IX. On a New Optical and Mineralogical Property of Calca- 

 reous Spar. By David Brewster, LL. D. F. R. S;. 

 Lond. & Edin. F. A. S. Edin. 



[Read April 30. 1816.] 



IT 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 Malus, 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. 



