too much illumination decidedly injures definition by 

 obliterating detail. 



Little experience is required to judge when the 

 condenser has its proper opening. When correct, the 

 image will stand out sharply defined without any 

 appearance of fogginess and as the diaphragm aperture 

 is reduced it will be noticeable by the decrease in the 

 amount of light. By removing the eyepiece and 

 looking at the back of the objective the relative aper- 

 ture of the condenser to that of the objective may be 

 easily seen, as the outlines of the diaphragm are 

 sharply defined. In testing for this, start with the 

 smallest aperture of the diaphragm and gradually 

 increase its diameter. If the opening in diaphragm 

 appears to have the same opening as the back of 

 objective, the condenser has the same angular aper- 

 ture. In the following instructions for the proper use 

 of light from the condenser the size of opening of its 

 diaphragm as it appears by viewing the back of the 

 objective is called apparent aperture. By experience 

 the following conditions have been found to give most 

 satisfactory results : 



With oil immersion objectives on bacteria use the 

 full opening of diaphragm. 



For diatoms reduce the apparent aperture to about 

 two -thirds opening in objective. 



