THE ENVIRONMENT 47 



for about 2j days it shines with constant in- 

 tensity; thereupon it begins to decline and in 

 approximately 4^ hours sinks to its minimum 

 of brightness; then it becomes gradually 

 brighter until after 4^ hours more it has re- 

 attained its full brilliancy. This behavior is 

 explained by the supposition that Algol is 

 accompanied by a dark star and that their 

 movements are such that a partial eclipse 

 occurs every 69 hours. Pickering has suc- 

 ceeded in calculating, upon the assumption 

 that the dark star as a whole intercepts the 

 rays of Algol, the approximate sizes, veloci- 

 ties, and orbits of these two stars, one of 

 which is quite invisible. Many similar phe- 

 nomena lead to similar conclusions regarding 

 other variable stars. 



It is apparent that such dark bodies, whether 

 extinct suns or planets, represent another 

 stage in celestial evolution. Their past his- 

 tories may be various, for there is still room 

 for much doubt as to the manner of formation 

 and origin of planets, but at any rate all are 

 probably derived from luminous stars or 

 planets through the process of cooling, with 

 its accompanying crust formation. Like their 

 earlier forms, they must therefore be made up 

 of matter as we know it, since when a heavenly 

 body puts on a crust, it does not change the 



