REPORT ON PHYSICAL, OPTICS. 325 



To account for these phenomena Newton sujjposed the ravs 

 of light to be inflected in passing bj^ the edges of bodies, by the 

 operation of the attractive and repulsive forces which the mole- 

 cules of bodies were conceived to exert on those of light at sen- 

 sible distances. Thus, the rays passing by the edges of a nar- 

 row opaque body are supposed to be turned aside by its repul- 

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

 the rays which pass at a distance from the body will be less de- 

 flected than those which pass close to it. The caustic formed 

 by the intersection of these deflected rays will be concave in- 

 wards 5 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 sup- 

 posed that the attractive and repulsive forces succeed one another 

 for some alternations ; 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 sepa- 

 ration of white light into its elements is explained, by supposing 

 that the rays which diflfer in refrangibility differ also in injiexihi- 

 lity ; the body acting alike upon the less refrangible rays at a 

 greater distance, and upon the more refrangible at a less di- 

 stance*. 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 he conceived to arise from a varia- 

 tion hi the density of the ether in the neighbourhood of the body ; 

 and, following the analogy of thin plates, he endeavoured to ac- 

 count for the coloured fringes by the vibrations of the ether 

 which are propagated faster than the rays themselves, and over- 

 take 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 

 of the circumstances of these phenomena, which are not only 

 unexplained by this theory, but which seem moreover irrecon 

 cileably at variance with it. 



If the phenomena of inflexion be the effects of attractive and 

 repulsive forces emanating from the interposed body, and if 

 these forces are the same, or even analogous to those to which 



• Optics, Book iii. Queries 1, 2, 3, 4. 



t December 21, 1675.— Birch's History of Royal Society, vol. iii. 



