ANOMALOUS TRICHROMATIC VISION 185 



They can only distinguish colours at the optimum intensity, and their 

 optimum is narrower than the normal and of greater intensity. 



(3) They are more dependent on luminosity differences, which 

 they are expert in translating into hue, thus showing a specially well 

 developed capacity for distinguishing differences in brightness of 

 coloured objects. 



(4) They need a considerably larger area of stimulation or visual 

 angle for the accurate perception of hue, a fact confirmed by Nagel 1 . 

 Diminution in field causes a marked diminution in their capacity for 

 distinguishing differences in hue. 



(5) They need longer time for the correct perception of colours, 

 a fact of great practical importance in certain occupations. 



(6) Fatigue for colour, and with it defective discrimination, occurs 

 more rapidly than in the normal. 



(7) They have a very marked increase in simultaneous contrast 

 effects as compared with the normal. Contrast depends to an excep- 

 tional extent upon the luminosities of the contrasting fields, and the 

 anomalous trichromatic colour thresholds are largely dependent upon 

 it. Nagel 2 also drew attention to the acuteness for contrast of the 

 partial deuteranopes. He found that a red and a yellow light, side 

 by side, of equal luminosity to the normal appeared different to them, 

 and they often called the yellow greenish or green, whereas in the 

 absence of the red light the yellow was correctly named. 



Guttmann found his own periphery values agreed with the normal. 

 Like Bonders, he calls anomalous trichromatic vision a " weakness ' 

 of colour vision, a term to which Nagel objects. Guttmann found that 

 the ' extreme anomalous dichromats ' nearly resemble complete 

 dichromats, and show diminished differential sensibility for hue, 

 rapid fatigue and increased perception of simultaneous contrast 

 effects. He also found the after-images of anomalous dichromats 

 agreed with those of the dichromats, but only with homogeneous lights. 

 With pigments the after-images appeared to resemble those of the 

 normal, and no accurate deductions could be drawn without comparison 

 lights, since the naming of colours is very liable to lead to mistakes. 



Koffka 3 , himself a partial protanope, confirmed Guttmann's general 

 conclusions for this group. They show the same greater sensitiveness 



1 Ztsch. f. Psychol. u. Physiol. d. Sinnesorg. xxxix. 1905. 



2 Klin. Monatsbl. f. Augenhlk. XLII. 356, 1904. 



3 Ztsch. f. Sinnesphysiol. XLHI. 123, 1908. 



