CHAPTEE XI 



SOME SPECIAL PROCESSES 



Photo-micrography by means of Ultra-violet Light. It has 



already been set out that the resolving-power of objectives 

 increases directly in proportion to any decrease in the wave- 

 length of the light used. To take full advantage of this, 

 Messrs. Zeiss have recently produced an apparatus, designed 

 by Dr. Kohler, in which the source of light lies in the region 

 of the ultra-violet ; and the wave-length of that light is such 

 that the resolving-power of the optical system is double that 

 of any objective used for visual work. 



It is well known that many organic substances are opaque, 

 or partially opaque, to ultra-violet light, although under 

 ordinary conditions of illumination no difference in trans- 

 parency of the component parts is to be discerned. It therefore 

 follows that by using such ultra-violet light for illuminating 

 purposes structure may be brought out, which by any other 

 method of illumination, or by any attempt at differential 

 staining, is invisible ; the effect being that although an object 

 may appear colourless by white light, it would in fact appear to 

 have colour differences if illuminated by ultra-violet light. 

 It is obvious that no direct observation of the image can in this 

 case take place, and that photography is the only means by 

 which a record of the structure of such objects can be obtained. 



The only method by which visible effects may be observed 

 is when the ultra-violet light exerts a very definite physiological 

 action upon living or similar objects which are under the 

 microscope. It is well known that ultra-violet light has a very 

 marked bactericidal effect, and that its physiological effect 

 on various living organisms and tissues is often well marked. 



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