The publication of Young's papers occasioned a fierce attack on him in the Edinburgh Review, from the pen of Henry Brougham, afterwards Lord Chancellor of England. Young replied in a pamphlet, of which it is said[1] that only a single copy was sold; and there can be no doubt that Brougham for the time being achieved his object of discrediting the wave-theory.[2]
Young now turned his attention to the fringes of shadows. In the corpuscular explanation of these, it was supposed that the attractive forces which operate in refraction extend their influence to some distance from the surfaces of bodies, and inflect such rays as pass close by. If this were the case, the amount of inflexion should obviously depend on the strength of the attractive forces, and consequently on the refractive indices of the bodies-a proposition which had been refuted by the experiments of s'Gravesande. The cause of diffraction effects was thus wholly unknown, until Young, in the Bakerian lecture for 1803,[3] showed that the principle of interference is concerned in their formation; for when a hair is placed in the cone of rays diverging from a luminous point, the internal fringes (i.e. those within the geometrical shadow) disappear when the light passing on one side of the hair is intercepted. His conjecture as to the origin of the interfering rays was not so fortunate; for he attributed the fringes outside the geometrical shadow to interference between the direct rays and rays reflected at the diffracting eige; and supposed the internal fringes of the shadow of a narrow object to be due to the interference of rays inflected by the two edges of the object.
The success of so many developments of the wave-theory led Young to inquire more closely into its capacity for solving the chief outstanding problem of optics—that of the behaviour of light in crystals. The beautiful construction for the extra-
