CHAPTER IX 



Infra-red and Ultra-violet Microscopy 



Infra-red and ultra-violet microscopy is a powerful means of inves- 

 tigation which finds numerous applications. It is well known that 

 there are many objects, transparent in visible light, which have ab- 

 sorption bands in the infra-red and the ultra-violet. In order to make 

 the object visible in the receiver, merely using a radiation whose 

 wave-length corresponds to one of the object's absorption bands will 

 achieve the result. For instance, living cells, transparent in visible 

 light, exhibit details in the ultra-violet because the 2700 A wave-length 

 radiations are absorbed by the nucleic acids. Moreover, ultra-violet 

 light enables reduction of the diffraction-disk diameter (Chapter L 

 equation 1) thus improving resolving power. Lastly, some substances, 

 under the effects of ultra-violet rays, radiate some visible light whose 

 colour frequently helps in identifying or discriminating these sub- 

 stances. 



1. MICROSCOPE OBJECTIVES IN INFRA-RED AND ULTRA-VIOLET 



MICROSCOPES 



Up to a wave-length of about 1-5/;, standard achromatic objectives 

 may be used but the image quality is not too satisfactory owing to 

 the chromatic variation of spherical aberration. This aberration is 

 more particularly prominent in powerful achromatic objectives. Better 

 resuhs are evidenced, in general, by apochromatic objectives whose 

 residual aberrations are not so marked. Designing glass achromatic 

 objectives transparent up to 3 ;t< and even 10 /< is feasible provided 

 that they be applied to a narrow spectral range. In fact, there are 

 several optical glasses, currently used, which are transparent up to 3 /v. 

 Beyond, special glasses or artificial crystals are required. 



Nevertheless, from a 1-5// wave-length approximately, application 

 of reflecting objectives is a suitable process. Such objectives are 



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