SPIRAL NEBULA PUISEUX. 147 



The object which first seemed to offer to Lord Rosse an unusual 

 character is numbered 51 in Messier's catalogue. It is to-day con- 

 sidered the most typical and the most curious of the spii-al nebulae. 

 If we examine how Lord Rosse drew it in 1850, we will find that the 

 rays do not come out from the nucleus in all directions but normally 

 and only from two diametrically opposite regions. The curvature, 

 pronounced at the start, decreases later but irregularly. One of the 

 spirals departing further from the center terminates in a secondary 

 bright nucleus. The principal spiral continues its path undisturbed 

 and completes at least a turn and a half before fading away. The 

 appearance of these structures, so fine, so geometrical, so prolonged, 

 gives the impression of a rapid whirling movement. 



Long aftei*wards, in 1878, Lord Rosso rctm'ned to tliis same object. 

 The general appearance remained the same, but the number of fila- 

 ments, their fineness and regularity of curvature seemed much 

 decreased. After mature examination, it appeared that the early 

 appearance had been judged too geometrical just as seems to be the 

 case with the canals of Mars. It looked as if now the principal spiral 

 expands into the secondary nucleus. 



Again, looking at the same object as photographed by Keeler at the 

 Lick Observatory, it is evident that the second dra\vdng of Lord Rosso 

 is the more faithful. But other important details are brought to 

 light. The junction of the two principal spirals with the main 

 nucleus is no longer radial but tangential. By theu* evident discon- 

 tinuity we are led to strongly doubt that they can be considered as 

 trajectories. Various points of the two spirals are the origin of inde- 

 pendent rays, each curved in the same sense as the main sj)iral but 

 with entirely dift'erent initial directions. At the starting points of the 

 secondary rays we always find a star, or if we look closer, a group of 

 stars. Upon a plate of the same nebula, taken by Dr. Isaac Roberts, 

 180 condensations were counted on the lines of the spmils. 



It is evidently weU in the presence of such immensely vast objects, 

 so different from any that we have at hand for experiments, to build 

 as much as possible on firm structural groundwork, neglecting no 

 evidence concerning their form, their structure, or distribution in 

 space. Thus armed, we may approach with less danger their fife 

 history and seek to know how these strange organisms are born and 

 how they grow. 



First, what can be stated as to the distribution of the spiral nebulae, 

 for instance with regard to that most natural plane of reference, the 

 mean plane of the milky way ? 



If we consider nebulae irrespective of class, wo can state on this 

 score a weU-defincd law. These objects show, as to theu' direction 



