THE PHOTOGRAPHY OF COLORS. 541 



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 inter- 

 vention of any chemical action, but simply by a series of lumi- 

 nous 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 



