by any substance 

 will be displaced toward the blue or red end of the spectrum, according 

 as the body emitting or absorbing the rays is moving towards or from us. 

 This method of determining the motions of stars to or from us, or their 

 velocity in the line of sight from us to the star, was first put into prac- 

 tice by Mr. — now Sir William — Huggins, of London. The method has 

 since been perfected by photographing the spectrum of a star, or other 

 heavenly body, side by side with that of a terrestrial substance, rendered 

 incandescent in the tube of a telescope. The rays of this substance pass 

 through the same spectroscope as those from the star, so that, if the 

 wave-lengths of the lines produced by the substance were the same as 

 those found in the star spectrum, the two lines would correspond in 

 position. The minute difference found on the photographic plate is the 

 measure of the velocity of the star in the line of sight. 



It will be seen that the conclusion depends on the hypothesis that 



