Page:Popular Science Monthly Volume 72.djvu/251

 that are red. This, of itself, is an undesirable condition, since the sign of danger should of all be most outspoken.

The disadvantage under which the red danger signal labors is, however, quite insufficiently expressed by saying that the ruby glass often virtually destroys fully four fifths of the light from a lantern flame already none too bright, and to this extent increases the liability that the most momentous of the signals will at some crisis be seen too late or not at all. Even the remaining portion is often far less effectual upon the eye than its physical quantity would lead us to expect. The importance of the matter for signaling will perhaps justify some further account.

If, by reliable devices of the laboratory, a semaphore light showing "white" be gradually reduced in brightness, a point can easily be found where the eye, grown accustomed to the dark, can just perceive the light. And when for comparison a railway ruby glass, or "roundel," is placed before the lamp, the observer now obtains no conscious impression at all. But instead of having to increase fivefold the brightness coming through the glass (as one might expect, knowing that the red glass is pervious, say, to but one fifth of the light of the flame), it is necessary to increase it no less than fourteenfold. Such an increase is the least I have found necessary when experimenting at night over a stretch of more than four thousand feet and when smoke gave a relative advantage to the red. Within the laboratory the red has never been perceptible until the light was increased eighteen times the brightness required for white. Such, however, are the most favorable experiments, and are by no means average ones. On the average it is necessary to increase the light as much as thirty times before any conscious impression at all is made by the light through the red glass. One of the subjects of this experiment—a man who would pass the usual tests for color-blindness—has still remained insensible to the red when the light is increased to seventy times what is needed for the white! Such facts as these show clearly that by merely looking at a cluster of railway signals, or even by taking the usual tests of their relative intensity or visibility when shining bright, we get no adequate idea whatever of the difficulty which the eye has with very feeble reds. And feeble reds are no great rarity in the actual conduct of trains. The many influences which render signal lights obscure thus act with a peculiarly fatal force upon the very color which is our chief reliance for the protection of life.

And the doubt thus raised regarding red is not allayed by the reports of railway accidents. For in these reports the frequency with which engineers fail to observe red signals at night is a most impressive fact. It is often impossible to tell assuredly why men at such times are unconscious of the danger sign; but even when allowance is