The microscope : its construction and management : including technique, photo-micrography, and the past and future of the microscope (1893)
Heurck, Henri van, 1838-1909 · London : C. Lockwood ; New York : D. Van Nostrand Co. · leaf 74
.These are the very same considerations which have excited long andanimated discussions among microscopists on what is called theaperture question. And one of the reasons which led ProfessorAbbe to abandon the idea of angular aperture, and adopt numericalaperture in its stead, which is applicable… … The apertometer, an apparatus placed on the stage of the microscope,consists of a thick semi-circular disk of flint-glass, 90 mill.(3 546 inches) in diameter. The diametral edge is bevelled so asto give the effect of a reflecting prism, and so send the lighthorizontally into the axis of the…
Microscopy
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