194 THE MICBOSCOPIST. 



therefore, will exhibit colors, as will those also in which the 

 thickness of the crystal is not below the proper standard. 



After the polarizing apparatus is adjusted, as before de- 

 scribed, the crystals properly mounted may be placed on the 

 stage, in the same way as ordinary objects. Some few vege- 

 table structures may be exhibited in the same manner, as the 

 siliceous cuticle of equisetum, starch, &c. Many animal 

 structures, as feathers, slices of quill, horn, &c., are best shown 

 by placing a film of selenite or mica beneath them, by which 

 they become intensely colored. If the film be of unequal 

 thickness, the colors will vary, 



"The application," says Mr. Quekett, "of this modification 

 of light to the illumination of very minute structures has not 

 yet been fully carried out, but still there is no test of diffe- 

 rences in density between any two or more parts of the same 

 substance that can at all approach it in delicacy. All struc- 

 tures, therefore, belonging either to the animal, vegetable, or 

 mineral kingdom, in which the power of unequal or double 

 refraction is suspected to be present, are those that should es- 

 pecially be investigated by polarized light. Some of the most 

 delicate of the elementary tissues of animals, such as the tubes 

 of nerves, the ultimate fibrillas of muscle, &c., are amongst 

 some of the most striking subjects that may be studied with 

 advantage under this method of illumination." 



To prepare Crystals for Polarized Light. Pour a few 

 drops of a saturated solution of the salt on a glass slide, 

 gently warm it over a spirit lamp, so^s to evaporate the 

 excess of fluid, taking care not to apply too much heat, lest 

 the water of crystallization be driven off and the salt become 

 opaque. The more slowly the crystallization is effected, the 

 better. 



The crystals should then be examined, and the best of them 

 mounted, either in the dry way (interposing a cell of paper, 



