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Color Filters or Color Screens 51
jected perpendicularly on to the filter by means of a cylindrical lens, which forms the image of a slit, this being brightly illuminated by the condenser of a projection lantern.
By means of the eyepiece of the spectroscope, that portion of the spectrum for which the filter should show the maximum of absorption is observed, and the filter is slowly displaced until this result is obtained. The portion corresponding to the colored region, which was illuminated at this moment is noted. On the other piece of glass the thickness el is measured. The weights of the dye being proportional to the thicknesses of the films, it is easy to calculate the weight per unit surface required for the depth of color corresponding to that we require :
e p el+p
e1 x e
When the coloration of the filter comes from two dyes, a plain and graduated filter are prepared from each. These latter are placed in juxtaposition and cemented with balsam, so that the deepest part of one comes next to the lightest part of the other. We thus obtain a filter in which the weights p1, p of each dye are inversely proportional. Passing this filter over the spectroscope slit, after we have located the region corresponding to the color required, it is not difficult by measuring the thickness of the gelatin on each plate, to find the ratio pl/p2.
R. J. Wallace6 proceeded in somewhat the same fashion, and prepared dye wedges of :
Hard gelatin 25 g.
Distilled water 1000 ccs.
Dye 2.5 g.
Seven cubic centimeters of this at a temperature of 55° C. were flowed on a plane glass strip, 50x250 mm., then laid aside to set in a drying chamber, with one end raised to a height of 5 cm. from the horizontal plane of the support, thus causing the gelatin to flow slowly toward the lower end. When dry a plane cover glass was cemented on with Canada balsam and the edges bound. A centimeter scale was then ruled on the glass with a diamond, and a series of spectrograms made, showing the absorption at every centimeter for constant exposures. From the negatives thus obtained, it was possible to calculate the composition of the required filter. Wallace referred to Monpillard's paper and stated that he found the method unsuitable, and considered that if the thickness of the film on the color-wedge was measured more delicately, say in the interferometer, then a much closer approximation might be arrived at than is possible with a micrometer. By a method of trial and error it was speedily found possible to arrive at an extremely satisfactory duplication. A plate of ordinary glass, of exactly the same size as the desired filter was taken, and flowed with a measured amount of the same solution, as used in