PROPERTIES OF THE RETINA. 353 



spectrum. 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 

 for the purpose of explanation of the principles involved is that of 

 Holmgren.! A number of skeins of wool are used and three test 

 colors are chosen, namely, (I) a pale pure green skein, which 

 must not incline toward yellow green; (II) a medium purple 

 (magenta) skein; and (III) a vivid red skein. The person under 

 investigation is given skein I and is asked to select from the pile 

 of assorted colored skeins those that have a similar color value. 

 He is not to make an exact match, but to select those that appear 

 to have the same color. Those who are red or green bhnd will see 

 the test skein as a gray with some yellow or blue shade and will 

 select, therefore, not only the green skeins, but the grays or grayish 

 yellow and blue skeins. To ascertain whether the individual is red 

 or green blind tests II and III may then be employed. 



With test II, medium purple, the red blind will select, in addi- 

 tion to other purples, only blues or violets; the green blind wiU 

 select as "confusion colors" only greens and grays. 



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

 1906, xU., 74. 



t Siven and Wendt, "Skandinavisches Archiv f. Physiologie," 14, 196, 

 1903, and 1905, 17, 306. 



t For details see the works of Holmgren and of Jefferies, already quoted. 

 23 



