BOTH because of its scientific interest and its practical bearings, this curious defect of the eyes occupies a large place in ophthalmological literature. Although it must have existed for centuries, the first case on record was discovered in the practice of a Dr. Tuberville in 1684. Nearly a hundred years later an English chemist by the name of Dalton, who was color-blind himself, and could see no difference between the color of a laurel leaf and that of a stick of red sealing wax, published the first accurate description of the condition. For this reason continental scientists gave it the name of Daltonism.
Although it would seem to be obvious that a condition of color blindness must be very dangerous, when it exists in persons responsible for the lives of others on railroads and steamers, it was not until the latter part of the nineteenth century, after much agitation by the medical profession, that its practical bearings were recognized. Owing to the remarkable tendency of color blindness to conceal itself both from the subject and his associates, managers of transportation companies distrusted the scientists and could not be brought to believe that such a defect could exist in persons who had been in their employ for years without its being discovered.
Sweden was the first country to pass a law forbidding the employment of any man upon a railroad until his color vision had been tested. This action was taken as the result of the investigations of Prof. Holmgren of the University of Upsala, who discovered thirteen color-blind men among 266 railroad employees, and his book on "Color Blindness in Its Relations to Railroads and the Marine," had the effect of concentrating the attention of the world upon the subject. Today most shipping and railway companies require employees whose duties include the recognition of variously colored lights and signals to submit to a special examination for color blindness.
There are various degrees of color blindness. The condition in which no color can be recognized, and the world looks like a steel engraving, is rare, and its existence is denied by some. In cases where this total lack of color perception has been recorded, there has also been a considerable reduction of visual acuity in other respects. Usually only one color is lost by the subject in this odd manner, but sometimes more than one. Thus the subject may be color-blind for red, for blue, for green, etc., as the case may be. The most common form is that in which red is deficient. Many theories have been advanced to account for color blindness, and it is generally supposed to be incurable, but the evidence at present available indicates that it is simply a functional trouble like errors of refraction. It has been relieved, even when of considerable degree, by the methods presented in this book. Practice should be taken to acquire this habit, even when there may be no other apparent trouble with or defect of vision.
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