DIFFRACTION. 105 



torily the curvilinear termination of the visible shadow, and 

 its excess above the geometric one. 



To account for the fringes which are parallel to the edge 

 of this shadow, Newton appears to have supposed the at- 

 tractive and repulsive forces to succeed one another for some 

 alternations ; and the molecules composing each ray, in their 

 passage by the body, to be bent to and fro by these forces, 

 in a serpentine course, and to be finally thrown off at one or 

 other of the points of contrary flexure. The intersection of 

 the rays thus thrown off at different points of the same ser- 

 pentine course will form a caustic or fringe ; so that each suc- 

 ceeding fringe will be produced by the rays which pass at a 

 given distance from the edge of the body. 



Finally, the separation of white light into its elements is 

 explained, by supposing that the rays which differ in refrangi- 

 bility differ also in inflexibility, the body acting alike upon 

 the less refrangible rays at a greater distance, and t upon the 

 more refrangible rays at a less distance. 



It is needless to comment upon the vagueness of these 

 explanations. Newton himself was dissatisfied with them, 

 and the subject fell from his hands unfinished. Still, how- 

 ever, the mere guesses of such a mind as that of Newton 

 must claim a deep interest ; and it was natural that among 

 his followers more weight should be attached to these con- 

 jectures, than he himself ever assigned to them. It seems 

 necessary, therefore, to advert to some of the circumstances 

 of the phenomena, which are not only unexplained by 

 this theory, but which seem moreover entirely at variance 

 with it, 



(121) If the phenomena of inflexion be the effects of at- 

 tractive and repulsive forces emanating from the interposed 

 body, and if these forces are the same, or even analogous 

 to those to which the reflexion and refraction of light are 



