176 MICRO-SPECTROSCOPE AND POLARISCOPE [CH. VI 



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

 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 shell. The black cross is strikingly 

 exhibited by the polariscope. Starch grains of other plants show 

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

 bring out the structural fearures so clearly. 



§ 250. Production of Colors. — For the production of gor- 

 geous colors, a selenite plate is placed anywhere between the polar- 

 izer and the analyzer. If properly mounted the selenite is very 

 conveniently placed on the diaphragm carrier of the Abbe illumin- 

 ator, 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 objects. 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 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 beautiful colors and the black 

 cross. (Clark.) 



It is very instructive and interesting to examine many organic 

 and inorganic substances with a micro polarizer. 



REFERENCES TO THE POLARISCOPE AND TO THE USE OF 

 POLARIZED LIGHT 



Anthony & Brackett, 133; Behrens; Behrens, Kossel und 

 Schiefferdecker; Carnoy, 61; Carpenter-Dallinger, 317, 1097; Clark; 

 Daniel, 494; Davis; v. Ebener, Gamgee; Halliburton, 36,272; Hogg, 

 133,729; Lehmann; M'Kendrick; Nageli und Schwendener, 299; 

 Quekett; Suffolk, 125; Valentin; Physical Review, I., p. 127. 

 Daniell, Physics for Medical Students. Nichols, Physics. 



MICRO-CHEMISTRY 



§ 251. During the last decade the microscope has become one 

 of the necessities of the expert chemist, and the signs of the times 



