318 DISCOVERY CH. 



often being joined by the force of mutual attraction to 

 one several degrees fainter. In the star Sirius this 

 difference is accentuated to a noteworthy amount ; for 

 while it is the brightest object in the stellar universe, its 

 companion is so faint that it can be seen only by using 

 telescopes of great light-grasping power. 



There is really nothing strange in this disparity when 

 the matter is logically considered. Why should we 

 measure mass in the universe by what we are able to 

 detect with our limited optical sense ? To a blind man, 

 or to an eye capable of seeing everything, the brilliant 

 star Sirius would not seem so overpoweringly great by 

 the side of the dark heavy body to which it is united ; 

 and if mortals possessed a sense capable of being affected 

 in proportion to substance, as the sense of sight is by 

 luminosity, the dark body would have been noticed as 

 soon as man turned his face towards the skies. For 

 though the faint star now seen to accompany Sirius 

 would need to have its brightness increased twelve 

 thousand times to equal the brilliancy of that gem of the 

 sky, it is nearly half as heavy when measured by the 

 standard of mass. 



There are other cases in which the companions of 

 bright stars are very faint, but heavy out of all pro- 

 portion to their brightness. As with Sirius, the astro- 

 nomer Bessel expressed the conviction that Procyon, 

 which rises about half an hour before it, had a dark 

 companion, disturbing its movements. Half a century 

 later, in 1896, this companion was detected by Prof. 

 Schaeberle, so we have here another example of a body 

 known to exist long before it was seen. 



The difference between mass and luminosity accounts 

 for the interval that elapsed between prediction and 

 discovery of each of the cases mentioned. The com- 



