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, 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; but I 
have lately repeated them with nine flat diamonds, for which 
I was indebted to Joun Rosison, 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 ofa plate of crystallised glass were held paral- 
lel to the fringes a d, cf, the difference of their effects was pro- 
duced, 
* See Phil. Trans. 1815, p. 29. 
