CH. 17} MICRO-SPECTROSCOPE AND POLARISCOPE 175 



put it under the microscope, remove the analyzer, focus the crystals 

 and then replace the analyzer and cross the Nicols. The field and 

 the crystals will remain dark. 



fB) Anisotropic or Doubly Refracting Objects. — Make a 

 fresh preparation of carbonate of lime crystals like that described 

 for pedesis (§ 164), 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 analyzer and focus the crystals, then replace the 

 analyzer. Cross the Nicols. In the dark field will be seen multi- 

 tudes of shining crystals, and if the preparation is a fresh one in 

 water, part of the smaller crystals will alternately flash and disap- 

 pear. By observing carefully, some of the larger crystals will be 

 found to remain dark with crossed Nicols, others will shine contin- 

 uously. If the crystals are in such a position that the light passes 

 through them parallel with the optic axis,* the crystals are isotropic 

 like salt crystals and remain dark. If, however, 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 crys- 

 tals will appear as if self-luminous in a dark field. The experiment 

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

 geneous immersion. 



As a further illustration of anisotropic objects, mount some 

 cotton fibers in balsam (Ch. IX), also some of the lens paper 

 (§ 125). These furnish excellent examples of vegetable fibers ; 

 Striated muscle fibers are also very well adapted for polarizing 

 objects. 



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

 the analyzer is rotated. An excellent subject for this will be found 

 in blood crystals. 



§ 249. Starch.— One of the 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 



*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 $164) are uniaxial 

 crystals. Borax crystals are bi-axial. 



