84 



THE USE OF THE MICROSCOPE 



proven < glare, would have a uiaxinium working apertun; 

 of perhaps 0.85 or 0.95. But with the same objective 

 and a corrected and properly adjusted condenser, a working 

 aperture of over 1.3 is achievable. With a large source 

 of light, the condenser aperture may be greater than 0.5 

 with the uncorrected condenser; but only one zone is in 

 focus at a time, and there is glare. 



B 



I)^ 



a^ 



I'.l 



CM 



— R.B.CM 



Fig. 18. — Sectional diagrams of three condensers. On the left is a dry, 

 uncorrected condenser. Above this is indicated roughly how the axial focus 

 differs for the center C and margin M of the condenser cone and for the different 

 colors R, B. In the center is a diagram of an immersion aplanatic condenser 

 corrected for yellow-green light by an aspheric surface on the lowest lens. An 

 accessory achromatic meniscus lens is seen below, which corrects the condenser 

 (made for ijarallol rays) for a near lamp and for water-immersion with a 1- 

 millimeter slide. Above is indicated that the lens gives a sharp focus beyond 

 the axis for one color, but has different focuses for different colors. On the right, 

 is an aplanatic achromatic condenser adjusted for water immersion and near 

 lamp by an additional achromatic lens below its diaphragm. Above is indicated 

 that there is one focus for the different colors and f(3r the different zones. 



4. The uncorrected condenser with a center stop does 

 not give a sufficiently bright nor a sufficiently achromatic 

 dark-ground illumination. Hence special dark-ground con- 

 densers are needed, in addition, even for medium powers. 



It seems obvious that the uncorrected condenser should 

 not be employed in the scientific use of the microscope. It 

 may be used for routine work, as Hartridge explained 

 (71), with an image of a large source of fight thrown by a 

 bull's-eye lens near its front focal plane, so as to send 

 nearly plane waves through the object from all sides 



