Ch. VIII] THE POLARISCOPE IN MICROSCOPY 269 



Use first a 16 mm. and then a higher power. The starch grains, many 

 of them, will be found in the potato cells. They have the general 

 appearance of a clam or oyster shell. The black cross is strikingly 

 exhibited by the polariscope. Starch grains of other plants show the 

 same, but the grains are generally smaller and therefore do not bring 

 out the structural features so clearly. 



§ 431. Production of colors. — For the production of gorgeous 

 colors, a selenite plate is placed anywhere between the polarizer and 

 the analyzer. If properly mounted the selenite is very conveniently 

 placed on the diaphragm carrier of the condenser, just above the 

 polarizer; an unmounted selenite may be placed over the ocular. 

 A thin plate or film of mica also answers well. 



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

 vivid colors in many object^. One of the most beautiful prepara- 

 tions and one of the most instructive also, may be prepared as follows: 

 Heat some xylene balsam on a slide until the xylene is nearly evapo- 

 rated. 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 wiU be shown beautiful colors and the black cross (Clark) . 



It is very instructive and interesting to examine many organic 

 and inorganic substances with a micro-polarizer. 



Collateral Reading 

 Chamot, Chemical Microscopy; Daniell, Principles of Physics; McMunn, 

 The Spectroscope in Medicine. 



