This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
The Camera, Etc. For three-color work certain modifications are necessary in the camera. Various forms have been devised, some of which make it possible to expose the three plates at once, but these latter have not been proven an unqualified success.
1169. Any ordinary stand camera may be made to do good service for this work. In the first place, a carrier arrangement must be provided for the light filters, so that the three filters may be framed up side by side, thus making it possible to pass them in succession either behind or in front of the lens. The writer favors the former as the most suitable position for them.
Arrangement Of Filters. It is usual to frame up the filters in the order of red, green, violet, and also to expose through them in the same order.
1171. For still-life work or the copying of paintings, the ordinary plate-holders will be quite suitable; but for portraiture, and other work requiring a quick change of the plates, a repeating-back arrangement should be provided for the camera, with a plate-holder in which the three necessary plates may be placed side by side. A means of quickly moving the plate-holder and filter-carrier, such as a coarse rack and pinion, would greatly facilitate the making of short combinations of exposures.
The Negatives. Having everything ready for practical testing, the ratio of the exposure of the plates must be discovered. If commercial bathed plates and light filters are being used, the exposure ratio for the three filters should be ascertained from the manufacturers, if it is not indicated on the plate package, or on an enclosed card.
Relative Exposures. When using plates home bathed with Pinacyanol-Pinaverdol take the following ratio as a rough test: Red, 2; green, 2; blue, 1. Thus, the blue will require about six times the exposure the plate would require without a screen, and the red and green twice as much as the blue. Put three plates into the plate-holder, carefully marking them 1, 2, 3, for future indication, and upon a color chart, which should include white and black patches, make strip exposures through each of the filters, keeping a careful record, and numbering each item. It will be necessary to fit a piece of cardboard into the back of the camera, in the center of which a half-inch slit has been cut, and through this make the exposure. Make all the exposures through one filter onto one plate, and after development compare the results. The strips which show the white patch of equal density, and the black patch of equal transparency on each plate, having received the correct exposures.
1174. On the negative made through the red filter, the red patch of the chart will be as opaque as the white patch, and the blue transparent. The green filter negative will have the red patch transparent and the blue and green opaque; and the blue filter negative will have the yellow patch transparent, and the red and blue opaque. Any other colors will be in various degrees of opacity in each case, according to the mixtures in their composition.
1175. Development - The most suitable developer for bathed plates is one of Metol-Hydroquinon. Any developer which would be likely to stain the film would probably alter the color values of the print.