94 COLOUR VISION 



stage the forward edge of the white sectors appears reddish, the back- 

 ward edge bluish. Fechner 1 , by a suitable arrangement of black and 

 white sectors obtained a pattern of yellow and blue. Subsequent 

 observers have succeeded in eliciting all gradations of colour. This 

 phenomenon is most easily seen in Benham's top 2 . It is probable 

 that successive induction (Section V, Chap, n) plays a prominent part 

 in these phenomena, and they are clearly of great importance in the 

 theory of colour vision. Exner 3 , Burch 4 and others have made 

 important observations on this subject, using coloured lights. 



More important for our present purpose is the relationship between 

 flicker and the conditions of stimulation intensity and nature of 

 stimulus, rapidity of stimulation, adaptation of the retina, and region 

 and area of the retina stimulated. Various methods of producing 

 intermittent stimulation other than by the usual rotating pigment- 

 coloured discs, have been devised by Rood 5 , Whitman 6 , v. Kries 7 , 

 Simmance and Abady 8 , Kruss 9 , Wild 10 , Ives 11 , and Watson 12 , and the 

 subject has been investigated not only by them but by others, notably 

 by Ferry 13 , Sherrington 14 , Haycraft 15 , 0. Griinbaum 16 , Polimanti 17 , 

 T. C. Porter 18 , Allen 19 , Kennelly and Whiting 20 , Dow 21 , Tufts 22 , 

 Millar 23 , and Morris Airey 24 . 



As regards the intensity of the stimulus, more rapid stimulation 

 is required for complete fusion with increased intensity. Up to a 

 certain point with alternate darkness and light as the intensity is 



1 Ann. d. Phys. u. Chem. XLV. 227 ; 1838. 



2 Bidwell, Prac. Roy. Soc. LX. 368, 1896; LXI. 268. 1897; Percival, Trans. Uphth. 

 Soc. xxix. 119, 1909. 



3 Arch.f. d. ges. Physiol. i. 375, 1868. 4 J. of Physiol. xxi. 431, 1897. 



6 Amer. J. of Sc. (3) XLVI. 173, 1893: (4) vm. 194, 1899; Science, vn. 757; vm. 11, 

 1898. 



8 Phys. Rev. III. 24L 1895. 



7 In Polimanti, Ztsch. f. Psychol. u. Physiol. d. Sinnesorg. xix. 263, 1899. 



8 Proc. Phys. Hoc. xix. 39. 



9 Phys. Zeitung, m. 65 ; Jl.f. Gas u. Wass. XLVII. 129, 1904. 



10 London Electrician, 1909 ; The Illuminating Engineer, i. 825, 1908. 



11 Philos. Mag. 1912. r - In Abney, p. 107. 



13 Amer. J. of Sc. (3) XLIV. 193, 1892. 14 J. of Physiol. xxi. 33, 1897. 



15 Ibid. xxi. 126, 1897. lfi Ibid. XXH. 433, 1898. 17 Loc. (.it. 



18 Proc. Roy. Soc. Lond. LXIII. 347, 1898 ; LXX. 313, 1902. 



19 Phys. Rev. xi. 257, 1900 ; xv. 1902 ; xxvni. 45, 1908. 

 2U The Illuminating Engineer, New York, u. 347, 1907. 



21 Proc. Phys. Soc. xx. 644. 1907 ; xxn. 58, 1910 ; Philos. M<KJ. 120, 1906; 58, 1910; 

 The Electrician, LVIII, 609, 1907. 



22 Phys. Rev. xxv. 433, 1907. 23 The Illuminating Engineer, New York, iv. 769, 1909. 

 24 J. Inst. Elec. Engs. XLIV. 177, 1910. 



