420 THE BEAUTIES OF NATURE CHAP, 
resent now what others once were, and what 
many will some day become. 3 
_ 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 
