48 



POLARISATION OF LIGHT. 



the yellow one, acquiring an accession 

 of blue light, becomes bluish white. 

 From p and p' to o, the ordinary image 

 is whitish, and the other deep blue ; but 

 the whiteness gradually diminishes to- 



wards o, where they ' are both almost 

 equally blue. 



The following Table contains the crys- 

 tals with two axes, in which Dr. Brew- 

 ster discovered the absorptive property. 



The same author has given (Phil. 

 Trans. 1819, p. 19) a list of various other 

 crystals which possess the property, but 

 our limits will not permit us to pursue 

 the subject any farther. 



The influence of heat in modifying the 

 absorptive action of crystals he found to 

 be very remarkable. Having selected se- 

 veral crystals of Brazilian topaz which 

 displayed no change of colour by expo- 

 sure to polarised light, he brought them to 

 a red heat, and thus communicated to 

 them the power of absorbing polarised 

 light. He then took a topaz in which one 

 of its pencils was yellow and the other 

 pink. A red heat acted more powerfully 

 upon the extraordinary pencil than upon 

 the ordinary one, discharging the yellow 

 colour entirely from the one, and pro- 

 ducing only a slight change on the pink 

 tint of the other. When the topaz was 

 hot, it was perfectly colourless, and ac- 

 quired the pink tint gradually while cool- 

 ing. By exposing it repeatedly to the 

 action of a very intense heat, he was un- 

 able either to modify or remove this 

 permanent tint. 



From several experiments which have 

 not yet been published, Dr. Brewster has 

 discovered that the colouring matter 

 itself, both in crystals with one and two 

 axes, possesses the property of double 

 refraction that this matter exhibits 

 hemitropism in crystals that are not 

 themselves compound and that the co- 

 louring matter has in some rare cases 



two axes of double refraction, while the 

 crystal itself has only one. The prose- 

 cution of this curious subject is likely to 

 throw much light on the constitution of 

 bodies. 



For an account of various interesting 

 phenomena relative to the distribution 

 of the colouring matter in topaz, the 

 reader is referred to the Transactions of 

 the Cambridge Philosophical Society, 

 vol. ii. See also Chap. XVL 



CHAPTER XIV. 



Double Refraction communicated to 

 Glass by Heat to Cylinders Rec- 

 tangular Plates Chromatic Thermo- 

 meter Effects of Crossed Plates 

 Spheres of Glass Tubes Unan- 

 nealed Plates of Glass Effects of 

 altering their Form Chromatic Ver- 

 nier Double Refraction produced 

 by Induration Lenses of Fishes. 



THE various phenomena described in the 

 preceding chapters were ascribed to 

 some unknown structure in the bodies 

 which produced them ; and it was never 

 even imagined that the power of pro- 

 ducing double refraction could be com- 

 municated artificially to glass and other 

 bodies. On the 8th April 1814, Dr. 

 Brewster communicated to Sir Joseph 

 Banks the remarkable fact that double 

 refraction could be imparted to glass 

 by the transient passage of heat through 

 its substance, and tlie letter contain- 



