28 REFLEXION AND REFRACTION. 



flexion ceasing altogether when this difference vanishes. How 

 is it, then, that some of the molecules obey the influence of 

 the repulsive force, and are reflected, while others yield to 

 the attractive force, and are refracted ? To account for this, 

 Newton was obliged to have recourse to a new hypothesis. 

 The molecules of light, in their progress through space, are 

 supposed to pass continually into two alternate states, or fits, 

 which recur periodically and at equal intervals. While in one 

 of these states, called the fit of easy reflexion, they are disposed 

 to obey the repulsive or reflective forces of any body which 

 they meet ; and, on the other hand, they yield more readily 

 to the attractive or refractive forces, when in the alternate 

 state, or fit of easy transmission. Now, the molecules com- 

 posing a beam of common light are supposed to be in every 

 possible stage of these fits, when they reach the surface ; 

 some in a fit of reflexion, and others in a fit of transmission. 

 Some of them, consequently, will be reflected, and others 

 refracted, and the proportion of the former to the latter will 

 increase with the incidence. 



To account for the fits themselves, Newton assumed the 

 existence of an ethereal medium, analogous to that of Huy- 

 gens, although he did not assign to it the same office. The 

 molecules of light were supposed to excite the vibrations of 

 this ether, just as a stone flung into water raises waves on its 

 surface. This vibratory motion was supposed to be propa- 

 gated with a velocity greater than that of the molecules ; so 

 as to overtake them, and impress upon them the disposition 

 in question, by conspiring with or opposing their progressive 

 motion. In one of his queries Newton has even calculated 

 the elastic force of this ether, as compared with that of air 

 in order that the velocity of propagation should exceed that 

 of light. 



(35) The hypothesis of the fits has lost much of its credit, 



