138 MICRO-SPECTROSCOPE AND POLARISCOPE. [CH. VI. 



preparation of carbonate of lime crystals like that described for pedesis 

 (§ 142), or use a preparation in which the crystals have dried to the 

 slide, use a 5 or 3 mm. objective, shade the object well, remove the an- 

 alyzer and focus the crystals, then replace the analyzer. Cross the 

 Nicols. In the dark field will be seen multitudes of shining crystals, 

 and if the preparation is a fresh one in water, part of the smaller crys- 

 tals will alternately flash and disappear. By observing carefully, some 

 of the larger crystals will ba found to remain dark with crossed Nicols, 

 others will shine continuously. If the crystals are in such a position 

 that the light passes through them parallel with the optic axis,* the 

 crystals are isotropic like the salt costal and remain dark. If, how- 

 ever, the light traverses them in any other direction the ray from the 

 polarizer is divided into two constituents vibrating in planes at right 

 angles to each other, and one of these will traverse the analyzer, hence 

 such crystals will appear as if self-luminous in a dark field. The experi- 

 ment with these crystals from the frog succeeds well with a 2 mm. ho- 

 mogeneous immersion. 



As further illustration of anisotropic objects, mount some cotton 

 fibers in balsam (Ch. VII), also some of the lens paper (§ 107). These 

 furnish excellent examples of vegetable fibers. 



Striated muscular fibers are also very well adapted for polarizing ob- 

 jects. 



As examples of biaxial crystals, allow some borax solution to dry 

 and crystallize on a slide ; use the crystals as object. As all doubly re- 

 fracting objects restore the light with crossed Nicols, they are some- 

 times called depolarizing. 



(C) Pleochroism. — This is the exhibition of different tints as the an- 

 alyzer is rotated. An excellent subject for this will be found in blood 

 crystals. 



(D) For the aid given by the polariscope in micro-chemistry, see 

 (Ch. VII). 



(E) See works on petrology and mineralogy for the application of 

 the micro-polarizer in those subjects. 



§ 218. Production of Colors. — For the production of gorgeous 

 colors, a plate of selenite giving blue and yellow colors is placed between 



*The optic axis of doubly refracting crystals is the axis along which the crystal 

 is not doubly refracting, but isotropic like glass. When there is but one such 

 axis, the crystal is said to be uniaxial, if there are two such axes the crystal is 

 said to be bi-axial. 



The crystals of carbonate of lime from the frog (see \ 142) are uniaxial crystals. 

 Borax crystals are bi-axial. 



