Excerpt from Hawkins Electrical Guide, Vol. 10: Questions, Answers and Illustrations; A Progressive Course of Study for Engineers, Electricians, Students and Those Desiring to Acquire a Working Knowledge of Electricity and Its Applications The emission theory assumes that luminous bodies emit, in all directions, an imponderabl'e substance which consists of molecules of an extreme degree of tenuity. These are propagated in right lines with an almost infinite velocity. Penetrating into the eye, they act on the retina and produce a sensation which is called vision. The undulatory theory assumes that all bodies, as well as the celestial spaces are filled with an extremely subtle elastic medium, fig. - Images produced by small apertures Showing the crossing of luminous rays at the aperture causing inversion of the image. 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.