ELEMENTARY PHOTO-MICROGRAPHY. 133 



and the English pattern 250 mm. (about 10 inches). 

 Objectives should be used which are designed for 

 the particular length of tube adopted. 



MICROMETER. A slip of glass ruled to scale, generally 

 hundredths or thousandths of an inch. Made for use 

 on the stage of microscope and also for the eyepiece. 



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. 



OPTICAL TUBE LENGTH. Usually 20 mm. more than the 

 mechanical tube length. Measured from the dia- 

 phragm of the eyepiece to the upper focal plane of 

 objective. 



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. 



