26o SCIENCE AND METHOD. 



appreciably diverted from its course? What do we 

 mean by near enough ? This is necessarily somewhat 

 arbitrary, but let us assume that it is the distance 

 from the Sun to Neptune, which represents a deviation 

 of about ten degrees. Supposing, now, that each of 

 our stars is surrounded by a danger sphere of this 

 radius, will a straight line be able to pass between 

 these spheres ? At the mean distance of the stars of 

 the Milky Way, the radius of these spheres will sub- 

 tend an angle of about a tenth of a second, and we 

 have a thousand million stars. If we place upon the 

 celestial sphere a thousand million little circles with 

 radius of a tenth of a second, will these circles cover 

 the celestial sphere many times over? Far from it. 

 They will only cover a sixteen-thousandth part. Thus 

 the Milky Way is not the image of gaseous matter, 

 but of Crookes' radiant matter. Nevertheless, as there 

 was very little precision in our previous conclusions, 

 we do not require to modify them to any appreciable 

 extent. 



But there is another difficulty. The Milky Way is 

 not spherical, and up to now we have reasoned as 

 though it were so, since that is the form of equilibrium 

 that would be assumed by a gas isolated in space. 

 On the other hand, there are clusters of stars whose 

 form is globular, to which what we have said up to 

 this point would apply better. Herschel had already 

 applied himself to the explanation of their remarkable 

 appearance. He assumed that the stars of these 

 clusters are uniformly distributed in such a way that 

 a cluster is a homogeneous sphere. Each star would 

 then describe an ellipse, and all these orbits would be 

 accomplished in the same time, so that at the end of 



