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.
(See Lens, Collinear.)
The first dry-plate process in practical use. Had the disadvantage, however, of being slow.
Simple solution of nitrate and cellulose. Colorless, or slightly-yellowish, neutral, syrupy liquid; when exposed in thin layers, evaporates and leaves colorless film. Used before the introduction of gelatin as a vehicle to hold the sensitive salts in emulsion. Its advantage over gelatin is that it allows of the making of an extremely dense negative, the film being very thin and practically devoid of grain. Although not in as general use as gelatin, it is employed for the making of both wet and dry plates when it is desired to secure specially clear, brilliant negatives. Is used almost exclusively in process work. (See Pyroxylin.)
A photo-mechanical printing process based upon the fact that bichromated gelatin, when exposed to the action of light, becomes insoluble. The bichromated gelatin is spread upon glass, metal, paper or other support and printed in daylight under a negative; the parts unaffected by light being etched away by soaking in a solution generally consisting of water, glycerine and sugar. The plate is then covered with a greasy ink, which latter adheres to the portions which have been etched away, and is repelled by the parts which were affected by the light, and which latter have absorbed water. Collotypes are printed on regular printing presses.
If a color filter in the form of a cell containing a dye or color in solution is being used, and it is desired to change and make a filter by coating a gelatined sheet of glass, the quantity of solid dye to use for the latter will be the same as required for the cell, if the surface area is the same; i.e. if the face of the cell is 4x4=16 square inches, and the quantity of dye used in the tank is 5 grains, then 5 grains of dye must be spread over 16 square inches of glass.
The easiest plan is to focus the camera on a white card. Expose sections of an orthochromatic plate, without the filter, doubling the exposure each time. To do this the slide in the plate-holder should be withdrawn about 1/2 inch for the first exposure, one inch for the second, etc. Then, expose another plate in the same way, using the color filter. Develop the two plates together, in the same tray, place in the fixing bath at the same time, and then compare the strips. It will be easily seen which strips coincide - the increase in exposure is then known.
A process reproducing natural colors. There are various processes in the course of experimentation, some of which have reached a practical basis; others, although producing excellent results, are limited more to the experimentalist and amateur than to the commercial worker.