544 THE POPULAR SCIENCE MONTHLY. 



mined by calculation that the distance between the sections is 

 about one four-thousandth of a millimetre ; and it is hence con- 

 ceivable 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 de- 

 termined 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 corre- 

 spond 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 accord- 

 ing 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 com- 

 merce, the sensitive coating of which is the result of an emul- 

 sion. When examined with the microscope, this washing usu- 

 ally exhibits a very coarse grain derived from solid particles of 

 perceptible matter, which are of considerable dimensions in pro- 

 portion 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- 



