DOUBLE STARS 163 



firmed by observation, and are readily noticed in the 

 sudden and very appreciable fall in pitch of the whistle 

 of an engine as it passes an observer at express speed. 



Light is also due to wave motion, being the result of 

 waves generated in, and transmitted by, the ether of 

 space ; colour, or, more definitely, position in the spectral 

 series, being determined, like the pitch of a note, by the 

 rapidity with which individual waves follow one another. 

 Owing to the great speed of light, i. e. that of the ether 

 waves, no change of colour that could be appreciated by 

 the eye could be produced by any velocity with which 

 we are acquainted, but the position of a ray in the photo- 

 graphed spectrum of the brighter stars can be measured 

 with such accuracy that the effect of an approach or re- 

 cession at the rate of a mile in a second can now be 

 detected. The spectrum of a star consists, like the 

 spectrum of the Sun, of a band of light displaying the 

 successive colours of the rainbow, but crossed by a 

 number of dark lines indicating the absorption of 

 definite rays by its atmosphere. In the case of an 

 approaching star these lines should consequently be 

 raised in pitch, or displaced towards the violet end of the 

 spectrum. By a motion of recession they should be 

 lowered towards the red. Such displacements are now 

 recognized in the spectra of a great number of stars, and 

 their study forms an important branch of astronomical 

 observation. From it, it is possible to deduce the com- 

 ponent of a star's velocity in the direction of the line of 

 sight. 



In 1889 Vogel applied this principle to the investiga- 

 tion of the problem of Algol. The spectrum of the star 

 was photographed at a quarter of its complete period 

 before its minimum, when, according to the eclipse 

 theory, it should be at S, receding from the Earth with 



M 2 



