Fi<;. 42. Spiral Xduila M Si I'rsii? Majoris. 

 THE NEBULAE. 



Hypotheses of stellar evolution usually assume 

 that the stars and stellar systems originate in the 

 nebulae, of which many excellent photographs, 

 showing much new and remarkable structure, 

 have been obtained on Mount Wilson. These 

 cloud-like objects, many of which are shown 

 by the spectroscope to consist of glowing gases, 

 occur in the widest variety of form. 1 he 

 vast majority, however, are of definite spiral 

 structure. Several nights of each month are de- 

 voted to a systematic photographic survey of the 



