354 OSCAR W. RICHARDS 



lengths of ultraviolet. Since monochromators are not convenient 

 sources, research continues toward the improvement of filters for iso- 

 lating the necessary regions of the spectrum. 



Even shorter wavelength radiation could be used for absorption 

 spectra reasons or to attain still better resolution. Between 200 m.n 

 and the wavelength region where image formation fails to take place, 

 however, the radiation is absorbed very strongly, even by air. This 

 necessitates working in a vacuum; also special photographic emul- 

 sions must be used. Little exploration has been done in this region 

 and its possible advantages are unknown, but the difficulties are 

 rather formidable. A simple X-ray microscope has been proposed 

 by Hamos and Engstrom {£4)- 



This is a specialized field that requires a good knowledge not onlj' 

 of microscopy but of radiant energy measurements. Ultraviolet 

 investigations should not be undertaken unless one is prepared to 

 master a complex and time-consuming technique. The research 

 program should be planned in advance of procuring equipment. 



D. OPAQUE SPECIMENS 



1. Direct Illumination 



Specimens too dense to transmit any light must be illuminated 

 from above. When low magnifications are adequate, light can be 

 reflected onto the specimen with a mirror or prism, or directed onto 

 the specimen from a small lamp having a focusable lens system to 

 concentrate the light where it is most useful. 



The microscopist of the previous century used a Lieberkiihn mir- 

 ror surrounding the objective to reflect light from the substage mir- 

 ror onto the top of the specimen. The Silverman Illuminator added 

 a Lumiline lamp to the reflector and was very useful. Another help- 

 ful source has a ring of small lamps that surrounds the objective {58). 

 A method that is useful for photomicrography of small objects is to 

 wrap some translucent paper in the shape of a cone around the ob- 

 jective; by directing the light from one or more lamps onto the out- 

 side of the cone, a diffused light falls on the specimen and harsh 

 shadows are avoided. In the case of higher power objectives that 

 focus so close to the specimens, the methods of Sections D3 and D4 - 

 become necessary. 



