XI. WHEN TO USE SPECIAL MICROSCOPES 363 



The polarization microscope is used to identify materials, espe- 

 cially minerals. Materials that possess a single, refractive index are 

 called isotropic and do not appear different in polarized light. Aniso- 

 tropic crystals have more than one refractive index and are charac- 

 terized by having one or more optic axes. From the observation and 

 measurement of their axes, refractive indices, and crystal habit, 

 microscopic specimens may be identified by reference to tables of 

 such data {3,27a,30a). 



Birefringent objects, and materials that become so under strain, 

 can be studied by the polarization colors they form, either with or 

 without retardation plates, quartz wedges, and other compensators. 

 The changes in birefringence with function, as the contraction of 

 muscle fibrils, may also be studied in this way. The membrane sur- 

 rounding red blood cells has been measured with the aid of a special 

 comparison polarization microscope {27h). Liquids that contain 

 asymmetrical particles become birefringent when forced to flow 

 through a capillary tube, or to move so as to orient the particles. 

 Solutions of proteins, myosin, tobacco mosaic virus, hemocyanins, 

 fibrinogen, and other materials have been studied with the polarizing 

 microscope {28). Skeletal elements of many organisms, fibers, hairs, 

 and many tissues of plants and animals are birefringent and the polari- 

 zation microscope is useful for their identification and analysis 

 {3, 6 (3rd ed.), 29, 30). 



A qualitative study of the images formed by anisotropic or bire- 

 fringent specimens requires only a minimum of equipment and train- 

 ing, but the use of a polarizing microscope as an analytical tool re- 

 quires considerable experience and skill. 



G. FLUORESCENT AND PHOSPHORESCENT SPECIMENS 



Many materials absorb one wavelength of radiation and reradiate 

 the energy at another, usually longer, wavelength. Such specimens, 

 when irradiated with invisible ultraviolet, may be seen by means of 

 the visible fluorescence emitted. Other materials may be treated 

 with selectively absorbable fluorescent chemicals, which is analogous 

 to a staining procedure, or a combination of fluorescent chemicals 

 and nonfluorescent dyes may be used {31a). Proteins and many 

 materials of natural occurrence are fluorescent. Great contrast may 

 be obtained with bacteria so that they appear bright and self-luminous 

 against a dark background, e.g., the identification of tuberculosis and 



