MiriiOSCUPICAL atOSSARY 285 



lijrht to a nearer and more convenient place (Abbe). Every loss by- 

 reflection at the surface of a lens diminishes the intensity by about 4 

 per cent. 



Interference. Coming together of wave fronts from the same point 

 of the source, in the same or different phase. In the same i)hase they 

 reinforce one another, in different phases they more or less neutralize 

 one another. 



Iris. (1) Circular regulatory diaphragm in the eye. (2) An adjust- 

 able circular diaphragm of metal pieces, usually near the lower (front) 

 focal plane of the condenser. 



Lens. A transparent colorless piece of glass of various kinds, of 

 quartz, or of fluorite; usually worked to accurate spherical surfaces. 

 Two or three single lenses may be cemented to form a doublet or triplet. 

 Lenses may be (1) convergent, collective, or convex; or (2) divergent, 

 dispersive, or concave. 



Lens, biconcave. Concave on both sides. 



Lens, biconvex. Convex on both sides. 



Lens, meniscus. Convex on one side, and concave on the other. 



Lens, plano-concave. Plane on one side and concave on the other. 



Light filter. A colored glass, or gelatin film, used to remove light 

 of certain wave lengths. 



Luminescence. Visible light coming from the transformation of 

 ultra-violet waves when absorbed by certain bodies. 



Limiinescence microscope. A microscope with a glassless mirror, 

 a quartz condenser, and quartz or uviol glass slides; for use with ultra- 

 violet screens and the arc light. 



Macula. The small area on the back of the eye where sight is fairly 

 sharp. It includes the fovea, where sight is sharpest. 



Magnification. Number showing linear increase of size of image 

 as compared with object, the image being always taken as at 250 

 millimeters (whether in focus there, or not). The magnification 

 number of an objective (with high powers, at 180 millimeters optical 

 tube length) is now used to designate that objective. The magnifica- 

 tion of the eyepiece (Huyghenian, or other) is reckoned as the increase 

 of the image the objective would cast without the eyepiece, the final 

 virtual image being supposed to be at 250 millimeters. 



Magnification, useless. Magnification over 1,000 times the working 

 aperture. Empty magnification. Mere enlargement. (Abbe.) 



Magnifier. A low-power lens or microscope. 



Micrometer, eyepiece. A disc of glass to fit on the diaphragm of 

 the eyepiece, with a scale photographed on it, usually of tenths of a 

 millimeter. Care should be taken that the right side is uppermost, 

 since the glass and cover-glass of the disc are of different thicknesses. 

 This scale is best used in an eyepiece provided with a lateral screw. 



