NEW APPLICATIONS OF LIGHT. 



47 



constructed for the purpose, Sir William Huggins, in 

 1868, found that such a change did occur in the case of 

 many stars, and that their rate of motion toward us or 

 away from us termed the radial motion could be cal- 

 culated. As the actual distance of some of these stars 

 has been measured, and their change of position annually 

 (their proper motion) determined, the additional factor 

 of the amount of motion in the direction of our line of 

 sight completes the data required to fix their true line 

 of motion among the other stars. 



This method of research has now been applied to 

 many double stars with great success, observations of 

 their spectra showing that in some cases they move one 

 toward and one away from us, as they must do if they 

 are revolving around their common centre of gravity in 

 an ellipse whose plane lies approximately in our direc- 

 tion. It has also brought to light the interesting fact 

 that some stars which appear singly in the most power- 

 ful telescopes are really double, since their spectra show 

 a shifting of spectrum lines, which after a considerable 

 time changes to an opposite direction, and by the period 

 occupied in the complete change of direction the time of 

 rotation of the component stars can be determined, 

 although one of the components has never been seen. 

 By this means the variable star Algol has been proved to 

 have a dark companion which partially eclipses it every 

 69 hours; and both Sirus and Procyon have been shown 

 to have dark or less visible companions, that of Sirius 

 being really just visible in the very best telescopes. The 

 unusual motions of Sirius have been long known, and 

 were supposed to be due to the presence of a companion, 

 which has now been shown to be the true explanation. 



