iar fact in regard to films—their wonderful and changing colors, what in scientific language is called "the colors of thin plates," because the same effect is produced in many cases where "film" is not exactly the word to use.
The colors of soap-bubbles furnish one of the most triumphant vindications of the wave theory of light, and offer to its opposers one of the hardest possible nuts to crack. A brief explanation of the wave theory of light and interference, so far as it bears upon our subject, will, it is hoped, be pardoned. It is an idea so familiar to those who have studied physics, and yet so difficult of conception to those who have not, that a few words seem necessary in a popular exposition of the colors of films.
Light is, of course, our name for the sensation, but back of the sensation there lie the physical conditions which are its cause. The theory of Newton, that light is caused by minute particles of matter shot out from the luminous body, stood the test of the simpler phenomena; but, when it came to the explanation of soap-bubble colors, his theory, even with the marvelous ingenuity which he brought to bear upon it, broke down. If light were matter, it is impossible to see how one light can be added to another light and produce darkness, which is sometimes the case; while, if it were motion, we can readily see how motion may be added to motion, and the result be rest. A sound can be so added to a sound as to produce silence, but the more familiar illustration is with waves of water. Two stones dropped into water will produce waves, and where these meet there are points at which the water remains at its original level. This is because at these points one set of waves tends to raise the water while the other set tends to lower it, and between the two it remains where it originally was. This occurs in some parts of the ocean where the tidal wave sweeps around an island and meets, one wave being half a length behind the other, in which case they simply neutralize each other. Where the crest of one wave would have been, the trough of the other would have been at the same time, and between the two impulses in opposite direction at the same moment the water remains unmoved, and there are no tides.
Darkness corresponds with this unmoved plane of water, and with silence in the case of sound-waves. If light were simple waves, as a result of such interference we would simply have darkness, and as a result of partial interference we would have all the gradations from darkness to light; but a light-wave is not a simple undulation, it is made up of innumerable vibrations of various wave-lengths, each of which corresponds with a color or tone. The resultant of all these motions combined is white light. Extinguish one rate of vibration, say the smaller waves which cause the sensation of blue, and we have a wave the resultant of all that
