For when the Danish astronomer, Romer, discovered that light travelled at the rate of somewhere about 186,000 miles per second it dawned upon people that it was scarcely believable that particles of any kind could by any means be made to move so fast. So they set about searching for a new explanation, and they found it in the idea that light was conveyed from the bright body to the observer's eye by means of waves, and as there cannot be waves of nothing they had to imagine a something to exist in all the vacant spaces of the universe capable of forming the waves of light. This something was called the luminiferous ether or light-bearing ether. We can neither see, feel, taste nor hear it. Our senses tell us nothing about it. Indeed, if it does really exist it must be so very different from anything that we do know by our senses that one is often tempted to doubt its existence. Still, it explains so many things which are otherwise unexplainable and enables us so correctly to reason from one phenomenon to another that our reason forces us to accept it as a fact, at all events until something better comes along. This wave theory in regard to light was finally set at rest by the curious discovery about a century ago by Dr. Thomas Young of London that if two lots of light were brought together in a certain way they produced darkness. Now if a ray of light were a stream of particles, two such rays would inevitably and always, if added together, produce a doubly brilliant light, and under no conceivable circumstances could they do anything else. But two lots of waves can, and do, under the proper conditions, neutralize each other so as to produce rest. This mutual action upon each other of two sets of waves can be very simply exhibited by two violin strings tuned to _nearly but not quite the same note_. If you have a violin handy, try it and you will find that when either string is plucked separately it gives a steady continuous sound, but if both be plucked at the same time they give a throbbing sound. That is because, periodically, as one string is coming up the other is going down, so that they neutralize each other, while at other times, owing to the fact that one is vibrating faster than its fellow, both are rising and falling together. When neutralizing each other there is a momentary silence, while in between the silences come the times when both are acting together and therefore producing a specially loud sound. And so as the vibrations of the faster keep gaining upon those of the slower string one hears a continual crescendo and then diminuendo repeated over and over again. So two sets of sound waves sometimes produce silence. And in like manner two sets of light waves can be made so to "interfere" (that is the technical term) that together they produce darkness.