HOMOGENEOUS FLUIDS BY POLARIZED LIGHT. 49 



crystal cut parallel to the axis, could be imitated. It consisted 

 of two glass parallelepipeds arranged at right angles, between 

 which the crystalline lamina was placed, so that the polarized 

 pencil of light suffered double total reflexion on leaving the 

 lamina as on entering it, but in a plane perpendicular to the 

 former, both these planes being inclined at an angle of 45° to 

 the axis of the crystal. This system of a crj'stalline lamina and 

 two glass parallelepipeds thus arranged, possesses the singular 

 property, that it can be turned upon its axis between the two 

 planes of extreme polarization, like a plate of rock-crystal cut 

 perpendicularly to the axis, without changing either the nature 

 or the intensity of the colours ; whilst by varying one of these 

 planes in relation to the other, all the various tints are obtained 

 which under similar circumstances are presented by plates of 

 rock-crystal cut perpendicular to the axis and by oil of turpen- 

 tine. Moreover, when the incident light has suffered double 

 total reflexion in a plane inclined at 45° to that of primitive 

 polarization, it is no longer coloured in traversing this apparatus, 

 in whatever azimuth it may be turned ; and when it suffers this 

 modification on leaving the apparatus, instead of receiving it at 

 its entrance, it takes, as does also the oil of turpentine in a 

 similar case, the same appearances as if it had been received im- 

 mediately upon the rhomboid of calcareous spar after leaving the 

 crystallized lamina. 



Lastly, when the incident light, after having been completely 

 depolarized by two successive reflexions before entering this 

 apparatus, is again at its exit twice totally reflected in a glass 

 parallelopiped, it is found to be again brought to a state of com- 

 plete polarization, as if the apparatus had not been used, or been 

 replaced by a tube filled with oil of turpentine. It would appear 

 then, from these numerous and varied phsenomena, that this ap- 

 paratus possesses all the optical properties of oil of turpentine. 

 This was also what I at first thought ; but a more attentive ex- 

 amination convinced me that a notable difference existed between 

 these two kinds of phaenomena. 



Having placed a glass parallelopiped at the^ extremity of a 

 tube 0"'50 in length, filled with oil of turpentine, so that the 

 rays which traversed it suffered double total reflexion parallel 

 to the primitive plane of polarization, I caused the extraordinary 

 image, which was of a violet red, to disappear by the interpo- 

 sition of a lamina of sulphate of lime, about 0'"'"-12 in thick- 



VOL. V. PART XVII. E 



