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Transparent. Allowing light, at loast of certain waw lengths, to 

 pass freely, so that an image (real or virtual) may be formed through it. 



Triplet. Three lenses used in combination, and usually cemented 

 together with balsam. 



Tube length. Optical tube length is the distance between the back 

 focal plane of the objective and the front focal plane of the eyepiece, 

 and is usually 180 millimeters, in high objectives. Mechanical tube 

 length is the distance between the shoulder of the objective screw and 

 the edge of the drawtube, including the nosepiece, and is usually 100 

 millimeters (170 millimeters for Leitz microscopes). 



Ultramicroscope. Arrangement for strongly illuminating submicro- 

 scopic particles on a dark field. Particles down to 0.004 micron may be 

 seen as diffraction discs of different l)rilliancy. 



Verant. Compound lens of low magnification, well corrected, for 

 use in viewing photographs, giving correct perspective. 



Virtual image. Points of intersection of the rays, if produced back- 

 wards, in the use of a magnifying (or a concave) lens. 



Vision. The perception of an image on the retina, especially on the 

 macula; and, for fine detail, on the fovea. 



Visual angle. The angle between two straight Unes drawn from the 

 ends of any object to the eye. 



Water immersion. Connection of objective or condenser with the 

 cover-glass or slide by a drop of distilled water. 



Wave length of light is different for different colors, and for the same 

 color in media of different refractive indices. 



Working distance. Actual distance between front mounting of 

 objective and upper surface of cover-glass, when object with standard 

 cover-glass is focused. 



Working aperture. Mean of objective aperture and used condenser 

 aperture. 



Wratten screens. Films of colored gelatin, containing measured 

 quantities of dye, cemented with balsam between two plane glass plates. 



Xylol. I'seful solvent for balsam and dammar, and for removing 

 innnersion oil from the objective and slide. 



Zones. Central circle and two or more annular areas around it, 

 in the aperture of an objective. Also applied to certain hitermediate 

 errors of spherical aberration, etc. 



