W. LeConte Stevens—Recent Progress in Optics. 377 
ArT. XXXIX.—Sfrecent Progress in Optics ;* by W. 
LeConte Stevens. Part [I. 
Color Photography. 
The conditions being now specified under which stationary 
light waves are produced, let us imagine common instead of 
monochromatic light, to be transmitted normally through a 
transparent sensitive film. Then a variety of stationary inter- 
ference planes are produced. This is the underlying principle 
of the process employed by Lippmann in Paris, who in 1892+ 
succeeded in obtaining a photograph of the solar spectrum 
in natural colors. Upon a surface backed with a reflecting 
mirror of mercury is a silver bromide albumen film, which has 
been treated with one or more aniline dyes to render it equally 
sensitive to waves of long and short period. After exposure 
and development the natural colors are manifested with bril- 
haney. Apart from the fundamental principle already ex- 
pressed, it can scarcely be said that the rationale of the process 
has yet been very fully and clearly explained. Lippmann 
recognizes the stationary wave systems, with maxima and 
minima of brightness in the film and corresponding max- 
ima and minima of silver deposit. If the incident light is 
homogeneous, a series of equidistant 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 certain 
amount of refiected energy which is a continuous periodic 
function of its 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 thick- 
ness of the layer, approaching zero as a limit, so long as this 
light is of different wave length from the homogeneous 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 composing the white light. By such mathematical con- 
siderations Lippmannt 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. 
* Address delivered by the Vice-President of Section B at the meeting of the 
American Association for the Advancement of Science, August 29. 
+ Comptes Rendus, cxiv, p. 961, and exv, p. 575. 
t Journal de Physique, 1894, p. 97. 
