236 



PROGRESS IN MICROSCOPY 



These three objectives are of the glycerine-immersion type and com- 

 plemented by a projection-type eyepiece and an achromatic condenser 

 of N.A. 8. 



2. OBSERVING AND RECORDING INFRA-RED 

 AND ULTRA-VIOLET IMAGERY 



Photography in the lower infra-red spectrum is feasible owing to 

 such sensitizers of photographic emulsions as dicyanine, neocyanine, 

 xenocyanine, and so forth. Such sensitizers have been used for dyeing 

 biological specimens. The photographic plate processed by the same 

 sensitizer has utmost sensitivity for the more-absorbed radiations. 

 This process furnishes highly contrasted negatives up to 1 -3 i-i (spectral- 

 sensitivity limit of photographic plates). Instead of registering the 

 images photographically, the infra-red image can be observed direct 

 on a fluorescent screen by means of a very simple image converter 

 also called frequency transformers. Figure 9- 12 shows schematically 

 an image converter: the semi-transparent photocathode C is set, on 



Fig. 9.12. Diagrammatic image converter. 



the inner wall of the vacuum-tube T, several millimetres away from 

 the anode A which is also a fluorescent screen. A potential difference 

 is originated between A and C. Infra-red photons, incoming on C, 

 release electrons which are accelerated by the electric field obtaining 

 between A and C. The electron pencil of rays appears on the fluores- 

 cent screen as a spot. Sharpness is not too good with such apparatus 

 and, in many converters, of which there are many types, the anode 

 is independent of the fluorescent screen. These apparatuses include 

 electrostatic lenses conjoining the photocathode C and the fluorescent 

 screen A. Highest spectral sensitivity attains approximately 1-5m. 

 Some devices are designed to obtain increased luminance of converted 

 images. 



