It was believed for a long time that light was a compact mass of tiny particles emitted by luminous bodies, which struck our eyes and so produced the phenomenon of vision. These particles or molecules were naturally thought to be extremely minute, and the objects illuminated by them were supposed to throw them off as if they were endowed with elasticity. Under this hypothesis, light was a material body. The illustrious Newton was the first propagator of this theory; the last was M. Biot, a French philosopher, lately dead.
The undulatory theory has now-a-days completely superseded the corpuscular hypothesis.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
