330 FOURTH UEPORT 1834. 



that the deviations produced in the dilTereiit species of simple 

 light, or the distances of the bands from the axis of the pencil, 

 were in all cases proportional to the lengths of the fits ; the mag- 

 nitude of the aperture remaining the same. The same analogy 

 was preserved in different media, the deviations varying in the 

 inverse ratio of the refractive indices of the media, or in tlie 

 direct ratio of the fits*. M. Pouillet adds, that they were un- 

 able to explain theselaws, having adopted the theory of emission t. 

 They are all simple consequences of the wave-theory. The in- 

 terval of the fits is exactly half the length of a wave, and the 

 true connexion between the place of the fringes and the latter 

 quantity had been already pointed out by Yonng. 



Mayer afterwards studied the phenomena of diffraction, but 

 without adding any new facts to those already known. As to 

 the theory, he adopted that of Newton, with some modifications. 

 With Newton, he ascribed the inflexion of light into the shadow 

 to the operation of an attractive force ; but, imwilling to admit 

 the existence of a repulsive force, he attempted to account for 

 deflexion hy ^he impact of the molecules reflected from the edge 

 against those which passed by it|. 



Fresnel at first adopted and developed Young's theory of 

 diffraction, and found that the general laws of the fringes, — the 

 dependence of their magnitude upon the length of a wave, and 

 upon the distances of the luminous origin and of the screen, — 

 were thus fully explained. It was shown, that as the position 

 of the screen is varied, the successive points at which the same 

 fringe is formed are not in a right line, but constitute an hyper- 

 hola ; and that when the distance of the luminous origin is les- 

 sened, the inclination of these hyperbolic branches considered 

 as coincident with their asymptots, augments, and the fringes 

 dilate in breadth §. Fresnel, however, was soon dissatisfied 

 with this theory. If the exterior bands had their origin in the 

 interference of the direct and reflected light, their intensity 

 should depend on the curvature of the edge ; it is found, on 

 the contrary, that the fringes formed by the back and by the 

 edge of a razor are precisely alike in every respect. As to the 

 other cases of diffraction, there were many phenomena, and 

 especially those exhibited in Newton's experiment with the two 

 knife-edges, which proved that the rays grazing the edges of tlie 

 body were not the only rays concerned in the production of 



* Biot, Trails de Physique, torn, iv., Supplement a I'Optiqiie. 



+ Elemens de Physique, torn. ii. p. 437. 



J Comm. Soc. Gottingensis Iiece?itiores, vol. iv. p. 49. 



§ Annales de Chimie, torn. i. p. 239. 



