AUXILIARY CONDENSER 105 



any other part of the optical apparatus, and that the occasions in which we have 

 found benefit were in reality due to the fact that the parallelism of the rays falling 

 on the substage condenser suited the specimen better than convergent ones. 



To centre this condenser, remove the objective and eyepiece and place its iris 

 diaphragm well dosed to a pin hole, over the lenses. The tiny sparkling point can 

 easily now be made central with the centre of the microscope tube. To complete 



Fig. 56 



the adjustment, return objective and eyepiece and move the milled heads until the 

 light is equally distributed over the field of view. 



(iv) How monochromatic light is ohtained is given in Chapter YL, Section I, (i), 

 paragraph 8 (page 113), so that need not be entered into here, but its use has yet to 

 be explained. 



Monochromatic light is of two kinds : 



(i) Partial, obtained by the use of glasses of different colour. 



(ii) Complete, by the use of prisms, or by troughs filled with special fluids, 

 (i) Partial monochromatic light is used for two purposes : 



A. To increase contrast, and 



B. To reduce it, 



0 



