THE PHOTOGRAPHY OF COLORS. 545 



terference. Sensitive collodion or albumen plates, wliicli have 

 the advantage of being continuous and transparent, are prefera- 

 ble. This choice of processes in sensitizing is, however, not abso- 

 lute. 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 photog- 

 raphy of colors to render his invention practicable. The reflect- 

 ing face of a plane metallic mirror is covered by the usual pro- 

 cess 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 transjoarent 

 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 lumi- 

 nous rays is only the repetition of what was produced in the ex- 

 periments 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. There- 

 fore, 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, 



VOL. XLT. 42 



