SPIRAL NEBULA PUISEUX, 149 



A moderate dispersion may be used Avith this class -wdthoiit weaken- 

 ing the lines of the spectrum too much. Keeler showed that the 

 brightest line does not occupy exactly the same position in all the 

 green nebulae. Natunxlly. these small differences are interpreted as a 

 sign of radial velocities. The 14 nebulae for which satisfactory results 

 have been obtained give for the radial component figures ranging from 

 18 to —64 kilometers per second. There is a predominance of nega- 

 tive values, evidently not because the green nebulae show a tendency 

 to approach us, but because the greater part of them which may be 

 easily observed are situated nearer the constellation Hercules toward 

 which our sun is moving, carrying us along with him. Contrary to 

 what is true of the nebulae in general, the majority of the green nebulae 

 lie in the mdky way. The existence of these gaseous bodies, owing 

 their light to a more or less extended mass of gas, has been considered 

 as furnishing the experimental basis formerly lacking for the Lapla- 

 cian hypothesis. 



Interesting as these results are, we will not dwell upon them as they 

 take us away from our subject. Indeed, of all the nebulae whose 

 spiral structure is beyond doubt, not one belongs in the class just 

 described. Not one is adapted to the determination of its radial 

 velocity. All of them, as well as the great majority of the faint 

 nebulae without defuiite form, shine with a white light wluch the 

 prism transforms into an apparently continuous spectrum. This 

 spectrum is too faint for the detection of absorption bands. However, 

 there is some justice in calling it purely stcUar. The white nebulae 

 owe the greater part of their light to the stars which are clustered 

 within them. As to the great nebula of Andromeda, which is the 

 brightest of the spiral nebulae, we may add that the majority of the 

 stars of its central portion are of the solar type. 



The contribution of the spectroscope to the study of the spiral 

 nebulas is on the whole somewhat restricted. The services rendered 

 by photography are, on the other hand, inestimable. The great 

 part taken by this method of study dates from the invention of the 

 sensitive bromo-gelatin plates. The green or gaseous nebulae, 

 whose light more strongly affects the photographic plate, brought 

 the first success. The photographs of Paul and Prosper Henry, of 

 Isaac Roberts, and of Keeler early showed evidence of a physical 

 relationship between the stars and the nebulae, even in the case of 

 the gaseous nebuke. Tliis connection is yet closer in the spiral 

 nebulae, of which we will now speak exclusively. 



About the year 1900 they were looked upon as rare and scattered 

 objects. Keeler undertook to form a collection of the most remark- 

 able nebulous objects and was led to the two following unexpected 

 conclusions: First, many nebulae formerly classed as globular, annular, 

 85360°— SM 1912 11 



