Physiological reviews Author:American Physiological Society (1887- ) Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: PHOTO-ELECTRIC CURRENTS IN THE EYE CHARLES SHEARD Southbridge, Massachusetts The effect of the stimulus of light on the retina is perceived by the brain a... more »s a visual sensation. The process or processes by which the ether-wave disturbance causes this visual impulse is still very obscure. As a matter of fact the whole of the field of photo-chemical action is still in its infancy. The process of making an ordinary negative by exposing a dry plate in a camera to light and the manner of developing and fixing such a plate are mechanically easy of accomplishment. But ever since the discovery that an invisible light effect could be developed into a strong image by the application of suitable reducing agents, the constitution of the invisible or so-called latent image has been the subject of study and controversy and no wholly satisfactory explanation of the effects of radiation upon silver salts has been presented. The process by which the ether disturbance causes a visual impulse may be ascribed to a, chemical action, 6, molecular strain and c, electrical action. According to the chemical theory it is presumed that certain visual substances (or possibly substance) in the retina are affected by light and that vision originates from metabolic changes produced in these visual substances. It is supposed that the metabolic changes consist of two phases; the upward, constructive or anabolic phase, and the downward, destructive or katabolic change. These anabolic and katabolic changes in various visual substances are supposed to produce the variations of sensation of light and color. This theory is complex. Numerous objections have been urged against its acceptance; for it is difficult, for instance, to see how this very rapid visual process can be due to a comparatively slow chemic...« less