Excerpt from A Treatise on Photographic Optics The object of this treatise is to provide an account of the principles of Optics, so far as they apply to Photography, in a form which is of scientific value, while not of too abstruse a nature to place it beyond the reach of all but the professional mathematician or physicist. I have attempted to steer a middle course between giving too much mathematics and giving none at all; the former course would restrict the book to a few, while the latter would deprive it of all real value. To make the mathematics as intelligible as possible, most of the results have been illustrated by worked numerical examples, and symbolical results have been expressed in words. The chapter on aberration necessarily contains a certain amount of algebraical formulae, too many perhaps for many readers, but care has been taken to explain clearly, by means of diagrams, the principles which underlie the formulae. I have to thank Captain Abney for giving me permission to use many of his diagrams, Major Darwin for allowing me to quote from his paper on lens testing at Kew Observatory, and the publishers of Photography Annual for allowing me to copy the tables contained in 127, 151, and 156;also I owe it to the kindness of the Sandell Plate Company that I am able to give an example of a test film for the speed of plates. My thanks are also due to the Rev. W. F. H. Curtis, who first suggested to me to write, and has given me many valuable suggestions and much help. 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."