250 POPULAR SCIENCE MONTHLY 



fied in whatever way its new work might require. Considerable inter- 

 vals 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 feed- 

 ing 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 at- 

 tempts 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 suc- 

 cess 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 



