COLOR-BLINDNESS, ACIIROMATOPSIA, or DALTONISM. An incurable defect of vision, owing to which some persons are unable to distinguish certain colors. The name Daltonism is after Dalton, the English chemist, who suffered from the defect. Acquired color-blindness is a symp tom of diseases of the optic nerve and retina. Congenital color-blindness usually affects both eyes, and is often hereditary. It is found in from 3 to 4 per cent, of men and less than 1 per cent. of women. It occurs in eyes whose power of vision is otherwise (as to form and distance) perfect. It is usually partial, being a failure to distinguish one or two of the funda mental colors—red, green, and blue. The eyes of persons having this defect of vision have been carefully examined after death without the dis covery of any peculiarity. Colo•-blindness there fore has its seat in the sensorium, not in the visual apparatus. The Hering theory is that the retina contains three pairs of visual materials— w hite-black, red-green. and blue-yellow. Color blindness is accounted for by the supposed ab ence of one or two of these substances. Accord ing to the Young-Helmholtz theory, there are three primary color-perceptions—for red, green, and violet. In the absence of one of these, a color appears composed of the others. The most com mon forms of color-blindness are red blindness, green blindness, and red-green blindness. To detect the defect, the method of having the patient name colors is not satisfactory, because colors may be differentiated by apparent differ ences of brightness. Professor Holmgren of Upsala, Sweden, devised a series of test wools which furnish the best means of recognizing de fects. These are skeins of wool of certain colors (`test colors'), various tints and shades of the same colors, and so-called 'confusion colors' When the patient attempts to match the colors with the other skeins, the confusion colors are often added also, and it may be noticed that there is some hesitancy in making the selec tions.
The question has received serious legislative attention, and in most of the States of the Union stringent laws have been passed regarding the examination of the vision of all who depend on colors for their guidance. It is a crime in color blind persons to pursue any calling when their defect, known to them, is liable to injure others, and it might be added that it is foolish for a color-blind painter, tailor, or milliner to attempt to compete with those who have perfect vision.
The safety of the traveling public depends in large measure upon the accuracy with which green or red signals are observed by employees of railways and ships. Yet in hut few countries is the matter of examination of color-sense de manded by statute. In Sweden. since 1877, only men with normal color-vision have been employed in the railway service. In Holland the Govern ment controls the matter efficiently. In Italy, while there is no special law, the employees are tested. In France, in the absence of law about examination, Holmgren's test is usually em ployed. In Germany examination of color-sense is prescribed by law. In Austria, on the State railroads, examination is made. In England there is no Government regulation, and the tests employed by certain companies are inefficient. Only three of the States of the United States— Ohio, Massachusetts, and Alabama—have enact ed laws to control the matter. In some other States the railroad commissioners formulate regulations requiring the examination of em ployees, as in New York State. That the travel ing public does not demand stringent laws com pelling examination of color-sense in all coun tries is astonishing, and can be explained only by the fact that the prevalence of color-blind ness and the possibility of its detection have been understood by the people only since 1875.
In 1892 there were employed by the different railroad companies of the United States a total of 821,415 men, while the number of miles of railroad reached 171,563.25. Accepting 3.69 as the percentage of color-blindness among men, the startling number of 30.310 color-blind men were in positions of some sort among the em ployees mentioned. In some cases the propor tion of employees with color-blindness is very small, owing to the adoption by companies of adequate tests and the proper selection of men. In 1880 the officers of the Pennsylvania Rail road invited Dr. William Thomson, professor of ophthalmology in Jefferson Medical College, Philadelphia, to advise and assist them in the examination of the sight of their 40,000 em ployees, scattered over more than 5000 miles of track, of whom 12,000 were actually dependent upon colored signals for their guidance. A sys tem was devised by which me-n attached to each division of the road were examined, each in his OW/1 locality, by intelligent laymen, with Holm gren's skeins, arranged on what is called 'Thom son's stick.' and the cooperation of employees was secured. Reports were sent to the surgical expert, whose decision, after a final examination, was decisive. About 4.2 per cent. were found de fective in colo•-sense. At a later examination, of 25•158 men, only 481, or about 2 per cent., were found color-blind, many men having left the company's employ before being detected, thus being able to secure positions on roads not re quiring colo•-tests. In 1887 the same system of examination was adopted by the Philadelphia and Reading Railroad, with about 15,000 em ployees on 2200 miles of track, after a serious conflict between officials and employees, which was settled by Dr. Thomson. Of these men, 3.5 per cent. were declared color-blind. The Mid land Railway and the London and Southwestern in England adopted the same method. In 1896, from responses to a circular addressed to one hundred of the most important railroad cor porations of the country, controlling in all 1°_9, 9i0 miles, inquiring if examinations were made as to color-blindness of employees, it was learned that 35 roads. controlling 54,465 miles, were using Thomson's or other methods: 31 roads, con trolling 29,423 miles. were making no test. No reply was received from 34 roads, controlling 46, 077 miles. The New York Central Railroad began examining its employees about 1890, requiring them to report for tests of sight at either New York or Buffalo at stated times. This method being found disadvantageous, two physicians were em ployed about 1899, whose duty it is to travel over the system, examining and reexamining the men from time to time in their own localities.
This road, in .Tune, 1901, employed about 40,000 men, and controlled 7137.31 miles of track, in clusive of the leased lines, such as the New York and Putnam; West Shore: Fall Brook; Boston and Albany, etc.; but exclusive of the great lines, such as the Lake Shore and Michigan Southern, included in the 'Vanderbilt system.' Consult: Holmgren, De la c('eitr' des couleurs (Sweden, 1877) ; Joy Jeffries. Color-Blindness (Boston, 1S79) ; Swanzy, Diseases of the Eye, appendix i. (Philadelphia, 1896) ; and Thomson and Weiland, "Detection of Color-Blindness," in Morris and Oliver's System. of Diseases of the Eye, vol. ii. (Philadelphia, 1897).