DIFFRACTION. 53 



unexplained by this theory, but which seem moreover irrecon- 

 cileably at variance with it. 



If the phenomena of inflexion be the effects of attractive and 

 repulsive forces emanating from the interposed body, and if these 

 forces are the same with, or even analogous to, those to which the 

 reflexion and refraction of light are ascribed in the theory of 

 emission, it will follow that they must exist in different bodies in 

 very different degrees ; so that the amount of bending of the rays, 

 and therefore the position of the diffracted fringes, should vary 

 with the mass, the nature, and the form of the inflecting body. 

 Now it is clearly ascertained, on the contrary, that all bodies, 

 whatever be their nature or the form of their edge, produce under 

 the same circumstances fringes identically the same; and in fact 

 the partial interception of light, caused by the interposition of an 

 obstacle of any kind, seems to be the only condition on which the 

 character of the phenomenon depends. Grravesende seems to have 

 first observed that the nature or density of the body had no effect 

 upon the magnitude of the diffracted fringes ; and the fact has 

 since been confirmed in the fullest manner by almost every inquirer 

 in this branch of experimental science. One of the ablest suppor- 

 ters of the theory of emission has admitted that the inflecting 

 forces, if such exist, must be independent of the chemical nature 

 of the inflecting body, and altogether different in their nature 

 from those to which, in the same theory, the phenomena of re- 

 flexion and refraction are ascribed.* To ascertain whether the 

 form of the edge had any effect upon the fringes, Fresnel took two 

 plates of steel, the edge of each of which was rounded in one half 

 of its length and sharp in the remaining half, and placed the 

 rounded portion of one edge opposite the angular part of the 

 other, and vice versa. If, then, the position of the fringes depended 

 on the form of the surface, the effect would thus be doubled, and 

 the fringes appear broken in the middle. They were found, on 

 the contrary, to be perfectly straight throughout their entire 

 length.f 



Biot, Precis elementaire, Tol. ii. p. 473, 3 Edit. 



t Me'moire sur la Dif, -action, p. 370. The Bulletin Unirersrl for February, 1828, 

 contains some animadversions on this part of Fresnel's optical labours, in a ; 

 signed by the secretary of the Academy of Sciences of St. Pctersburgh, and purporting 

 to be an official reply to some remarks in a former number of the Bulletin .on the pro- 

 gramme of the prize questions proposed by the Academy. The writers hare con- 



