352 OSCAR W. RICHARDS 



adjustment for best focus in the green and in the red and to continue 

 to turn the fine adjustment in the same direction for a distance equal 

 to twice the difference between the red and green foci (18). Since 

 some wooden plate holders and the hard rubber plate holder slides 

 are transparent to this radiation, metal ones should be used, or others 

 tested to make certain that they are satisfactory. 



Visual observation is possible by means of the 1P25 converter 

 tube. The microscope image focused onto the sensitive surface of 

 the tube releases electrons and the rest of the tube is an electrostatic 

 microscope which focuses the electrons onto a fluorescent screen. 

 The fluorescent image can be observed through a magnifier {18a). 



Red stained specimens and some natural reddish materials reveal 

 considerable detail with infrared microscopy. The dark chitin of 

 many insects and fossil graptolites have been successfully examined 

 in this manner. Thicker sections may be used than with shorter 

 radiation. Staining the specimen with kryptocyanine or neocyanine 

 and using photographic film sensitized with these same dyes makes 

 good use of the absorption bands to reveal any detail selectively 

 stained. 



Infrared microscopy has also been found useful for the examination 

 of colored coral skeletons, silver-impregnated nerve tissue, kidney 

 sections, capillaries and injected capillaries in tissue sections, em- 

 bryos stained with silver nitrate or with carmine, plant cell walls, 

 woody structures, calcareous algae, and textile fibers. Too little 

 infrared microscopy has been done to permit a critical evaluation at 

 the present time {18, p. 275). 



2. Ultraviolet ^Microscopy 



Proteins, nucleic acids, and other interesting constituents of bio- 

 logical materials have absorption bands in the ultraviolet region, 

 which permit the microscopic identification and quantitative meas- 

 urement of these substances within single cells. In addition the in- 

 creased resolution resulting from the shorter wavelength has made 

 the method useful for examination of those materials that were just 

 below the limit of resolution with the ordinary microscope in the 

 visible region. 



The source of ultraviolet radiation may be a resonance (cold 

 cathode) or other type mercury arc, or a metallic spark {e.g., with 

 rotating cadmium electrodes). Since glass is opaque to short wave- 



