A third and most important analogy between sound and light is now to be noted; and it will be best understood by reference to something more tangible than either. When a stone is thrown into calm water a series of rings spread themselves around the centre of disturbance. If a second stone be thrown in at some distance from the first, the rings emanating from both centres will cross each other, and at those points where the ridge of one wave coincides with the ridge of another the water will be lifted to a greater height. At those points, on the contrary, where the ridge of one wave crosses the furrow of another, we have both obliterated, and the water restored to its ordinary level. Where two ridges or two furrows unite, we have a case of _coincidence_; but where a ridge and a furrow unite we have what is called _interference_. It is quite possible to send two systems of waves into the same channel, and to hold back one system a little, so that its ridges shall coincide with the furrows of the other system. The "interference" would be here complete, and the waves thus circumstanced would mutually destroy each other, smooth water being the result. In this way, by the addition of motion to motion, _rest_ may be produced. [Sidenote: LIGHT ADDED TO LIGHT MAKES DARKNESS.] In a precisely similar manner two systems of sonorous waves can be caused to interfere and mutually to destroy each other: thus, by adding sound to sound, _silence_ may be produced. Two beams of light also may be caused to interfere and effect their mutual extinction: thus, by adding light to light, we can produce _darkness_. Here indeed we have a critical analogy between sound and light--_the_ one, in fact, which compels the most profound thinkers of the present day to assume that light, like sound, is a case of undulatory motion.