Dr. Brewster on the laws of polarisation, &c. 203 
of lime and mica, and afterwards analysed with a prism of 
calcareous spar, they exhibited the most beautiful comple- 
mentary colours. Without knowing what had been done by 
M. Arago, the same discovery was made by me in 1812; 
but though the general fact which each of us discovered was 
the same, yet I had the good fortune, by a peculiar mode of 
observation, to examine the phenomena seen along the real 
or apparent axes of crystals, and have thus been led to the 
results contained in the following paper, — results which, as 
will be seen in the sequel, could not possibly have been 
obtained by the mode of observation employed by M. Arago, 
and afterwards by M. Biot. 
The last of these philosophers has examined this class of 
phenomena with singular diligence and ingenuity. By a 
series of fine experiments, he has established many funda- 
mental points in the science, and has associated his name 
with this branch of physics, as one of its most distinguished 
and successful cultivators. The crystals which M. Biot 
examined were 
Calcareous spar. Tourmaline. 
Rock crystal. Feld-spar. 
Beryl. Arragonite. 
Phosphate of lime. Topaz. 
Sulphate of lime. 
Sulphate of barytes. 
Sulphate of strontian. 
Mica. 
But his experiments for investigating the laws of moveable 
polarisation were principally made with sulphate of lime , on 
account of its perfect transparency, and the facility with 
which it can be separated into thin plates. The general 
result of these experiments is, that all the preceding crystals, 
with the single exception of certain specimens of mica, have 
