color-blindness

Century Dictionary and Cyclopedia

1. n. Incapacity for perceiving colors, independent of the capacity for distinguishing light and shade, and form. It is not a mere incapacity for distinguishing colors (for this might be due to want of training), but an absence or great weakness of the sensations upon which the power of distinguishing colors must be founded. Colorblindness may be total, that is, the absence of all perception of colors as such, independently of light and shade, all colors appearing simply as shades; or partial, the entire or partial inability to distinguish particular colors independently of difference of light and shade. The most common form of the latter defect is the inability to perceive red as a distinct color, red objects being confounded with gray or green, and next in frequency is the inability to perceive green. The color which to a normal eye is complementary to the defective color appears as gray; and a mixture of white and black (gray) of the proper luminosity certainly cannot be distinguished by the color blind from the defective color (red or green). The results of statistical inquiries as to the prevalence of color-blindness show its existence in from 2 to 6 per cent. of males, while among women the number of cases seems to be considerably under 1 per cent. Also called daltonism and achromatopsia.
2. n. Since it is estimated that one man in twenty-five is color-blind, the importance of recognizing this defect in railroad or maritime employees, whose occupation requires the ability to distinguish quickly between signals of different colors, is evident. Many European governments and a few of the United States now have laws regulating the examination of applicants for such positions. The large railroad companies on their own account test the vision of new men and require all employees to submit to reëxamination at stated intervals. Numerous tests have been devised for this purpose, but practically only two are used in official examinations. One of these is nearly always founded on the wool test of Holmgren. In this a hundred or more skeins of wool, all different in color or shade, are placed in a pile, and the applicant is required to select first all the skeins corresponding in color to a light green skein. The test is then repeated with a rose-pink skein, and in some cases with a bright red one. The color-blind person hesitates in making his selections and matches the colors incorrectly. The other test consists in the use of a lantern so arranged as to show the light through one or more disks of glass, the color, size, and brightness of which can be varied so as to simulate lantern signals under different conditions of distance, fog, smoke, etc. The wool test is sometimes modified by having the skeins suspended side by side from a stick, so that the selections may be made more quickly. Instead of skeins of wool, colored blocks, small glass tubes filled with colored powders, or slips of paper of different colors, are sometimes used, or colored letters are printed on a colored background in such a way that some of them cannot be distinguished from the background by color-blind persons. In official examinations the acuteness of vision for form and the hearing are usually tested, as well as the function of color-perception. It should be added that the wool and lantern tests are by no means adequate tests of color-blindness at large. Many cases of red-green blindness are missed by the wool test; and the lantern test merely shows whether or not the employee can distinguish the particular colors used under the particular conditions of observation, not whether or not he is partially color-blind. To detect the less obvious cases of partial color-blindness, recourse must be had to some instrument of precision, such as Hering's colorblindness tester, in which the color-tone and brightness of complementary color-pairs can be accurately and measurably adjusted.

Wiktionary

1. n. American Alternative form of colour-blindness.