OTHER THEORIES 295 



heptachromics, who interpose indigo between blue and violet, usually 

 map out 22 29 such patches. Konig, Lord Rayleigh, and all previous 

 observers found a much greater discrimination sensibility (v. p. 30), 

 and the very careful researches of Steindler confirm their results. 

 There is a fundamental fallacy here which requires explanation. The 

 discrepancy is borne out by the protocols of the author's investigations 

 of 14 dichromics 1 , in whom the number of monochromatic patches varies 

 from 11 to 2. " There may be shortening of the red or the violet end 

 of the spectrum ; there may be defective perception for some of the 

 other spectral rays ; the luminosity curve may have its maximum at 

 a different place from the normal ; .there may be defective perception 

 when the image on the retina is diminished in size ; and the size of the 

 neutral region is very variable 2 ." If so many varieties of one type of 

 colour blindness occur it is difficult to imagine how they can be explained 

 by any theory which is not a mere generalisation of facts, of little value 

 for purposes of classification. Edridge-Green summarises his view r s of 

 dichromics as follows : "(1) There are many degrees and varieties of 

 dichromic vision. (2) There are not two well-defined varieties of 

 dichromic vision, there are innumerable gradations connecting the two. 

 (3) In many cases precisely the same errors are made both by those 

 with and those without defective perception of red, when the rays for 

 which there is defective perception are not involved. (4) All dichromics 

 are not equally colour-blind, that is, one may have a much better hue 

 perception than another. (5) Dichromic vision may be associated with 

 defects of light perception which are also found in cases in which the 

 vision is not dichromic. (6) Dichromics may have a perception of 

 shade and a luminosity curve similar to the normal. (7) Many dichro- 

 mics match very accurately, their colour perception being sufficient for 

 the purpose when the colours are not too close in the spectrum. (8) The 

 degree of colour blindness varies with the state of health. (9) Colour 

 discrimination is diminished as a whole in dichromic vision. (10) Dichro- 

 mic vision appears to be due to a defective power of colour differentiation 

 probably corresponding to an earlier state in evolution of the colour sense. 

 (11) The two colours seen are red and violet." 



Edridge-Green's conclusions with regard to anomalous trichromatic 

 vision are thus summarised by him 3 : 



" (1) Trichromic vision is not synonymous with anomalous trichro- 

 matism. (2) Many persons with otherwise normal colour perception 



1 Ophthalmoscope, XH. 1914. - Loc. cit. p. 77. 



3 Proc. Roy. Soc. Lond. B, LXXXVI. 164 1913. 



