COLOUR VISION AND COLOUR-VISION THEORIES 477 



colour-blind in accordance with the number of primary colours 

 which they see in the spectrum. If the normal-sighted be 

 designated hexachromic, those who see five colours may be 

 called pentachromic ; those who see four, tetrachromic ; those 

 who see three, trichromic ; those who see two, dichromic ; and 

 the totally colour-blind. There are many degrees included in 

 the dichromic class. There may or may not be a neutral band, 

 and this is widest in those cases approaching most nearly to 

 total colour-blindness. 



The fact of this gradation of colour perception has now been 

 definitely recognised. The old classification of red-blindness, 

 green-blindness, etc., has no meaning — experts examining the 

 same case may diagnose it differently. The late Dr. Pole, who 

 was colour-blind (a simple dichromic), was examined by Maxwell, 

 who stated that he was completely red-blind, and subsequently 

 examined by Holmgren, who pronounced him to be completely 

 green-blind ! (6). Shortening of the red end or violet end of 

 the spectrum is a distinct defect from defective colour discrimina- 

 tion (7). A normal-sighted person when examined with my 

 spectrometer with a bright spectrum marks out about eighteen 

 monochromatic divisions, those with defective colour discrimi- 

 nation mark out a fewer number in proportion to their defect 

 (8, 9, 10, 11, 12). The dichromic (13, 14) see two colours in the 

 spectrum, red and violet, with a neutral division of varying size 

 between the two colours. The trichromic (15, 16, 17) see three 

 colours in the bright spectrum, red, green, and violet. The 

 orange and yellow regions are seen as red-green and the blue 

 region as green-violet. Here we have persons who have three 

 sensations who are to a certain extent colour-blind. Sir William 

 Ramsay and Sir J. J. Thomson belong to this class. A trichromic 

 in conditions of difficulty becomes dichromic. As the colours 

 are farther apart in the colour-blind, simultaneous contrast is 

 increased. 



IV. The Evolution of the Colour Sense (18) 



It is obvious that the sense of light must have been developed 

 first, and then the sense of colour. Let us consider the evolu- 

 tion of the colour sense in accordance with the difference of 

 wave-length. First there will be a colourless spectrum, then 

 a spectrum with a tinge of red at one end and a tinge of violet 

 at the other, then the red and violet will encroach on the white 



