SIGHT AND HEARING OF RAILWAY EMPLOYJES. 437 



sense, or at one metre shows yot> ^^ ^^ one third of a metre or one 

 foot shows but g-J-jj- of color-sense, where many fail to distinguish the 

 color even at this short distance : finally, by coarse tests of colored 

 glasses placed in front of large gas-lights, and by flags shown near at 

 hand. No color-blind man has lost his place without the satisfaction 

 of a professional examination, and a full demonstration of his defects 

 in most instances even to his own satisfaction. Should he fail to see 

 the red, for example, at five metres, of the large opening in Donders's 

 instrument, or of a gas-light with a red glass before it, calling it green, 

 he would be directed to obey the green signal and approach it slowly, 

 walking up to it until when within one metre or less he might perhaps 

 recognize it as the danger-signal, when too near to prevent an accident. 

 Color-blindness, it must be remembered, is in some respects like deaf- 

 ness, and with its various degrees there are different possibilities of 

 disaster. 



No excitement has arisen, no interference with the business of the 

 road, no color-blind man has escaped detection, very few mistakes 

 have been made by the examiners, not a single word has been changed 

 in the instructions, and there is nothing now to amend, except perhaps 

 to make the color-stick into a smaller, more elegant, and self -registering 

 instrument. 



One simple test not hitherto mentioned has been used in the first 

 moment of the writer's examinations, by placing a piece of cobalt-blue 

 glass in front of the man's eye, and directing him to look at a gas-light 

 of moderate size like a candle, at twenty feet distance ; this glass trans- 

 mits both blue and crimson light, and the normal eye sees a rose-colored 

 flame surrounded by a blue halo, while the color-blind sees no red, but 

 describes it as composed of two shades of blue only. 



That the color-blind depend upon the relative intensity of the 

 lights to distinguish them is shown by the fact that, if over a white 

 light we place a medium shade of London-smoke glass, it will probably 

 be called " green," while a deeper one will be called " red." In like man- 

 ner, if a red glass, then a green one, are placed before the light alter- 

 nately, and then tints of red and green of other depths, the man will 

 often call one red, red, and the other red, green, or vice versa. In the 

 display of flags that have been in use, a very bright or clean red one 

 having been correctly called, if it be thrown carelessly near by so that 

 it can be compared by the man, and another somewhat soiled be shown, 

 he may pronounce the latter green, and adhere to the opinion even 

 when he takes it in his hand, being misled by the brightness of the 

 cleaner one, and the relative dullness of the other. 



This photograph-picture of the color-stick gives in its tints only 

 various ones of gray, since, as we know, colors are incapable of being 

 rendered by this color-blind process, whereas color-blind men have lost 

 but the reds and greens, preserving perfectly the power to see yellows 

 and blues. If, therefore, we were to paint blue that part of the print 



