THE DUPLICITY THEORY 207 



sensitiveness for rays of short wave-length as compared with those 

 of long wave-length, red showing no appreciable increase. The 

 third characteristic is the cause of the Purkinje phenomenon. The 

 characteristics of rod vision will therefore be (1) total colour blindness, 

 (2) maximum sensitiveness in the situation of the green of the photopic 

 spectrum, and shortening of the red end, (3) a very high degree of 

 adaptation. 



The theory explains satisfactorily the apparent deviations from 

 Newton's law of colour-mixtures described by the earlier investigators. 

 Of these, Hering 1 and v. Kries and Brauneck 2 held that colour equations 

 were independent of the intensity of the light. Konig and his pupils 3 

 disagreed with this conclusion, and v. Kries found it necessary later to 

 modify his opinion 4 . The photopic values of the matches are essentially 

 cone-values. At lower intensities rod- and cone-values are mixed, and 

 at the lowest only rod-values persist. The results of Konig and his 

 pupils are partly vitiated by lack of attention to adaptation, but more 

 to wandering fixation and the use of objects whose retinal images 

 surpass the rod-free limits. The striking examples brought forward by 

 Ebbinghaus and Mrs Ladd-Franklin (v. p. 60) are easily explained by 

 the duplicity theory, and indeed strongly support it. v. Kries 5 extended 

 these observations and explained the anomalous results of other observers. 

 He showed that it was theoretically possible to obtain a photopic 

 " white ' : equivalent to a scotopic " white," i.e., a " white ' : which 

 would lose only in subjective intensity on diminution of the physical 

 intensity. The difference in rod-values between a homogeneous scotopic 

 white and a mixture-white are so slight in normal colour-vision that 

 the results are ambiguous ; they are much more conclusive in dichromats. 

 The theory also accounts for the " wandering of the neutral point ' : 

 in dichromats on altering the physical intensity of the light. In most 

 of the observations referred to above macular pigmentation is a disturb- 

 ing influence (cf. Hering). In general form it may be stated that 

 matches valid for high intensities become invalid for low intensities and 

 dark-adaptation in the sense that the mixture which possesses the 

 greater rod-value exhibits the greater luminosity in tone-free scotopic 

 vision (v. Kries). The regional peculiarities of scotopic as compared 

 with photopic vision afford very strong evidence in favour of the 



1 Lotos, vn. 1886 ; Arch. f. d. ges. Physiol. LIV. 277, 1893. 



2 Arch. f. Physiol. 79, 1885. 



3 Brodhun, Ztsch. /. Psychol. u. Physiol. d. Sinnesorg. v. 323, 1893 ; Tonn, op. cit. vn. 

 279, 1894. 



4 v. Kries, op cit. ix. 81, 1896, etc 6 Ibid. ix. 81, 1896 



