mined by calculation that the distance between the sections is about one four-thousandth. of a millimetre; and it is hence conceivable that, the naked eye not being able to take in such small intervals, the sensation is one of a uniform light. But while the naked eye is impotent, the photographic plate is not. So M. Lippmann thought, when he conceived the idea of utilizing the phenomenon of interference to produce, not in the open air, but on the sensitive photographic plate, the stratifications formed alternately by the luminous and dark lines. By this process the luminous impression of the object photographed will appear only on the sections where the light is bright, while no action will take place in the dark strata.
If, then, we seek to reproduce photographically a body of many colors, each of these colors will find in the thin sections determined by these stratifications the place corresponding to the thickness of each of them. Red will find sections of six hundred and twenty millionths of a millimetre, and violet sections of four hundred and twenty-three millionths of a millimetre, to correspond to the thickness of the luminous stratum producing these colors. So with all the other simple colors, and consequently with the constituent parts of the complex colors. In developing the sensitive plate thus impressed, its thickness will be formed of a series of leaves of photographic silver, separated from one another by distances infinitely small and differing exactly according to the color which has impressed the plate placed behind the objective. We understand, then, that those leaves constitute precisely the organ of reproduction of colors, without which they would have to be colored by themselves. In practical operation it is necessary to prevent any object in the photographic stratum from hindering the fixation or accumulation of the colors in these virtual sections, which are to produce the colors by reflection as the liquid films of the child's soap bubble produce them.
It is necessary, therefore, before everything else, to exclude the ordinary bromide-gelatin or chloride-gelatin plates of commerce, the sensitive coating of which is the result of an emulsion. When examined with the microscope, this washing usually exhibits a very coarse grain derived from solid particles of perceptible matter, which are of considerable dimensions in proportion to the wave-length of a color-stratum. They obstruct that stratum completely, deform its reflecting planes, and prevent all communication of chromatic phenomena. These plates could no more produce the thin strata corresponding to the colors to be photographed than a stone sixteen feet thick can be worked into a wall of three feet. The plates of commerce are, besides, usually opaque and can not be traversed by the direct wave and the reflection wave which are to produce the phenomenon of in-
