ELEMENTARY PHOTO-MICROGRAPHY. 97 



NUMERICAL APERTURE (N.A.) A term employed for 

 measuring the ray-receiving capacity of an objective. 

 Must not be confounded with diameter or size, for, 

 unlike the telescope, the smaller objectives have 

 usually the larger aperture. The comparison of 

 apertures is made not by angles but by their sines, 



OBJECT GLASSES (O.G.), OBJECTIVES OR POWERS. The 

 lenses used for the first magnification of the image. 

 In selecting objectives the following points should be 

 considered : Definition, aperture or resolving power, 

 depth of focus or penetration, working distance or 

 actual distance between front lens and object, flatness 

 of field, and freedom from colour. 



OBLIQUE ILLUMINATION. Light passed to the object from 

 only one edge of the condenser at an angle with axis 

 of microscope. 



OPTICAL CENTRE OR FOCAL CENTRE. A point from which 

 focal measurements must be made. Sometimes it is 

 within the lens and sometimes outside. 



OCULAR. (See EYEPIECE.) 



ORTHOCHROMATIC. The quality of rendering correct colour 

 values. 



PENETRATION OR DEPTH OF Focus. The power to give a 

 distinct image above or below the exact focal plane. 

 The penetrating power of an objective decreases with 

 the increase of both numerical aperture and magnify- 

 ing power, inversely as the numerical aperture and 

 also inversely as the square of the magnifying power. 



POWER. Magnifying powers of lenses are estimated by 

 comparing their focal lengths with the distance of 

 distinct vision, and this has been generally agreed 

 should be arithmetically expressed by reference to 

 visual magnitude seen at loin, distance. Thus xoin. 

 divided by the focal length will be approximately the 



