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 Theory. The beautiful colors of the spectrum demonstrate to us that the composition of white light is not simple, but a combination of various colored rays. This being so, it is clear that these rays may be divided into series and means employed by which they may be isolated one series from another. This is briefly the root idea of Three-Color Photography.
1137. The visible spectrum is divided into three parts by means of light filters, each of which absorb all the colored rays of one series. The light filters employed consist of dyed gelatin or other celluloid between two glasses; and the colors employed are respectively, red, green and blue-violet. The red filter has the faculty for absorbing all the blue series of rays, the green filter absorbs all the red series, and the blue-violet filter all the yellow series of rays in the light. That is to say, no yellow rays can pass through the blue-violet filter; neither can any red rays pass through the green filter, or blue rays through the red filter. Therefore, as only the rays passing through the light filters to the photographic plate can have any effect upon the sensitive surface, it follows that the absorbed colors would appear upon the developed negatives in various degrees of transparency, and that they would each print black upon an ordinary monochrome printing basis.
1138. Instead of printing in black, however, if we make from each negative a transparent print of each of the colors represented by the absorption of our filters, i. e., low and blue, we are able to reconstruct the colors of nature by superimposing the three prints one over the other, the transparency of each allowing the necessary shades of the lower prints to be reflected through the top one. so presenting to the eye all the various combinations of colors and tints which are seen in the objects photographed.
The Light Filters. Before three-color work can be attempted, a set of three filters must be obtained, either from one of the various commercial sources, or by home construction; and for the latter several methods will now be given.
Requisites For Home-Made Filters. A number of carefully selected squares of thin glass all cut to the desired size, usually 2 to 3 inches square; a supply of hard photographic gelatin of good quality; some fresh Canada balsam; six small stoppered bottles; three larger ones; a few ounces of alcohol; and a color chart, consisting of squares of colored paper, pasted onto a piece of cardboard. The following colors are most useful for the purpose: Blue, scarlet, yellow, green, orange, and purple.
1141. Obtain a small quantity of each of the following dyes, which are specially prepared for photographic purposes: Victoria blue, Naphthol green, New Rapid Filter green, Filter Yellow K, Rose Bengal. If necessary, Brilliant Acid green may be substituted for Rapid Filter green.
1142. Make 1-100 alcohol stock solutions of all the dyes. This is done by dissolving 10 grains of the dye in 2 ounces of alcohol. Keep the bottles well stoppered and distinctly labeled.
1143. Take one-half ounce of gelatin and soak for three hours in cold water. Then, with several changes, carefully wash the viscous mass, in order to free it of any foreign matter that may affect the dyes. Heat in a water bath until dissolved; make up the resulting solution to five ounces with hot water, and filter through a wad of cotton-wool previously wet in boiling water.
1144. Carefully clean a number of selected glasses with alcohol, taking every precaution to avoid dust particles settling upon them afterward.