COLOR-BLINDNESS. 93 



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 sul- 

 phate 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 ludi- 

 crous 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 com- 

 panies 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 at- 

 tached 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 re- 

 cently examined the employees of the Upsala-Gefle road, showing 

 that, out of two hundred and sixty-six individuals, eighteen were af- 

 flicted 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 under- 

 stood if we accept the hypothesis first brought forward by the English 

 physicist Young, and now subscribed to by the leading scientific ob- 

 servers 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 



