found that the addition of one part of white light to 360 parts of pure colored light produced a change which was perceptible to the eye; smaller amounts failed to bring about this result. It was also ascertained that mingling pure colored light with from 120 to 180 parts of white light caused it to become invisible, the hue being no longer distinguishable from white. Differences in luminosity as small as 1⁄120 to 1⁄180 could under favorable circumstances be perceived. It hence followed that irregularities in the illumination or distribution of pigment over a surface, which were smaller than 1⁄180 of the total amount of light reflected, could no longer be noticed by the eye. Experiments with red, orange, and blue disks were made on the sensitiveness of the eye to changes of tone or refrangibility; thus the combination of the blue disk with a minute portion of the red disk altered its hue by moving it a little toward violet; on reversing the case, or adding a little blue to the red disk, the tone of the latter moved in the direction of purple. Similar combinations were made with the other disks. Aubert ascertained, in this way, that recognizable changes of tone could be produced by the addition of quantities of colored light as small as from 1⁄100 to 1⁄300 of the total amount of light involved. From such data he calculated that in a solar spectrum at least 1,000 distinguishable tones are visible. But we can still recognize these tones when the light producing them is subjected to considerable variation in brightness. Let us limit ourselves to 1,000 slight variations, which we can produce by gradually increasing the brightness of our spectrum, till it finally is ten times as luminous as it originally was. This will furnish us with a million tones, differing perceptibly from each other. If each of these tones is again varied 300 times, by the addition of different quantities of white light, it carries up the number of hues we are able to distinguish as high as 300,000,000. In this calculation no account is taken of the purples, or of colors which are very bright or very faint, or mixed with very much white light. For these it will hardly be extravagant to demand another 100,000,000; we reach thus the astonishing conclusion that the human eye under favorable circumstances is able to distinguish as many as 400,000,000 different hues!
