CH. II] 



LIGHTING AND FOCUSING. 



41 



penter-Dallinger, pp. 254, 263). 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 objec- 

 tive is put into position, the front lens being next the object. As will 

 be seen below (§ 79-80), the full aperture of an objective can rarely be 

 used, and for histological preparations perhaps never, so that an objec- 

 tive of greater equivalent focus, i. e., lower power is used than the one 

 on the microscope. 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 necessary 

 for lighting different objects for the same objective and for lighting with 

 objectives of different apertures. An excellent condenser of this form 

 has been produced by Zeiss (Fig. 41 ). It has a total numerical aper- 

 ture of 1. 00, and an aplanatie aperture of N. A. 0.65. 



Fro. 41. Zeiss' Achromatic Conden- 

 ser, c.s c s. Centering screws for 

 changing the position of the condenser 

 and making its axis continuous with 

 that of the microscope. A segment of 

 the condenser is cut away to sho:u the 

 combinations of tenses. For very low 

 powers the upper lens is sometimes 

 screwed off. There is an iris dia- 

 phragm between the middle and lower 

 combinations. (Zeiss' Catalog, No. 

 3°) 



§ 77. Centering the Condenser. — To get the best possible illumina- 

 tion for bringing out in the clearest manner the minute details of a mi- 

 croscopic 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 although 

 daylight may be used with almost equal facility : The object is placed 

 on the stage and lighted with the edge or face of the flame and then 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 of stiff, 

 black paper, care must be taken, however, to make the opening exact- 

 ly central. This is best accomplished by putting the paper disc over 

 the iris or metal diaphragm and then making the hole in the center of 



