59 



the planetesimal hypothesis offers a possible means 

 of accounting for their origin and development. 

 We are fortunate in being able to cooperate with 

 Chamberlin and Moulton, the authors of this 

 hypothesis, in a study of the structure of spiral 

 nebulae. The small spirals are of peculiar interest, 

 and here the loo-inch telescope should be of service 

 in extending our knowledge of the structural de- 

 tails of objects now barely recognizable as spiral 

 in form. 



MAGNITUDE OF THE UNIVERSE. 



In our study of the sun as a typical star and in 

 this investigation of nebulae as the probable source 

 of stellar existence, we are thus engaged on two 

 phases of the evolution problem. Let us now see 

 how the use of the 6o-inch reflector in cooperation 

 with Kapteyn has contributed in the same direc- 

 tion. A question of prime importance in astron- 

 omy is the magnitude of the universe. As our 

 telescopes increase in power we reach farther and 

 farther into space. As the sphere of vision en- 

 larges, each increase in its diameter must mean 

 the addition of a larger number of stars than the 

 previous equal increase produced, because it in- 

 volves the addition of a larger volume of space. 

 This reasoning is based on the assumption that 

 the stars are distributed nearly uniformly through 

 space, at least as far as we can penetrate it, and 

 that no light is lost in transmission. 



In practice we find that each successive advance 

 into a region of more distant, and therefore (on 



