94 PRESSURE OF LIGHT 



Then 



io 



if the whole momentum is absorbed. But it is an 

 exceedingly high estimate to suppose that y^ of the 

 sunlight is stopped. The mass repelled even on this 

 high estimate of absorption cannot exceed 



IO 



Behind a square kilometre or io 10 sq. cm. there cannot 

 be more than 



io 10 



= io d grammes 



io' 



or i kilogramme of matter. 



We thus obtain a superior limit to the amount of 

 matter in a comet's tail on the light-pressure theory. If 

 we suppose that the tail is io 7 kilometres long, and that 

 it consists of absorbing spheres of density i and radius 

 io~ 5 cm. each, and if we suppose that n = io, the maxi- 

 mum number v of particles behind a square centimetre 

 is given by 



V . 7TIO~ 15 = I0~ 7 , 



or v y- io 8 



47T 



As the number of c.c. behind i sq. cm. in a length of 

 io 7 kilometres is io 12 , the number of particles per c.c. 

 N is 



-- -- 

 io 12 473- 1 o 4 



or about 25 per cubic metre. 



