280 THE USE OF THE MICROSCOPE 



refraction between glass and air. This gives a large loss after 20 or more 

 refractions (Koehler, Metzner). No condenser or other apparatus can 

 make an image brighter than the actual source of light. 



Brilliancy of image. Freedom from fog or glare, and sharpness of 

 definiti(jn. 



Camera. (1) Closed box for exposing a photographic slide. (2) 

 Apparatus for superposing the microscope image on the image of an 

 external object. Called "camera lucida," or better, "drawing camera." 



Capacity of objective. Its power to define small details ; which depends 

 on its aperture, and on the accuracy of correction. 



Cardioid condenser. A two-reflection convex-concave dark-field 

 condenser for special work, mostly on objects in water (Siedentopf). 



Cassegrain condenser. A two-reflection, concave-convex, dark- 

 field condenser for high-aperture immersion objectives, on objects in 

 balsam or in immersion oil (Nelson). 



Centering ring. An arrangement with two screws and a spring, or 

 an excentric ring (Zeiss), for moving short-focus condensers or objectives 

 the fraction of a millimeter required to accurately center them. 

 Useful for adjusting objectives as well as for condensers. 



Chromatic aberration (see aherration). 



Chromatic condenser. Term usually applied to an uncorrected 

 condenser. 



Circle, aperture (see condenser, objective, and eyepiece). 



Coarse motion of the microscope, by rackwork. (Also called coarse 

 adjustment.) 



Color corrections. Corrections for dift'erences of refraction or 

 dispersion, in different wave lengths. 



Coma. Aberration of oblique cones of light, due to departure from 

 Abbe's sine law. 



Combination, of two or more separate lenses to form a system, such as 

 an objective or condenser. 



Compensating eyepiece. Eyepiece with a doul^let or triplet in place 

 of one lens, calculated so as to equalize the magnification for different 

 colors in the image formed by apochromatic objectives (Abbe). 



Compensating the imused eye. By a disc of ground glass. 



Concave. Si)herically hollow. 



Concentric condenser. Dark-field condenser of Jentzsch. 



Condenser. Combination of lenses to cast a large enough image of 

 the source of light on the object-field, together with a large enough 

 aperture. 



Condenser, achromatic. A condenser corrected at least for axial 

 chromatic aud spherical aberration, by lenses of two kinds of glass. 



Condenser, achromatic aplanatic. A ccjndenser, preferably immer- 

 sion, corrected for axial chromatic and spherical aberrations; and also 

 following the sine law, so that a sufficiently wide area of the image of the 



