and while a few hints can be given to guide the begin- 

 ner, practice will, after all, lead to the best results. 

 The purpose of the diaphragm is to modify the amount 

 of light and by its aid obtain results in the definition 

 of the object which without it are impossible. Much 

 will depend upon the density of the object, the intensity 

 of illumination, and the power of the objective. As 

 already stated the plane mirror should be used with 

 the low powers. The diaphragm of any form should 

 be close to the upper surface of the stage, and if 

 possible adjustable with respect to the object. This 

 is to prevent any amount of extraneous light from the 

 mirror reaching the object and not to shut out any of 

 the useful light. 



Use an opening in the diaphragm of about the same 

 size as the front lens in the objective. As a rule this 

 will be found to give a super-abundance of light, 

 especially in the low power objectives, and by reducing 

 the aperture it will be found that there will be an 

 increased differentiation in the object. If in medium 

 and high power objectives the illumination is insuf- 

 ficient, it is not due to the size of the diaphragm, 

 unless this lies at a considerable distance from the 

 object, but to the insufficiency of illumination. The 

 diaphragm should be reduced to a point where the 

 amount of illumination will be perfectly comfortable to 

 the eye. Very often better results are obtained by 

 varying the distance of the diaphragm from the object 



130 



