DARK GROUND ILLUMINATION 



183 



chromatic effects are even then obtained by the use of the 

 new dark ground illuminators, in most of which light is 

 passed to the objective by reflection only. Fig. 56 show r s 



Fig. 56 



LEITZ' DARK GROUND ILLUMINATOR 



Leitz' dark ground illuminator. The Nelson dark ground 

 condenser, Fig. 57, made by C. Baker, involves refrac- 



Fig. 57 

 NELSON'S DARK GROUND CONDENSER 



tion, but the results given by it compare very favourably 

 with those given by the reflecting illuminators. Its cor- 

 rections may not be so accurate as those of some of the 

 reflectors, but it is easier to work with, as it is not quite 

 so sensitive to errors in centration, and does not require 

 so intense an illuminant for photographic purposes. 



The reflecting condensers of different makers vary in 

 construction, though the principles involved are the same. 



