Prof. Stokes on the Polarization of Diffracted Light. 161 



being supposed perpendicular to the plane of polarization^ pro- 

 vided the dift'raction be conceived to take place in the Jirst 

 medium, or in other words, just before the light reaches the 

 grating ; but they would not at all fit the hypothesis of vibra- 

 tions parallel to the plane of polarization. I put forth some 

 considerations, founded on probable reasoning, to show that the 

 supposition of diffraction taking place in the^^V*^ medium was 

 in accordance with the physical circumstances of the case. So 

 decided was the result obtained, that it seemed to me a strong 

 argument in favour of the hypothesis that the vibrations are 

 perpendicular to the plane of polarization, though I still felt the 

 necessity of repeating the experiments under varied circum- 

 stances. 



But since the appearance of M. Holtzmann's researches the 

 state of the question is changed. I have no reason to doubt the 

 correctness of his results, while on the other hand the result I 

 myself obtained was far too decided to be passed by. The con- 

 clusion which, in the present state of the question, seems to me 

 most probable is, that the polarization of light diffracted at a 

 large angle is, in fact, influenced by the nature of the diffract- 

 ing body. The subject demands a much more extensive expe- 

 rimental investigation, in which the circumstances of diffraction 

 shall be varied as much as possible. I hope to have leisure to 

 undertake such an investigation : meanwhile it would be prema- 

 ture to offer any decided opinion. It seems to me, however, 

 worthy of attentive consideration, whether a glass grating may 

 not offer a fairer experiment for the decision of the question as 

 to the direction of vibration in polarized light than a smoke 

 grating, inasmuch as in the former we have to do with an unin- 

 teiTuptcd medium, glass, the surface of which is merely rendered 

 iiTegular, whereas in the latter the problem is complicated by 

 the existence of two distinct media, glass and soot, placed alter- 

 nately. I call the layer of soot a medium, for though no light 

 can pass through any sensible thickness of it, we must not con- 

 clude from that that it is without influence on the light which 

 passes excessively close to it. 



I have not mentioned the effect of oblique refraction in the 

 experiments of M. Holtzmann, because if it were allowed for, the 

 character of the results obtained would remain unchanged, the 

 magnitude of the observed effect would only be somewhat dimi- 

 nished. 



Pembroke College, Cambridge, 

 February 20, 1857- 



