COLOR BLINDNESS 99 



himself as 'violet', could yield only white since yellow and violet are 

 complementary. For such an individual, proper amounts of any two 

 wavelengths which were not on the same side of his neutral point could 

 be mixed as complementaries to make white. 



Now, the above is actually a fair description of one kind of dichrom- 

 atic vision, called 'protanopia' in the older terminology since it was sup- 

 posed to result from the lack of the first (protos = first) of the three 

 component processes of trichromatic vision. Another, much more com- 

 mon, type is 'deuteranopia' (from deuteros = second) . This form we can 

 represent by shifting the green curve in Figure 32 to lie on top of the 

 red one. The deuteranope experiences no shortening of the spectrum at 

 the red end, and his neutral point is nearer the red end than that of the 

 protanope (though neither of the actual neutral points is quite where it 

 ought to be as theoretically called for by the diagram.) Otherwise, his 

 experiences are about the same : two hues only, with one at either side of 

 the neutral point; the same white region at the neutral point; and the 

 same white or gray sensations from stimuli which appear to the normal 

 as purple. 



A condition much like dichromasy occurs, as a rarity, in one eye only. 

 The individual is then able to tell us what he sees with that eye in terms 

 of the trichromatic visual performance of his normal eye. Usually, he 

 reports that the spectrum contains only yellow and blue, not violet as 

 described above; but such pathological cases could not be expected to 

 duplicate perfectly the situation in true Daltonism. 



Theoretically, two other kinds of dichromasy are possible, but only 

 one of them has been found (or else the two have been confused) : 

 'tritanopia' is so extremely rare that it has not had proper study. We 

 could represent its two possible versions by aligning the green curve of 

 Figure 32 with the violet, or the violet curve with the green one. The tri- 

 tanope's neutral point, depending, would then coincide with either the 

 protanopic or deuteranopic one. In the latter case, the spectrum would 

 be shortened at the violet end. In either case, the only possible hue- 

 experiences, it would seem, would be red and blue. The shortened spec- 

 trum of at least some tritanopes seems to have been noticed by the older 

 investigators and recognized in the common name of the condition, 

 *blue-blindness'. Tritanopia can be simulated in some individuals by ex- 

 cessive absorption of short-wave light in an abnormally rich macular 

 pigmentation (see Chapter 8, section D), or in an extremely yellow, 

 pre-cataractous lens; and also by the yellowing of vision in jaundice 



