92 Modern Microscopy 



of such condensers. They designed two of apochromatic 

 form, one having a N.A. of I'O for use dry and the other 

 an oil immersion having a N.A. of T40. Some of the 

 condensers before referred to possess aplanatic apertures 

 slightly in excess of those by Powell and Lealand, but they 

 are beautifully corrected, and those who may be wishing 

 to have condensers in keeping with their apochromatic 

 objectives would find these admirable. Still, the value of 

 a sub-stage condenser is not to be reckoned by its total 

 numerical aperture, but by the solid cone that it will 

 transmit, or, in other words, by its perfection of correction ; 



FIG. 26. IRIS DIAPHRAGM AS FITTED TO THE CONDENSER CARRIER. 



for its aplanatic cone alone can be employed for critical 

 illumination. 



If, therefore, an achromatic condenser gives a superior 

 aplanatic cone to an apochromatic for practical work, it 

 may generally be considered preferable, and, seeing that 

 this is done, and that the achromatic form is very much 

 less costly than the apochromatic, the microscopist may 

 with assurance take the former, and be satisfied that he 

 can perform with it all that possibly can be done. 



In Fig. 25 the achromatic condenser that is shown is 

 mounted on a carrier for the sub-stage. It is provided with 

 an iris diaphragm similar to that illustrated in Fig. 26, by 

 means of which any desired aperture may be quickly and 



