MICROSCOPE ACCESSORIES 221 



position. If one of the prisms is rotated so that the 

 light rays passing through it are not parallel with 

 those passing through the other prism, a dark field 

 results, and the nicols are said to be in the crossed 

 position. If the mounting of the polarizer is graduated 

 the prism should be so arranged that fields of maxi- 

 mum darkness are visible at 90 and at 270 degrees. 



The polarizing apparatus is used in the determi- 

 nation of optical and crystallographic properties of 

 different substances. For these observations the 

 prisms should be set in a crossed position. All 

 transparent and translucent materials may be classi- 

 fied according to the characters they exhibit when 

 viewed between crossed nicols. Isotropic or singly 

 refracting substances do not polarize or change the 

 direction of light rays, and therefore show no change 

 when rotated between crossed prisms. Crystals of 

 the isometric type and most amorphous substances 

 are isotropic. The isotropic vegetable tissues and 

 cell contents include cork, epidermis, parenchyma, 

 inulin and oil globules. Anisotropic or doubly refract- 

 ing substances polarize or change the direction of 

 light rays and therefore become alternately light 

 and dark upon rotation between crossed nicols. Most 

 crystalline chemical substances, excepting those of 

 isometric form, are anisotropic. The vegetable tis- 

 sues and cell contents which act similarly to aniso- 

 tropic substances include fibers, 'vessels, calcium 

 oxalate crystals, starch and mucilage deposits. 



In certain anisotropic substances the property 

 of double refraction is barely apparent and only 

 neutral gray tints are visible upon rotation. In these 

 instances a thin plate of selenite is interposed between 



