50 LIGHTING AND FOCUSING [ CH. II 



most perfect of all seems to be the apochromatic of Powell and Lea- 

 land (Carpenter-Dallinger, p. 302). To attain the best that was 

 possible many workers have adopted the plan of using objectives as 

 condensers. A special substage fitting is provided with the proper 

 screw and the objective is put into position, the front lens being 

 next the object. As will be seen below (94, 95), the full aperture 

 of an objective can rarely be used, and for histological preparations 

 perhaps never, so that an objective of greater equivalent focus, i e., 

 lower power, is used for the condenser than the one on the micro- 

 scope. It is much more convenient, however, to have a special 

 condenser with iris diaphragm or special diaphragms so that one 

 may use any aperture at will, and thus satisfy the conditions neces- 

 sary for lighting different objects for the sa'me objective and for 

 lighting with objectives of different apertures. An excellent con- 

 denser of this form has been produced by Zeiss (Fig. 47). It has a 

 total numerical aperture of i.oo, and an aplanatic aperture of 0.65. 



FiG. 47. Zeiss' Achromatic Con- 

 denser, c. s. c. s. Centering screws 

 for changing the position of the con- 

 denser and making its axis continuous 

 with that of the microscope. A seg- 

 ment of the condenser is cut away to 

 show the combination of lenses. For 

 very low powers the upper lens is 

 sometimes screwed off. There is an 

 iris diaphragm between the middle 

 and lower combinations. (Zeiss' 

 Catalog, ) 



92. Centering the Condenser. To get the best possible 

 illumination for bringing out in the clearest manner the minute de- 

 tails of a microscopic object two conditions are necessary, viz. : The 

 principal optic axis of the condenser must be continuous with that 

 of the microscope (see frontispiece) and the object must be in the 

 focus of the condenser, i.e. , at the apex of the cone of light given 

 by the condenser. 



The centering is most conveniently accomplished as follows al- 

 though daylight may be used with almost equal facility. A very 

 small diaphragm is put below the condenser. (If the Zeiss achro- 

 matic condenser is used, the diaphragm of the Abbe illuminator 

 serves for this. If there is no pin-hole diaphragm one can be made 



