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50 History of Three-Color Photography
becoming darker. The impurity causing this trouble is an unstable substance, as the gelatin improves if allowed to stand in a swollen condition for from 24 to 48 hours. Storage of the dry gelatin also appears to act favorably; and it was found that some sheets, taken from the middle of a packet, had a stronger bleaching action than those from the outside.
Sulfurous acid is frequently used to bleach the raw stock, or to neutralize the last traces of "bleach" when chloride of lime is used.
A radicle means of making such a gelatin fit for use is to add, before incorporating with the dyes, about 0.5 to 1 ccs. of a 1 per cent solution of iodine to every 10 g. of dry gelatin. This proportion is usually enough; but in any case it is easy to test with a little fast red, and if on drying the color is a clear red, the gelatin is fit for use. One may, of course, titrate the iodine in the usual way.
Whilst the various formulas recorded will save any trouble as to the composition of a filter in ordinary practice, the following note by F. Monpillard5 may be useful, as by its help any given description of filter can be reproduced : the principle of the method is simple. A given weight of the dye is dissolved in a given volume of aqueous gelatin solution. If the same volumes of this be distributed over the same areas, the same weight of the dye will occur per unit area, and the two colors will be identical. In practice the dyed solution is spread on the surface of an optically worked glass, and, after drying the film of gelatin is covered by a glass, also optically worked, and the two cemented with Canada balsam. If planeparallelism of the filter is necessary, the two outside surfaces are worked.
In the case of preparing a filter to possess certain properties previously decided on — i. e., absorption of certain particular parts of the spectrum — it is necessary to adjust with the greatest precision the weights of the dyes before coating. To avoid the disturbing influences of the gelatin, Canada balsam, etc. — factors which are far from negligible — but also to avoid errors, which arise from differences in the absorptions of the dyes, according as the film is wet or dry, the following method has been devised : Taking first the case of a single dye, the colored gelatin is applied to two glasses, one placed horizontally and the other inclined so as to form an incline of 2 in 100. The volume of solution applied to the first glass having been determined with care, the weight p, of coloring matter per unit surface is known. After drying, the gelatin film has a thickness, e.
The gelatin having been flowed over the inclined surface and dried, the latter is divided into two parts along its length, and to one of these strips a white glass is cemented with balsam. A filter is thus obtained, constructed in the same way as the one required, but with the weights p1, p2, p3 ... of dye per unit area varying in the same proportion as the thicknesses el, e2, ez ... of the gelatin film. This filter is then placed in an apparatus by means of which it can be moved in front of the slit of a spectroscope. The latter is illuminated by a narrow beam of light, pro