May 25, 1917] 



SCIENCE 



527 



Fig. 16. The North America Nebula, photographed by Baruard with the 10-inch Bruce Tele- 

 scope of the Yerkes Observatory. 



tern being non-luminous, or at least invis- 

 ible. 



2. There must be an enormous amount of 

 comet material distributed throughout 

 space. The astronomers discover only a 

 negligibly small proportion of the comets 

 which pass near the center of our solar sys- 

 tem. Maj^ not comet materials exist also in 

 abundance in the systems of the other stars 1 



3. Students of meteors have established 

 that the separate little bodies which collide 

 with the earth 's atmosphere and are respon- 



sible for the so-called shooting stars are 

 stupendous in numbers. It has been esti- 

 mated that as many as twenty or thirty 

 millions of such bodies collide with the 

 earth every twenty-four hours. How in- 

 comparably and inconceivably greater must 

 be the total number in our solar system. 

 The power of these countless particles to 

 obstruct the passage of light is not negli- 

 gible. There may be as many such particlei= 

 on the average around the other stars m 

 our stellar system. 



