PROPERTIES OF THE RETINA. 327 



to be so rare as a congenital and permanent condition that no 

 exact study of it has been made. 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* 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 nor 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. f 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 blind 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 addition 

 to other purples, only blues or violets ; the green blind will select as 

 "confusion colors" only greens and grays. 



With test III, red, the red blind will select as confusion colors 

 greens, grays, or browns less luminous than the test color, while the 

 green blind will select greens, grays, or browns of a greater brightness 

 than the test. 



Monochromatic Vision. A number of cases of total color 

 blindness have been carefully examined. | It would seem that in 

 such individuals there is an entire loss of color sense, they possess 



*Siven and Wendt, "Skandinavisches Archiv f. Physiologie," 14, 196, 1903. 

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

 t Grunert, "Archiv fur Ophthalmologic," 56, 132, 1903. 



