NEWTON AND THE COLOURS OF THE SPECTRUM 251 



of the evidence bearing on the matter. It is little known, 

 at least to the English-speaking peoples, because Newton's 

 Opticks has not been printed in English since 1 730, and because 

 the part of the Lectiones Opticce bearing on the subject has never 

 been printed in English. Newton's Opticks is for sale at present 

 only in the German translation. 



We shall first of all consider Newton's colour vision. This 

 explanation has been put forward by Edridge-Green . In his 

 theory of colour vision he does not accept the usual classification 

 of red-blind, green-blind, etc., but divides colour vision into 

 seven classes — monochromic (totally colour-blind), dichromic, 

 trichromic, tetrachromic, pentachromic, hexachromic, and hep- 

 tachromic — according as the subject sees one, two, three, four, 

 five, six, or seven distinct colours in the spectrum. According 

 to his theory normal colour vision is hexachromic ; the normal 

 individual sees the six colours, red, orange, yellow, green, blue, 

 violet. The normal individual sees, of course, intermediate 

 shades between these colours, but these six colours appear to be 

 primary. The heptachromic class see indigo as a primary colour, 

 and have a decidedly better colour perception than the hexa- 

 chromic. It is not merely a matter of colour nomenclature ; 

 the heptachromic really see something at this region in the 

 spectrum, which the hexachromic do not see. Newton, accord- 

 ing to Edridge-Green, was a heptachromic. 



I was at first somewhat sceptical about this explanation, 

 because Newton states in a passage to be afterwards quoted in 

 full that his eyes were not so critical for distinguishing colours 

 as the eyes of his assistant ; also I gathered from Edridge- 

 Green 's papers that heptachromics occurred rarely, and the 

 probability of both Newton and his assistant being hepta- 

 chromic seemed to me too small. But out of eighteen cases — 

 members of the staff and advanced students of the university — 

 that I have recently examined I find that three at least quite 

 definitely see indigo as a separate colour, and there is a fourth 

 case doubtful. The others are normal. So that the peculiarity 

 seems to be fairly common. 



The examination was made at first by projecting a spectrum 

 about eight inches long on a screen. The student under 

 examination was asked whether he saw a colour between blue 

 and violet with as much right to a special name in the spectrum 

 as orange has, if so how would he describe it, and what were 



