ON LIGHT. 269 



this, and all subsequent changes of direction and velo- 

 city, are held, on this theory, to be effected by attractive 

 or repulsive powers resident in the bodies on which the 

 light-corpuscules fall (or, which comes to the same thing, 

 in the corpuscules themselves), and from which they are 

 either reflected, if the repulsive powers be too strong to 

 permit their penetration; or in which they are refracted, 

 if they are able to enter and make their way among the 

 particles of the refracting body. Colour, according to 

 this theory, is accounted for by specific diversity among 

 the luminous particles ; and difference of refrangibility, 

 by differences in the intrinsic energy of the acting forces 

 as determined by the specific nature of the molecules, 

 or, which comes to the same, by a difference of propor- 

 tion between their moving force and their inertia. This 

 is one of the many weak points of the theory. It runs 

 counter to the only analogy which the observation 

 of nature furnishes. It is as if the sun should be sup- 

 posed to attract a planet of lead and one of cork with 

 different accelerating forces ; or as if, here on earth, a 

 lump of platina and a lump of iron should be supposed 

 to acquire different velocities in falling through the same 

 space. It runs counter, too, to the original assumption, 

 that when first emitted from a luminous body, in their 

 passage through empty space, all the coloured particles 

 move with equal velocities, and have therefore been 

 equally accelerated by the emitting forces. That they do 

 so, we know from astronomical observation. The Aber- 

 ration of all the coloured rays is the same. Were it not 

 so, every star seen through a highly magnifying telescope 



