648 PHYSIOLOGY 



any interference with the appreciation of luminosity. According to 

 Edridge-Green a normal individual will name six distinct colours in the 

 spectrum red, orange, yellow, green, blue, violet. Such an individual, 

 when made to map out the spectrum in the manner indicated on 

 p. 642, will distinguish about eighteen monochromatic patches. 

 A few individuals will place another colour, which has been called 

 indigo, between the blue and the violet, and will mark out from 

 twenty-two to twenty-nine monochromatic patches. 



' Colour-blindness ' may be brought about by one of two con- 

 ditions : (a) a shortening of the red or violet end of the spectrum ; 

 (b) absence of power to discriminate between the colours in the 

 spectrum. The former condition may be present with complete 

 power of discrimination between the different parts of the spectrum 

 which are visible. Thus in normal individuals the limit of the visible 

 red spectrum is between X 760 and X 780. In a certain number 

 of cases it is found that the spectrum is not visible beyond x 700 

 with bright light, or beyond X 620 with dim light. Such cases may 

 be said with truth to suffer from red blindness, and they will be 

 unable to see a red lantern or appreciate its colour unless the red 

 light is mixed with a considerable amount of orange. They may 

 be detected by testing their power of mixing colours. A rose colour 

 consists of a mixture of a violet and red light. In an individual 

 with a shortened red end of the spectrum only the violet element 

 of the rose would be visible, so that he would be inclined to class 

 it with the blues rather than with the reds. Cases also occur in 

 which there is a shortening of the violet end of the spectrum, but 

 they have little or no practical importance. 



Of the second class of cases, distinguished by deficiency of power 

 of discriminating colours, all grades are known. A very large pro- 

 portion of individuals, as much as 20 per cent., present a power of 

 distinguishing colours which is below normal, and Edridge-Green 

 distinguishes these various classes (calling the normal person hexa- 

 chromic) as pentachromic, tetrachromic, trichromic, and dichromic. 

 If we regard a spectrum in very dim light it appears grey. With a 

 slight increase in luminosity we can make out two colours, red at 

 one end and violet at the other. On further increasing the luminosity 

 the spectrum appears trichromic, being composed of the colours 

 red, green, and violet. In colour-blind individuals this limitation 

 of the colours distinguished applies to all strengths of luminosity. 

 Thus the dichromic sees only red and violet, the trichromic sees red, 

 green, and violet. It is the dichromic cases to which the name of 

 ' colour-blind ' has been chiefly applied, the trichromic cases having 

 received the name of * anomalous trichromats ' (on the Young- 

 Helmholtz theory). The ordinary ' red- blind ' person is generally 



