POLARISATION OF LIGHT. 



incident on the crystallised plate M N, 

 bavin? its principal section or one of its 

 neutral axes incline:! 45 to the plane in 

 which R A is polarised. This ray will 

 be divided by double refraction into two 

 rays oppositely polarised, viz. the extra- 

 ordinary ray C D or E, and the ordinary 

 ray B F or O. As these rays differ in 

 velocity, the one ray will be "behind the 

 other, and they will consequently inter- 

 fere, being polarised +45 and 45 to 

 the plane of primitive polarisation. Let 

 these rays be now received on a doubly 

 refracting 1 prism of calcareous spar P Q, 

 having its principal section in the plane 

 of primitive polarisation, and they will 

 be aofain doubled, 'forming: the four pen- 

 cils DL, FK, DH, FG, all of equal 

 intensity, of which H S and G T are 

 parallel, and L V, K X. The compound 

 pencils, H G T S and L K X V will con- 

 sist of two systems of ray, O e and E e, 

 and O o and E o, the one of each system 

 following the other at a given distance, 

 the distance between O e and E e being 

 d, and by law 5, the distance between 

 O o and E o being <fc4 undulation. But 

 as each of the rays of these two systems 

 have similar polarisations, they will by 



lawl , p. 38, interfere and produce the com- 

 plementary colours corresponding to the 

 intervals d and d-\-% undulation. If the 

 ray R A is polarised in a plane at right 

 angles to what it was in the preceding 

 case, it will suffer exactly the same di- 

 vision ; but the intervals of retardation 

 will now be d % undulation and d, so 

 that the two pencils will exchange co- 

 lours. 



CHAPTER XL 



New Species of Doubfe Refraction in 

 Analcime. 



THE remarkable mineral called Anal' 

 cim.fi, or Cubizite, has been ranked by 

 mineralogists among those which have 

 the cube for their primitive form ; and 

 hence, if this were the case, we should 

 expect to find it without double refrac- 

 tion. By attentive observation, however, 

 no distinct cleavage planes can be ob- 

 served, and the remarkable optical struc- 

 ture of the mineral confirms us in the 

 opinion that its crystallographic struc- 

 ture is still unknown. 



The most common form of this mine- 

 ral is the icosatetrahedron shown \nfig. 

 41. This solid is contained by twenty- 



Fig. 41. 



four equal and similar trapezia, and 

 may be considered as derived from the 

 cube by three truncations on each of its 

 angles, inclined 144 44' 8" to each of 

 its" faces, and 146 26' 33" to one an- 

 other. 



If we suppose the original cube to be 

 divided by planes passing through all 



the twelve diagonals of its six faces, it 

 will be reduced, as shown mfig. 42, into 

 twenty-four irres^ilar tetrahedrons. The 

 same planes will divide the iccsitetiahe- 

 dron, Jig. 41, into twenty-four similar 

 pentahedrons, two of whose planes are 

 placed at right angles to each other, 

 having for their common section one .of 



