Page:Popular Science Monthly Volume 21.djvu/96

88 green, a certain proportion of the other colors (red and violet)—as certain shades of what we call green do—will cause, when presented to such a green-blind individual, the sensation of white of diminished intensity. When the solar spectrum is placed before him, there should be a gray or neutral band at the line which divides the two colors which are unmistakably distinguishable; and, in the green-blind, it is nearer the red end of the spectrum than in the red-blind.

When the violet is the lacking fiber, we have phenomena analogous to those where the red fiber is missing, though, of course, there are differences in details.

In accordance with this theory, therefore, there can be no color-blindness, in the strict acceptation of the term, except when all the color-fibers are lacking; because all colors produce an impression of some kind, though it may not be the one experienced by those of normal color-perception. There is, however, a marked confusion of the various colors, and by the special character of this confusion one kind of color-blindness is differentiated from another.

In making an examination for the diagnosis of color-blindness, nomenclature, or the naming of the colors which are presented to the person to be examined, is entirely discarded. It has been found that an individual maybe able to name the several colors correctly, and yet make mistakes when called upon to match them; and, on the other hand, he may not be able to name a single color correctly, and yet make no serious mistakes in "matching." The method of comparison is therefore the only one which should be adopted in making examinations for color-blindness.

The method of Professor Holmgren, which is the simplest and, on the whole, the most convenient, consists in placing on a table before the examinee a large assortment of skeins of colored worsteds. A "sample" skein of light-green is laid to one side, and the individual is told to select from the pile all the skeins which are of the same color—lighter or darker. If he places by the sample a shade of any other color but green, he is color-blind. This examination, however, does not fix the particular color to which he is blind, and, in order to find the color which is lacking in his chromatic scale, a purple or rose-colored skein is laid aside as a sample and the confusions he makes here are supposed to fix the diagnosis. If he matches the purple with blue and violet or one of them, he is red-blind. If, however, he selects the greens and grays, he is green-blind. Violet-blindness (which is very rare) is recognized by a confusion of red, purple, and orange in the test with the purple skein.

Another plan for employing the comparative method is to have two solar spectra, one above the other, the upper of which is movable. A colored band is isolated in the fixed spectrum, and the upper spectrum is moved until what is supposed to be the same color is immediately above it. Or, the isolated band may be matched with a skein of