160 ON THE OPTICAL PROPERTIES OF MURIATE OF SODA, 



effect as o when turned 90° round. The preceding phenome- 

 na were exhibited in every specimen that had a considerable 

 thickness. 



My experiments on muriate of soda were made with large 

 masses of various sizes, from half an inch to three inches in 

 length. They all exhibited the same properties as fluor-spar, 

 the depolarising axes being coincident with the diagonals of 

 the square faces, and the neutral axes with their sides. In the 

 largest pieces, the polarised tint was a fine blue y with a pale 

 yellow for its complementary colour, and the oppositely pola- 

 rised portions produced by different parts of the mass, were 

 arranged in streaks parallel to one of the diagonals AC of the 

 cubical face ABCD, as represented in Fig. 4. Similar pheno- 

 mena were exhibited in large pieces of transparent alum. 



In my first experiments on the Diamond *, the spe- 

 cimens which I employed had very uneven surfaces j but I 

 have lately repeated them with nine flat diamonds, for which 

 I was indebted to John Robison, Esq. Almost all these spe- 

 cimens depolarised the light in separate spots, of an irregu- 

 lar shape, and the depolarising portions had opposite struc- 

 tures, like the specimens of muriate of soda and fluor-spar 

 which have already been described. The appearance of these 

 diamonds, when exposed to polarised light, is represented in 

 Fig. 5. One of them, however, had a more perfect crystalli- 

 sation, and exhibited four parallel fringes, as shewn in Fig. 6. 

 The tints were a white of the first order. When the inte- 

 rior fringes of a plate of crystallised glass were held paral- 

 lel to the fringes a rf, cf, the difference of their effects was pro- 

 duced, 



* See Phil. Trans. 1815,. p. 29. 



