144 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1924 



choice as between one point of the equator and another if the various 

 minor factors were absent, but when these minor factors come into 

 play, a discrimination at once takes place. Assuming, as seems 

 likely, that the tidal irregularities are the minor factors which de- 

 termine the choice of points for the ejection of matter, mathematical 

 investigation shows that the ejection of matter will take place from 

 the two antipodal points on the equator at which the tide is highest. 

 The equator being slightly elliptical, these points are of course the 

 ends of its major-axis. After the nebula has passed its critical land- 

 mark, shown in Plate 1, Figure 1, its shape ought to be similar to the 

 lenticular figure which formed the landmark, but with the additional 

 feature of matter streaming out from two antipodal points on its 

 equator. 



This describes exactly what is observed in the spiral nebulae. Plate 

 1, Figure 2 (N. G. C. 5866) shows a nebula in which the ejection 

 of matter is just beginning; we notice the bulge along the equator 

 and the dark band which we may assume represents ejected matter 

 which is already cooling. Plate 2, Figure 1 (N. G. C. 4594) exliibits 

 a more advanced state of development; and Plate 2, Figure 2 (N. 

 G. C. 891), a still later one in which the ejected matter already 

 dwarfs the central nucleus in size, although probably not in total 

 mass. 



In all these figures we are looking at the nebulae very approxi- 

 mately edge-on. Plate 3, Figure 1 (M. 51) shows the well-known 

 " whirlpool " in Canes Venatici, a nebula which may be very similar 

 physically to that shown in Plate 2, Figure 2, but we see it face on ; 

 we are looking along its axis of rotation. Again the central nucleus 

 occupies only a small part of the picture. Plate 3, Figure 2 (M. 101) 

 and Plate 4, Figure 1 (M. 81) shows two nebulae, the evolution of 

 which has proceeded still further, so much so that in the last of 

 these there is very little nucleus left, and by far the greater part of 

 what we see is what we believe to be ejected matter. 



In both of these last two nebulae it will be seen that the arms of 

 ejected matter proceed from two antipodal points, exactly as re- 

 quired by dynamical theory. So far we have spoken of the matter 

 in these arms as ejected matter because theory has suggested this 

 interpretation, but we need not be satisfied with theory; there is 

 very direct observational evidence on the point. Various astrono- 

 mers, especially Van Maanen, have detected motion in the arms of 

 many nebulae, including the three shown in Plate 3, Figures 1 and 2, 

 and Plate 4, Figure 1. Their observations show that the arms are in 

 truth jets of matter coming out of the nucleus. Plate 4, Figure 2 

 shows the motion found by Van Maanen for about 100 points in the 

 nebula M. 81, the arrows showing the motion in a period of 1,300 



