TRANSACTIONS OF THE SECTIONS. 15 



In attempting to give the Section some idea of the nature and 

 singularity of the results which I obtained, I shall omit all details 

 and confine myself to the statement of the general phenomena. 



When light is reflected at the separating surface of two media, 

 the lowermost of which is a doubly refracting one, the reflected ray 

 is exposed to the action of two forces, one of which is the ordinary 

 reflecting force, and the other a force which emanates from the in- 

 terior of the doubly refracting crystal. When the first medium is 

 air, or even water, the first of these forces overpowers the second; 

 and in general the effects of the one are so masked by the effects 

 of the other that I was obliged to use oil of cassia, — a fluid of high 

 refractive power, — in order that the interior force of the calcareous 

 spar, which I wished to examine, might exhibit its eftects independ- 

 ently of those which arise from ordinary reflexion. The separating 

 surface therefore which I used had a small refractive power, and the 

 reflecting pencil is so attenuated, especially in using polarized light, 

 that it is almost impossible to use any other light than that of the sun. 



When a pencil of common light is reflected from the separating 

 surface of oil of cassia and calcareous spar, the general action of the 

 spar is to polarize a part of the ray in a plane perpendicular to that 

 of the reflexion, and thus to produce hy reflexion the very same effect 

 that other surfaces do by refraction. 



On the face of calcareous spar perpendicular to the axis of the cry- 

 stal the effect is exactly the same in all azimuths, but in every other 

 face the eff'ect varies in diff"erent azimuths and depends upon the in- 

 clination of the face to the axis of double refraction . On the natu- 

 ral face of the rhomb common light is polarized in the plane of re- 

 flexion in 0° of azimuth, or in the plane of the principal section ; 

 but at 38° of azimuth the whole pencil is polarized at right angles 

 to the plane of reflexion, and in other azimuths the effect is nearly 

 the same as I have stated in my printed paper. 



In order, however, to observe the change which is actually pro- 

 duced upon light it is necessary to use two pencils, one polarized 

 -f- 45°, and the other — 45° to the plane of incidence. The planes 

 of polarization of these pencils are inclined 90° to each other, and 

 the invariable effect of the new force is to augment that angle in the 

 same manner as is done by a refracting surface, while the tendency 

 of the ordinary reflective force is to diminish the same angle. Hence 

 I was led to make an experiment in which these opposite forces 

 might compensate one another. I mixed oil of olives and oil of cassia 

 till I obtained a compound of such a refractive power that its action 

 in bringing together the planes of polarization should be equal to the 

 action of the new force in separating them. Upon reflecting the com- 

 pound pencil from this surface I was delighted to find that the incli- 

 nation of the planes was still 90°, and I thus obtained the extraor- 

 dinary result of a reflecting surface which possessed no action what- 

 ever upon common or upon polarized light. 



The action of the new force when the plane of reflexion coincides 

 with the principal section of the crystal is obviously inexplicable by 



