42 LIGHTING AND FOCUSING [CH. II 



It is of the greatest advantage to have the sub-stage condenser 

 mounted so that it may be easily moved up or down under the stage. 

 The iris diaphragm is so convenient that it should be furnished in all 

 cases, and there should be marks indicating the N. A. of the condenser 

 utilized with different openings. Finally the condenser should be 

 supplied with central stops for dark-ground illumination (§92) and 

 with blue and neutral tint glasses to soften the glare when artificial 

 light is used (§ 89, 93). 



Condensers or Illuminators fall into two great groups, the 

 Achromatic, giving a large aplanatic cone, and Non-achromatic, 

 giving much light, but a relatively small aplanatic cone of light. 



§ 80. Achromatic Condenser. — It is still believed by all expert 

 microscopists that the contention of Brewster was right, and the con- 

 denser to give the greatest aid in elucidating microscopic structure 

 must approach in excellence the best objectives. That is, it should be 

 as free as possible from spherical and chromatic aberration, and there- 

 fore would transmit to the object a very large aplanatic cone of light. 

 Such condensers are especially recommended for photo-micrography by 

 all, and those who believe in getting the best possible image in every 

 case are equally strenuous that achromatic condensers should be used 

 for all work. Unfortunately good condensers like good objectives are 

 expensive, and student microscopes as well 'as many others are usually 

 supplied with the non-achromatic condensers or with none. 



Many excellent achromatic condensers have been made, but the 

 most perfect of all seems to be the apochromatic of Powell and Lealand 

 (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 (§ 83-84), 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. <?. , lower power is used for 

 the condenser than the one on the microscope. It is much more con- 

 venient, 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 aperture of 1.00, and an aplanatic aper- 

 ture of 0.65. 



