|American Microscope Makers|
The first appliance was a deep plano-convex lens, centrally mounted below the object and having its center of curvature in the object place. Afterwards, I adopted a plano-cylindrically convex lens, equal to a hemisphere less the thickness of the object slide, which was placed in immersion contact with the base of the slide so that the object itself formed the center of curvature of this illuminating lens. Around the convex surface of this central lens moved a shutter to regulate and limit the access of light, and it was provided also with a small plano-concave lens which, applied by its concave to the convex surface of the larger lens by immersion contact, cancelled the refracting surfaces and allowed a perpedicular beam of light to reach the suitably immersed object without refraction.
The device in a more complete form is represented in the annexed figure, where P is the basilar plate of the whole traverse system, having a circular groove and track in which the carriage, C, moves. On a projecting arm, A, of the carriage, C, are mounted whatever appliances are to be used to modify or direct the light upon the traverse lens, T, in the direction of the object at the center of the system.
In the figure, the concave lens, N, is shown in position on the arm. Thus situated, the interior convex and concave surfaces being of no effect, the two exterior plane surfaces of the traverse system constitute a prism, and every slightest movement of this concave faced 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 degrees, 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 1/4 inch focus and about 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. The arm, A, has a swing of 70 degrees of arc each way from the axis of the microscope. An index line is marked on the bevelled edge of the carriage 10 degrees 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 eyepiece with luminous field, would expess 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.
Robert B. Tolles
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