More than two hundred years ago Sir Isaac Newton discovered that a triangular glass prism would transform a beam of sunlight into a beautiful band of color. If the prism is held in a beam of sunlight which enters a moderately lighted room, there will appear on the walls, ceiling or floor, here and there, as the glass is moved, beautiful spots in rainbow colors. If the room is darkened by shutters, and only a small beam of light is admitted through a very narrow slit and the prism properly adjusted to receive this beam of light, a beautiful band of variegated colors may be thrown on to a white ceiling or screen, and this effect is called a prismatic solar spectrum. A perfect solar spectrum once seen under favorable conditions in a dark room is a sight never to be forgotten.
The accompanying illustration shows the relative positions of the parts named. A is the beam of light as it enters the room. B is the triangular prism. The dotted lines represent groups of rays extending to the vertical band of colors indicated by the letters V for violet at the top, then blue, green, yellow, orange to red at the bottom.
The explanation of this phenomenon is that the beam of sunlight is composed of a great number of different kinds of rays, which in passing through the prism are refracted or bent from their direct course, and some are bent more than others, the red least of all and the violet most. It is supposed that light is propagated by waves or undulations in an extremely rare substance termed ether which is supposed to occupy all space and transparent bodies. These waves are thought to be similar to sound waves in the air or the ripples on the smooth surface of a pond when a pebble is thrown into it. Because so many of the phenomena of light can be satisfactorily explained by this theory, it has been very generally adopted by the scientists. The amount that rays of light are refracted from a straight line in passing through a prism is in proportion to the number of waves or undulations per second, and in inverse proportion to the length of the waves. The red waves are refracted the least and are the longest, while the violet rays are refracted the most and are the shortest.
Whether this theory of the spectrum formation is absolutely correct or not, the fact is established that the colors found in a prismatic solar spectrum are always the same under the same conditions and the order of their arrangement is never changed. By means of the quality of spectrum colors called the wave length, a given color can always be located in the spectrum, and hence if a spectrum color is selected as a standard it can always be determined by its recorded wave length.
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 [Pg 18] 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.