126 DARK-GROUND ILLUMINATION 



with water lightly tinted with copper sulphate, an arrangement which has 

 the advantage of absorbing the heat rays and so prevents deterioration of 

 the preparation from that cause. 



The image of the source of light must be formed on the mirror : to secure 

 this, a sheet of white paper may be placed upon the surface of the mirror 



FlG. 115. Illumination with Nernst lamp and lens. 



which is then moved about until the image is clearly defined. The mirror 

 should be uniformly illuminated and the whole surface covered with light- 

 To get the light arranged satisfactorily requires prolonged manipulation, 

 so that for clinical work where time is an important consideration the 

 apparatus should be arranged beforehand. It is of great advantage in this 

 connexion to have an optical bench, for with it the respective positions of 

 the light, the lens and the microscope can be found once for all. Roughly 

 speaking, the lamp, the lens and the mirror are placed at a distance of 15 to 

 20 cm. from one another according to the apparatus used. The tube of the 

 microscope should be vertical. 



B. Centering. 



The dark-ground illuminator, whether placed on the stage of the microscope 

 or arranged in the place of the Abbe condenser, must be centered. The method 

 by which this is done will depend upon whether the apparatus is above or 

 below the stage. 



(a) Dark-ground illuminators fixed in the collar ordinarily carrying the 

 Abbe condenser must be so arranged that the lower flange is close up against 

 the collar, and the upper surface just below the upper surface of the stage. 



Using a low-power objective and looking down the tube of the microscope, 

 the centre of the apparatus should be brightly illuminated without shadows 

 or halos. If the field be not bright, adjust the lateral screws (fig. 113) until 

 the lighting appears quite uniform. 



(b) In those forms which are made for use on the stage, first fix the apparatus 

 with the clips and then, using a low-power objective and looking down the 

 tube, take hold of it on each side with thumb and finger and move it about 

 gently until the centre appears brightly and uniformly illuminated. 



