An Illuminating Ti^averse-Lens. By Bolert B. ToUes. 389 



concave facet lens on the traverse lens T would give a different 

 prism to infinite variety. In this arrangement, the concave mirror 

 can be used in the ordinary manner and condense light enough 

 upon the object for all ordinary purposes. The full interior 

 aperture of a dry objective would be reached at the very convenient 

 obliquity of 41'"', i. e. at less than the critical angle, or angle of 

 total internal reflection between crown-glass and air. L is a 

 double-convex condensing lens, that may be placed at about its 

 principal focal distance from the object. 



For a condenser, with the size of apparatus as drawn in the 

 figure, a simple lens of 1|^ inch focus and about ten (10") degrees 

 of aperture is convenient, and if the lens is movable along the arm A, 

 it can be focussed readily on the object, the position being fixed by 

 inspection. This would be well for parallel rays. If diverging 

 rays are used another lens of two or three inches focus, mounted on 

 the arm A, will conveniently take up the rays from the radiant at 

 the distance of the focus of this supplementary lens. 



The plate P is graduated on its circular edge, as in the figure, 

 to two degrees, and the arm A has a swing of seventy degrees of 

 arc each way from the axis of the Microscope. An index-line is 

 marked on the bevelled edge of the carriage 10^ from the axis of 

 the condenser, which must be added to or subtracted from the real 

 obliquity of the illuminating rays. 



It is obvious that any observation made and duly recorded as 

 to its conditions, as of obliquity of incidence of illuminating pencil 

 or ray, form of the pencil or beam, focal length and distance of 

 the condenser, such observation could be successfully repeated. 

 The record of the obliquity of the most oblique rays reaching the 

 object directly and giving view of it at the eye-piece with luminous 

 field, would express the " balsam " aperture, or more correctly, the 

 half interior aperture of the objective when the front lens of the 

 objective and the traverse system are of glass of similar refraction. 



Having thus the " balsam " angle we readily calculate or learn 

 the corresponding angle for glycerine, or water, or any medium of 

 which we have the index of refraction. A corresponding notation, 

 perhaps for air, might be engraved in juxtaposition on the basilar 

 plate. 



