Dr . Brewster on the laws of polarisation, &c. 
the rings shown in fig. 2, I have been able to prove it by 
direct observation. The exact parallelism of the sides of the 
two rhomboids, when their surfaces are not perfectly flat, 
can easily be obtained by separating them with a piece of 
soft wax, and observing the perfect coincidence of the 
reflected images from the two adjacent surfaces ; or by look- 
ing at the system of rings, and altering the position of the 
crystals till they become quite perfect. I have obtained a 
similar result by combining a plate of beryl with a plate of 
calcareous spar, for the system of rings will always be the 
same as would have been produced by two plates of beryl, 
one of which was the plate employed, and the other a plate 
which gave rings of the same size as the plate of calcareous 
spar. 
But when we combine the system of rings produced by a 
crystal of zircon, &c. with the system produced by calcareous 
spar, a very different effect is produced. The system of rings 
instead of being diminished is increased, and is equal to the 
system which would have been produced by a thin plate of 
calcareous spar, whose thickness is equal to the difference of 
the thicknesses of the plate of calcareous spar employed, and 
another plate of calcareous spar, that would give rings of the 
same size as those given by the zircon, &c. alone* This result, 
which I succeeded after much labour in obtaining experimen- 
tally, will also be obtained by substituting ice in place of 
zircon. Quartz cannot be employed in these experiments, as 
the system of rings is never complete, on account of the 
secondary tints which M. Biot discovered along its axis. If 
the plate of zircon, &c. gives a system of rings of the very same 
size as those of calcareous spar, the one system will be com- 
