fied in whatever way its new work might require. Considerable intervals may be between the lights, and yet from a distance they will seem a continuous line; and as for length, experiment both in the laboratory and over a distant stretch at night shows that the main directions of such a line can be caught by the normal eye when the length is about a thousandth of the distance from which it is to be read. Three or four lights in a row about five feet in length would thus suffice for giving an engineer his signal a mile before he passed the post. A space signal given in some such way would naturally require more feeding of electricity or gas or oil—than does the single small wick flame that now gives forth the colored beam. But on the whole it would perhaps be well to spend fuel and light rather than life.
The mere imaging of a generous and glowing line to give the signal, will at once quiet some grumbling doubts that come from failures hitherto. Certain older attempts to use the space principle for railway signals at night could easily bring misgiving if the real cause of the failure had lain in the space sense itself. But, in fact, there has been no readiness to use the amount and extent of light needed for a proper signal. One of our roads tried to guide its trains by means of two lights whose changing position with reference to each other should give the sign to the engineer; and this signal was found unsatisfactory because these two lights blended into one when looked at far away. But for the most part the attempts have been confined to spreading out into a band, by means of a reflector, the light of a single lamp—a band of light that was too faint to be well seen when the reflector became dimmed by smoke or the corrosion of the weather. Such crude attempts to make a spatial signal were of course foredoomed to failure, and give no reason to distrust the perception of space itself. They simply prove that sufficient brilliancy of light must be maintained, and that this brilliancy must be stretched to sufficient length—conditions which can certainly be fulfilled at least wherever there are electric lights.
One can speak with greater confidence because of the practical success of such spatial signals in another field. The use of the luminous line is already well established in the navy. Two movable arms, each provided with a row of incandescent lights, here rapidly convey, by the direction in which they point, their message from ship to ship, or to the shore. And even with comparatively short lines of light, their position is legible by the unaided eye at a considerable distance. The outcome here gives ample reason to believe that it would be possible to apply the same general method to the railway service.
The advantages of relying on our space-perception instead of on the color sense will probably in time be recognized as far outweighing whatever difficulties there may be in the change. The new plan would