PROFESSOR STOKES, ON THE DYNAMICAL THEORY OF DIFFRACTION. 5 



the experiments were made on light which was diffracted in passing through the grating. 

 The results appeared to be decisive in favour of Fresnel's hypothesis. In fact, theory shews 

 that diffraction at a large angle is a powerful cause of crowding of the planes of vibration of 

 the diffracted ray towards the perpendicular to the plane of diffraction, and experiment pointed 

 out the existence of a powerful cause of crowding of the planes of polarization towards the 

 plane of diffraction ; for not only was the crowding in the contrary direction due to refraction 

 overcome, but a considerable crowding was actually produced towards the plane of diffraction, 

 especially when the grooved face of the glass plate was turned towards the incident light. 



The experiments were no doubt rough, and are capable of being repeated with a good deal 

 more accuracy by making some small changes in the apparatus and method of observing. 

 Nevertheless the quantity with respect to which the two theories are at issue is so large that 

 the experiments, such as they were, seem amply sufficient to shew which hypothesis is 

 discarded by the phenomena. 



The conclusive character of the experimental result with regard to the question at issue 

 depends, I think, in a great measure on the simplicity of the law which forms the only result 

 of theory that it is necessary to assume. This law in fact merely asserts that, whereas the 

 direction of vibration in the diffracted ray cannot be parallel to the direction of vibration in the 

 incident ray, being obliged to be perpendicular to the diffracted ray, it makes with it as small 

 an angle as is consistent with the above restriction. This law seems only just to lie beyond the 

 limits of the geometrical part of the theory of undulations. At the same time I may be 

 permitted to add that, for my own part, I feel very great confidence in the equations of motion 

 of the luminiferous ether in vacuum, and in that view of the nature of the ether which would 

 lead to these equations, namely, that in the propagation of light, the ether, from whatever 

 reason, behaves like an elastic solid. But when we consider the mutual action of the lumi- 

 niferous ether and ponderable matter, a wide field, as it seems to me, is thrown open to 

 conjecture. Thus, to take the most elementary of all the phenomena which relate to the action 

 of transparent media on light, namely, the diminution of the velocity of propagation, this 

 diminution seems capable of being accounted for on several different hypotheses. And if this 

 elementary phenomenon leaves so much room for conjecture, much more may we form various 

 hypotheses as to the state of things at the confines of two media, such as air and glass. 

 Accordingly, conclusions in favour of either hypothesis which are derived from the comparison 

 of theoretical and experimental results relating to the effects of reflection and refraction on the 

 polarization of light, appear to me much more subject to doubt than those to which we are led 

 by the experiments here described. 



In commencing the theoretical investigation of diffraction, I naturally began with the 

 simpler case of sound. As, however, the results which I have obtained for sound are of far 

 less interest than those which relate to light, I have here omitted them, more especially as 

 the paper has already swelled to a considerable size. I may, perhaps, on some future occasion 

 bring them before the notice of this Society. 



