482 



JAMES CLERK MAXWELL. 





these two individuals, producing constant and measurable 

 differences in the apparent colour of objects." 



With those colour-blind persons whom Maxwell first ex- 

 amined only two slits were required to produce a colour 

 chromatically identical with white ; while the spectrum, a 

 little on the red side of the line F, appeared to be identical 

 with white. " From this point to the more refrangible end 

 the spectrum appears to them ' blue.' The colours on the 

 less refrangible side appear all of the same quality, but of 

 different degrees of brightness ; and when any of them are 

 made sufficiently bright they are called ' yellow.' " Thus 

 a colour-blind or dichromic person, in speaking of red, green, 

 orange, and brown, refers to different degrees of brightness 

 or purity of a single colour, and not to different colours. 

 This colour he calls yellow. 



Of the three standard colours the red appears to them 

 " yellow," but so feeble that there is not enough in the whole 

 red division of the spectrum to form an equivalent to make up 

 the standard white. The green at E appears a good " yellow," 

 and the blue at f from F towards G appears a good " blue." 



It was for these researches that Maxwell received the 

 Eumford Medal of the Eoyal Society in 1860. 



The only cases of colour-blindness which Maxwell met 

 with in the course of his earlier experiments were those of 

 persons blind to red rays. This, however, is not the only 

 kind of colour-blindness known. Some appear to be de- 

 ficient in the blue (or violet) sensation. To these persons 

 bright yellow appears white, and the neutral line is a line 

 drawn through the yellow and the point corresponding to 

 the pure blue (or violet) sensation. In a valuable paper 

 published in the Proceedings of the Royal Society, vol. xxxi. 

 p. 302, M. Frithiof Holmgren, Professor of Physiology in 

 the University of Upsala, gave an account of a case of red 

 x blindness, and a case of violet-blindness in each of which 

 only one eye was really colour-blind, the vision of the other 

 being nearly normal. This peculiarity enabled the observer 

 to compare the experience of colours gained through one eye 



