ATTEMPTS TO DISCOVER THE LAWS OF OTHER PHENOMENA. 79 



the same kind as it would Lave been, to discover all the inequalities of 

 tlie moon's motion without the aid of the doctrine of gravity. We 

 will enumerate some of the phenomena which thus employed and 

 perplexed the cultivators of optics. 



The fringes of shadows were one of the most curious and noted of 

 such classes of facts. These were first remarked by Grimaldi 1 (1665), 

 and referred by him to a property of light which he called Diffraction. 

 When shadows are made in a dark room, by light admitted through a 

 very small hole, these appearances are very conspicuous and beautiful. 

 Hooke, in 1672, communicated similar observations to the Royal 

 Society, as " a new property of light not mentioned by any optical 

 writer before ;" by which we see that he had not heard of Grimaldi's 

 experiments. Newton, in his Opticks, treats of the same phenomena, 

 which he ascribes to the inflexion of the rays of light. He asks (Qu. 

 3), " Are not the rays of light, in passing by the edges and sides of 

 bodies, bent several times backward and forward with a motion like 

 that of an eel ? And do not the three fringes of colored light in sha- 

 dows arise from three such bendino-s ?" It is remarkable that Newton 



O 



should not have noticed, that it is impossible, in this way, to account 

 for the facts, or even to express their laws ; since the light which pro- 

 duces the fringes must, on this theory, be propagated, even after it 

 leaves the neighborhood of the opake body, in curves, and not in straight 

 lines. Accordingly, all who have taken up Newton's notion of inflex- 

 ion, have inevitably failed in giving anything like an intelligible and 

 coherent character to these phenomena. This is, for example, the case 

 with Mr. (now Lord) Brougham's attempts in the Philosophical Trans- 

 actions for 1796. The same may be said of other experimenters, as 

 Mairan" and Du Four, 8 who attempted to explain the facts by supposing 

 an atmosphere about the opake body. Several authors, as Maraldi, 4 

 and Comparetti, 5 repeated or varied these experiments in diflx-rei.t 

 ways. 



*/ 



Newton had noticed certain rings of color produced by a glass spc-u- 

 lum, which he called "colors of thick plates," and which he attempted 

 to connect with the colors of thin plates. His reasoning is by no 

 means satisfactory ; but it was of use, by pointing out this as a ease in 

 which his " fits " (the small periods, or cycles in the rays of light, of 



1 Physico-Hathesis, de Lumine, Coloribus et Iride. Bologna, 1665. 



2 Ac. Par. 1738. 3 Memoires Presentes, vol. v. * Ac. Pa'. 1723. 

 6 Observations Opticce de Luce Inflexd et Coloribus. Padua, 17S7 



