merely require us to suppress the worse half of the present composite set of block signals—the half, which relies on color—and to render universal its better portion which already signals by direction. The present system would thus be simplified and fulfilled, rather that annulled, and there would be no need of training engine drivers to an unfamiliar code. And while the perception of the trend of a line of light requires that the refractive power of the eye shall be normal or shall be corrected by the use of glasses, the engineer's work even now demands that his spatial vision shall be keen, and thus no innovation would be made.
But even were there many objections to the use of spatial signals, they must be grave indeed to outbalance the fact that a line of light not only frees us from the treachery of the color sense, but gives a symbol that is distinct from the usual lights of the window or the street, and at a stroke renders well-night impossible those accidents that come from mistaking foreign lights for block signals. Moreover, we should then have a system wherein danger would be indicated at all times as clearly and as unmistakably as safety, whereas in the present code the red danger signal can too readily remain unseen. An important advantage besides would be that signals of the kind here proposed could hardly, by influence of smoke, or fog, or storm, be made to seem the very contrary of what they really were. A green light may look whitish, or a yellow light red, by mere conditions of the air. But a vertical line can not well be made to appear horizontal, or a horizontal diagonal, by smoke or fog. Its message might be cut off entirely, but could not readily be distorted into its fatal opposite. In this, as in so many other ways, a change of usage commends itself to the critical sense.
In urging that we no longer rely upon the color faculty for the safety of our trains, I have spoken almost exclusively of those difficulties which color offers to eyes that are entirely normal and sound. And upon such facts the main objection to the present system may well be based; for they are strong enough in themselves to condemn our usage and to demand that it be changed. But the reasons so far given are immeasurably strengthened by the existence of color-blindness and other defects of the sense of color. There are some men, it is true, who believe that the danger from this source is entirely averted by the current examination of engineers. No one would wish needlessly to lessen faith in such examinations. And yet it should be more widely known that defects of color vision are not always easy for physicians to detect; far less are they for laymen. And since many dangerous cases are known to slip through the meshes of medical examiners elsewhere, it is reasonably certain that the same is true with us. Dr. Stadfeldt, of Copenhagen, in a recent examination of 295 pilots (who, like engine-