420 THE BEAUTIES OF NATURE chap. 



resent now what others once were, and what 

 many will some day become. 



For, in addition to the luminous heavenly 

 bodies, we cannot doubt that there are count- 

 less others invisible to us, some from their 

 greater distance or smaller size, but others, 

 doubtless, from their feebler light ; indeed, we 

 know that there are many dark bodies which 

 now emit no light, or comparatively little. 

 Thus in the case of Procyon the existence of 

 an invisible body is proved by the movement 

 of the visible star. Again, I may refer to the 

 curious phenomena presented by Algol, a 

 bright star in the head of Medusa. The star 

 shines without change for two days and thir- 

 teen hours ; then in three hours and a half 

 dwindles from a star of the second to one of 

 the fourth magnitude ; and then, in another 

 three and a half hours, reassumes its original 

 brilliancy. These changes led astronomers to 

 infer the presence of an opaque body, which 

 intercepts at regular intervals a part of the 

 light emitted by Algol ; and Vogel has now 

 shown by the aid of the spectroscope that 

 Algol does in fact revolve round a dark, and 



