MICROSCOI'KWL (!I/)SSARY 281 



jsourre is fairly free from aberrations and ^ixcs alxnit. the same iipcrtun; 

 all over (Metz). 



Condenser aperture circle. Iniaf^e of condenser iris seen on the 

 back of tlie objeeti\'e filled with uniform light. 



Condenser, aspheric. An immersion condenser, of three lenses of one 

 kind of glass, with the largest lens corrected by grinding to an empirical 

 curve flatter near the margin, so that the spherical aberration is corrected 

 for the whole combination, and one color. Also called "aplanatic" 

 condenser. (Zeiss. Bausch and Lomb.) 



Condenser, chromatic. Usually a two-lens condenser similar to that 

 first made by Abbe, and described in 1873. It was made for the use of 

 direct or oblique cones of less than 0.3 aperture. It is now usually 

 replaced, in scientific high-power microscopy, by corrected condensers, 

 especially for high powers. (Abbe, 3.) 



Conjugate focus, or image. Corresponding, or coordinate, real or 

 virtual, meeting place of the light rays from points, after passing a 

 corrected lens or lenses. 



Contrast in microscopical image. The same as in ordinary 

 photography. 



Cornea. The front transparent part of the eyeball. 



Corrected lens (or condenser). Commonly made of two or three 

 components of different glasses, arranged so that the spherical and 

 chromatic aberrations are much lessened. (Usually called an achro- 

 matic lens.) 



Correction collar. Screw collar to approximate the one or two front 

 to the two or more back lenses of an objective (or condenser) for thick 

 covers, and separate them for thin covers (or slides). Nearly essential 

 for high dry, and high water-immersion objectives. (Carpenter, 42.) 



Corrections of a lens system require long calculation and many trial 

 tracings of rays through the different zones of the lenses. Spherical 

 corrections are attained by properly spacing and ordering the differently 

 curved surfaces of the various lenses of different kinds of glass which are 

 to compose the objective, and changing, in the calculations, both curves 

 of each lens, while keeping the focus. Chromatic corrections depend on 

 the appropriate balancing of the dispersions at convex and concave 

 surfaces of different glasses. In a corrected objective or condenser, 

 the distances of the component simple lenses, or cemented doublets and 

 triplets, are important factors in the calculations. (Abbe.) 



Cover-glass. Thin flat glass, best of hard crown, cut into pieces 

 usually less than 50 by 20 millimeters. The cover-glass should be 

 0.17 milUmeter thick, or slightly less, for nearly all purposes. The 

 R.I. is to be about 1.52. For dark-ground illumination, thick plane- 

 parallel covers are sometimes made of glass or quartz. For counting 

 blood corpuscles, covers 0.4 millimeter thick have to be used. 



Cover-glass gage. Best in the screw form with "feeler" head. 



