PROPERTIES OF THE RETINA. 335 



scribe some of their after-images as red, the after-image of' indigo- 

 blue, for example, but that they describe none as green. The 

 after-image of purple, for instance, which to the normal eye is 

 bright green, is described by them as gray blue or pale blue. 

 From the descriptions given it is probable that the color vision 

 of the so-called red-blind is not by any means the same in all cases, 

 but exhibits many individual differences. The green-blind are 

 also, according to recent descriptions, red-green blind; they also 

 confuse reds and greens and in the spectrum are conscious of only 

 two color qualities, namely, yellow and blue. They differ from 

 the red-blind in that the red end of the spectrum is not shortened, 

 and the maximum luminosity, as with the normal eye, is placed 

 in the yellow. In the matching and combination of colors they 

 show distinct differences from the red-blind, so that though re- 

 sembling the latter in general features, they differ obviously in 

 some details. Violet blindness (tritanopia) seems to be so rare 

 as a congenital and permanent condition that no very exact study 

 of it has been made. In cases of acquired tritanopia resulting 

 from pathological changes it is reported that the violet end of 

 the spectrum is colorless (neutral) and that a neutral band appears 

 also in the yellow-green region of the spectrum.* By the ingestion 

 of santonin it is said that a condition of this kind may be produced 

 temporarily. The violet end of the spectrum is shortened and 

 white objects take on a yellowish hue. The conditions produced 

 by santonin are evidently more complex than can be explained 

 by simply assuming that the violet color sense is lost. Recent 

 observers f state that the drug produces a condition of yellow 

 vision, outside the fovea, in the daylight, and a condition of 

 violet vision with yellow-blindness, but no red- or green-blindness, 

 in dim lights. 



Tests for Color Blindness. Although the vision of the red and 

 the green blind is deficient as regards green and red colors, it will 

 be found in many cases that they recognize these colors and name 

 them correctly, having adopted the usual nomenclature and adapted 

 it to their own standards. In order to detect the deficiency they 

 must be examined by some test which will compel them to match 

 certain colors. Under these circumstances it will be found that 

 along with correct matches they will make others which to the nor- 

 mal eye are entirely erroneous. A great number of methods have 

 been proposed and used to detect color blindness. The simplest 

 perhaps is that of Holmgren. :j: A number of skeins of wool are used 



* Collins and Nagel, " Zeitschrift f. Psychol. u. Physiol. d. Sinnesorgane," 

 1906, xli., 74. 



fSiven and Wendt, " Skandinavisches Archiv f. Physiologie," 14, 196, 

 1903, and 1905, 17, 306. 



J For details see the works of Holmgren and of Jeffries, already quoted. 



