Download or read book The Ophthalmoscope, 1910, Vol. 8: A Monthly Review of Current Ophthalmology (Classic Reprint) written by Sydney Stephenson and published by Forgotten Books. This book was released on 2019-02-11 with total page 504 pages. Available in PDF, EPUB and Kindle. Book excerpt: Excerpt from The Ophthalmoscope, 1910, Vol. 8: A Monthly Review of Current Ophthalmology The eye has often been likened to a camera, but it is really a complete photographic apparatus. Light falling upon the eye is refracted by the cornea, and brought to a focus by a lens which by the power of accommoda tion can be adjusted for varying distances. The amount of light entering the eye is regulated by the iris, which acts as a movable diaphragm, contracting when the light is strong and dilating when it is feeble. The image of an object is brought to a focus on the sensitive layer of the retina, the layer of the rods and cones which corresponds to the sensitive plate of a camera. It is with this sensitive layer that _we are chiefly concerned this evening, SO I will describe it and the rods and cones in detail. The rods and cones are situated at the external part of the retina, that is, the portion of the retina farthest from the front of the eye. Light has therefore to pass through all the other layers of the retina before it reaches the rods and cones. As you will see by the picture which is thrown on the screen, the rods and cones each consist of an outer and an inner segment. The rods and cones project into a thin layer of fluid which is kept in its place by a membrane, the external limiting membrane. The most external layer of all of the retina is a layer of hexagonal cells containing black pigment; these cells throw out processes which surround the external segments of the rods when the eye is exposed to light and are retracted when the eye is in darkness. The outer segments of the rods contain a purple substance, the visual purple, which is photo-chemically sensitive to light. The outer segments of the cones are made up of a series of strongly refractile discs, which gradually diminish in circumference. The following is the theory which I hold with regard to the perception of light and colour. A ray of light impinging on the retina liberates the visual purple from the rods and a photograph is formed. The rods are concerned only with. The formation and distribution of the visual purple, not with the conveyance of light impulses to the brain. 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.