POLARIZATION. 185 



When the position of the depolarizing crystal is 

 such that the plane of the polarized light coincides 

 with the direction of the optic axis or axes, the light 

 is not doubly refracted nor polarized in certain parts ; 

 hence these parts appear white if the plane of the 

 polarizer and analyzer coincide, and black if they be 

 crossed. A crystal cut at right angles to its optic 

 axis, with its length directed towards the polarizer 

 and analyzer, is in this position, and it exhibits alter- 

 nately a black and white cross, with one or two sets 

 of concentric rings of complementary colours at each 

 quarter of a rotation. 



To prepare crystals for examination by polarized 

 light, a little Epsom salt, nitre, or borax should be 

 dissolved in water, a drop placed upon a slide, and 

 dried at a gentle heat. The crystals should then be 

 mounted in balsam, and viewed as transparent objects. 



To see the cross and rings, the crystals should 

 be sawn or cut across transversely, the ends being 

 polished on a strained piece of silk moistened with 

 water, and the sections mounted in balsam. 



The property of doubly refracting and polarizing 

 light is not confined to crystalline substances, being 

 also possessed by many organic bodies, for the de- 

 tails of which I must refer to the article Polariza- 

 tion in the ' Micrographic Dictionary/ 



