52 REPORT ON PHYSICAL OPTICS. 



light to be inflected in passing by the edges of bodies, by the ope- 

 ration of the attractive and repulsive forces which the molecules 

 of bodies were conceived to exert on those of light at sensible 

 distances. Thus, the rays passing by the edges of a narrow 

 opaque body are supposed to be turned aside by its repulsion ; 

 and as this force decreases rapidly as the distance increases, the 

 rays which pass at a distance from the body will be. less deflected 

 than those which pass close to it. The caustic formed by the in- 

 tersection of these deflected rays will be concave inwards ; and 

 as none of the rays pass within it, it will form the boundary of 

 the visible shadow. To explain the alternations of darkness and 

 light beyond this, Newton appears to have supposed that the 

 attractive and repulsive forces succeed one another for some alter- 

 nations; and that the molecules composing each ray, in their 

 passage by the body, are bent to and fro by these forces, " with a 

 motion like that of an eel," and are finally thrown off at one or 

 other of the points of contrary flexure. The separation of white 

 light into its elements is explained, by supposing that the rays 

 which differ in refrangibility differ also in inflexibility, the body 

 acting alike upon the less refrangible rays at a greater distance, 

 and upon the more refrangible at a less distance.* In one of his 

 letters to Oldenburgh,f Newton advances a more refined theory of 

 diffraction. The bending of the ray near the edge of the obstacle 

 ne conceived to arise from a variation in the density of the ether 

 in the neighbourhood of the body ; and, following the analogy of 

 thin plates, he endeavoured to account for the coloured fringes by 

 the vibrations of the ether, which are propagated faster than the 

 rays themselves, and overtake them at the middle of the curved 

 portion of the trajectory they describe. 



It is needless to comment upon the vagueness of these expla- 

 nations. Newton himself was dissatisfied with them, and the 

 subject fell from his hands unfinished. Still, however, the mere 

 guesses of such a mind as that of Newton must possess a high 

 interest, and we are not to wonder that among his followers more 

 weight should be attached to these explanations than he himself 

 ever gave them. It seems necessary therefore to advert to some 



the circumstances of these phenomena, which are not only 



* Optia, Book iii., Queries 1, 2, 3, 4. 



t December 21, W5.-JKreh; History tf the Royal Society, vol. iii. 



