A Study on the Performance of "Night-Glasses" (Classic Reprint)

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Excerpt from A Study on the Performance of "Night-Glasses"Assuming, as Mr. Martin does, that the pupil of the eye in the dusk has a diameter of about 8 mm., it is evident that, so long as the magnifying power is not greater than the ratio of the diameter of the o.g. And the diameter of the pupil, the brightness of the retinal picture is practically the same no matter what the magnifying power maybe, but as soon as this limit is exceeded and the magnifying power made greater than this ratio, then the brightness of the retinal picture falls off with increase of magnifying power. In spite, however, of this falling off in brightness increased visibility still results, within limits, from increased magnifying power. The problem of the night-glass therefore practically resolves itself into the determination of the extent to which, under given light conditions, magnifying power may be put into a telescope at the expense of the reduction of the effective diameter of the pupil, and the consequent reduction of the brightness of the retinal picture.Frederic J. Cheshire, Director of the Technical Optics Department, Imperial College of Science and Technology, South Kensington.About the PublisherForgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.comThis book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully, any imperfections that remain are intentionally left to preserve the state of such historical works.