By WILLIAM THOMSON, M.D.,
PROFESSOR OF OPHTHALMOLOGY, JEFFERSON MEDICAL COLLEGE.
SHORTLY after the demonstrations of Professor Holmgren, in Sweden, of the dangers in transportation to persons and property on land and sea from color-blindness, the writer called the attention of the officers of the Pennsylvania Railroad to the subject; and, at the request of the president, Mr. Thomas A. Scott, and the vice-president, Mr. Frank Thomson, he undertook to solve the problem of eliminating these dangerous men from their service. To his first statement that there were probably four per cent of men incapable of distinguishing unerringly between red and green flags by day, or lights by night, it was responded that their signals alone would detect such men, and force them from their places, since, as we all know, the most imperative orders in railway administration are transmitted through the visual organs, in the white of "Safety," the green of "Caution," and the red of "Danger"; and it was considered by the officers of the road impossible for men color-blind to pass the thousands of signals in daily use on their thousands of miles of road without detection. A very slight search dispelled this idea, and a demonstration of the defect before the Society of Transportation Officers of the Pennsylvania Railway aroused the members of it to the dangers to be feared, and led to the appointment of a special committee to aid the writer in completing a system which would have practical value, with the Gen-
- Read before Section B of the American Association for the Advancement of Science, at Philadelphia, September 8, 1884.