40 PROFESSOR STOKES, ON THE DYNAMICAL THEORY OF DIFFRACTION. 



supposed wholly or nearly to neutralize each other. But if we adopt the other hypothesis we 

 shall be obliged to suppose that in the oblique emergence from the glass, or in something else, 

 there exists a powerful cause of crowding towards the plane of diffraction, that is, in the 

 manner of reflection, sufficient to neutralize the great crowding in the contrary direction 

 produced by diffraction, which certainly seems almost incredible. 



The nearly uniform distribution of the planes of polarization of the diffracted light shews 

 that the two streams of light, polarized in and perpendicular to the plane of diffraction 

 respectively, into which the incident light may be conceived to be decomposed, were diffracted 

 at emergence from the glass in very nearly the same proportion. This result appeared to 

 offer some degree of vague analogy with the depolarization of light produced by such sub- 

 stances as white paper. This analogy, if borne out in other cases, might seem to throw some 

 doubt on the conclusiveness of the experiments with reference to the decision of the question 

 as to the direction of the vibrations of plane-polarized light. For the deviation of the light 

 from its regular course might seem due rather to a sort of scattering than to regular diffraction, 

 though certainly the fact that the observed light was very nearly plane-polarized does not at 

 all harmonize with such a view. Accordingly, I was anxious to obtain a case of diffraction in 

 which the planes of polarization of the diffracted light should be decidedly crowded one way 

 or other. Now, according to the explanation above given, the approximate uniformity of 

 distribution of the planes of polarization in the first two experiments was due to the antago- 

 nistic effects of diffraction, (according to FresnePs hypothesis respecting the direction of 

 vibration,) and of oblique emergence from the glass, or irregular refraction, that is, refraction 

 produced wholly by diffraction. If this explanation be correct, a very marked crowding 

 towards the plane of diffraction ought to be produced by diffraction at reflection, since dif- 

 fraction alone and reflection alone would crowd the planes in the same manner. 



To put this anticipation to the test of experiment, I placed the grating with its plane 

 perpendicular to the incident light, and the grooved face towards the polarizer, and observed 

 the light which was diffracted at reflection. Since in this case there would be no crowding of 

 the planes of polarization in the regularly reflected light, any crowding which might be observed 

 would be due either to diffraction directly, or to the irregular reflection due to diffraction, or, 

 far more probably, to a combination of the two. 



The experiments indicated indeed a marked crowding towards the plane of diffraction, but 

 the light was so strong at the minimum, for most positions of the pointer of the polarizer, that 

 the observations were very uncertain, and it was evidently only a rough approximation to 

 regard the diffracted light as plane-polarized. The reason of this was evident on consideration. 

 Of the light incident on the grating, a portion is regularly reflected, forming the central image 

 of the system of spectra produced by diffraction at reflection, a portion is diffracted externally 

 at such an angle as to enter the eye, a small portion is scattered, and the greater part enters 

 the glass. Of the light which enters the glass, a portion is diffracted internally at such an 

 angle that after regular reflection and refraction it enters the eye, a portion diffracted at other 

 angles, but the greater part falls perpendicularly on the second surface. A portion of this is 

 reflected to the first surface, and of the light so reflected a portion is diffracted at emergence 

 at such an angle as to enter the eye. Thus there are three principal images, each formed by 



