406 Prof. J. H. Poynting on Radiation Pressure. 



that radiation pressure has not yet succeeded in fully ex- 

 plaining the eccentricities o£ this comet. But comets are 

 vague creatures. As Mr. Plummer suggests, we hardly know- 

 that we are looking at the same matter in the comet at its 

 successive returns ; and I still have some hope that the want 

 of success is due to the uncertainty of the data. 



There is one more effect of this radiation pressure which is 

 worthy of note : its sorting action on dust particles. If the 

 particles in a dust cloud circling round the sun are of different 

 sizes or densities, the radiation accelerations on them will 

 differ. The larger particles will be less affected than the 

 smaller, will travel faster round a given orbit, and will draw 

 more slowly in towards the sun. Thus a comet of particles 

 of mixed sizes will gradually be degraded from a compact 

 cloud into a diffused trail lengthening and broadening, the 

 finer dust on the inner and the coarser on the outer edge. 



Let us imagine, as an illustration of this sorting action, 

 that a planet, while still radiating much energy on its own 

 account, while still in fact a small sun, has somehow captured 

 and attached to itself as satellite a cometary cloud of dust. 

 Then, if the cloud consists of particles of different sizes, while 

 all will tend to draw in to the primary, the larger particles 

 will draw in more slowly. But if the larger particles are of 

 different sizes among themselves, they will have different 

 periods of revolution, and will gradually form a ring all 

 round the planet on the outside. Meanwhile the finer particles 

 will drift in, and again difference in size will correspond to 

 difference in period and they too will spread all round, forming 

 an inner fringe to the ring. If there are several grades of 

 dust with gaps in the scale of size, the different grades will 

 form different rings in course of time. Is it possible that here 

 we have the origin of the rings of Saturn ? 



The Radiation Theory is only just starting on its journey. 

 Its feet are not yet clogged by any certain data, and all 

 directions are yet open to it. Any suggestion for its future 

 course appears to be permissible, and it is only by trial that 

 we shall find what ways are barred. At least we may be 

 sure that it deals with real effects and that it must be taken 

 into account. 



