180 COLOUR VISION 



Areal Effects. Nagel 1 , who was a deuteranope, found that he was 

 only completely dichromatic for small fields. With larger fields, 

 involving the extra-foveal regions, he became partially trichromatic. 

 By fatiguing his eyes with red and orange lights he became completely 

 dichromatic. His perception of red was much better than that of 

 green, possibly due to red not causing strong contrast effect. Good 

 dark adaptation had the same effect as fatiguing with red. He found 

 a similar condition in 30 other dichromats. Stilling 2 regards Nagel's 

 observations as erroneous. 



The Colour Sensations of Protanopes and Deuteranopes. Abney has 

 given illustrations of the names applied by dichromats to the various 

 parts of the spectrum. Since they have only two true colour sensations 

 the distinctions are based chiefly upon variations in luminosity, and 

 are therefore often contradictory. Most observers think that the two 

 sensations experienced correspond most closely to normal yellow and 

 blue 3 . Uncomplicated cases of unilateral congenital colour blindness 

 would afford valuable evidence, but most such cases recorded are of 

 doubtful value. One case for example was regarded by Holmgren 4 

 as a typical protanope with shortening of the red end of the spectrum, 

 but v. Hippel 5 found that he could see the rubidium line y. Both agree 

 that his colour sensations were yellow (D line) and blue (indium or 

 caesium line). Holmgren recorded a second case and Becker 6 a third. 

 Hayes 7 has recorded a case of a woman with partial protanopia in one 

 eye and normal vision in the other. So far as I am aware no other 

 congenital cases have been recorded, and pathological cases are value- 

 less for the purpose under consideration. 



TRITANOPIA. Cases of tritanopia or so-called blue-blindness are rare 

 and mostly due to disease. Since the defect causes little risk of con- 

 fusion of red and green lights it is not of much practical importance. 

 It is, however, of considerable theoretical importance, and a bibliography 

 of the literature on the subject is appended. Goethe 8 referred to the 

 condition as "akyanoblepsia 9 ." Konig 10 investigated five pathological 



1 Arch.f. Anat. u. Physiol. 543, 1907 ; Ztsch. f. Sinncsphysiol. XLIV. 5, 1909. 



2 Ztsch. f. Sinnesphysiol. XLIV. 371, 1909. 



3 Pole, Phil. Trans. Roy. Soc. Land, CXLIX. 322, 1859. 

 1 Centralbl.f. d. mod. Wiss 898, 913, 1880. 



5 Arch. f. Ophth. xxv. 2, 205, 1879 ; xxvi. 2, 176, 1880 ; xxvii. 3, 47, 1881. 



6 Centralbl.f. Augenhlk. 353, 1888. 7 Amer. J. of Psychol. xxn. 369, 1912. 

 8 Zur Farbenlehre, 1798. <J Konig, p. 4. 



10 p. 396 ; Sitz, d. Akad. d. Wiss. Berlin, 718, 1897. 



