232 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
which make up white light, and therefore light of any other color. 
If these three colors are combined, their relative intensities being kept 
the same, they will produce white light. If they are used as filters, 
however, and interposed successively in the path of a beam of light, 
white or colored, they will absorb it completely. But if each screen 
is interposed singly in the path of a beam of light, it will transmit all 
simple radiations belonging to the group of which it is the represen- 
tative, and absorb all those of the other two groups. ‘Thus if the eye 
were placed in the path of a beam of light, and filters of the three 
fundamental colors interposed one by one, the eye would receive three 
distinct sensations of color corresponding in each case to the portion 
of the original beam transmitted by the screen in question. The prob- 
lem to be solved is to find a means of registering these three distinct 
sensations so that they can be combined and transmitted simultane- 
ously, giving the sensation of the original beam. Ducos du Hauron 
and Cros accomplished this by making three negatives each with a 
different filter, and therefore each containing the record of the radia- 
tions belonging to its particular group in the object photographed. 
To combine these three records, each negative was printed on a sen- 
sitized fi'm and the resulting print dyed the color complementary to 
the color of the screen with which its negative was made. Thus the 
film made with the negative taken with the orange screen was dyed 
blue, the film from the ‘negative taken with the violet screen dyed 
yellow, end the film from the negative taken with the green screen 
dyed red. These films were then developed by washing, which re- 
moved all the emulsion covered by the dark parts of the negative, 
that is, the parts affected by the light transmitted through the screen 
in the original exposure. By superposing the three films made in 
this way and looking through them the object was seen reproduced in 
its original colors. 
I. E. Ives has produced beautiful results with superposed films, but 
his most valuable work in color photography has been in the direc- 
tion of positives on glass for the photo-chromoscope, and for lantern 
projections. 
M. Lippmann in 1891-92 succeeded in making the first permanent 
direct photograph in color (afterwards improved by Lumiere). The 
details of this beautiful process were published in a previous Smith- 
sonian report (1901). 
Diffraction-grating color photography was invented by Prof. R. W. 
Wood in 1899 and published in the Philosephical Magazine, from 
which the following extract is taken: 
If a diffraction grating of moderate dispersion and a lens be placed in the 
path of a beam of light coming from a linear source, and the eye be placed in 
