170 COLOUR VISION 



the facts, for owing to the very low stimulus value of red for the 

 protanope he requires much more red in his red-blue match of a blue- 

 green homogeneous hue than does the deuteranope for the same amount 

 of blue. Hence a protanope confuses a slightly bluish red (in the 

 physical sense) with a dark green, e.g., scarlet with olive green, whereas 

 a deuteranope matches a much bluer red with a green which is of about 

 equal brightness to the normal. " The red which appears to match 

 a given green differs markedly both in colour-tone and intensity in 

 protanopes and deuteranopes " (v. Kries). 



We have considered the conspicuous difference in dichromatic 

 vision from trichromatic vision. Its conspicuous similarity in an 

 important respect was pointed out by Seebeck in 1837, viz., that all 

 colour mixtures which appear equal to the normal eye, also appear 

 equal to the colour-blind. This statement is too general, and the 

 following is more accurate, all colour equations valid for normal vision 

 are also valid for dichromatic vision, or colour matches which are valid 

 for the trichromatic are also valid for the dichromatic eye. If this 

 statement can be fully substantiated it follow^s that the dichromat 

 possesses no variable which the trichromat lacks, but lacks a variable 

 which the trichromat possesses. In other words, dichromatic vision 

 IS, Q> reduction form (v. Kries) of normal vision, and not a fundamentally 

 different kind of vision. 



V. Kries^ has made a very extensive series of observations of the 

 equivalence of trichromatic and dichromatic matches, and they 

 strongly support the validity of the law. Especially is this the case 

 for the red end of the spectrum, where macular pigmentation is least 

 disturbing and where the typical differences in the two groups of 

 dichromats are most obvious. Monochromatic yellow is matched by 

 a red (670-8 yLfx) and yellow-green (550 /z/x) mixture. As the ratio of 

 red to green is altered a suitable intensity of yellow can always be found 

 which will match the mixture. The match for the protanope, however, 

 is not valid usually for the deuteranope. For the former a very red 

 mixture matches a dark yellow and a very green mixture a bright 

 yellow. The deuteranope finds the mixture too bright in the first 

 case and too dark in the second. When the mixture is such that it 

 exactly matches the yellow for both dichromats it also matches for 

 trichromats, and that with extraordinary exactitude. In fact when 

 we succeed in finding matches with which both protanopes and 



1 Ztsch.f. Psychol, u. Physiol, d. Sinnesorg. xiii. 274, 1897. 



