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242
THE ADVANCE OF PHOTOGRAPHY
is made for the colour screens to be held in front of the
lens, and a dark slide is used which will hold the three
plates. Sanger-Shepherd, however, supplies a dark slide
with repeating back in which all three negatives can be
taken upon one plate.
This is illustrated in diagram (103). The three screens —
blue, green, and red — which are seen on the right are
clipped in a frame to the front of the dark slide, which
stands next. They are then placed with the plate in the
repeating-back case of the camera, and are passed one
after another in front of the single opening seen in the
figure. By this means
it is absolutely certain
that light of one colour
only reaches any third
of the plate, and the
result is that by this
means three negatives
of the object can be
obtained which differ
only in their respective Fig. 103. , J ... r
5 densities.
The three negatives are illustrated by the figure to the left of the illustration.
Cameras with repeating backs are, of course, inconvenient, and, in fact, impossible when the object it is desired to photograph is moving, for, of course, in whatever form the camera is made three exposures must be made. This has been one great drawback in three-colour photography.
Several attempts have therefore been made to produce a camera in which all three negatives can be exposed at the same time. E. T. Butler has devised one in which the principle involved is the same as that of the Ives Kromskop1 (p. 241). Others have been invented by Sir
1 See also description by Dr Mees, Journ. of Photog., July 1908.