NEWTON S PRINCIPIA. 151 



not equable, another curve will be described ; yet, that in 

 either case the sine of the angle of incidence (or that made 

 with the plane where they enter the medium), is to the sine 

 of the angle of refraction (or that made with the plane 

 they emerge from) in a given ratio ; that the velocities 

 before incidence and after emerging are inversely as the 

 sines of incidence and refraction ; and that if the velocity 

 after incidence is retarded, and the line of incidence inclined 

 more towards the plane of the refracting medium, the 

 small bodies will be reflected back at an angle equal to 

 that of incidence. 



He then remarks on the inflexion and deflexion which 

 light suffers in passing, not through, but by or near bo 

 dies, as discovered by Grimaldi *, and as confirmed by his 

 own experiments. He shows that the rays are bent most 

 probably in curve lines, the nearest rays towards the bend 

 ing body, the furthest rays away from it ; and he infers 

 that, in refraction and reflexion, a similar curvilinear bend 

 ing takes place somewhat before the actual point of re 

 fraction and reflexion. He further mentions the colours 

 formed by flexion, as three coloured fringes or bands, 

 &quot; tres colorum fascias.&quot; I, however, long ago showed 

 (Phil. Trans. 1797, Part II.) f that this is not the real fact; 

 having found that a much greater number of these fringes 

 are formed by flexion, and that they are, like the pris 

 matic spectrum, images of the luminous body. This ex 

 periment has been repeated by Sir David Brewster and 

 others ; nor can any doubt be entertained that there are 

 innumerable fringes decreasing in breadth, and in the 

 breadth of the dark intervals between them, until they 

 become evanescent. But as if it were the fate of all this 



* Grimaldi termed it diffraction. 



f In Phil. Trans., 1850, and Mem. Inst. de France, 1854, are my other 

 papers on Inflexion, showing the same phenomenon, as well as the different 

 flexibility of the rays. 



L 4 



