58 



LIGHTING WITH A SUBSTAGE CONDENSER [Ch. II 



ihereforc would transmit to the object a very large aplanatic cone of 

 light. Such condensers are especially recommended for photo-microg- 

 raph}' by all, and those who believe in getting the best possible image 

 in every case are equally emphatic that achromatic condensers should 

 be used for all work. Unfortunately good condensers like good ob- 

 jectives 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 (Car- 

 penter-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 posi- 

 tion, the front lens being next the object. As 

 will be seen below (§ 106-107), the full aperture 

 of an objective can rarely be used, and for his- 

 tological 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 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 objectives of different apertures. 



§ 102. Non-achromatic condensers. — Of the non-achromatic 

 condensers or illuminators, the Abbe condenser or illuminator is the 

 one most generally used. From its cheapness it is also much more 

 commonly used than the achromatic condenser. It consists of two or 

 three very large lenses and transmits a cone of light of 1.2 N.A. to 

 1.40 N. A. (fig. 41, 46), but the aberrations, both spherical and chro- 

 matic, are very great in both forms. Indeed, so great are they that 

 in the best form with three lenses and an illuminating cone of 1.40 



Fig. 39. Achrom- 

 atic Substage Con- 

 denser. 



(From Watson's 

 Catalogue). 



7, 2, j The lens ele- 

 ments making up the 

 condenser. While the 

 lenses are larger, 

 the general construc- 

 tion of a substage 

 condenser is like an 

 objective, /represent- 

 ing the front, w the 

 middle, and e the back 

 combination of the 

 objective. 



