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Excerpt from Cantor Lectures on Photography and the Spectroscope It often happens that a second patch of white light, comparable to that formed, is te quired. Advantage is taken of the fact that from the first surface of the first prism P', a certain amount of light is reflected. Placing a lens. L5, in the path of this reflected beam, and a mirror, G, another square patch of light can be thrown on the same screen as that on which the first is thrown, and this second patch may be made of the same size as the first patch if the lens, L5, be of suitable focus, and it can be superposed over the first patch if required. We have now a square white patch upon the screen, from the te-combination of the spectrum. If I wish to diminish the bright ness of this patch, there are at least two ways in Which I can accomplish it. First, by closing the slit of the collimator, and, second, by the introduction of rotating sectors, M, which can be Opened and closed at pleasure during rotation in the path of the beam. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This 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.

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