57 



FIG. 43. Spiral Nebula M 101 Ursae Majoris. 



small nebulae, which have been catalogued in 

 great numbers by Herschel and others, but never 

 studied, except in a moderate number of cases, 

 under adequate telescopic power; in fact, in most 

 instances the existing (visual) records give no con- 

 ception whatever of the true form of these curious 

 objects. At the same time the spectra of many of 

 these nebulae are being photographed. 



Keeler's discovery, that there are at least 100,000 

 spiral nebulae, left no doubt of their fundamental 

 importance in the scheme of stellar evolution. The 

 development represented by their spiral forms is 



