16. By saturation we mean the attribute of color-sensations by virtue of which they appear in all possible stages of transition to sensations of pure brightness, so that a continuous passage is possible from every color to any point in the series of whites, greys, and blacks. The term "saturation" is borrowed from the common method of producing these transitional colors objectively, that is, by the more or less intense saturation of some colorless soluble with color-pigment. A color may be ever so saturated, yet it is possible to think of a still greater saturation of the same color-tone, and, on the other hand, pure brightness always denotes the end of the series of diminishing grades of saturation for any color whatever. A degree of saturation may, therefore, be thought of as an attribute of all color-sensations, and, at the same time, as the attribute by which the system of color-sensations is directly united with that of sensations of pure brightness. If, now, we represent some particular sensation of white, grey, or black by the central point of the color-circle, all the grades of saturation that can arise as transitional stages from any particular color to this particular sensation of pure brightness, will obviously be represented by that radius of the circle which connects the centre with the color in question. If the shades of saturation corresponding to the continuous transitional stage, from all the colors to a particular sensation of pure brightness are thus geometrically represented, we have the system of saturation-grades as a circular surface whose circumference is a system of simple color-tones, and whose centre is the sensation of pure brightness, corresponding to the absence of all saturation. For the formation of such a system of saturation-grades any point whatever in the series of sensations of pure brightness may be taken, so long as the condition is fulfilled that white is not too bright or the black too dark, for in such differences in both saturation and color disappear. Systems of saturation which are arranged about different points in the series of pure brightness, always have different grades of brightness. A pure system of saturation, accordingly, call be made for only one particular grade of brightness at a time, that is, for only one point in the series of sensations of pure brightness. When such systems are made for all possible points, the system of saturation will be supplemented by that of grades of brightness.
17 . Brightness is just as an attribute of color-sensation as it is of achromatic sensations, and is in this case, too, at once a quality and degree of intensity. Starting from a given grade, if the brightness increases, every color approaches white, in quality, while at the same time the intensity increases; if the brightness decreases, the colors approach black in quality, and the intensity diminishes. The grades of brightness for any single color thus form a system of intensive qualities, analogous to that of pure brightness, only in place of the achromatic gradations between white and black, we have the corresponding grades of saturation.
From the point of greatest saturation there are two opposite for variation in saturation: one positive, towards white, accompanied by an increase in the intensity of the sensation, and the other negative, towards black, with a corresponding decrease in intensity. As limits for these two directions we have, on the one hand, the pure sensation white, on the other, the pure sensation black; the first is at the same time the maximum, the second the minimum of intensity. White and black are in this way opposite extremities of the system of sensations of pure brightness, and also of the system of color-sensations arranged according to grades of brightness. It follows obviously that there is a certain medium brightness for every color, at which its saturation is greatest. From this point, the saturation diminishes in the positive direction when the brightness increases, and in the negative direction when the brightness decreases.
The grade of brightness most favorable for the saturation is not the same for all colors, but varies from red to blue, in such a way that it is most intense for red and least intense for blue. This accounts for the familiar phenomenon that in twilight, when the degree of brightness is small, the blue color-tones -- of paintings, for example -- are still clearly visible, while the red color-tones appear black.