Color Vision

Color vision is a component of complex visual perception that provides our consciousness with the ability to distinguish the coloration of visible objects. This ability to distinguish color is often also called color sense. In its manifestations, two categories are distinguished: 1) the colorless series from the brightest white to the deepest black with all the intermediate shades of gray, and 2) the series of colored tones corresponding to the solar spectrum. This latter series can be transformed into a circle through purple tones not contained in the spectrum and formed by mixing red and violet. Thus, in the spectrum, on a relatively small space, are collected all those light rays that cause the entire range of color sensations. If we associate the qualitative aspect of sensations with the wavelength of simple (monochromatic) spectral rays, then the boundaries of the range are given (according to Helmholtz) by a wavelength of 761.7 mμ (near the Fraunhofer line A) - the most extreme reds still visible to the eye, and 310.8 mμ (near the line K) - the most extreme ultraviolet rays. Within these limits, however, a very large number of color shades are distinguished, because the transitions from one color tone to another occur in the spectrum with great gradualness. In addition to coloration depending on the length of light waves, color vision enables the distinction of 1) brightness, determined by the energy of the ray, and 2) saturation, depending on the amount of mixed white rays. In the solar spectrum under daylight conditions, yellow color is the brightest, followed by green and red, and blue and violet are the darkest. Spectral colors appear to us as the most saturated of all observed in nature, however, they can become even more saturated for us through simultaneous or successive contrast. Thus, red (even spectral), when viewed next to green or immediately after it, appears more saturated, dense, than when viewed alone. Color vision occurs only at medium light intensities. With a significant increase in brightness, not only does the brightness of spectral colors increase, but at the same time their coloration and saturation change, so that ultimately the sensation of coloration disappears and all colors become colorless, with the maximum brightness still remaining in the yellow color area. With sufficient reduction in illumination brightness (twilight vision), the coloration of the spectral band becomes colorless, but with varying brightness in different areas, with the maximum of this latter shifting to the area corresponding to the yellow-green part of the spectrum (Shaternikov). However, even under daylight conditions, color vision is fully mediated only by the central parts of the retina (see Perimetry) - they are surrounded by a zone mediating the perception of only yellow and blue colors - and finally the periphery of the retina enables the distinction only of different brightnesses of colorless light. The absence of color vision in twilight vision and the presence of a central scotoma under these conditions gave Kries (J. v. Kries), following Schultze and Parinaud (M. Schultze, Parinaud), grounds for the so-called theory of duality, according to which the perceiving apparatus in twilight, colorless vision is the rods, and in daylight, associated with color vision - the cones, which however also react to colorless light of sufficient brightness. - As early as Newton (1675), based on the analogy between auditory and color sensations, divided the visible solar spectrum into 7 sections, the width of which he took proportional to the intervals of the musical scale. These sections correspond to Newton's 7 basic colors, showing that by mixing them on a rotating disk, the original white (gray) color is obtained. Thus, all the diversity of color vision was reduced to a limited number of basic colors, various combinations of which could cause all the others. The entire set of colors and their shades that we perceive can be represented spatially in the form of two cones converging at their bases (Fig. 1). Section AB gives a circle (Fig. 2), divided into 8 sectors corresponding to Newton's 7 colors + purple. On planes parallel to the base of the upper cone will lie tones with an admixture of white color, increasing in the direction of the apex, where there will be a white point. On the planes of the lower cone will lie tones with an admixture of black color (red + black = brown; orange + black = brown; green-blue + black = olive, etc.), and at its apex - a black point. - A further step in the doctrine of color vision was made by Thomas Young (Th. Young, 1807), one of the founders of the wave theory of light. Young reduced Newton's system to 3 basic colors: red, green and blue and expressed

Figure 2.

the assumption of the presence in the retina of three kinds of nerve endings, one for each of the primary colors. Simultaneous, sufficient and equal irritation of all three endings forms the basis of the sensation of white, i.e. mixed light. Gray is merely various degrees of irritation of the same endings, while black is the sensation of a state of rest. Experimental substantiation and further development of Young's theory was received in the works of Maxwell and especially Helmholtz in the study of color mixing. The latter was achieved either by superimposing spectral colors (from the same or two spectra) or by rapidly alternating (30-50 times per second) effects on the same spot of the retina with colored sectors of rotating disks. Maxwell gave a very convenient form to the disks, allowing easy change in the number, size and color of the sectors. If two disks of different diameters are made to rotate on the same axis, one can, for example, compose a small disk from white and black sectors, and a large one from red, green, and blue sectors, selecting the size of the latter so that the gray of the center and periphery is the same, i.e. to compose a color equation. For example, 100° white + 260° black = 165° red + 122° green + 73° blue, or 141° green + 219° red = 73° yellow + 52° white + 235° black. Helmholtz showed that if to Newton's 7 colors are added 3: yellow-green, blue-green, and purple, then all colors can be arranged in 2 columns: 1) red, orange, yellow, yellow-green, green; 2) blue-green, blue, violet, purple. Colors located in both columns at the same level, for example yellow and blue, give white (resp. gray) when mixed and are called complementary. By mixing the extreme colors of the column, all intermediate colors of the column can be obtained depending on the proportion of the mixed colors, for example from the first column red and yellow-green will give orange or yellow, from the second column blue and purple will give blue or violet. The color of the mixture will be the more saturated the closer the mixed colors are to each other. From mixing colors that are further apart than complementary ones, purple tones are obtained. Here the color of the mixture will be the more saturated the more distant the mixed colors are from each other. Thus, the entire sum of color sensations, caused both by simple rays of the spectrum and by their mixtures, can be reproduced by mixing three spectral colors, two of which must lie at the ends of the spectrum, while the third somewhere in the middle. Helmholtz took red, green, and violet as the primary colors and, satisfying the principle of specific energy, together with Young recognized the presence in each color-perceiving point of the retina of three kinds of nerve endings. Each of them is excited not only by its own, so to speak, color (i.e. rays of a certain wavelength), but also by the other two, but only to a much lesser degree (see Vision). This circumstance explains the non-maximal saturation (see above) of spectral colors and their transition with increasing color intensity into white color. Color blindness (see Daltonism) is considered as the loss (or weakening in anomalous trichromats) of the function of the corresponding perceiving elements. Successive contrast is explained by the fatigue of the strongly excited elements, which determines the vision of white or gray in the complementary color. Simultaneous (simultaneous) contrast is explained psychologically as a deception of judgment. The Young-Helmholtz theory establishes a number of regularities connecting the effect of objective irritants with the function of the visual apparatus, in the central parts of which physiological processes arise that are the correlate of the corresponding sensations. The latter can arise in the central part of the visual apparatus also without external influence, under the influence of one or another endogenous factors (see Hallucinations, theories). A weak point of the theory is its interpretation of the sensation of white and black, the psychological explanation of simultaneous contrast, the vision of yellow in red-green blindness, cases of blindness to yellow and blue colors, and finally the distribution of color-sensitive zones of the retina.

The theory of opposite colors (1872) by E. Hering, based on the analysis of sensations, considers black not as the influence of a state of rest, but as a sensation opposite to white, and distinguishes 4 primary colors in the spectrum: primary red, yellow, green and blue, pairwise opposite, because in red we never feel an admixture of green, in blue - yellow. In accordance with this, Hering assumes the presence in the visual apparatus of 3 visual substances: white-black, red-green and yellow-blue. But based on his general biological views, according to which in living substance continuously and in parallel processes of decay, dissimilation and restoration of losses, assimilation, proceed, Herring assumed that similar processes occur in his visual substances, with the dissimilation of the white-black substance forming the basis of the sensation of white, and assimilation - black; dissimilation of yellow-blue gives the sensation of yellow, assimilation - blue; dissimilation of red-green gives the sensation of red, assimilation - green. These relationships are represented in the scheme of fig. 3. In accordance with these ideas about 3 pairs of opposite colors, the color body, according to Hering, should have the form of a double pyramid, as shown in fig. 4. Herring further assumes that the dissimilation of any of the substances favors its assimilation and vice versa, and this interaction explains simultaneous contrast. In addition he assumes that "white valence" is inherent in the dissimilation of both colored substances. Hering's theory satisfactorily explains the above difficulties of Helmholtz's theory, however for it it is difficult to explain protanopia and deuteranopia (see Daltonism), as well as the sensation of white in conditions of daylight and twilight vision.

In view of the unsatisfactoriness of both basic theories, there exist many attempts to improve each of them; however, none of these attempts has received sufficiently wide recognition.

Figure 3.