30 COLOUR VISION 



Purity is " the freedom of the colour from admixture with white light," 

 but he says " when I speak of a colour being mixed with white light, 

 I have a different meaning from that which is signified by most writers 

 on colour." His explanation should be read carefully 1 , but he appears 

 to use the term " white," which denotes a physiological sensation, in the 

 sense of a physical property of the light. 



CHAPTER II 



THE DISCRIMINATION OF HUE IN THE SPECTRUM 



Observations on the discrimination of hue in the spectrum have 

 been made by Mandelstamm 2 , Dobrowolski 3 , Peirce 4 , Konig and 

 Dieterici 5 , Uhthoff 6 , Brodhun 7 , F. Exner 8 , Steindler 9 , Edridge-Green 10 

 and others. Of these, the most accurate and complete are those of 

 Steindler. 



The maximum discrimination sensibility for hue, i.e. the smallest 

 difference in wave-length (S\) which gives rise to appreciable difference 

 in colour-tone, occurs in the yellow and the blue-green. It has already 

 been remarked (p. 28) that there are regions at each end of the spectrum 

 in which differences of hue are no longer appreciated. Two neighbouring 

 wave-lengths in these regions may appear to differ slightly in hue, but 

 only owing to differences of intensity. When the intensities are suitably 

 modified the differences disappear. 



Konig and Dieterici found three maxima, I at 440 /A//, II at 490 //,//, 

 and III at 570 up (Dieterici) or 590 pu (Konig). Of these the greatest 

 discrimination sensibility was II. The minima were at 450 pp between 

 I and II, and at 540^ between II and III. These results were obtained 

 by the method of mean error (v. p. 19). (Fig. 4.) 



1 Colour-blindness and Colour Perception, 2nd cd. London, 1!)09, p. 60. 



2 Arch.f. Ophlh. xin. 2, 399, 1867. 3 Arch. f. Ophth. xvm. 1, 0(5, 1872. 



4 Ame.r. J. of Sc. xxvi. 299, 1883. 



5 Ann. d. Physik, xxn. 579, 1884; Arch. f. Ophth. xxx. 2, 158, 1884; Konig, 

 Ztsch. f. Psychol. u. Physiol. d. Sinnesonj. vui 375, 1895 : in Konig, pp. 23, 105, 3(37. 



6 Arch.f Ophth. xxxiv. 4, 1, 1888. 



7 Ztsch. f. Psychol. u. Physiol. d. Sinnesorg. m. 89, 1892. 



8 Sitz. d. Wiener Akad. cxi. ii a, 857, 1902 



9 Sitz. d. Wiener Akad. cxv. ii a, 115, 1906. 10 Vide infra, Part in 



