408 SLIPHER— NEBULA. 



Because of its bearing on the evolution of spiral nebulse it is de- 

 sirable to know the direction of rotation relative to the arms of the 

 spirals. But this requires us to know which edge of the nebula is 

 the nearer us, and we have not as yet by direct means succeeded in 

 determining even the distance of the spiral nebulae. However, in- 

 direct means, I believe, may here help us. It is well known that 

 spiral nebulse presenting their edge to us are commonly crossed by 

 a dark band. This coincides with the equatorial plane and must 

 belong to the nebula itself. It doubtless has its origin in dark or 

 deficiently illuminated matter on our edge of the nebula, which ab- 

 sorbs (or occults) the light of the more brightly illumined inner 

 part of the nebula. If now we imagine we view such a nebula 

 from a point somewhat outside its plane the dark band would shift 

 to the side and render the nebula unsymmetrical — the deficient edge 

 being of course the one nearer us. This appears to be borne out by 

 the nebulse themselves for the inclined ones commonly show this 

 typical dissymmetry. Thus we may infer their deficient side to be 

 the one toward us. 



When the result of this reasoning was applied to the above cases 

 of rotation it turned out that the direction of rotation relative to 

 the spiral arms was the same for all. (The nebula N.G.C. 4594 

 is unfortunately not useful in this as it is not inclined enough to 

 show clearly the arms.) The central part — which is all of the nebulae 

 the spectrograms record — turns into the spiral arms as a spring turns 

 in winding up. This agreement in direction of rotation furnishes a 

 favorable check on the conclusion as to the nearer edge of the 

 nebulse, for of course we should expect that dynamically all spiral 

 nebulae rotate in the same direction with reference to the spiral arms. 

 The character and rapidity of the rotation of the Virgo nebula 

 N.G.C. 4594 suggests the possibility that it is expanding instead of 

 contracting under the influence of gravitation, as we have been 

 wont to think. 



As noted before the majority of the nebulse here discussed have 

 positive velocities, and they are located in the region of sky near 

 right ascension twelve hours which is rich in spiral nebulse. In the 

 opposite point of the sky some of the spiral nebulae have negative 

 velocities, i. e., are approaching us ; and it is to be expected that 



