BEFORE considering the various processes of color photography, it is essential to explain the formation of color, and the difference between the additive and sub-tractive methods of color reproduction.
The source of all color is the spectrum or ribbon of colors formed by the dispersion of white light. When a narrow slice of white light is passed through a prism, it is spread out into a band of various colors, ranging from red through orange, yellow, green and blue to violet. This is nothing more than an artificially made rainbow, and while the colors enumerated above are the principal ones, they insensibly merge one into the other so that there are innumerable intermediate tints.
In Fig. 1 is shown a normal spectrum, giving approximately the distribution of the colors. It will be seen that there are numerous transverse lines, designated by the letters of the alphabet, which always occur in the same places, that is to say in the same color regions, no matter what the method of spectrum production may be, for there are other ways of forming spectra besides the use of a prism, such as by means of diffraction gratings. These lines are called the Fraunhofer lines, from the physicist who first mapped them out, and who assigned to them the letters by which they are known. They may be looked upon as convenient milestones or data posts, which enable us to identify any color at once. For instance, the E line occurs in the middle of the green and if we were to designate the color of a dress material or a dye as similar in color or hue to the E line, there would be no question as to its exact shade; whereas the mere term "green" conveys no definite meaning. It is very common practice to define a position in the spectrum with regard to these lines, and we may meet with such terms as B 1/2 C or E § 2/3, which also define the color, as one has merely to locate these particular points to determine at once the particular color designated.
All natural objects possess the property of absorbing more or less of white light and reflecting the remainder, and to this property their characteristic colors are due. The absorption may be general and equal for every part of the spectrum, when we obtain light of lowered luminosity or brightness which is called grey. If the absorption is complete, then obviously we have a total suppression of all color and light and the resultant is black. But if one region of the spectrum is absorbed more than another, that is to say, if the object selectively absorbs part of the spectrum, the sum of what is left is color.
A concrete example of the selective and absorptive power of objects is afforded by a very simple experiment.
Nearly fill a black vulcanite dish, such as is used for developing, with a solution of ammonio-sulphate of copper, which can be made by dissolving cupric sulphate in distilled water (about a ten per cent solution will do), and adding strong ammonia until the whitish precipitate which is formed at first is redissolved. A very rich blue solution is formed, and if this is poured into the dish to the depth of about three-quarters of an inch, it will appear quite black. But if a sheet of opal glass or white cardboard is placed in the solution at an angle, so that one end rests on the edge of the dish and the other on the bottom, it will be seen that the solution is actually colored and that the color shades off from the white of the opal glass through all shades of blue to almost black at the other end. This experiment proves in the first place that there must be some reflective surface capable of sending back to our eyes the incident fight, and in the second place it also shows that the thicker the stratum of colored substance the deeper the color. In the case in point, the white light incident on the liquid penetrates to some depth, and, without the opal glass, reaches the black surface of the tray. As this reflects no light, the solution appears black; but on the insertion of the glass the incident light meets its surface, which reflects the whole of the light, except that absorbed or suppressed by the liquid. It is obvious that the light actually traverses a double thickness of the solution at any point, first through one distance to the glass and then through the same thickness to the surface again, so that the depth or intensity of color is due to a double absorption.
The processes involved in the above experiment are continually taking place with all objects. Light penetrates below their surfaces to a greater or less depth, and meets a reflective layer; if the substance possesses selective absorption, then the reflected light is colored.
The absorption or suppression of a particular spectral region gives color, and this is roughly shown in the accompanying diagram, Fig. 2. It will be seen that with the exception of green, all the colors are due to the suppression of one region only, while in this case there are two absorption bands, one at each end of the spectrum.
On the left of the above diagram is given the color that is absorbed and on the right the residual color. It will at once be seen that while we are accustomed to talk of yellows, blues, crimsons, etc., these are not simple colors, but the sum of those spectral regions that are left when a certain band is cut out. It is also evident that if the right and left hand colors be added together the result must be white light, because we are merely restoring that which was temporarily removed. Two such colors, which, when mixed together, form white light, are said to be complementary colors. There are innumerable possible pairs of complementary colors, for it may be said that with the exception of the greens every line or point in the spectrum has its complementary. In the case of green the real complementary lies outside the spectrum, and can only be matched by assuming that instead of the spectrum being a straight line, it is actually a circle with the red and violet overlapping, forming the magentas or crimsons, in which lie the complementaries to green.
If we take two of the complementaries shown above, number II, in which blue is suppressed, with yellow as the resultant color, and number VI, which is violet-blue, and if we assume that we can match these in pigments with lemon chrome and ultramarine, and mix these pigments together, we shall find that we obtain, not white, but a more or less dirty olive-green. This seeming paradox leads us to another fundamental fact, which is that an admixture of pigments does not give the same results as an admixture of lights. Although this is a much misunderstood subject it is really very simple, but we will defer the complete explanation till we have cleared a little more ground.
The length and breadth of the spectrum are, like a camera image, dependent entirely on the instruments used. Everybody knows that the picture made with a vest-pocket camera may include as much of a given subject as a 10 x 12 print; the size of the result is determined by the focal length of the lens and the size of the plate or film, and is not dependent on the subject. It is obvious that if we ignore for the time being the Fraunhofer lines and divide our spectrum from the violet to the red end into equal steps, starting with wave length 4000 for the former and limiting the red to 7000, then we have 3000 different measurable steps which actually differ one from the other in color. Although our eyes are not sufficiently sensitive to differentiate between such fine gradations, yet they are capable of recognizing a great many, about 128 in all. This being the case, the question arises as to how we become conscious of color. Have we, as it were, 128 strings or wires in our eyes, each of which responds to one of these small spectrum steps, or is there some simpler system? Apparently the system that answers all theoretical requirements is a three-wire one. It has been established, principally by the researches of Young and Helmholtz, that we have but three nerve fibrils in the retina, the sensitive surface of the eye, which respond to all the colors of the spectrum. By the simultaneous and equal excitation of these three nerve fibrils we receive the sensation of white, and those of other colors by the action of two of them.