172 COLOR-BLINDNESS IN ITS RELATION TO 



if absolutely limited to the visual sense. Many dififerent ways suggest 

 themselves of varying the signals by form and arrangement in space: 

 large brilliant surfaces arranged in dififerent forms; several small lights 

 grouped in different positions with reference to each other; lights sim- 

 ply disposed, but differing in value as signals according to their number, 

 or else illuminated figures of simple colors, and produced by different 

 movements, and so on. A practical difficulty seems to be connected 

 with such a system ; the signals require the illuminated surfaces to be 

 large enough, or placed at distances considerable enough between the 

 luminous points, to appear distinctly afar off; now the larger such a fig- 

 ure, having the outlines marked with luminous points, the greater the 

 risk that a portion of it bo hidden by other objects intervening between 

 it and the eye. These two inconveniences must be apparent, particu- 

 larly if these surfaces and illuminated figures have to be placed at every 

 point where colored lanterns are found, as for example on locomotives 

 and cars. 



If the system of signals were based upon form, and all persons dis- 

 charged from the service of railways who, in consequence of an imperfec- 

 tion of vision, could not clearly and decidedly distinguish tbese signals at 

 a distance, the proportion of such would be larger than that of the color- 

 blind. To form an idea of the different capacities of the normal eye in 

 the various senses just mentioned, we will recommend a very simple ex- 

 periment. Take something, colored i)aper, for example ; make some plain 

 figures, such as letters, one of which must be attached vertically to a 

 large black or white surface. To prevent any distraction from subject- 

 ive influences, let some one else select and attach this letter, while the 

 observer stands at such a distance that even with the eye directed 

 toward it but a single object can be seen. If the letters are small, it is 

 not necessary that the distance should be very great. Then if the sym- 

 bol be slowly approached, with the eye fixed on the colored surface, the 

 following observations in the order in which the different impressions 

 succeed each other will be made. When the letter is first perceived, 

 neither form nor color can be distinguished ; nothing is seen but a point 

 or patch darker than the background, if that be white, or lighter, if it 

 be black. The fllrst attribute remarked, as the distance diminishes, is its 

 color. When the color is very distinct, it is necessary to approach con- 

 siderably nearer before the form is perceptible, that is to say, before this 

 letter can be read, and its name given. This simple experiment clearly 

 shows that the eye, as far as it is possible to compare itscapacitiesin differ- 

 ent directions, is first sensible to the relative intensity of light, then to 

 color, and finally to form. It would be necessary, of course, to consider 

 various circumstances relative to the choice of color, form, etc., if the 

 experiment should be made with exactness to serve as a basis for a 

 scientific demonstration. It would appear therefore that a system of 

 signals based on the power of distinguislung light from darkness is less 

 suitable than the one now in use ; not only because this is based upon 



