MICROSCOPICAL GLOSSARY 283 



Eyelens. Back (upper) lens of eyepiece. 



Eyepiece. Detacha})le niau;iiificr of iinaii;e cast by objective. (1) 

 Witli a field-lens, and the image between the two lenses (lluyghenian. 

 periplane, etc.). (2) With the image below the lenses, and no field lens 

 (Ramsden, orthoscopic, high compensating). 



Eyepiece circles. Seen at eyepoint. Images of condenser aperture 

 circle, and objective aperture circle. 



Exit pupil of microscope. The full (Ramsden) circle above the eye- 

 piece (only the central part of which is usually bright) (Abbe). 



Field lens. Lower lens of Huyghenian eyepiece. 



Finder. A low objective used in searching. 



Fine motion of the microscope, by micrometer screw, or by cogwheels, 

 etc. (Also called "fine adjustment.") 



Fixation by the eyes. Directing and converging the eyes so that the 

 image of the fixed spot comes on the fovea in both eyes. 



Flare. In photography, reflection of light from lens surfaces on to the 

 sensitive plate. 



Flint glass. Glass containing lead. 



Flooding with light, by opening the condenser iris beyond the ai)erture 

 of the objective; the extra aperture acting as dark-field illumination. 



Fluorescence. Change of wave length of light absorbed and then 

 emitted by certain substances, such as eosin. 



Fluorite. Natural calcium fiuoride crystallized in the cubical system. 

 Only water-clear pieces, crystallized without strain, are useful for lens 

 making. Its R. I. and dispersion render it more suited to correct the 

 chromatic (and hence also some spherical) aberrations than any artificial 

 glass. (Abbe.) 



Fluorite objectives. Contain fluorite, and have the secondary 

 spectrum nearly absent, but not the color differences of spherical 

 aberration. With yellow-green screen they are excellent. 



Focus. (1) Point or plane to which plane waves are caused to con- 

 verge (or from which they diverge) after passing a lens or combination. 

 Distance of this point from the principal plane (focal distance). (2) 

 Any point where a real or virtual image or its object may be. 



Fog. An apparent haze over the object, due usually to smears on 

 glass surfaces. 



Fovea. The small slightly depressed spot near the center of the 

 retina; where the cones (about 5,000) are close together, and vision is 

 most distinct. 



Front lens, of condenser or objective. (1) Strictly, the one the light 

 first meets. (2) By analogy with the ol^jective, the small lens of a 

 condenser is sometimes called the front lens (Spitta). 



Glare. Term used to include all cases of extraneous light in the image 

 (except flooding, which is caused by greater aperture in the condenser 

 than in the objective). Beck distinguishes glare due to reflections in 



