THE PRIMARY COLORS.

ABSTRACT OF PAPER READ BY G. H. MORTON, JUN.

Of all the interesting phenomena concerning color, resulting from recent scientific researches, probably the hypothesis that red, green and violet are the three primary color sensations, attracts most attention, because of its apparent antagonism with the fact that red, yellow and blue are the three primary colors of pigments.

All colors are really sensations, caused by the action of light on one of the divisions of the retina, the so-called layer of rods and cones. It has only recently been discovered, however, by Professor Max Schultz, that both the rods and the cones have each their peculiar function, and though probably both serve as elements of light, it is more especially the function of the rods, whilst the perception of color is due, possibly exclusively, to the cones. These cones appear to be divided into three sets; one set being stimulated by the strongest vibrations of light produces the sensation of red, another set acted upon by the vibrations of medium strength produces green, and the third set responding to the short and weak vibrations produces the sensation of violet. Red, green and violet are therefore termed primary colors. Intermediate vibrations affect two sets of cones simultaneously, and consequently produce compound or secondary colors. Upon the three sets of cones being excited together, in their proper proportions, the sensation of white is the consequence. If this theory be true, we may assume the existence of a color-sense wherever we find the cones, and to mark it absent wherever they are absent. A defect in their arrangement probably accounts

for color-blindness. It is well known that color-blind persons are usually insensible to the stronger, red-producing, vibrations, though cases occur when they are blind to other colors and keenly sensible to the red; indeed, the partially color-blind, as a rule, appreciate more intensely the colors they are able to perceive than persons possessing the normal sense of color. The number of persons affected, more or less, with color-blindness is surprising; according to Professor Rood it has been estimated that in England about one person in every eighteen has an imperfect color-sense, though it is remarkable that the affliction is almost exclusively peculiar to the male sex, women being comparatively free from it. Often the color-blind are for years unconcious of their defect, and it is very difficult to demonstrate the fact to them. When we consider the theory of the rods and cones, and are aware that a slight difference in their arrangement causes a proportionate difference of colorperception, and knowing that we are all subject to a slight difference, it would appear probable that each individual is affected by the vibrations of light to a different degree, or, in other words, that possibly no two persons see colors exactly alike.

Color combinations may be demonstrated in different ways; perhaps the simplest is by employing two lanterns, and projecting colored light from both on to a white screen. The two color-discs, when superposed, produce one color; the sum of the two separate discs. Taking the primary colors, red and green produce the sensation of yellow; red and violet produce pink; green and violet produce greenish-blue.

The resulting colors, yellow, pink and greenish-blue, are the secondary colors, and it follows that the brightness, or lightness, of any of these colors must be greater than that of any of the primaries, because of the nearer approach to

white caused by the combination; yellow, for instance, will be as bright as the brightness of red and green added together; white will be as bright as the brightness of the three primaries added together.

The different vibrations of which white light consists are capable of producing every possible color sensation, yet we cannot say that all color is due to them alone. Were it not for the power which natural bodies have of selecting their own particular color, or rather colorproducing vibrations, the phenomena would be incomplete. No natural body creates color; it simply absorbs a part of the light thrown upon it, and rejects the remainder. It is the portion so rejected, and not that retained, which determines the color.

Intercepting the white beam of light from the lantern by red glass, we prevent the vibrations which produce the sensations of violet, blue, green and greenish-yellow from passing through. The red glass absorbs all these rays; consequently the red rays alone are left, and we have a red disc on the screen. We will now from our second lantern project a greenish-blue light, this color being as nearly as possible the sum of the colors absorbed by the red medium. By moving the lanterns in order that this color may be, as it were, superposed, or added, to the red, we produce white, thus proving that the red glass had absorbed all those rays of which the bluish-green consists, and these rays had only to be given back to reproduce the original white disc. Repeating this experiment, but with blue and yellow lights, white is also produced, and not green, as might be expected from our experience of the result obtained by a mixture of blue and yellow pigments. By passing a beam of light through the blue and yellow glass together, placing both colors in the one lantern, green is produced, because the blue and yellow glass are not only transparent to their respective

colors, but also to the green, which is contiguous to both; but the blue glass absorbs the yellow and less refrangible rays, and the yellow glass absorbs the blue and more refrangible rays, consequently green is the only color left to pass through both, and therefore when a beam of white light goes through these colors together, green is the only possible result. This is exactly what occurs in the mixture of pigments, and it is through the confounding of the mixture of lights and the mixture of pigments that many erroneous conclusions are arrived at.

So far absorption has been considered in connection with transmitted light; we have now, in order to show its connection with the colors of pigments, to consider it when combined with reflected light. The coloration of pigments, of flowers, of green leaves, indeed of all natural bodies, "is due to the combination of reflection with the phenomenon of absorption." * I have here a colorless glass vessel, containing a red solution, and of course it exhibits the same phenomena as colored glass; it requires light either transmitted or reflected to be seen, for when placed in front of the black screen it appears dark and colorless. By placing a white object in the solution, we notice that part of the light directed upon it undergoes absorption whilst passing through the fluid, and the portion left is reflected by the object, and passes back again to the eye. This light, therefore, has itself gone twice through the fluid, or double the distance of the object from the front surface of the glass vessel. It is evident that our red color is due to what is called "selective absorption," red being the only color not devoured, as it were, by the fluid. We will now add to the solution some powdered chalk, each particle of which may be regarded as a minute white object

* Stokes.

reflecting a small quantity of the light not absorbed; all I have said of the one white object applies also to these minute particles. All color pigments are composed of such minute objects, mixed with a vehicle which has the power of sifting the white light shed upon them, and selecting certain rays, whilst the particles reflect the remainder. The condition of the petals of flowers and of green leaves is just the same. A white lily, for instance, if it were quite smooth, would have the appearance of thin glass; it is, however, composed of a vast quantity of minute cells, so that it resembles finely-powdered glass, from each little particle of which light is reflected backwards and forwards, and there being nothing in the lily to cause the selection of one ray over another, the light is white. A scarlet geranium is similarly composed, but its particles, or cells, are infused with colored matter, which absorbs the green and blue rays, and the unabsorbed scarlet is reflected. All green leaves owe their color to the same cause; light passing backwards and forwards through a coloring matter which extinguishes the red rays, is received upon the retina as green.

A property of absorption, not only interesting, but of some importance to colorists, is the change of hue colors undergo when deepened or lightened. Each additional layer of a colored medium naturally absorbs more of the light not actually destroyed, and therefore deepens the color, until at last it, too, is altogether extinguished. Light, however, has been shown to consist of vibrations of different degrees of strength; this being the case, the stronger vibrations will continue to act after the weaker are quenched; or as the color deepens its hue alters. To explain this more clearly, I will quote from a lecture by Professor Stokes, F.R.S., who, for the sake of simplicity, presupposes two kinds of light, blue and red, having intensities respectively of 100 and 10. "There is, of course, a great predominance of blue over red.