58 KEPORT ON PHYSICAL OPTICS. 



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, unwilling to admit the ex- 

 istence of a repulsive force, he attempted to account for deflexion 

 by the impact of the molecules reflected from the edge against 

 those which passed by it.* 



Fresnel at first adopted and developed Young's theory of dif- 

 fraction, 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 hyperbola ; and 

 that when the distance of the luminous origin is lessened, the 

 inclination of these hyperbolic branches, considered as coincident 

 with their asymptots, augments, and the fringes dilate in breadth, f 

 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 experi- 

 ment with the two knife-edges, which proved that the rays grazing 

 the edges of the body were not the only rays concerned in the 

 production of the fringes, but that the light which passed by those 

 edges at sensible distances was also deviated, and concurred in 

 their formation.* 



Fresnel was thus led to seek a broader foundation for his 

 theory, and the result of his investigations is given in the able 

 memoir which was crowned by the French Academy in 1819. In 

 this memoir the laws of diffraction are derived from the two prin- 

 ciples to which the laws of reflexion and refraction are them- 

 selves referred the principle of interference and the principle of 

 Huygens. To apply these principles to the present case, Fresnel 

 supposes the surface of the wave, when it reaches the obstacle, to 



* Comm. Soc. G'Mingensis Recentiores, vol. iv. p. 49. 



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



J Memoires sur la Diffraction de la Lumitre, p. 368. 



