86 



COLOUR VISION 



the phenomenon " recurrent vision," and the observation was con- 

 firmed by Shelford Bidwell^. An allied phenomenon was described by 

 Purkinje^, rediscovered by A. S. Davis^, and is commonly called 

 "Bidwell's ghost"; Hamaker^ called it the "satellite", v. Kries "the 

 following image." The ghost is seen in typical form when a light of 

 moderate brightness is moved across a dark field of view at a moderate 

 speed while the eye remains at rest, appearing as a second dimmer 

 image following the primary image after a short interval, usually 

 about -^sec, of darkness. The subject has been specially studied by 

 Bosscha^, Charpentier^, Hess', v. Kries^, Samojloff^ Munki", Exner^i, 

 and McDougalli2. 



Fig. 34. 



Fia;. 35. 



Fig. 34. Appearance of a radial slit 2° in width and 7 cm. in length, with its mid-point 

 15 cm. from the centre of the disc, rotating at the rate of one revolution per 3" before 

 a glass illuminated by four acetylene-gas jets. (McDougall.) 



Fig. 35. Appearance with slightly increased speed of revolution — " Charpen tier's bands." 

 (McDougaU.) 



' Proc. Roy. Soc. Land. lvi. 1.02, 1894 ; Curioftitict of Light and Sight, London. 1899. 



2 Beobachtungen zur Physiol, d. Sinne, ii. 110. 1825. 



3 Lond. and Edin. Philosoi^h. Mag. xi.iv. 526, 1872. 



* Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xxi. 1, 1899. 



5 Arch.f. Ophth. xl. 2, 22, 1894. « 4^g^_ ./^ physiol. iv. 541, 1892 ; vi. 677, 1896. 



' Arch. f. d, ges. Physiol, xlix. 190, 1891 ; Arch. f. Ophth. XL. 2, 259, 1894 ; XLiv. 3, 

 445, 1897 ; Li. 2, 225, 1900 ; Arch.f. d. ges. Physiol, ci. 226, 1904; ovii. 290, 1905. 



8 Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xii. 81, 1896 ; Arch.f. Ophth. XLii. 3, 1896 ; 

 Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xxv. 239, 1901 ; xxix. 81, 1902 ; Arch. f. d. 

 ges. Physiol, cm. 167, 1904. 



* Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xx. 118, 1899. 



1" Ibid. xxra. 66, 1900. " Arch.f. d. ges. Physiol, cm. 1905. ^- Loc. cit. 



