PRESSURE OF LIGHT IN ASTRONOMY 71 



would be I if million million times the light- 

 pressure. Or, the ratio of light-pressure to gravi- 

 tative pull on each sphere increases in proportion 

 as the radius diminishes. If we imagine the sub- 

 division continued, when the earth was subdivided 

 into equal spheres, each with a radius one 47 

 million millionth of the radius of the earth, the 

 total light-pressure would equal the total gravitative 

 pull, and if each sphere had the same mean density 

 as the earth, viz. 5 '5, the equality would hold for 

 each sphere separately. The radius of each would 

 then be about 13*5 millionths of a centimetre. If 

 the radius were still further reduced, sunlight- 

 pressure would exceed gravitative pull and the sun 

 would push the sphere from it. 



If the push on a particle exceeds the pull at any 

 one distance from the sun, it will exceed it in the 

 same proportion at all distances, for both gravita- 

 tion pull and light push vary inversely as the 

 square of the distance, and as the particle is driven 

 away they both lessen together. If we suppose 

 the sphere to have the density of water, 5j times 

 less than that of the earth, pressure equals pull 

 when the radius is 5-5 times greater; that is, when 

 it is about 75 millionths of a centimetre about 

 the length of a wave of deep red light. 1 



Absorbing spheres of the density of water and 

 of this radius would be neither attracted to nor 

 1 Note 5, p. 90, 



