MICEO^COPICM ainssARY 287 



of the uoscpicce; and oik^ firm, at least, allows for this by (ittiiiy; an 

 appropriate adapter for a single objectiv(^ used without a revoh^ing 

 nosepiece. The true centering of the nosepiece is most inii^ortant. 



Object-field. That circular ])ortion of the object, the hnage of which 

 is seen in a given eyepiece with a given objective. No more than this 

 should be illuminated, or there is more or less glare. 



Objective. The chief lens combination of the compound microscope. 

 It receives light from the points of the object in cones of definite aper- 

 ture, and is usually so constructed (in high powers) as to throw its best 

 image at a distance of 180 millimeters from its back focal plane. 



Objective aperture circle. Seen at back of objective. Circle bound- 

 ing the aperture of all rays, direct or diffracted, passing through 

 objective. 



Objectives, dry. The low- and medium-power objectives, which 

 are separated from the cover-glass by air. In consequence of this, 

 the refraction at the upper surface of the cover, and at the plane surface 

 of the front lens, causes spherical aberrations; which, in objectives of 

 over about 0.85 aperture, cannot be properly corrected by subsequent 

 refracting surfaces. (Hence, these are now made with concave front 

 surfaces.) (Abbe, Conrady, Zeiss.) 



Objectives, low, medimn, and high, as to focal length (power), in the 

 ordinary compound microscope. These are, according to Carpenter, 

 low, if above 1 2.5-millimeter focus; medium, if above 5 millimeters and 

 if 12.5 millimeters or less in focus; and high, from 5 to 2 millimeters 

 (or even 1.5 millimeters) focal length. Thus, of the ordinary apochro- 

 matics, the 16-millimeter is low, the 8-millimeter medium, and the 

 others high-power. (See also Chap. II.) 



Objectives, low, medimn, and high, as to numerical aperture. Tak- 

 ing the multiplying factor of 23-2 corresponding to Carpenter's choice 

 of focal lengths, we should have low aperture, below 0.22; medium 

 aperture, from 0.22 to 0.56 ; and high aperture, from 0.56 to 1 .4. (See also 

 Chap. II.) 



Objectives, low, medivmi, and high, as to useful magnification. 

 (These of course follow the apertures.) The low magnifications 

 would be, following Carpenter's scheme, those below 220; the medium, 

 from 220 to 560; and the high from 560 to 1,400. This is for 1,000 times 

 the working aperture. (See also Chap. II.) 



Oblique illixmination. Usually applied to the lighting of the object 

 by a lateral beam near the margin of the objective, and of less than 0.3 

 aperture. This might be obtained by an eccentric diaphragm; but it 

 is rarely used now, since it gives incorrect pictures of all but certain fine 

 gratings. Annular illumination, and especially dark-field lighting, are 

 preferable. A nearly full cone usually includes the extreme oblique 

 pencils. 



Ocular. Eyepiece. 



