324 Dr BREWSTER on certain new Phenomena of Colour 



generally displays throughout the very same tint ; but in some 

 cases, the same parallelogram exhibits different colours at the 

 same angle of incidence, owing sometimes to the mixture of the 

 tints of superposed parallelograms, and sometimes to the variable 

 thickness of the space by which the colours are occasioned. 



The parallelograms which produce the colours now described, 

 may be crystallized laminae disseminated through the felspar, 

 and giving the colours of thin plates ; or they may be slender 

 crystals, which, like the veins of calcareous-spar, develope the 

 tints of polarised light ; or they may be crystallized cavities, ei- 

 ther entirely empty, or containing solid, fluid, or gaseous sub- 

 stances. 



The exceeding toughness of the mineral renders it impracti- 

 cable to obtain good cleavage planes, passing through the paral- 

 lelograms, for the purpose of shewing their interior, or of dis- 

 charging their contents, as I succeeded in doing while examining 

 the topaz cavities, so that I had no other resource but that of op- 

 tical analysis. 



As it was necessary to examine the light transmitted through 

 the parallelograms, I detached a very thin splinter from the mi- 

 neral, and placed it in Canada balsam * between two plates of 

 glass. It was so thin at one edge, that it did not give the co- 

 lours of polarised light, and at its greatest thickness, it developed 

 only the red of the third order. It had fortunately only one 

 stratum of parallelograms, so that their reflected and transmit- 

 ted tints could be observed with the greatest distinctness. The 

 reflected tints were uncommonly brilliant and pure, but the 

 transmitted ones were very faint, and of a yellowish, reddish, or 

 greenish-brown colour, varying with the obliquity of the incident 

 ray. I now placed the splinter on the base of a prism, with Ca- 



* Oil of Cassia would have been preferable in other cases, but as it has a colour 

 of its own, and disperses light so powerfully, it was unsuitable where delicate tints 

 were to be observed. 



