Manchester Memoirs, Vol. I. (1906), No. 1. 13 



Let me first take perhaps the simplest case. We 

 have said that the particles must be small. Now Clerk- 

 Maxwell pointed out years ago how the radiation of light 

 would exert a pressure on bodies receiving the light, 

 which would be quite insensible for large bodies but 

 might become important for very small ones. This 

 remark has scarcely received proper attention until 

 recently, but in the last year or two Professor Poynting* 

 and others have stated in definite form the amount of 

 this light pressure, and shown that for bodies smaller 

 than two wave lengths of light the pressure may be so 

 great as to counterbalance the sun's gravitational attrac- 

 tion. If so, the particles may be continually repelled 

 from the sun, instead of attracted. Repulsion could, of 

 course, also follow from electrical action of the sun. For 

 our purpose these two hypotheses can be considered 

 together. Supposing, then, particles to be repelled from 

 the sun, how far would their distribution fit in with the 

 observed law of brightness ? To a certain extent, the 

 supposition looks promising ; starting from the surface, 

 where the particles may be densely packed, they are spread 

 over a larger and larger surface as they travel outwards. 

 Moreover, since they are travelling with ever-increasing 

 velocity, their density will further diminish owing to 

 this fact. Further, the light received by each diminishes 

 with increasing distance from the sun whether the light 

 be simply scattered sunlight or the incandescent light of 

 the particle itself, heated up as it must be by the intense 

 radiation from the sun : but this is taken account of in 

 Professor Schuster's paper and the deduction we made 

 from it, whereby we converted the law of brightness into 

 a law of density of particles, viz., as the inverse 4^ power 



* Phil. Trans. ^ Series A, vol. 202, pp. 525-552; and J'roc. Physical 

 Society of London, vol. 19 ; also Phil, ^fag., April, 1905. 



