76 PRESSURE OF LIGHT 



into the sun. The velocity is less by I in 2 X 

 74,000, and the year is therefore greater for it by 

 I in 148,000 of our year, or by about 210 seconds, 

 say 3^ minutes. If the particle has a radius 

 i/iooo of a centimetre the effect, being 1,000 

 times greater, will lengthen out its year by about 

 58 hours, or nearly 2\ days. 



If, then, there were a group of particles of 

 diameters from a centimetre downwards, and so 

 thinly scattered that their mutual actions could be 

 neglected, they would gradually get sorted out, 

 the larger ones going ahead of the smaller ones, 

 and ultimately the particles would form a ring 

 round the sun. 



This would be still true it the particles were 

 moving in an elliptic orbit instead of the simple 

 circular orbit which we have supposed. 



If the orbit is elliptical, another effect of light- 

 pressure comes in, which we may term the 

 "Doppler Reception Effect." 



Let fig. 33 represent the orbit. As a particle is 

 moving through P, say towards the apse A, and is 

 so lessening its distance from the sun, it is meeting 

 the stream of momentum flowing from the sun, 

 and is therefore receiving more momentum each 

 second and experiencing a greater sunlight pres- 

 sure than if it were at rest or were moving in a 

 circular orbit. But when the particle has passed 

 the apse A and is moving, say at Q, away from 



