Excerpt from A Dictionary of Photography There are es1tain forms of reflecting and refracting surfaces and lenses in which a particular pencil is reflected or refracted without aberration. The cases with which the photographer is chiefly con cerned are those of the parabolic and spheroidal mirrors. All rays incident on the surface of a parabolic reflector, in directions parallel to its axis, are reflected in directions which pass through its focus and all rays incident on the surface of a prolate spheroid, in direc tions which pass through one of its foci, are reflected in directions which pass through the other focus. 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.