78 PRESSURE OF LIGHT 



If a group of particles of different sizes is moving 

 in an ellipse, the ellipse tends to become more cir- 

 cular for all, but the effect in a given time is greater 

 for the smaller particles, and there is a sorting 

 action, reducing the ellipticity for them more 

 rapidly than for the larger particles. 



There is a third effect of light-pressure on a small 

 particle which we may term the " Doppler Emission 

 Effect," and this must be manifested as a force 

 which always opposes the motion of the particle. 

 The particle is heated by the sun on the side ex- 

 posed to sunlight, and if it is small enough the heat 

 is conducted through to all parts rapidly enough 

 to make it all at practically the same temperature. 

 If it is at the earth's distance from the sun, and if 

 it absorbs all the radiation falling on it, it will be 

 at a temperature nearly the same as the mean of 

 the Earth's surface, say about 15 C. It is sending 

 out as much radiation as it receives when at this 

 temperature. But the waves it sends out in the 

 direction towards which it moves are shorter than 

 those which it sends out sideways ; and these again 

 are shorter than those which it sends out back- 

 wards. This is seen from fig. 34, where A, B, C, D 

 are successive positions of the particles and 

 WA, WB, We, WD the positions at a given instant 

 of the waves sent out by the particle when it is at 

 A, B, C, D respectively. As shown in the first 

 chapter, there is more energy in the shorter waves 



