nately bright and dark. These rings are not equally distant from one another; they center at the point of contact of the two glasses. By employing simple lights of different natures we can see the diameters of the rings increase or diminish according to the different wave-lengths of the lights used. It appears, therefore, from this experiment that if we illuminate the glass with white light we shall have the superposition of the effects obtained with different simple lights. In such case the colors can not coincide, and then, instead of having a system of alternately dark and light rings, we shall have rings iridescent with all the colors of the rainbow; and this is precisely what is produced in the soap bubble. The important fact in the phenomenon is that the color varies according to the thickness of the film. In this experiment we are dealing with natural colors, produced without the intervention of any chemical action, but simply by a series of luminous phenomena which we shall shortly explain. M. Lippmann's invention rests upon this principle.
If you blow out a soap bubble it reflects violet as it issues from the pipe; then, becoming larger—that is, the film becoming thinner—it reflects blue, then green, yellow, and finally, when the film has reached its thinnest, red. In this experiment we can perceive what is the real origin of colors. They are only the successive notes of the luminous gamut, as musical notes are formed by the gamut of the scale of sounds. Newton arbitrarily counted seven colors in the spectrum, so that he might make it display as many colors as there are principal notes in the musical scale.
Like sound, light is propagated by undulations through space. This transmission of vibratory motion is carried on with great swiftness, passing through the distance from the sun to the earth in eight minutes. Aside from the difference in velocity, light-waves are like sound-waves. The simple colors are for light what musical notes are for sound. In this way Fresnel, in his theory of undulations, explains the difference in the coloring of the different parts of the spectrum.
Every sound is caused by a vibrating body engendering waves which reach our ear and produce the sonorous sensation in it. But all sounds are not identical. Every one can distinguish an acute note from a grave note. In studying the characters of acuteness and gravity of sound, the conclusion has been reached from experiment that the sounds emitted by a vibrating body are higher the more rapid the vibrations, or the more there are of them in the same time. Each length of wave corresponds to each sound peculiar to it, and is in inverse proportion to the number of vibrations. Since the acute sounds result from the more numerous waves, their waves are shorter and closer than
