ON LIGHT. 271 



attractive and -a repulsive sphere one within the other 

 the particles of light being repelled while passing through 

 the outer or repulsive sphere, and attracted when arrived 

 within the internal or attractive one. These forces are 

 supposed immensely energetic, and to decrease with such 

 excessive rapidity as to be absolutely insensible at any, 

 the very smallest, distance appretiable to our senses. In 

 virtue of this repulsive force, the surface of any mate- 

 rial body may be conceived as coated (metaphorically 

 speaking) with a film of repulsive power, off which, as 

 from an elastic cushion, the luminous particles may be 

 imagined to rebound : in which case, according to the 

 known laws of elastic rebound, the angle of reflexion 

 (perfect elasticity being supposed) would be equal to 

 that of incidence, and the velocities before and after re- 

 flexion equal. 



(55.) Reflexion, then, is easily and readily explained 

 on this theory. In fact, it is explained too well. For it 

 will be at once asked, how, on such suppositions, there 

 can be such a thing as partial reflexion. Since all the 

 luminous particles of a ray arrive at (suppose) a plane 

 surface in the same direction and with the same velocity; 

 whatever happens to one, the repulsive force, being the 

 same, must happen to all. This is another weak point of 

 the corpuscular theory; and to escape from the difficulty 

 so created, it becomes necessary to supplement the 

 original hypothesis of luminous particles with another, 

 converting those particles into mechanisms of a peculiar 

 nature, of which the simplest conception that can be 

 formed is to suppose them as it were minute magnets 



