92 THE USE OF THE MICROSCOPE 



an aplanatic condenser not corrected for color, with no 

 color screen, or with the less dense yellow-green screens 

 Nos. 66 and 56, or with daylight glass, has to be carefully 

 focused for yellow or yellow-green; which special focusing 

 is not needed if an achromatic condenser is used, or if a 

 dark green screen like Wratten No. 58 is employed. 



Regulation of the Iris of the Condenser. — On the regula- 

 tion of the iris of the condenser depends much of the 

 performance of the objective. It is, as already stated, 

 assumed that the effective aperture of the microscope is 

 equal to half the sum of the whole aperture of the objective 

 and the used aperture of the condenser (as shown on the 

 back of the objective with a small source of light). With 



ooo 



JL 4 J. 



2 5 10 



Fig. 20.- Diagrams of the back of an objective with a condenser cone of 

 one-fourth, one-half, four-fifths, and nine-tentha of the aperture of the objec- 

 tive in question. 



a small light-source, and a well-adjusted objective and 

 condenser (that is, adjusted for tube length, thickness 

 of cover and slide, etc.), a cone of light of nearly the full 

 aperture of the high-power objective (say, nine-tenths) 

 can be regularly employed on well-stained objects, in 

 appropriate media. At any rate, more than Nelson's four- 

 fifths aperture cone (101) can be used, as Beck has shown 

 (33). Hence, the condenser should be capable of giving 

 nine- tenths of the aperture of the highest objective (see 

 Fig. 20). It need not be capable of more, if not used for 

 dark field. 



If the iris of the condenser is opened to give a greater 

 aperture than that of an objective of lower aperture, 

 there occurs the flooding of details with a glare which 

 eliminates many of them. For the extra aperture of the 

 condenser lets in marginal rays, which give dark-ground 

 illumination, as usual; and the result is that l)lack hues 



