126 RECENT PROGRESS IN OPTICS. 



long and short period. After exposure and development the natural 

 colors are manifested with brilliancy. Apart from the fundamental 

 principle already expressed, it can scarcely be said that the rationale 

 of the process has yet been very fully and clearly explained. Lipp- 

 mann recognizes the stationary wave systems, with maxima and minima 

 of brightness in the film and corresponding maxima and minima of 

 silver deposit. If the incident light is homogeneous, a series of equi- 

 distant parallel planes of equal photographic efficiency are produced in 

 the film. If the plate after development is illuminated with white 

 light, then to every point within the film there comes from below a cer- 

 tain amount of reflected energy which is a continuous periodic function 

 of the distance from the reflecting surface. The total reflected light of 

 any color becomes then represented by the integral of this periodic 

 function for the entire thickness of the layer. The solution of this 

 integral brings the result that the intensity of the reflected light 

 decreases with increasing thickness of the layer, approaching zero as 

 a limit, so long as this light is of different wave length from the homo- 

 geneous light employed for illumination of the plate. Only light of 

 the same wave length, or of an entire multiple of this, maintains a 

 finite value. A similar consideration applies to each of the hues com- 

 posing white light. By such mathematical considerations, Lippmann 

 (Journal de Physique, p. 97, 1894) reaches the conclusion that the light 

 reflected from the plate must have exactly the same relations of wave 

 length as that with which the plate was illuminated. 



For the Lippmann photographs, which at first required a very long 

 exposure, and could even then be satisfactorily viewed at only a single 

 definite angle, it is now claimed that an exposure of only a few seconds 

 is needed, and that the colors are visible at all angles of incidence so 

 long as the plate is moist (Journal de Physique, p. 84, 1894). But, like 

 the daguerreotypes of fifty years ago, they are incapable of multiplica- 

 tion, and, great as is the scientific interest connected with them, it 

 seems scarcely probable that they can long continue to hold an impor- 

 tant place practically. The problem of ascertaining definitely the cause 

 of the return of a color the same as that which falls upon a given sur- 

 face may seem to be solved mathematically, but the mastery of the 

 physical conditions required to produce a single colored negative, from 

 which may be had any desired number of positives with varied hues 

 accurately reproduced, is still in the future. From the very nature of 

 stationary light waves it does not appear probable that the Becquerel 

 method as improved by Lippnianu will give the means of multiplying 

 copies of a single picture. Wiener has lately published an elaborate 

 research upon this subject (O. Wiener, Wiedemann's Annalen, pp. 225- 

 281, June, 1895), in which he recognizes the necessity for the employ- 

 ment not of interference colors but rather of what he calls body colors 

 (Korperfarben) due to chemical modification of the reflecting surface. 

 M. Carey Lea (American Journal of Science, p. 349, May 1887), in 1887 



