of light producing darkness at their meeting-point. If, on the other hand, rays of light consist of waves of some kind in a medium, then, as we have seen in the case of water ripples and air waves, it is quite possible for two wave-trains to annihilate each other’s effect at a certain point, if a hollow of one wave-train reaches that place coincidently with a hump belonging to the other. Accordingly, the experiment of producing _interference_ between two sets of light rays so that they destroy each other is a strong argument in favour of the view that light must consist in some kind of wave-motion existing in a medium susceptible of supporting it, filling all space, and existing in all transparent bodies. This medium we call the _luminiferous æther_. The term “æther,” or “ether,” has been in use for many centuries to express the idea of something more rare, tenuous, or refined than ordinary matter. The classical writers employed it to describe the space above the higher regions of the atmosphere, which was, as they; supposed, occupied by a medium less palpable or material than even air itself. Thus Milton, speaking of the downfall of the enemy of mankind (“Paradise Lost,” Book I. line 44), says— “... Him the Almighty Power Hurled headlong, flaming, from the _æthereal_ sky With hideous ruin and combustion down.” But although poets and philosophers had made free use of the notion of an æther, or even assumed the existence of several æthers, the conception did not become a serious scientific hypothesis until it was experimentally shown by Young that the phenomena of optics imperatively demand the assumption of such a medium in space which is not ordinary matter, but possesses qualities of a special kind, enabling it to have created in it waves which are propagated with the enormous velocity of nearly one thousand million feet a second. The proofs which have accumulated as to the validity of this hypothesis to explain optical effects show that this medium or æther must exist, not only in free space, but also in the interior of every solid, liquid, or gaseous body, although its properties in the interior of transparent bodies are certainly very different from those which it possesses taken by itself. This æther fills every so-called vacuum, and we cannot pump it out from any vessel as we can the air. It occupies, likewise, all celestial space, and suns and stars float, so to speak, in an illimitable ocean of æther. We cannot remove it from any enclosed place, because it passes quite easily through all material solid bodies, and it is for the same reason intangible, and it is not possessed of weight. Hence we cannot touch it, see it, smell it, taste it, or in any way directly appreciate it by our senses, except in so far as that waves in it of a certain kind affect our eyes as light.