378  W. LeConte Stevens—fecent Progress in Optics. 
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 expo- 
sure 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.* 
But like the daguerreotypes of fifty years ago they are incapa- 
ble of multiplication, and great as is the scientific interest con- 
nected with them, it seems scarcely probable that they can 
long continue to hold an important place practically. The 
problem of ascertaining definitely the cause of the return of a 
color the same as that which falls upon a given surface may 
seem to be solved mathematically, but the mastery of the 
physical conditions required to produce a single colored nega- 
tive, 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 
Lippmann will give the means of multiplying copies of a single 
picture. Wiener has lately published an elaborate research 
upon this subject,t in which he recognizes the necessity for the 
employment not of interference colors but rather of what he 
calls body colors (kérperfarben) due to chemical modification 
of the reflecting surface. M. Carey Leat in 1887 obtained a 
rose-colored form of silver ‘“ photochloride” which ‘in the 
violet of the spectrum assumed a pure violet color, in the blue 
it acquired a slate blue, in green and yellow a bleaching infla- 
ence was shown, in the red it remained unchanged.” But in 
the absence of any means of fixing these colors a promising 
prospect brings disappointment. 
While it is abundantly possible that colored illumination 
upon suitable color-receptive materials can give rise to similar 
body colors, we are still far from having these materials under 
control. There seems at present to be greater promise in 
another and quite different application of optical principles. 
The suggestion appears to have first been made by Maxwell§ 
in 1861, that photography in colors would be possible if sensi- 
tizing substances were discovered, each sensitive to only a 
single primary color. Three negatives might be obtained, 
one in each color; and three complementary positives from 
these, when superposed and carefully adjusted, would present 
a combination that includes all the colors of nature. In 1873 
H. W. Vogel in Berlin discovered that silver bromide, by 
treatment with certain aniline dyes, notably eosene and cyanine 
* Journal de Physique, 1894, p. 84. 
+ O. Wiener, Wiedemann’s Annalen, June 1895, pp. 225-281. 
¢ This Journal, May 1887, p. 349. 
§ Royal Institution Lecture, May 17, 1861. 
