OPTICAL PHENOMENA OF CRYSTALS. 229 



branched or plumose appearance, and assume beautiful 

 colours in polarized light when a plate of sulphate of 

 lime is placed beneath them. The microscope shews the 

 colour to reside principally in the outline, but to the 

 naked eye the whole film appears coloured. As these 

 films may be obtained of large size, the phenomenon can 

 be well seen by the unassisted eye. 



A very interesting experiment, and one which throws 

 much light upon the cause of these appearances, is to 

 transmit a beam of polarized light very obliquely through 

 a small prism of nitre immersed in gum, and viewed 

 with the microscope. Its outline then generally exhibits 

 two colours instead of one ; for, while the edge of the 

 prism which is on that side from whence the ray of light 

 comes, is, for instance, of a red colour, the opposite edge 

 will appear green. Reverse the polarization of the light, 

 and these colours are exchanged one for the other. This 

 observation enables us to explain the origin of the pheno- 

 menon in a satisfactory manner, and to shew why it only 

 occurs in crystals possessing strong double refraction, 

 like nitre, in which the refractive indices of the two rays 

 are materially different. 



When a ray of common light is incident upon such a 

 crystal, and therefore divides itself into two rays oppo- 

 sitely polarized, both rays are transmitted through the 

 central parts of the crystal, which are bounded by parallel 

 planes, or by planes approaching to parallelism. But 

 when the bounding planes of the crystal are much hi- 



