CH. II] LIGHTING AND FOCUSING 



53 



with the condenser. And the size of the diaphragm must be 

 directly as the aperture of the objective. That is, it is just the 

 reverse of the rule for diaphragms where no condenser is used 

 (§ 76) ; for there the diaphragm is made large for low powers, and 

 consequently low apertures, while with the condenser the diaphragm 

 is made small for low and large for high powers as the aperture is 



Ob] Ob] 



o 



Fig. 5? Fig. 53 



Figs. 52-53. Figures showing the dependence of the objective upon the 

 illuminating cone of the condenser (Nelson) . 



Fig. 52 (A). The illuminating cone from the condenser (Ilium.). This 

 is seen to be just sufficient to fill the objective (Obj. ). 



(B.) The back lens of the objective entirely filled with light, shozving that 

 the numerical aperture of the illuminator is equal to that of the objective. 



Fig. 53 (A). In this figure the illuminating cone from the condenser 

 (Ilium.) is seen to be sufficient lofill the objective (Obj.) . 



(B.) The back lens of the objective only partly filled with light, due to the 

 restricted aperture of the illuminator, 



greater in the high powers of a given series of objectives. It is 

 very instructive to demonstrate this by using a 16 mm. objective 

 and opening the diaphragm of the condenser till the back lens is 

 just filled with light. Then if one uses a 3 or 4 mm. objective it 

 will be seen that the back lens of the higher objective is only partly 

 filled with light and to fill it the diaphragm must be much more 

 widely opened. 



With a condenser, then, the diaphragm has simply to regulate 

 the aperture of the illuminating cone, and has nothing to do with 

 lighting a large or a small field. 



With the condenser there are two conditions that must be ful- 

 filled, — the proper aperture must be used, and that is determined 

 by the diaphragm, and secondly the whole field must be lighted. 

 The latter is accomplished by using a larger source of light, as the 

 face instead of the edge of a lamp flame, or by lowering or raising 



