terference. Sensitive collodion or albumen plates, which have the advantage of being continuous and transparent, are preferable. This choice of processes in sensitizing is, however, not absolute. The pre-eminently important point is that the sensitive plates have no grains, or that the grains be of negligible size—that is, of dimensions inferior to half the length of wave that corresponds to the color.
Without going into operative details we can easily represent to ourselves the process employed by the inventor of the photography of colors to render his invention practicable. The reflecting face of a plane metallic mirror is covered by the usual process of sensitizing with an impressionable stratum of albumen or collodion and chloride or bromide of silver. If a light-ray of any simple color is made to act upon this, it occupying, consequently, a determined place in the gamut of simple colors, there results that the incident rays will traverse the sensitive and transparent stratum, will be reflected on the polished surface, will return backward, and will meet on their return the rays that are coming. There will then be formed two luminous waves a direct wave and a reflected wave—and these, meeting, will produce interferences. We shall see that what is created in the projection of these luminous rays is only the repetition of what was produced in the experiments of Colonel Savart by the projection of the sonorous vibrations on a wall.
In the photography of colors the space in front of the mirror is filled with parallel planes alternately bright and dark, in such a way that every two of the bright planes are separated from one another by a distance equal to half a wave-length—that is, to the four-thousandth part of a millimetre. There results from this the creation of a large number of these planes in the thickness of the sensitive stratum. In short, this sensitive coating, already very thin, is divided, as the sheet of paper we have mentioned would be, into a number of layers infinitely thinner.
Only the brightest planes could impress the sensitive layer, and in the course of photographic development this impression will be revealed in a black color, while the sections corresponding to the dark planes will not be impressed. If, then, employing the process of ordinary photography, we dip the developed plate into hyposulphite of soda, all the matter sensitive to light and not changed will be dissolved in it, and there will persist on the plate only the infinitely thin sections of reduced silver, and those at the points where the bright planes had fixed themselves. Therefore, the whole thickness of the photographic stratum will be divided into sections by planes of metallic silver parallel to one another and separated by a distance equal to half a wave-length of the simple color which has impressed the plate. These planes,
