Motion picture projection; an elementary text-book (1921)

118 MOTION PICTURE PROJECTION hinder the motion of the stars and planets. Huygens assumed such a medium to exist, and called it ether. The second difficulty in the way of the wave theory of light, was that it seemed to fail to account for the fact of straight line propagation. Sound waves, water waves and all other forms of waves with which we are familiar bend readily around corners, while light apparently does not. It was this difficulty chiefly which led many of the famous philosophers, including the great Sir Isaac Newton, to reject the wave theory and to support the projected particle theory. Within the last hundred years, however, this difficulty has been completely removed and in addition other properties of light have been discovered, for which the wave theory offers the only satisfactory explanation. If the wave theory is to be accepted, we must conceive with Huygens, that all space is filled with a medium, called the ether, in which the waves can travel. This medium cannot be like any of the ordinary forms of matter ; for if any of these forms existed in interplanetary space, the planets and the other heavenly bodies would certainly be retarded in their motion. As a matter of fact, in all the hundreds of years during which astronomers have been making accurate observation of the motion of heavenly bodies no such retardation has ever been observed. The medium which transmits light waves, must therefore have a density which is infinitely smaller even in comparison with that of our lightest gases. The existence of such a medium is now universally assumed by physicists. Just as sound waves are disturbances set up in the