72 PRESSURE OF LIGHT 



repelled from the sun. Smaller spheres would be 

 repelled, and finally would be driven away alto- 

 gether. If they reflected back some of the light, the 

 repulsion would be thereby increased, and a drop 

 of water of radius a hundred thousandth of a centi- 

 metre would be repelled probably ten or twelve 

 times as much as it would be attracted. But the 

 law of inverse proportionality to the radius does 

 not hold when the radius becomes small compared 

 with a wave-length of the repelling light. Certain 

 diffractive effects, pointed out by Swartzschild l 

 come into play, and at a certain radius, not far 

 from that last given, the ratio of pressure to pull 

 begins to fall off, and falls off rapidly. 



If, then, there are dust particles in the solar 

 system a hundred thousandth of a centimetre 

 across, and not much more dense than water, they 

 will be vigorously repelled, and will be driven right 

 out of our system. 



The formation of comets' tails, which are almost 

 always directed away and nearly straight away 

 from the sun, has been ascribed to light-pressure. 



Euler used light-pressure to explain them long 

 ago, but without any clear demonstration that 

 light exerted pressure. A few years after Maxwell 

 had published his theory of the pressure, Fitz- 

 gerald 2 revived the explanation, applying it on the 



1 Kgl Bayer, Ak. d. Wess, XXXI. 293 (1901). 

 ? Scientific Writings , p. 108. 



