POLARISATION OF LIGHT. 



57 



cannot fail to have observed that the 

 forces which produce them have the 

 same character and the same intensity 

 in all parallel directions. The tints or 

 the systems of rings, or the separation of 

 images, are identically the same through 

 whatever portion of a crystalline plate 

 the ray is transmitted, provided it has 

 always" the same inclination to the axis 

 or axes of the crystal. 



In composite or hemitrope crystals 

 there is an apparent deviation from this 

 law. If one crystal adheres to another, 

 so that their axes are not parallel, the 

 system or systems of rings seen through 

 a plate formed out of the adhering crys- 

 tals will not have the same position, nor 

 will the tints, or the separation of the 

 images, be the same in parallel directions. 

 This kind of composite structure some- 

 times presents very curious phenomena, 

 as in nitre, arragonite, calcareous spar, 

 harmotome, arseniate of iron, aplome, 

 scolezite, feldspar, sulphato-tricarbonate 

 of lead, and various minerals ; but it is 

 particularly beautiful in Brazilian topaz 

 and certain crystals of sulphate of 

 potash. 



A very particular account of the com- 

 pound structure of Brazilian topaz has 

 been given by Dr. Brewster in the Cam- 

 bridge Transactions, vol. ii. It consists 

 of a central rhomb inclosed within four 

 other crystals, as \nfig. 68. The whole 



Fig. 68. 



hibits the tesselated structure shown in 



Jig. 69, when placed in the apparatus 



Fig. 69. 



fg. 39. By inclining the plate the tes- 

 selae give different tints, which have a 

 very fine effect. 



In all these cases, and in many others 

 that might be adduced, each of the tes- 

 selae have all the properties of separate 

 crystals. They have all the same po- 

 larising force, and exhibit the same pro- 

 perties at equal inclinations to the axes of 

 each, so that their optical structure and 

 properties are exactly such as might 

 have been predicted from a knowledge 

 of their crystallographic structure. 



From all these structures that of the 

 tesselated apophyllite differs in a very 

 remarkable manner. The doubly re- 

 fracting force varies in different parts of 

 the crystal, and this variation takes 

 place with such admirable symmetry in 

 relation to the faces of the crystal, that 

 it produces, when exposed to polarised 

 light, the most beautiful phenomena that 

 have ever been witnessed. 



The apophyllite from Faroe most com- 

 monly crystallises in right-angled prisms 

 like C D, Jig. 70. If we remove the 

 Fig. 70. 



have often the same colour, but very 

 often the central rhomb is of one colour, 

 while the external tesselae are of a different 

 colour ; and in some rare cases one of 

 the external tesselse has been pink while 

 the other three were yellow. These ex- 

 ternal tesselae are often divided into a 

 number of minute laminae, whose prin- 

 cipal sections are not parallel, the prin- 

 cipal section of the innermost being in- 

 clined no less than 10 or 11 to that of 

 the outermost. 



The bipyramidal sulphate of potash, in 

 place of being a simple crystal, as Count 

 Bournon supposed, has been shown by 

 Dr. Brewster to be a compound one, and 

 when a plate is cut out of it perpendi- 

 cular to the axis of the pyramid, it ex- 



