86 COLOUR VISION 



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

 firmed by Shelf ord Bidwell 1 . An allied phenomenon was described by 

 Purkinje 2 , rediscovered by A. S. Davis 3 , and is commonly called 

 "Bidwell's ghost"; Hamaker 4 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 5 , Charpentier 6 , Hess 7 , v. Kries 8 , Samojloff 9 , Munk 10 , Exner 11 , 

 and McDougall 12 . 



Fig. 34. Fig. 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." 

 (McDougall.) 



1 Proc. Roy. <S'oc. Land. LVI. 132, 1894 ; Curiosities of Light and Sight, London. 1899. 



2 Beobachtungen zur Physiol. d. Sinne. u. 110. 1825. 



3 Land, and Edin. Philosoph. Mag. XLIV. 526, 1872. 



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



5 Arch.f. Ophth. XL. 2, 22, 1894. 6 Arch, dc Physiol. iv. 541, 1892 ; VI. 677, 1896. 



7 Arch. /. d, ges. Physiol. XLIX. 190, 1891 ; Arch. f. Ophth. XL. 2, 259, 1894 ; xi.iv. 3, 

 445, 1897 ; LI. 2, 225, 1900 ; Arch.f. d. yes. Physiol. ci. 226, 1904; evil. 290, 1905. 



8 Ztsch. f. Psychol. u. Physiol. d. Sinnesorg. xn. 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. 



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



1(1 Ibid. xxin. 66, 1900. n Arch.f. d. ges. Physiol. cm. 1905. 12 Loc. cit. 



