supposed that light, therefore, like sound, might be due to wave motion, but if it were wave motion, there must have been a medium to propagate the waves. In order to account for this wave motion, he supposed all space to be filled with a luminiferous Aether, which would be to his light waves what air is to sound waves. In this conception he was supported by Euler the mathematician, and in 1690 he was able to give a satisfactory explanation of the reflection and refraction of light, on the hypothesis that light was due to wave motion in the Aether. It was not, however, till the advent of Thomas Young, that the undulatory or wave theory reached its perfection, and finally overthrew its competitor the corpuscular theory. Young made himself thoroughly acquainted with wave motion of all kinds, and applied his knowledge and experience to the phenomena of light, and from the analogies so obtained, he gradually built up the undulatory theory, and gave to it a foundation from which it has not yet been moved. Young made use of the same aetherial medium in order to propagate the wave motion of light in the same way that Huyghens did. From that conception, the Aether has been gradually perfected, until we have the conception which has been presented to the reader in Chapter IV., in which I have endeavoured to show that this aetherial medium is matter, but infinitely more rarefied and infinitely more elastic, but notwithstanding its extreme rarefaction and elasticity, it possesses inertia, because it is gravitative. It is this Aether, then, that is concerned in the propagation of light, and is the universal medium which is to light what air is to sound. Young, therefore, having applied himself to the wave motion of sound, from such researches was able to explain the physical cause of colour, and that phenomenon termed interference. We will therefore look at wave motion, in order to understand the wave theory of light.