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.
51. It is often difficult for the uninitiated to understand and appreciate the principles involved in correct rendering of colors in monochrome (one color). Messrs. Wratten and Wainwright, Ltd., of Croyden, England, in their booklet "Real Orthochromatism," give a very clear explanation of the subject, and the following is, in substance, taken from a portion of the article:
52. To make the principle most clear, one should be in possession of a small piece of dark blue and one of dark green (not green-yellow) glass, celluloid, or gelatin. (The glass may be procured from a dealer in stained glass.) If the blue glass is held up in front of one eye and any scene observed through it, it will be noticed, in the first place, that the screen has removed all color from the scene - that in fact all things are one color, namely, blue. But if you consider the scene a little longer, you will see that not only have all things lost their color, but that different colors have lost brightness to different extents, and the relative brightnesses of the scene are wrong. The bright yellows, reds, and yellow-greens have become dark, while the usually dark blues and violets have become light.
53. Now, if you photograph the scene on an ordinary plate, and then compare a print from the negative with the scene as it looks through your blue glass, you will see that the plate gives exactly the same incorrect rendering; that it sees bright yellows and reds dark, and dark blues and violets bright. And if it is a landscape with blue sky and light clouds, you will find that the screen and the plate agree in making those clouds invisible. Now it is in order that you may get those yellows and blues of the same degree of brightness as the eye sees them that you are so frequently advised to use orthochromatic plates and screens.
54. You have probably been convinced that it is desirable to avoid this objectionable sensitiveness of a plate to blue and insensitiveness to all the other colors, and you have decided to try what is known as orthochromatic or isochromatic plates - that is, plates which are sensitive to other colors than blue. But there are difficulties. In the first place, it is not yet practicable to make plates which are not very much more sensitive to blue than to other colors (and your eye is very much less sensitive to blue than to other colors), so that you must put in front of your lens a piece of yellow glass or gelatin, which will stop the blue light before it gets to the plate. This yellow screen must increase the exposure the plate will need. If it does its work properly, it will increase the exposure very much. Most orthochromatic plates are about twenty times as sensitive to blue as to green, so that if this matter is to be put right, the screen which cuts out the blue light will require about twenty times the usual exposure for the plate.
55. Also, you are accustomed to developing plates by a red light, so that the plates you use must not be sensitive to red. This means that the plate will still be blind to red. If you hold a green glass in front of your eye, you will see how things look to a plate which is sensitive to green, and is being used with a yellow screen strong enough to cut out the excess of blue light, but is not sensitive to red. You will see at once that the reds are not right; they are still black. In fact, to call the plate orthochromatic was wrong, because "orthochromatic" infers that correct color rendering is obtained, and no red-blind plate can give that. So that for real orthochromatism you must use a red sensitive plate, and that means that you cannot develop in red light. We will go back to that presently.
56. But you are probably a photographer who photographs landscapes mainly, and there are not many reds in landscapes. Take two pieces of colored fabric, a bright green and a yellow, and look at them through your green screen.
You will see at once that compared with the yellow the green has become too light. The cause of this needs a little explanation.
57. An object is yellow because it absorbs the blue from white light, and does not reflect it, so that all the rest of white light, except blue, is what we call yellow; that is to say, yellow is white light minus blue. Gas light, for instance, is yellow because it does not contain as much blue as the daylight with which it is compared. White light contains red and green and blue, so that if we take away blue from white light, we get only red and green light left; that is, red light and green light together make yellow light. An object is yellow because it reflects the red and the green portions of white light, but not the blue portion; therefore, a red-blind plate will not see the red in yellow, but only the green, and consequently it will see a bright green object as too light compared with a yellow object, just as you see it through the green screen.
58. If a red-sensitive plate is used, then it records not only the green in yellows, but also the red, and consequently yellows do not appear the same as green - they are lighter. Thus one of our difficulties has been overcome entirely by using a plate sensitive to all colors, or panchromatic. The other difficulty with regard to exposure is removed by the same procedure.
59. The only satisfactory method of obtaining a fast red sensitive plate is to bathe the plate in a solution of dye. Ordinary orthochromatic plates are sensitized in the course of manufacture by adding a dye to the emulsion, but this has not yet proven satisfactory, and the "Panchromatic" or "Trichromatic" plate owes its success to the fact that it is sensitized by bathing. In this plate the sensitiveness to other colors than blue is so great that a screen increasing exposure only three times gives superior results to one requiring twenty times increase on green-sensitive or "red-blind" plates.