20 SCIENCE PROGRESS 



nucleus is actually lenticular in shape, and the rotation of 

 certain spiral nebulae has recently been conclusively estab- 

 lished by Slipher. Moreover, such evidence as is available 

 supports the view that the matter in the arms is in motion 

 outwards from the nucleus and not inwards to it. The matter 

 is in all cases thrown off from two opposite points on the peri- 

 phery, and not in the form of a ring, as conjectured by Laplace. 

 This could be accounted for by supposing the existence of 

 tidal forces from neighbouring masses, a comparatively small 

 force only being required to cause the ejection of matter when 

 the mass reaches its critical state of equilibrium. Thus the 

 matter will be ejected in two symmetrical filaments. The 

 characteristic equiangular spiral shape of the arms cannot at 

 present be accounted for in a fully satisfactory manner. 



Jeans further finds that the arms will become unstable and 

 break up into several masses if they are on a sufficiently mas- 

 sive scale, but that if their mass is small the gravitational 

 attraction is not sufficient to enable them to cohere together, 

 and they will diffuse away into space. In the former case, 

 the nuclei will form at approximately regular intervals, and 

 such nuclei are actually found in the arms of most spiral 

 nebulae. The order of distance apart of these nuclei can be 

 calculated, and also their masses. Jeans finds that the masses 

 will be comparable with the mass of our sun, so that it is very 

 improbable that the planets of the solar system could have 

 been formed in this way. If a nebula of mass as small as our 

 sun had thrown off arms they would have dissipated into 

 space. 



The work of Jeans has therefore considerably advanced 

 our knowledge of the nebular hypothesis, and it appears prob- 

 able that while the sequence of events pictured by Laplace 

 may have happened in the case of the spiral nebulae, they could 

 not have happened in the case of the solar system, to explain 

 the existence of which the hypothesis was formulated. 



The Application of Photoelectric Photometry to Astronomy. — 

 Although astronomy is the oldest of the sciences, it is never- 

 theless true that many of the principal advances which have 

 been made in it of late years have been due to the application 

 of new methods. It is only necessary to instance the applica- 

 tion of photography, which has made possible the rapid deter- 

 mination, amongst other things, of the positions of faint stars, 



