INFRA-RED AND ULTRA-VIOLET MICROSCOPY 



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perfectly achromatic thus ensuring image focusing in visible light. 

 They also have a long working distance. An objective of numerical 

 aperture 065 has a working distance of approximately 13 mm whereas 

 that of a lens objective does not exceed 0-5 mm for the same aperture. 

 Therefore phenomena can be observed with a magnifying power 

 unattainable with lens objectives. Such is the case, for instance, when 

 observing an inaccessible object through the peep-hole of a furnace. 



Fjg. 9.1. Schwarzschild-type reflecting objective. 



Reflecting objectives are based on the two types of assembly shown 

 in Figs. 9.1 and 9.2. In the objective shown in Fig. 9.1 (Schwarzschild- 

 type objective), the light diffracted by the object A is first reflected, 

 by the concave spherical mirror M, then by the convex spherical 

 mirror m, on to the image A' through the aperture T, provided in the 

 mirror M. In the objective shown in Fig. 9.2 (Cassegrain or Newton 

 type) the fight is reflected either first on a flat or convex mirror m 



Fig. 9.2. Cassegrain type reflecting objective. 



and then on the concave mirror M. In both arrangements, the rays 

 located in the half-cone angle Uo are stopped and occlusion of a portion 

 of the incident beam occurs. The occlusion ratio of a mirror objective 

 is defined as foflows: sinwo/sinz/. This ratio is higher in objectives 

 of the second type and is the reason why Schwarzschild-type objectives 

 are now preferred. 



