154 MICRO-SPECTROSCOPE AND POLARISCOPE [CH. VI 



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 an- 

 alyzer, hence such crystals will 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 ( 256), also some of the lens paper ( 114;. These 

 furnish excellent examples of vegetable fibers. 



Striated muscle fibers are also very well adapted for polarizing 

 objects. 



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

 and crystallize on a slide ; use the crystals as objects. As all doubly 

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

 times called depolarizing. 



(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. 



227. Production of Colors. For the production of gorgeous 

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

 the polarizer and the object. If properly mounted, the selenite is very 

 conveniently placed on the diaphragm carrier of the Abbe illuminator, 

 just above the polarizer. A thin plate or film of mica also answers 

 well. 



It is not necessary to use selenite or mica for the production of the 

 most glorious colors in many objects. One of the most beautiful pre- 

 parations, and one of the most instructive also, may be prepared as 

 follows : Heat some xylene balsam on a slide until the xylene is nearly 

 evaporated. Add some crystals of the medicine, sulphonal and warm 

 till the sulphonal is melted and mixes with the balsam. While the 

 balsam is still melted put on a cover-glass. If one gets perfect crystals 

 there will be shown not only the most beautiful colors, but the black 

 cross with perfection. (Clark). 



*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 \ 151) are uniaxial crystals. 

 Borax crystals are bi-axial. 



