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DARK-GROUND ILLUMINATION 



of the system of lenses, but merely illuminates particles when on a dark back- 

 ground and so renders them more easily visible. 



The researches of Siedentopf and Zsigmondy, afterwards extended by 

 Cotton and Mouton, have been taken up by optical instrument makers, who 

 have constructed and are daily improving the apparatus necessary for dark- 

 ground illumination. 



1. The application of dark-ground illumination 

 to micro-biology. 



Whatever the form of apparatus employed, the dark-ground illuminator 

 does not appear likely to be of assistance in the study of infinitely small 

 things, such as the so-called "invisible micro-organisms," [for the simple 

 reason that objects less than 0'1/x are not resolved. They are seen just as 

 stars are seen, which subtend no appreciable angle, but are visible because 

 their image forms such an intensely bright point of light on part of the apex 

 of one retinal cone that they become visible. Such minute objects appear as 

 bright points in the field of vision surrounded by light and dark diffraction 

 rings ; they have neither shape nor form.] 



The instrument is, however, of considerable practical value in that it affords 

 more favourable conditions than are obtainable with the ordinarily illuminated 

 microscope stage for the examination of material in the fresh unstained con- 

 dition. The dark-ground illuminator renders cells and organisms easily 

 visible in the living condition with their natural movements unimpaired. 

 The valuable aid afforded by the instrument in the rapid diagnosis of certain 

 micro-organic diseases, and particularly of syphilis, has been demonstrated 

 by Landsteiner and Mucha, by Gastou and others. 



2. The construction of the dark-ground illuminator. 



The essential features of the dark-ground illuminator. The dark back- 

 ground and the powerful illuminant that it is necessary to apply can be 

 realized in several ways. 



A. Zeiss' diaphragm. The simplest and cheapest method sufficient more- 

 over in the majority of cases for purposes of clinical diagnosis is to use an 



FIG. 113. Dark-ground illuminator for fixing below the stage. 



ordinary microscope fitted with an Abbe condenser (N.A. 1'40), a dry lens 

 (7 or 8) and a high eyepiece (Zeiss' 12 or 18 compensating ocular) : the 



