NEBULAE. 



By V. M. SLIPHER, Ph.D. 



{Read April 13, 1917.) 



In addition to the planets and comets of our solar system and 

 the countless stars of our stellar system there appear on the sky 

 many cloud-like masses — the nebulae. These for a long time have 

 been generally regarded as presenting an early stage in the evolu- 

 tion of the stars and of our solar system, and they have been care- 

 fully studied and something like 10,000 of them catalogued. 



Keeler's classical investigation of the nebulje with the Crossley 

 reflector by photographic means revealed unknown nebulae in great 

 numbers. He estimated that such plates as his if they were made 

 to cover the w^hole sky would contain at least 120,000 nebulae, an 

 estimate which later observations show to be considerably too small. 

 He made also the surprising discovery that more than half of all 

 nebula are spiral in form ; and he expressed the opinion that the 

 spiral nebulae might prove to be of particular interest in questions 

 concerning cosmogony. 



I wish to give at this time a brief account of a spectrographic 

 investigation of the spiral nebulas which I have been conducting at 

 the Lowell Observatory since 1912. Observations had been previ- 

 ously made, notably by Fath at the Lick and Mount Wilson Observ- 

 atories, which yielded valuable information on the character of 

 the spectra of the spiral nebulae. These objects have since been 

 found to be possessed of extraordinary motions and it is the obser- 

 vation of these that will be discussed here. 



In their general features nebular spectra may for convenience 

 be placed under two types characterized as (I.) bright-line and 

 (II.) dark-line. The gaseous nebulas, which include the planetary 

 and some of the irregular nebtilae, are of the first type; while the 

 much more numerous family of spiral nebulas are, in the main, of 

 the second type. But the two are not mutually exclusive and in the 



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