44 



POLARISATION OP LIGHT. 



tions -which are composed of opposite 

 veins." 



In a large amethyst the veined por- 

 tions were perfectly colourless, while the 

 sectors corresponding to M c N, &c. fig. 

 48, were of a pale yellow colour, and in 

 another specimen, one half of which was 

 yellow, and the other lilac, the yellow was 

 obviously a portion of one of the sectors ; 

 while the lilac portion consisted wholly of 

 alternate veins, so minute as to destroy 

 almost wholly the rotatory structure. 



The property of quartz, in virtue of 

 which it produces circular polarisation, 

 was supposed, by M. Biot, to reside in 

 the ultimate particles of silex ; but if this 

 were the case, the same property would 

 be possessed by silex in all its forms, 

 which Dr. Brewster found not to be the 

 case, after a minute examination of opal, 

 tabasheer, and other siliceous minerals. 

 He likewise found that the rotatory pro- 

 perty was not possessed by quartz, which 

 had been deprived of its crystalline struc- 

 ture by fusion ; and Mr. Herschel 

 sought for it in vain in a solution of 

 silica in potash, though the silex previ- 

 ously exhibited circular polarisation. 



Conceiving that this property might 

 be related to some crystalline structure 

 in quartz, Mr. Herschel examined dif- 

 ferent crystals of the quartz Plagiedre 

 of Hauy, which possesses unsymmetrical 

 faces, x, x, x, x', x', x', (see fg. 10. 

 p. 6,) which always lean in one uniform 

 direction round the summit A, but some- 

 times to the right and sometimes to the 

 left. After an examination of fifty-three 

 different crystals, he found that the direc- 

 tion in which they turned the planes of 

 polarisation was invariably the same as 

 the direction in which the plagiedral 

 faces x, x, leant round the crystal ; so 

 that even, if these faces were microscopic, 

 the sight of one of them would enable 

 us to predict the direction of rotation in 

 a plate cut from it. Hence Mr. Herschel 

 concluded that whatever be the cause 

 which determines the direction of rota- 

 tion, the same has acted in determining 

 the direction of the plagiedral faces. 

 Apatite and some other crystallised mi- 

 nerals exhibit also plagiedral and unsym- 

 metrical faces ; but as they do not pos- 

 sess circular polarisation, we must infer 

 that the latter is not a necessary conse- 

 quence of the structure which exhibits 

 the former. 



In order to determine if the colours of 

 circular polarisation were produced by 

 the interference of the rays formed by 



double refraction, M. Fresnel made the 

 following experiment. He took a prism 

 of right-handed quartz, ABC, (Jig. 49,) 



Fig. 49. 

 15 B D 



having its faces A B, B C equally in- 

 clined to R R, the axis of the prism, 

 along which the circular polarisation is 

 seen. The refracting angle ABC was 

 150. As the ray R R, incident at 

 Q, could not emerge from the face B C, 

 he cemented on the faces B A, B C, 

 two halves of another prism exactly 

 similar to the first, but cut from a crystal 

 of left-handed quartz. These two halves 

 are shown at A B E, C B D, and are dis- 

 tinguished by the sign . As the ray 

 RR will pass through all these three 

 prisms in a line parallel to their axes of 

 double refraction, it cannot be separated 

 into two by the ordinary refracting forces ; 

 but if it is influenced by another doubly 

 refracting force belonging to circular 

 polarisation, it will be first divided into 

 two pencils by the left-handed prism 

 A E B : but when these two pencils enter 

 the right-handed prism A B C at Q, their 

 angular separation will be doubled, owing 

 to the prism ABC having an opposite 

 circularly polarising structure. When 

 the two rays come to P, and enter the 

 prism BCD, their angular separation 

 will be again doubled, so that the ray will 

 finally emerge in two pencils. By this 

 ingenious contrivance, M. Fresnel suc- 

 ceeded in separating the two pencils 

 produced by the force of circular polari- 

 sation. 



When either of these pencils is ex- 

 amined by a doubly refracting prism, 

 it is doubled like common light, and none 

 of the two pencils ever vanish during the 

 revolution of the doubly refracting prism ; 

 but that they differ from commonlight M. 

 Fresnel proved by the following experi- 

 ment : (fig. 50.) If the two pencils R P 

 produced along the axis of quartz are 

 made to fall perpendicularly on the face 

 A B of a parallelepiped of crown glass, 

 A B C D, whose refractive index is 1.51, 

 and whose angles ABC, ADC, are 

 each 54^, they will suffer total reflec- 

 tion at Q and S, and will emerge in the 

 direction S T. The two emergent pencils 

 will now be found completely polarised. 



