MICUOSCOI'ICAL dLOSSARV 279 



Aplanatic. (1) Lens or combination corrected for a fairly large visual 

 field by observation of the sine law (Abbe). (2) Used erroneously for 

 combinations corrected mainly for axial spherical aberrations 

 (Woodworth). 



Aplanatic condenser. Corrected by the sine law either by concave 

 flint lenses, aspheric surfaces, or certain combinations of lenses of one 

 refractive substance (Metz, Zeiss, von Rohr). 



Apochromatic. Applied to objectives containing usually one or more 

 lenses of fiuorite and lenses of the Jena glasses; and having corrections 



(1) for the secondary chromatic aberrations, and (2) for spherical aberra- 

 tions of two different colors and all zones of the objective. The ordinary 

 achromatic objective has only the primary chromatic aberrations 

 corrected, and is corrected spherically for only one color, showing 

 secondary colors. (Abbe.) 



Aspheric. Condensers or bull's eyes, etc., corrected for spherical 

 aberration (for one color) by an empirically made aspheric surface, 

 ground flatter towards the margin (Zeiss, Bausch). 



Astigmatism. Having different foci in different azimuths. Applied 

 to the eye (with deformed cornea, etc.), or to oblique pencils through an 

 uncorrected lens. 



Axis, optical. The line joining the centers of the curved or plane lens 

 surfaces. 



Back lens. The last lens passed by the light in an objective or 

 condenser. (This is the strict use of the term.) 



Balsam, Canada. Said to be the resin from two species of American 

 conifer. The R.I. of the hard resin is given as 1.56. The hard resin 

 is to be dissolved in xylol. The R.I. of the actual balsam in which 

 the specimens are mounted varies with the amount of xylol unevaporated . 

 A standard mounting medium might be prepared of known R.I. and 

 dispersion. Sections can well be mounted in thickened cedar oil, 

 which has the correct refractive index and dispersion for oil-immersion 

 objectives. (R.I. = refractive index.) 



Biconcave. Hollow on both sides. 



Biconvex. Bulging on both sides. 



Binocular magnifier or microscope. Arrangement for vision with 

 both eyes; either stereoscopic, as in the low-power magnifier with 

 rhcjmbohedral prisms and simple lenses, and in the low-power CJreenough 

 with Porro prisms and twin objectives; or non-stereoscopic with a 

 semi-reflecting silvered or platinized surface, and one objective (Abbe, 

 Czapski, Jentzsch). 



Borate glass. Jena glass containing borates. 



Brightness of the image is due (1) to the intensity of the light, and 



(2) to the aperture of the objective, the magnification being unaltered. 

 Probably more than 4 per cent of light is lost by reflection at every 



