DICHROMATIC VISION 163 



and also the imanalysed white light. It is found that there are two 

 groups of dichromats, as was first pointed out by Seebeck 1 , agreeing 

 amongst themselves but differing from each other in the proportions 

 of the two mixed lights required for the matches. These are the 

 protanopes and deuteranopes, of whom the latter are more common 

 (23 : 11, Nagel). 



Now, since white light can be matched by the dichromat with a 

 suitable mixture of two monochromatic lights, and since all spectral 

 colours can be matched by mixing the same two colours in various 

 proportions, it follows that there is some spectral colour which will 

 match white. This point is called the neutral point of the dichromatic 

 spectrum. 



Konig 2 first made systematic examinations of the colour-mixtures 

 of the colour-blind with spectral colours. He was followed by v. Kries 

 and Abney 3 . 



The principle of the method adopted by Konig had been previously 

 applied on a less extensive scale by Bonders and van der Weyde 4 . 

 Konig and Dieterici divide the dichromatic spectrum into three regions 

 as compared with the five regions of the trichromatic spectrum. If 

 L and L 3 are two spectral lights from the end regions, and L 2 is any 

 spectral light from the middle region then 



L 2 = aL -+ bL 3 



where a and b are two coefficients determined experimentally. In other 

 words any spectral colour can be matched by the mixture of appro- 

 priate quantities (a and b) of two colours selected from the end regions 

 of the spectrum. If for L^ a red (645 P,/JL) is chosen, and for L 3 a blue 

 (460 up or 435 //,/z), then Konig and Dieterici found that for protanopes 

 no blue is necessary to match any colour from about 550 ^ to the end 

 of the spectrum on the red side, hence b = 0. Similarly no red is 

 needed to match any colour from about 460 p-p, to the violet end, and 

 a = 0. 



A more complicated method, ensuring greater accuracy, was to 

 divide up the spectrum into fractions, determining the matches for 



1 Ann. d. Physik, XLII. 177, 1837. 



2 Konig and Dieterici, Sitz. d. Akad. d. Wiss. Berlin, 805, 1886 ; Ztsch. f. Psychol. u. 

 Physiol. d. Sinnesorg. iv. 241, 1892 ; Konig, Brit. Assoc. 1886. 



3 Much of Abney's work covers the same ground as that of v. Kries. Since, however, 

 it is expressed throughout in terms of the Young-Helmholtz theory and was inspired by 

 that theory detailed consideration of it will be left until Part III 



4 Arch. f. Ophth. xxvra. 1, 1, 1882. 



112 



