Therefore it seems possible to establish certain standards of color by a series of definitely located portions of the solar spectrum and in the system here presented six have been chosen, namely red, orange, yellow, green, blue and violet. These six are more distinctly recognized than the others, and from them by combination in pairs of colors adjacent in the spectrum all the other colors can be very closely imitated, and hence these six are selected as the spectrum standards. In these standards the most intense expression of each color is chosen i.e. the reddest red, greenest green, etc. which by the closest scientific investigation have been located by their wave lengths so that if they are in doubt in future they can be re-determined by individuals or if disputed, may be corrected by any authoritatively established congress, selected for the purpose. The wave lengths of our six standards are represented by the following numbers in ten millionths of a millimeter. Red, 6571; Orange, 6085; Yellow, 5793; Green, 5164; Blue, 4695; violet, 4210. Having thus scientifically established these unchangeable standards the attempt is made to secure the best possible pigmentary imitation of each.
To any one who has ever compared a piece of colored material with a good presentation of a spectrum color, it is unnecessary to say that the result in an attempt to match the spectrum color with the material or pigmentary color is a very weak approximation, but the one thing aimed at is to secure nearly as possible the same kind of color. For example in the red, it is the aim to obtain the same kind of red, by which we mean the same location in the spectrum, i.e. a red neither more orange nor more violet than the reddest spot in the spectrum. This selection must be based on a purely æsthetic perception or impression of color. The same is true of each of the six standard colors, as for example, for orange we select the location which has seemed to a large number of good judges to best represent the feeling of orange as between the quite well defined red on one hand and the equally definite narrow band of yellow on the other, and it is quite wonderful what unanimity of opinion there is on this particular color which would naturally seem to be the one most doubtful in its location. On the other side of the yellow the green seems to offer little difficulty and the pure Paris or emerald green is very nearly the standard. The violet being at the other end of the spectrum is as easily decided as the red, but the blue between the green and violet is not so easily determined, because, from the best blue the hue runs so imperceptibly into the violet on one side and the green on the other. Pure ultramarine blue is the nearest approach to the spectrum standard of blue of any of the permanent pigments, but even this is a trifle too violet.
For educational purposes papers coated with pigments afford at once the purest colors and the most economical and useful material, and on this plan a line of colored papers has been prepared for color instruction in the kindergartens and primary schools in imitation of the above described spectrum standards.
From the pure spectrum standards it is possible by reflected light to combine the two standards to produce a color between them, for example if two small mirrors are held in a spectrum one at the "red" and the other at the "orange" and the two reflected on to the same spot on a white surface, the result is a color between the red and the orange. So also if we mix red and orange pigments together we may produce colors between the two which may be termed orange-red or red-orange; but unfortunately there is no means known by which we can measure the proportion of the red and orange color-effect which is produced by any given mixture of these two pigments, because color-effect cannot be measured by the pint of mixed paint or the ounce of dry pigment.
The Color Wheel and Maxwell Disks.
We, however, have another means for measuring color effect which just in this emergency seems providential. It is a fact well known to every boy that if he rapidly whirls a lighted stick the fire at the end produces the effect of a circle of light, which phenomenon is explained by a quality of the eye called retention of vision, by which the impression made by the point of light remains on the retina of the eye during an entire rotation. It is a fact, based on the same quality of vision, that if one color is presented to the eye, and instantly replaced by another the effect is a combination of the two colors. Therefore if one-quarter of the surface of a disk of cardboard is covered with orange paper and three-quarters with red paper, and then the disk placed on a rapidly rotating spindle, the color effect is a mixture of red and orange, and the effect is exactly in proportion to the angular measurements of the two sectors, so that if the circumference is divided into 100 equal parts the resultant color will be definitely represented by the formula "Red, 75; Orange, 25."
Less than forty years ago an English scientist named J. Clerk Maxwell while making experiments with such painted disks happily conceived the idea of cutting a radial slit in each of two disks from the circumference to the center so that by joining the disks they could be made to show any desired proportion of each and hence they are called Maxwell disks. With such disks made in the six pigmentary standards red, orange, yellow, green, blue and violet, the intermediate pigmentary spectrum colors may be very accurately determined by combination and rotation. If we give to each of these standards a symbol as R. for red, O. for orange, Y. for yellow, G. for green, B. for blue, V. for violet, we then have the basis for a definite nomenclature of colors in imitation of the pure spectrum colors. As all pigmentary or material colors are modified by light and shade thus producing in high light tints and in shadow shades of the colors, we must seek for some means of imitating these effects, and fortunately find them in white and black disks. If with a standard color disk we combine a white disk we may have a line of tints of that color, and with a black disk, shades. Giving this white disk a symbol of W. and the black disk N. we complete our nomenclature. We cannot use B for black because B has already been used for blue, and therefore we use N. for niger, the Latin word for black.
The Bradley System of Color Instruction.
Briefly stated then this system of color instruction is comprised under the six general heads: Spectrum Standards; Pigmentary Standards based on the spectrum standards; Maxwell Rotating Disks in the pigmentary standards and Black and White; a Color Nomenclature based on the accepted standards and their disk combinations; and Colored Papers and Water Colors made in accordance with these standards.
For spectrum standards, six definite locations expressing the natural æsthetic or psychological impressions of red, orange, yellow, green, blue and violet are selected. Six standards are chosen instead of a larger number as for example twelve, because for the purpose of a nomenclature the smaller number is more convenient than a greater number. The six are selected rather than three, four or five, because while in the consideration of colored light alone the smaller number would possibly suffice to form by combinations imitations of all other colors, any number smaller than six is entirely inadequate to form by pigmentary or disk combinations fairly good expressions of the corresponding spectrum color combinations.