104 



brought into the optical axis or passed diametrically 

 across the condenser, giving different degrees of 

 obliquity. 



The vertical adjustment is obtained by sliding in 

 the sub-stage. 



The best form is that which is provided with an 

 Iris diaphragm, which, when open, gives full volume 

 of light but may be reduced to nearly a pin-hole 

 opening. Then an addition which carries it laterally 

 under the condenser by rack and pinion, and another 

 rack and pinion for vertical adjustment either on the 

 sub-stage bar or as part of the condenser. As the suc- 

 cessful utilization of the condenser_depends upon the 

 nicety with which the various adjustments can be 

 accomplished, they can be none too complete. 



Centering. In using the condenser the first con- 

 dition is the proper centering. For this purpose 

 the pin-hole cap should be placed over the top and 

 focused upon by means of a medium 

 power objective. The condenser 

 should be fixed so that the opening 

 will be in the center of the field. For 

 Fig. 24. objectives under 1.0 numerical aper- 

 ture it is not necessary to bring the condenser in 

 immersion contact with the slide ; for objectives of 

 wider aper-ture, however, it should be done by first 

 placing a drop of immersion fluid on the top lens and 

 then placing the slide in position on the stage, raise 

 the condenser until the fluid comes in contact with 

 the lower surface. 



