888 THE SENSE OF SIGHT 



Repeated attempts have been made to harmonize the facts of 

 color-blindness with the hypotheses outlined in the preceding para- 

 graphs. But, inasmuch as this is almost impossible, it seems permis- 

 sible to adopt a perfectly empirical classification and to state that 

 one group of color-blind is characterized by an absence of the power 

 to perceive colors, while the other experiences merely a difficulty 

 in distinguishing colors. The first possess achromatopsia and the 

 latter dyschromatopsia, but even the former condition is rarely 

 complete, excepting in cases of definite pathological changes in the 

 optic nerve. 1 Consequently, even the achromatopic person is 

 capable of recognizing one or more fundamental colors. Attention, 

 however, should be called to the fact that the absence of a particular 

 color from the spectrum does not imply that perception of its lumi- 

 nosity has been interfered with. A person may well be able to recognize 

 the spectrum throughout its entire length and yet be unable to dis- 

 tinguish more than two colors, say, red and violet. 



Upon the basis of the Helmholtz theory, we may divide color- 

 blindness into blue, green and red-blindness. The most common of 

 these is the red-blindness, in which the red end of the spectrum is consid- 

 erably shortened. A person so afflicted confounds light red colors with 

 dark green and cannot see a dark-red square upon a black background. 

 In fact, the most typical cases show a green-blindness and are capable 

 of distinguishing only the yellows and blues. Consequently, the red, 

 orange yellow, and green appear to them merely as different shades of 

 yellow, while the green is perceived as gray and the indigo, violet, and 

 purple seem blue. A person afflicted with green-blindness confounds 

 light-green with dark red and does not recognize a dark green square 

 upon a black background, but can perceive a red square upon black. 

 In many cases, however, they also show a certain interference with 

 the red end of the spectrum and hence, are really green-red blind, 

 although they differ from the red-green blind in certain minor par- 

 ticulars. A person afflicted with blue-blindness, sees only red and 

 green and confounds blue with green, purple with red, orange with 

 yellow, and violet with yellow-green. This condition indicates a 

 shortening of the violet end of the spectrum. Blue-blindness of a 

 temporary kind may be produced by the ingestion of santonin. 



Color-vision is commonly tested by means of a number of skeins 

 of wool, exhibiting three colors, namely, a pale pure green, a medium 

 purple, and a vivid red (Holmgren). The person suspected to be 

 color-blind is asked to match the pale green skein. If red or green 

 blind, he will recognize this skein as gray with some admixture of 

 yellow or blue and will match it not only with the green skeins but 

 also with those possessing a grayish yellow or blue color. If he is 

 then asked to match the medium purple skein, he will select either 



1 Siven and Wendt, Skand. Archiv fiir Physiol., xiv, 1903, 196, and Grunert, 

 Archiv fur Ophtbalm., liii, 1903, 132. 



