161 



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 rela- 

 tive aperture of the condenser to that of the objec- 

 tive may be easily seen, as the outlines of the 

 diaphragm are sharply defined. If the opening 

 in diaphragm appears to have the same opening 

 as the back of objective, the condenser has the 

 same angular aperture. By experience the fol- 

 lowing conditions have been found to give most 

 satisfactory results : 



In oil immersion objectives on bacteria use tlic 

 full aperture of condenser. 



On diatoms reduce the aperture to about tu'o- 

 thirds opening in objective. 



In Jiistological and other dense objects use an 

 aperture equal to about one -half t lie opening of back- 

 lens in objective. 



Oblique Light With Condenser. Oblique 

 light may be obtained by setting the mirror in 

 such a position that the light reflected from it 

 shall enter the condenser only at one side, leaving 

 the balance of it unused. This, however, is only 

 advisable when the condenser mounting has no 

 other provision for obtaining oblique light. In 

 the mountings however, having such provision, it 

 is obtained by redticing the opening in diaphragm 

 so that it shall correspond in size to the back 



11 



