88 THE POPULAR SCIENCE MONTHLY. 



sides 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 nor- 

 mal 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 examina- 

 tions 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 spec- 

 trum is moved until what is supposed to be the same color is immedi- 

 ately above it. Or, the isolated band may be matched with a skein of 



