COLOR BLINDNESS 97 



hues at all are seen, hence is the only type which should ever have been 

 called color blindness at all. Vision is restricted to white, grays, and 

 black, and the condition had best be called 'achromatic vision'. It seems 

 nearly always to be due to the congenital absence, or a gross defective- 

 ness, of the cones, for along with it there are usually to be seen : (a) low 

 visual acuity both scotopically and photopically; (b) a central scotoma or 

 blind spot where the bouquet of foveal cones should be ; (c) a nystagmus 

 or uncontrollable fluttering of the eyeballs owing to the lack of this cen- 

 tral fixating region; and (d) photophobia or light-shyness, owing perhaps 

 to an excess of rods, occupying the spaces where cones should be. 



In 'anomalous trichromatic vision', some one spectral region appears 

 less bright than it does to the normal person, and the individual requires 

 more of such light, mixed with some other color, to match an inter- 

 mediate color. An individual who, say, perceives green weakly must mix 

 more green with less red than the normal individual, in order to match 

 a standard yellow. This condition is not color blindness — it would much 

 better be called color weakness. 



These color-weak individuals have poor hue-discrimination and an in- 

 creased perception-time for colors. They fatigue rapidly for colors, which 

 seem to them to fade upon continued observation; and to identify some 

 colors they require them in larger areas, with greater intensity and satur- 

 ation, than the normal. Anomalous trichromates probably outnumber all 

 other kinds of so-called color-blinds, but since they less often get into 

 difficulty through unfortunate selections at the neckwear counter, they 

 usually live and die without ever knowing of their peculiarity. 



The conspicuous and familiar color-blind type is the dichromate or 

 Daltonist, whose confusion of red and green is proverbial — and also 

 hereditary, in a sex-linked fashion which keeps the defect a rare one in 

 females. One white man in twenty-five is a dichromate, but only one 

 white woman in twenty-five hundred. The dichromate is so called because 

 he requires only two primaries, instead of three, to mix and match any 

 and all hues and white. It so happens also that he can experience only 

 two hues instead of the large number* of the normal trichromate; but 

 the prefix (di = two) on his label does not refer, to this latter fact. The 

 dichromate is not color-blind — he is color-poor. 



^Usually taken as 160-180; but these are the discriminable hue-and-saturation complexes. 

 Similarly, a dichromate can distinguish a large number (about 60) of spectral regions, tut 

 chiefly through saturation-difrerences. 



