540 PEOFESSOR J. H. POYNTIXG OX RADIATIOX IX THE SOLAR SYSTEM ; 
The o-ravitative acceleration towards the siin at the distance of the earth is about 
0'59 centiin./sec.n Then we have 
Radiation pressure __ __ 
Gravitation pull U X X 0'59 
Tlie two will be efiual when 
-3 
Up X 0-59 
If Ave put 
p = 1 ; S = 0-175 X 10~; U = :3 X Kd'^ ; 
we £;'et 
a = 74 X 10~h 
Tills is the well-known result that a body of diameter about two wave-lengtlis of 
red light would be equally attracted and repelled if we coidd assume that a surface 
so small still continued to absorb. But, of course, when we are getting to dimensions 
comparable with a wave-length that assumption can no longer be made. 
It is not, I think, equally well recognised that if the radiating body is diminished 
in size, the radiation pressure due to it also decreases less rapidly than the gravitative 
pull which it exerts. For the radiation decreases as the square of the radius of the 
emitting body and its gravitative pull as the cube. 
We can easily compare the radiation and gravitation forces between tuo bodies, 
if for simplicity we assume that their distance apart is very great compared with the 
radius of either. 
Let AB, fig. 3, be two spheres with full radiating surfaces. Let their radii be 
B 
a, h and let their centres oo' lie d apart. II this distance is great compared vith a 
and h, each may lie regarded as receiving a parallel stream from the othei. 
Let A send out a normal stream N c/w per square centimetre tlirough cone dw, 
while B sends out NG/w. 
B receives the stream of cross section Trlr or the angle of the cone is TTh-f'd-, and it 
issues virtually from area ttci', for at B, A will appear as a uniformly brignt flat disc. 
Then the total force on B is 
77cnN Trh^ _ 7Ta'lrP\. 
' U~ ^ cf “ 
wliere 
II = ttN. 
