world as it is seen by some color-blind persons is to hold up before the eyes a glass vessel with flat, parallel sides, filled with a solution of sulphate of copper. We shall then be pretty much in the same condition as a red-blind person.
The inconveniences which color-blind people must frequently be exposed to are manifest. Numerous stories are told of the most ludicrous mistakes made, especially by red-blind persons: of a tailor, for instance, who mended a black coat with a piece of red cloth; of a hunter who bought red cloth to have made what he supposed would be a green hunting-jacket. The story of the tailor shows how this malady, or, rather, constitutional defect, may do injury to men in their professional capacity. But the consequences that may possibly arise from it are of a far more serious nature when the safety of a large number of human beings is dependent on the color-vision of a single individual. This is the case with railroad operatives, who must be able without fail to tell one signal from another; and, as of late years the conviction has gained ground that color-blindness is far more common than it was formerly supposed to be, the railroad companies are warned more emphatically from year to year by scientific men to see to the eyes of their employees. Some of the European Governments are beginning to turn their attention to this important matter (all the more important because railroad-signals are usually red and green, and red-blindness is the most common form of the failing), and the Swedish Government has lately directed the physicians attached to its state roads to examine all the operatives on these roads, with a view to the detection of the presence of color-blindness. The first fruit of this order is a report by Professor Holmgren, who recently examined the employees of the Upsala-Gefle road, showing that, out of two hundred and sixty-six individuals, eighteen were afflicted with the malady to a degree sufficiently high to incapacitate them entirely for service on the road. The prevalence of the disease varies in different countries, the highest percentage being found in England, where, according to a statement made by Professor von Bezold, in his "Theory of Color," republished in this country in an English translation, one out of every eighteen persons is said to be afflicted with it. Among men, as before remarked, the disease is more common than among women.
The cause of total or partial color-blindness may easily be understood if we accept the hypothesis first brought forward by the English physicist Young, and now subscribed to by the leading scientific observers of all countries. According to Young, all the phenomena of color-vision are due to the (hypothetical) presence of three different kinds of nerve-fibers in the retina—that is to say, in that part of the eye on which the reflected images of the objects of the outer world are projected as upon a screen, and through the agency of which the sensations produced by the impressions so received are transmitted to