. Refraction and motility of the eye, with chapters on color blindness and the field of vision . e as much likethose in the service as possible. For this purpose coloredglass discs are used revolving in front of a lantern, of suchsize and brilliancy that they shall subtend the same angleand have approximately the same brilliancy as the signallamps on the road. The Williams or Friedenberg testlanterns are good types. Xo attempt is necessarily made tohave the patient name the colors shown, but only to recog-nize immediately their significance as indicating a cleartrack, danger, etc. The illumina
. Refraction and motility of the eye, with chapters on color blindness and the field of vision . e as much likethose in the service as possible. For this purpose coloredglass discs are used revolving in front of a lantern, of suchsize and brilliancy that they shall subtend the same angleand have approximately the same brilliancy as the signallamps on the road. The Williams or Friedenberg testlanterns are good types. Xo attempt is necessarily made tohave the patient name the colors shown, but only to recog-nize immediately their significance as indicating a cleartrack, danger, etc. The illumination should be so arrangedthat it can be increased or diminished 1>\ approaching orwithdrawing the light or by the interposition of differentshades of smoked or ground glass. 328 REFRACTION AND MOTILITY OF THE EYE. The commonest forms of color blindness are those inwhich red and green are confused and this would be par-ticularly dangerous in railway employees, since these arethe commonest colors employed in signals. A man may becolor-blind and still be able to distinguish a red light from. Fig. 105. a green in the testing room. The lights do not seem to himto diifer materially in color, but he distinguishes at once bytheir varying brilliancy. For instance to the red-blind thegreen signal is bright green, while the red one is the samecolor but very much darker, and he immediately identifiesit as red simply from its darkness. Evidently, if we showhim a green light and then, covering it for a second, reduce COLOR-BLINDNESS. 329 the illumination, the color-blind person will get the impres-sion of red while normally he should still be conscious ofgreen. If he is green-blind;, the green light will appearsimply as a differently illuminated red, and both these con-ditions are often approximated in the service when thebrilliancy of signals is interfered with by atmospheric con-ditions. A man who makes no mistakes in a carefully con-ducted test of this kind would be competent enough f
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