THE MIXTURE OF PURE-COLOUR STIMULI 41 



\ = 670 590 580 575 570 560 555 550 540 535 530 

 1 0-991 0-986 0-981 0-971 0'966 0'962 0-951 0'946 0'941 



X= 520 510 505 500 490 480 474 470 464 454420 

 0-905 0-800 0-770 0-740 0-700 0'680 0'677 0-075 0-672 0'670 



Owing to this absorption one would expect variations in colour mixtures, 

 and it is indeed found that, for example, in matching a homogeneous 

 yellow with a mixture of red and green, some people require slightly 

 more green than others, so that a given match appears too green to some, 

 too red to others. If such people make the match eccentrically, so that 

 the images fall just outside the macula, where there is no yellow pigment, 

 their matches agree 1 , v. Frey and v. Kries 2 exhaustively investigated 

 the matches of numerous normal-sighted individuals (students) and 

 found that the deviations were such as would be expected from pig- 

 mentary absorption. This variation in individuals is therefore purely 

 physical, and must not be confounded with allied variations which 

 are due to physiological abnormalities. (See Part II, Chap, in.) Since 

 the physiological abnormalities very rarely affect the blue end of the 

 spectrum, a ready method of distinguishing the effects of macular 

 pigmentation from them is by measuring their match between spectral 

 blue-green and a mixture of blue and green, v. Kries 3 found that 

 reduction of blue by minimum and maximum macular pigmentation 

 was in the ratio 1 : 0'31, the corresponding ratio for green (517^) 

 being 1 : 0'5. (See also Section IV, Chap, n.) 



The Influence of Lenticular Pigmentation. With advancing years 

 the crystalline lens becomes tinged with a yellow pigment. The effect 

 is similar to that of macular pigmentation, but does not manifest itself 

 until late adult life. Konig 4 estimated the coefficients of transmission 

 of monochromatic lights by the lens of a man, aet. 55. 



1 Cf. Hering, 'Arch. f. d. ges. Physiol. LIV. 277, 1893 ; Breuer, Ztsch. f. Psychol. u. 

 Physiol. d. Sinnesorg. xm. 464, 1897. 



2 Arch,/. Anal. u. Physiol. 336, 1881. 



3 Ztsch. f. Psychol. u. Physiol. d. Sinnesorg. xm. 284, 1896. 



4 Konig, p. 346. 



