MECHANICAL PRESSURE OF LIGHT; ITS CONSEQUENCES. 233 



pended disc in a bulb containing a vacuum. This vacuum 

 was attained with the greatest care by first exhausting the 

 bulb to the highest degree possible and then freezing out 

 the residue of mercury vapour. In such a vacuum the disc 

 was repelled on the impact of the light-beam and its repul- 

 sion was measured by its torsional effect on the suspending 

 wire. This repulsive pressure of the light was found very 

 nearly equal to that calculated so long in advance by Clerk- 

 Maxwell. Since Lebedew's demonstration, Nichols and 

 Hull have repeated his work with greater exactness and 

 there is now no shadow of doubt as to the fact that it has 

 Maxwell's value. This light pressure at the distance of the 

 earth from the sun is small, not quite a milligram per 

 square metre of the earth's surface, or, put roughly, 70,000 

 tons on the whole earth. Were we to consider only the 

 effect of the impact on large bodies our interest would not 

 proceed very far, but things take on a different complexion 

 when we notice the remarkable effect of size on the relation 

 between the light pressure and weight or gravitational 

 attraction. The light pressure is applied only on the sur- 

 face and is proportional to the surface while weight or the 

 pull of gravitation, on the other hand, affects the whole 

 body. 



Suppose we divided a sphere, such as a cannon ball, into 

 eight equal spheres. The sum of the surfaces of these eight 

 spheres would be twice that of the original sphere while 

 the weight of gravitative pull would remain the same. If 

 we continued the process of division until the spheres were 

 the size of the smallest shot, the total sum of their sur- 

 faces would be enormous compared with the original sphere 

 while the weight would again be equal to that of the can- 

 non ball. If we continued the division on and on we should 

 eventually come to a body so small that the ratio of its 



