COLOR IN ANIMALS. See COLORATION PaorEctivz.
Achromatropsia, popularly known as Daltonirm, a singular affection, producing an inability to distin guish one color from another, and in certain rare cases to discern color at all, the eye per ceiving only light and shade, or black and white. Although recognized for centuries, its modern study dates from the time when the famous English chemist, Dr. Dalton, brought it into notice by publishing in 1794 an account of his own case as marked by this peculiarity. More recently, Dr. George Wilson of Edin burgh also examined minutely into this phe nomenon, and collected many strilcing instances. From the result of these investigations it would appear that color-blindness is much more com mon among men than among women, and that of the former 1 in 20 is unable to discern the nicer shades of color, and 1 in 50 to dis tinguish certain primary colors from one an other. The colors most liable to be confounded are red and brown with green, purple and green with blue, red with black, light hues of all sorts with white, and dark shades with black. Color-blindness may be either con genital or acquired. The latter is believed to be an affection of the optic nerve and retina, may result from disease or accident and may be caused by the excessive use of tobacco, alcohol or other drugs. It sometimes happens that persons having contracted color-blindness may be able to distinguish colors accurately when they are near at hand, while they are not able to do so when they arc at a distance. Con genital color-blindness is frequently hereditary and generally exists in both eyes. Sometimes it ousts only in a slight degree, at others it is found in a marked degree, while in others it is complete; generally, however, it is only partial; that is, the person affected fails to distinguish properly one or two of the funda mental colors, red, green and blue. Based on these known facts and on extended investiga tions and experiments, several theories have been advanced to account for and explain color blindness. One of these, the Hering theory, maintains that the retina contains three pairs of visual materials, namely, white-blacic, red green and blue-yellow. According to this
theory color-blindness results from one or more of these materials. The Young-Helmholtz theory, on the other hand, maintains that there exist in every normal person three primary perceptions, red, green and violet, in the ab sence of any one of which the sense of color is defective; and the resulting missing color seems to be composed from the others. This theory is based partially on the fact that the most common forms of color-blindness are green, red and red-green blindness. Various tests have been arranged for the detection of color-blind ness, the best of which are those devised by Professor Holmgren of Upsala, Sweden. They consist of skeins of wool of certain 'test colors,* including various tints and shades of the same colors. These the person under ex amination is required to match.
Very stringent laws have been passed in most modern civilized countries to prevent color-blind people from occupying positions where their defective vision might endanger the lives of others. Sweden was one of the first countries to make laws for the protection of the traveling public against the danger aris ing from the employment of people affected with color-blindness in the railway service. As early as 1877 a law was passed in Sweden re quiring that only people of normal vision for color should be employed in the railway service of the country. In the United States, Great Britain (and all her colonies), Germany, France, Italy and Austria provisions of some kind have been made to protect the traveling public from the danger of color-blind em ployees in the service of the railways. Consult Holmgren,