of Crystalline Reflexion described by Prof. Stokes. 259 



plane of incidence, the reflected light is polarized in the 

 perpendicular plane, and vice versa. When I first obtained 

 this result, I thought it applicable without reservation in the 

 actual experiment, and on trial was disappointed to find that 

 the reflected light was nearly unpolarized, even when the 

 incident light was fully polarized, whether in the plane of 

 incidence or in the perpendicular plane. When, however, 

 the angle of incidence wets diminished, the expected phenomenon 

 was observed, provided that the original polarization were in, 

 or perpendicular to, the plane of incidence. If the original 

 polarization were oblique, the reflected light was not fully 

 polarized, even though the angle of incidence were small*. 



Further consideration appeared to show that the loss of 

 polarization usually observed could be explained by the de- 

 polarizing action of the layer of crystal through which the 

 light passes, both on its way to the reflecting plane and on its 

 return therefrom. As is shown in the preceding paper, 

 this depolarizing action does not occur when the angle of 

 incidence is small, and the polarization in, or perpendicular to, 

 the plane of incidence. It seems scarcely too much to say 

 that the theory not only explains the laws laid down by Stokes, 

 but also predicts a very peculiar law not before suspected")". 



The theory, as so far developed, is indeed limited to inci- 

 dences in the two principal planes. It could probably be treated 

 more generally without serious difficulty ; but there seems 

 no reason to suppose that anything very distinctive would 

 emerge. It is not unlikely that the intensity would prove to 

 be proportional to the square of the sine of the angle between 

 the planes of incidence and of symmetry. If this theory 

 be accepted — and I see no reason for distrusting it — the 

 brilliant reflexion cannot be explained as due to a single 

 twin stratum. The simplest case which we can consider 

 is when the angle of incidence is small and the polarization 

 in or perpendicular to the plane of incidence. There is 



* Whatever the angle of incidence, the arrangement of crossed nicols 

 may sometimes be conveniently applied in order to isolate the light under 

 investigation from that reflected at the front surface of the crystalline 

 plate. In the observations described in the text the crystal was mounted 

 with Canada balsam between thick plates of glass, so that there was no 

 difficulty in observing separately the various reflexions. At small angles 

 of incidence the coloured image is at its brightest when the analyzing 

 nicol is so turned that the white image (reflected from the glass) vanishes, 

 and vice versa, the incident light being polarized in, or perpendicularly to, 

 the plane of incidence. 



t The wording of Prof. Stokes's description is perhaps a little am- 

 biguous, but I gather that he did not examine the result of a simultaneous 

 operation of polarizer and analyser. 



S2 



