580 PHYSIOLOGY 



Most of the data with regard to colour-blindness have been worked out with 

 reference to the Young-Helmholtz theory, and have therefore been inter- 

 preted in accordance with this hypothesis. 



It is very difficult, however, to harmonise the facts of colour-blindness 

 either with this or with Hering's hypothesis. It is better therefore to 

 abandon hypotheses altogether and to adopt a purely empirical classification 

 of colour- vision, as has been done by Edridge- Green. This observer points 

 out truly that very marked colour- blindness may be present without any 

 interference with the appreciation of the luminosity of any part of the 

 spectrum. A person may be able to see the spectrum up to its extreme red 

 end, and yet distinguish in the spectrum -only two colours, which we may 

 call red and violet. We may, in fact, regard discrimination of colour differ- 

 ence as superadded to and evolved later than the appreciation of light. 

 Discrimination may show various degrees of deficiency without any inter- 

 ference 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. 575, 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 conditions : 

 (a) a shortening of the red or violet end of the spectrum ; (6) 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 proportion 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 hexachromic) as pentachromic, tetra- 

 chromic, 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 



