272 ON LIGHT. 



having attractive and repulsive poles, and during their 

 progress through space revolving round their own centres 

 about axes not coincident with the direction of their 

 motion. Under such circumstances it is clear that some 

 might arrive at the reflecting surface with the attractive 

 pole foremost others with the repulsive. The former 

 would be attracted, and escape the reflective action ; the 

 latter repelled, and therefore subjected to it. Or, with- 

 out making any supposition as to the sort of mechanism 

 by which such a result might be attained, we might con- 

 tent ourselves with assuming, as Newton (the framer of 

 this hypothesis) did, that the particles of light, through- 

 out their whole progress through space, pass periodically 

 through a succession of alternating physical states or, 

 as he called them, " fits " " of easy reflexion and easy 

 transmission :" the only objection to such a form of 

 statement being, that it conveys no clear physical con- 

 ception to the mind. 



(56.) The particles so escaping reflexion are conceived 

 to have penetrated within the limit of the repulsive, and 

 to have entered that of the attractive forces, while yet at 

 some inconceivably minute distance outside of the actual 

 surface of the medium. Their movement of approach 

 therefore to the surface is accelerated by the attractive 

 force whose resultant direction is perpendicular to the 

 surface, and when they have arrived within the medium 

 so far that all further action ceases (by the counteraction 

 of equal and opposite forces on all sides) each of them 

 will have undergone the total amount of acceleration 

 due to the attractive force in the direction of that force, 



