234 



Mr. H. F. NeivalL 



[Feb. 9, 



a tail which is repelled and held repelled from the sun. That has 

 always been regarded as a mystery, and Arrhenius attempts to explain 

 it by saying that the comet's tail consists of small particles which are 

 repelled under the influence of the light of the sun more than they 

 are attracted by the mass of the sun. Schwarzschild, the professor of 

 astronomy at Gottingen, intervenes, and tells us that the magnitude 

 of the forces involved in explaining this repulsion produced by light 

 must not be calculated for the smaller particles according to the same 

 law as that which holds good for the larger particles. For the larger 

 particles throw clean shadows behind them, while the smaller particles 

 necessarily allow the Ught waves to creep round by diffraction into 

 the shadow. The smaller particles thus throw only ill-defined 

 shadows, and the radiation has a much smaller repulsive effect under 

 those circumstances than is the case with larger bodies where no such 

 diffractive effect is shown. He calculates what the true conditions 

 are for particles of various sizes, and his results are summarised in 

 this diagram. The ratio of the Hght pressure to gravitation is 

 represented by the length of the ordinate. For instance, the point 

 at the top of the curve indicates that the light pressure is twenty 



SCHWARZSCHILD'S 



LIGHT' PRESSURE nATfn 

 GRAVITATION ^^ ' l^- 



X«0-6J 



times as great as the attraction exerted by the sun in pulling small 

 particles, of diameter equal to about a third of a wave-length of 

 light, towards the surface of the sun. Along the line E L, the hght 

 pressure is just equal to the attraction produced by gravitation. 

 Bodies of the size that would ])e acted upon equally by light pressure 

 and gravitation would be neither repelled nor drawn into the sun. 



