Page:Popular Science Monthly Volume 26.djvu/453

Rh, or at one metre shows or at one third of a metre or one foot shows but  of color-sense, where many fail to distinguish the color even at this short distance: finally, by coarse tests of colored glasses placed in front of large gas-lights, and by flags shown near at hand. No color-blind man has lost his place without the satisfaction of a professional examination, and a full demonstration of his defects in most instances even to his own satisfaction. Should he fail to see the red, for example, at five metres, of the large opening in Donders's instrument, or of a gas-light with a red glass before it, calling it green, he would be directed to obey the green signal and approach it slowly, walking up to it until when within one metre or less he might perhaps recognize it as the danger-signal, when too near to prevent an accident. Color-blindness, it must be remembered, is in some respects like deafness, and with its various degrees there are different possibilities of disaster.

No excitement has arisen, no interference with the business of the road, no color-blind man has escaped detection, very few mistakes have been made by the examiners, not a single word has been changed in the instructions, and there is nothing now to amend, except perhaps to make the color-stick into a smaller, more elegant, and self-registering instrument.

One simple test not hitherto mentioned has been used in the first moment of the writer's examinations, by placing a piece of cobalt-blue glass in front of the man's eye, and directing him to look at a gas-light of moderate size like a candle, at twenty feet distance; this glass transmits both blue and crimson light, and the normal eye sees a rose-colored flame surrounded by a blue halo, while the color-blind sees no red, but describes it as composed of two shades of blue only.

That the color-blind depend upon the relative intensity of the lights to distinguish them is shown by the fact that, if over a white light we place a medium shade of London-smoke glass, it will probably be called "green," while a deeper one will be called "red." In like manner, if a red glass, then a green one, are placed before the light alternately, and then tints of red and green of other depths, the man will often call one red, red, and the other red, green, or vice versa. In the display of flags that have been in use, a very bright or clean red one having been correctly called, if it be thrown carelessly near by so that it can be compared by the man, and another somewhat soiled be shown, he may pronounce the latter green, and adhere to the opinion even when he takes it in his hand, being misled by the brightness of the cleaner one, and the relative dullness of the other.

This photograph-picture of the color-stick gives in its tints only various ones of gray, since, as we know, colors are incapable of being rendered by this color-blind process, whereas color-blind men have lost but the reds and greens, preserving perfectly the power to see yellows and blues. If, therefore, we were to paint blue that part of the print