Excerpt from The Theory of Modern Optical Instruments: A Reference Book for Physicists, Manufacturers of Optical Instruments, and for Officers in the Army and Navy 29 Power. The Dioptre 30 Spectacle Nomenclature 31 Convergence 32 Power and Vergences for any Centred System of Spherical Surfaces. 33 Summary of the Results for a System referred to the Principal Points 34 Focal Point Vergence 35 Nodal Point Vergence 36 The Curvature Of the Translators' Appendix to Chapter III. 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.