294 COLOUR VISION 



The observations on fatigue are at variance with those of Burch, 

 Abney and others. 



The " cerebral " part of Edridge-Green's theory is founded on the 

 principle of psychophysical units and series which dates from the time 

 of Fechner. The reasoning upon which it is based will be found in the 

 early chapters of his book on Colour-blindness and Colour-perception. 

 According to the theory the colour-sense has been evolved in stages. 

 First there was only a sensation of light. When colours were first 

 recognised only red and violet — the two ends of the spectrum — were 

 seen, and a spectrum seen by such an eye was red at one end, violet at 

 the other, the colours merging in the centre. Such eyes are called 

 " dichromic." In the next stage of evolution a third colour appeared 

 between the other two, viz., green : these persons were " trichromic." 

 In the next stage a fourth colour, yellow, appeared between the red and 

 green (" tetrachromic "). In the next stage a fifth colour, blue, was 

 seen between the green and violet (" pentachromic "). In the next 

 stage a sixth colour, orange, was seen between the red and yellow. 

 Persons having this type of vision are " hexachromic " and the majority 

 of people to-day belong to this class. The highest development which 

 has yet been reached is that of the " heptachromic," who in addition to 

 the other colours distinguish a seventh colour between the blue and 

 violet, viz. indigo. 



Colour blindness is atavistic, and all stages are represented amongst 

 the colour-blind. The " dichromics " correspond most clearly to what 

 we have called dichromats for the sake of distinction from Edridge- 

 Green's nomenclature. The anomalous trichromats, on this theory, 

 include tri-, tetra-, and pentachromics. 



In addition to these classes of the colour-blind there are others 

 distinguished by shortening either of the red or the violet end of the 

 spectrum. Such cases show a shift in the position of their psycho- 

 physical units towards the unshortened end. 



Light perception and colour perception are therefore according to 

 this theory quite distinct^, and cases of colour blindness can be divided 

 into two classes, according as the defect is one of light perception, or 

 one of colour perception or differentiation without any defect in light 

 perception : both defects may be present in the same individual. 



Edridge-Green finds that the normal hexachromic, who describes 

 six colours in the spectrum — red, orange, yellow, green, blue, violet, — • 

 will map out about 18 monochromatic patches. The supernormal 



1 Proc. Roy. Soc. Lond. B, lxxxii. 458, 1910. 



