Who would suppose that absolute darkness may be derived from two rays of light! Yet such is the fact. If two rays proceed from two luminous points very close to each other, and are so directed as to cross at a given point on a sheet of white paper in a dark room, their united light will be twice as bright as either ray singly would produce. But if the difference in the distance of the two points be diminished only one-half, the one light will extinguish the other, and produce absolute darkness. The same curious result may be produced by viewing the flame of a candle through two very fine slits near to each other in a card. So, likewise, strange as it may appear, if two musical strings be so made to vibrate, in a certain succession of degrees, as for the one to gain half a vibration on the other, the two resulting sounds will antagonize each other and produce an interval of perfect silence. How are these mysteries to be explained? The Delphic Oracle of science must again be consulted, and among the high priests who officiate at the shrine, no one possesses more recondite knowledge, or can recall it more instructively than Sir David Brewster. "The explanation which philosophers have given," he observes, "of these remarkable phenomena, is very satisfactory, and may easily be understood. When a wave is made on the surface of a still pool of water by plunging a stone into it, the wave advances along the surface, while the water itself is never carried forward, but merely rises into a height and falls into a hollow, each portion of the surface experiencing an elevation and a depression in its turn. If we suppose two waves equal and similar, to be produced by two separate stones, and if they reach the same spot at the same time, that is, if the two elevations should exactly coincide, they would unite their effects, and produce a wave twice the size of either; but if the one wave should be put so far before the other, that the hollow of the one coincided with the elevation of the other, and the elevation of the one with the hollow of the other, the two waves would obliterate or destroy one another; the elevation, as it were, of the one filling up half the hollow of the other, and the hollow of the one taking away half the elevation of the other, so us to reduce the surface to a level. These effects may be exhibited by throwing two equal stones into a pool of water; and also may be observed in the Port of Batsha, where the two waves arriving by channels of different lengths actually obliterate each other. Now, as light is supposed to be produced by waves or undulations of an ethereal medium filling all nature, and occupying the pores of the transparent bodies; and as sound is produced by undulations or waves in the air: so the successive production of light and darkness by two bright lights, and the production of sound and silence by two loud sounds, may be explained in the very same manner as we have explained the increase and obliteration of waves formed on the surface of water."