268 THE POLARISCOPE IN MICROSCOPY [Ch. VIII 



analyzer and cross the Nicols. The field and the crystals will remain 

 dark. 



(2) Anisotropic or doubly refracting objects. — Make a fresh 

 preparation of carbonate of lime crystals like that described for pedesis 

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

 sUde ; use a 5 or 3 mm. objective, shade the object well, remove the 

 analyzer, 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 crystals will alternately flash and disappear. By observing 

 carefuUy, some of the larger crystals will be found to remain dark 

 with crossed Nicols, others will shine continuously. If the crystals 

 are in such a position that the Hght passes through parallel with the 

 optic axis (§ 429a), the crystals are isotropic like salt crystals and 

 remain dark. If, however, the hght 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 wiU appear as if self- 

 luminous in a dark field. The experiment with these crystals from 

 the frog succeeds well with a 2 mm. homogeneous immersion. 



As a further illustration of anisotropic objects, mount some cotton 

 fibers in balsam (Ch. X), also some of the lens-paper (§ 158). These 

 furnish excellent examples of vegetable fibers; striated muscle fibers 

 are also very well adapted for polarizing objects. 



(3) Pleochroism. — This is the exhibition of different tints as the 

 analyzer is rotated. An excellent subject for this will be foimd in 

 blood crystals. 



§ 429a. The optic axis of doubly retracting 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 § 209) are uniaxial 

 crystals. Borax crystals are bi-axial. 



§ 430. Starch. — One of the most important uses of a polariscope 

 is for the study of starch. Starch gives a characteristic black cross 

 which rotates as the analyzer is rotated. Make a thin slice of fresh 

 raw potato with a razor or other sharp knife and mount it in water. 



