The undulatory theory has now-a-days completely superseded the corpuscular hypothesis. It was first started about the year 1660 by the Dutch philosopher Huyghens, who has left behind him numerous treatises on optics, and the properties of light, as well as a curious account of the inhabitants of the other members of the solar system, including a minute description of the various planetary manners and customs. At the beginning of the present century, Fresnel showed, by the most brilliant discoveries the superiority of this theory, and shortly after Arago confirmed him in his demonstrations. According to the undulatory hypothesis, light is not a mass of molecules emitted by a luminous body, but simply the vibration of an elastic fluid which is conceived to fill the whole of space. A comparative example may assist you in understanding this theory more clearly. If you throw a stone into a smooth piece of water, there will form around the point where the stone fell, a series of circular undulations, starting from the centre and gradually enlarging themselves. If a loud noise is suddenly heard, the same effect is produced round the point from whence the sound proceeds. A series of waves are formed which spread not only horizontally, as on the surface of the water disturbed by the stone, but in every direction. In fact, in the case of sounds, the waves are so many gradually increasing spheres. In the case of light, when a luminous body is placed in space, the ether which surrounds it is thrown into a state of vibration, and the motion is immediately propagated in all directions, with extreme velocity. It is these undulations that produce upon our eyes the sensation of light. We may therefore say that light, like sound, is movement, while darkness, like silence, is absolute rest. Many people still believe that light is propagated instantaneously, and cannot bring themselves to imagine that we do not see a flame the moment we light it, but only an instant after. I have myself spoken to well-educated people possessed of good judgment and a certain amount of elementary knowledge, who could never bring themselves to believe that we see the stars, not as they now exist, but as they appeared at the particular moment when the luminous wave by which we are enabled to perceive them left their surface, and which only reaches us after travelling through space a certain number of years, days, or hours, according to their distance. It is extremely useful and interesting to form a correct idea upon the way in which light is propagated.