IX. Ona New Optical and Mineralogical Property of Calca- 
reous Spar. By Davin Brewster, LL. D. F. BR. S:. 
Lonp. & Enry. F. A. S. Epi. 
[Read April 30. 1816.] 
T has been long known to those’'who have studied the opti-- 
cal properties of Calcareous Spar, that there are numerous: 
specimens of that mineral which form a multiplicity of images,. 
affected with the most brilliant colours. These colours were: 
considered by Matvs, and other philosophers, as the colours: 
of thin plates, and were supposed to be produced by a film of 
air, included in accidental fissures or fractures within the crys- 
tal. I have already shewn*, from numerous experiments, that 
this opinion is erroneous, and that the various. colours with 
which the images are affected, arise from the transmission of 
polarised light, through: an. extremely thin film or stratum of 
calcareous spar, which has different thicknesses in different 
specimens, but which has an invariable position, being always’ 
parallel to a plane passing through the longer diagonals of the 
rhomboidal faces. — 
Since that paper was written I have obtained some very per- 
fect specimens of calcareous spar, which have enabled’ me to: 
obtain several new and interesting results. One of these speci-- 
mens: 
* See Phil. Trans, 1815, p. 270. 
