HOMOGENEOUS FLUIDS BY POLARIZED LIGHT. 57 



h <o, the light will revolve from left to right when the principal 

 section of the particles is to the left of their plane of entrance ; 

 and from right to left when this plane is to the left of the prin- 

 cipal section. 



It is clear from this, that when polarized light traverses suc- 

 cessively two fluids which cause the light to rotate in contrary 

 directions, the effects produced by the one upon each kind of 

 rays are subtracted from the elFects produced by the other; so 

 that with homogeneous light the extraordinary image is made 

 to disappear completely, by lengthening or shortening one of 

 the tubes. But it maj' happen with white light that this com- 

 pensation is impossible, if, for instance, the variations of the 

 double refraction of the different rays do not follow the same 

 law in both fluids ; for then the relation of the lengths, which 

 produce exact compensation for one species of rays, would not 

 produce it for another. 



To complete the theory which I have just set forth, there re- 

 main to be explained two phaenomena described at the com- 

 mencement of this memoir. When polarized light has suffered, 

 at an azimuth of 45°, the modification produced by double total 

 reflexion before traversing the oil of turpentine, it no longer 

 gives rise to colours ; and when it only undergoes this modifi- 

 cation after passing out of the tube, the tints of the two images 

 remain constant during the rotation of the rhomboid of calca- 

 reous spar with which they are observed, and they only vary in 

 intensity in passing into perfect whiteness, as those of the cry- 

 stalline lamina? cut parallel to the axis. 



The cause of the first phsenomenon is very simple : the light 

 then undergoes only one kind of refraction in the liquid. In 

 fact, we have seen that the rays polarized parallelly or perpen- 

 dicularly to the principal section of a particle, after having suf- 

 fered, on leaving it, the modification in question, can only un- 

 dergo a single kind of refraction in the following particle. The 

 polarized light, thus modified, can only be refracted in one single 

 manner in the oil of turpentine, and ought to produce, conse- 

 quently, but one single system of waves. 



I am now about to consider the case when the light only 

 undergoes this modification on leaving the tube. Let P P' be 

 the primitive plane of polarization. We have seen that the 

 action of the particles upon the luminous vibrations was always 

 the same in whatever azimuth their axes were turned. We 



