may result from the superposition of several simple vibrating motions. In general, the coloring of bodies results from the diffusion of the light-rays which illuminate them. The bodies absorb a part of the rays and reflect others. The mingling of the reflected rays produces on the eye the impression of a definite tint. A cloth appears red to us because it reflects chiefly the red light and absorbs all the other colors. If it reflects all the solar rays as they are, it appears white to us; if, instead of reflecting them, it absorbs them, it appears black.
The origin of colors, therefore, we see, depends upon a physical or mechanical and not on a chemical cause. The white light which comprises them all is only the resultant of the infinity of the colors that exist and succeed one another in gradation from the red to the violet. This may be easily perceived by letting a ray of sunlight pass through a crystal cut in facets.
To comprehend fully the direction of M. Lippmann's thoughts before hitting on the photography of colors by the application of the theory of vibratory motions, we must say a little more concerning the phenomena of interference. When two sound-waves meet, there occurs, according to certain specific conditions, either an amplification,of the sound by their combination or a destruction of it by their collision. The principle of the interference of sound was demonstrated by Colonel Napoleon Savart in 1839, by an experiment which is not so well known as it should be. This sagacious officer placed in front of the principal wall of the citadel in which he was garrisoned a bell which he rung by striking it with a hammer. The bell thus became the center of a direct wave which was propagated to the wall of the citadel and reflected from it. In other words, the action of the sound was brought to bear upon the wall, which sent it back to the starting point and thus could give rise to the phenomenon of interference. Some among the soldiers stationed along the line between the bell and the wall observed a distinct re-enforcement of the sound; while others, placed exactly at the points of interference, heard nothing.
What passed in Colonel Savart's experiments is reproduced in the same manner with light-vibrations. Just as sound added to sound may produce either silence or amplification of the sound, so light added to light may produce darkness or amplification of the luminous effect. When direct light falls upon a mirror, it meets on the way the light that was previously reflected, and wherever the vibrations agree in direction the brightness is increased, whereas it is extinguished wherever they are opposed to one another. The space in front of the mirror will therefore be divided into successive sections or stratifications. In some, the light will be of its highest brightness; in others, on the other hand, there will be complete darkness. It can easily be deter-
