157 



blue glass for moderating- light, or stops for dark 

 ground or oblique illumination. A screw motion 

 gives a serviceable means of adjustment. 



The most complete form is that shown in Fig. 41, 

 which has adjustments for obtaining every modifi- 

 cation and character of illumination, with rack and 

 pinion for vertical adjustment and swinging out of 

 the way the condenser and iris diaphragm if it is 

 not desired to use these. 



A condenser while useful may be abused so as to 

 do more harm than good and we deem it proper 

 to give some instructions in its use, which we trust 

 may be of service. 



Use only plane mirror with the condenser. 



Centering the Condenser. Every condenser 

 should have a centering arrangement, so as to 

 bring its axis coincident with that of the objective. 

 In the simple form of microscopes in which the 

 character of work is not critical, the condenser is 

 sufficiently centered for all ordinary requirements, 

 but even here it should be possible to center if 

 conditions should demand it. 



In the more complete apparatus a pin hole cap 

 to aid in centering the condenser and fitting over 

 the upper part of the mounting should be supplied, 

 and after focusing upon the opening in the cap 

 with the low power objective, bring into the center 

 of field with the centering screws. Closer adjust- 



