Excerpt from A Treatise on Simple and Compound Ophthalmic Lenses: Their Refraction and Dioptric Formul , Including Tables of Crossed Cylinders and Their Sphero-Cylindrical Equivalents By virtue of the deflection or bend alluded to, the incident ray, i, must include a different angle, a, with the perpendicular p1 from that, B, of the refracted ray e1 e2, and it is by the trigonometrical values S and S1 of these angles, which have been found to bear a constant proportion to each other, that we are enabled to give expression to the amount of deflection sustained by a ray in passing from one medium to another. 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.