io 7 4 VISION. 



was no recurrent image in a case of night blindness, and in one of total 

 colour-blindness. 



Hamaker 1 has recently brought forward evidence which shows that 

 the phenomenon is more complex than has been supposed. In addition 

 to the recurrent image, several observers have seen others, and Hamaker 

 clearly distinguishes four stages : the primary original image, followed 

 immediately by a short " tail " ; a secondary image, separated from the 

 former by a dark interval ; a tertiary image, lasting for two or three 

 seconds ; and a quaternary image, which is probably identical with the 

 ordinary negative after-image. Some of the discrepancies between 

 different observers may have been due to confusion between the 

 secondary and tertiary images. The influence of adaptation and of other 

 conditions is not the same for the two images. 



In this and previous sections, four distinct phenomena have been dis- 

 tinguished which are ordinarily classed together as after-images, namely, the 

 negative image due to local adaptation ; the quasi-pathological positive image ; 

 the persistence due to inertia ; and the recurrent image of Purkinje. These are 

 four quite distinct conditions, and it has been a great source of confusion in 

 the past, that they have all had the same name. The term after-image should 

 be confined to the phenomenon due to local adaptation, while the other three 

 phenomena should be termed the persistent positive image, retinal persistence, 

 and the recurrent image. A fifth phenomenon, which is sometimes confused 

 with after-image, is the memory-image, which in many individuals (of the 

 visual type) not only remains after the impression, but may be recalled. 



Charpentier's band. — Another phenomenon pointing to the com- 

 plexity of the sensory curve, is that known as Charpentier's dark 

 band. 2 If a black disc which has a white sector is rotated slowly in 

 strong illumination, the side of the white sector which first penetrates 

 the black ground shows a black band separated from the black ground 

 by a similar white band. The band occurs about ^ of a second after 

 the beginning of the passage of the white, and lasts about the same time. 

 It may often be seen to be followed by a series of similar less distinct 

 black bands at uniform distance. Charpentier ascribes these bands to 

 retinal oscillations, which he supposes to have a constant rapidity, each 

 total oscillation lasting about -£% of a second. The oscillations are 

 supposed to be of two kinds, one more rapid being polarised in the 

 direction of the yellow spot, while the other radiates in all directions. 



Other appearances with intermittent stimulation. — Some other 

 phenomena of interest occur with intermittent retinal stimulation. The 

 flicker which occurs before complete fusion is, at a certain stage, of a 

 glittering character, and gives the appearance of a brightness considerably 

 greater than that of the continuous sensation: the Talbot-Plateau law 

 being only true after complete fusion. Briicke 3 described the maximum 

 brightness with black and white sectors as occurring when there were 17 '6 

 stimuli per second, and ascribed the appearance to the reinforcement of 

 each white sector by the positive (recurrent) image of the preceding sector. 



Exner i described certain subjective appearances with intermittent 

 stimulation as occurring especially with red, green, and blue light, while 

 other colours appeared to be decomposed by this means, violet light 



1 Ztschr. f. Psychol, u. Physiol, d. Sinnesorg., Hamburg u. Leipzig, 1899, Bd. xxi. S. 1. 



2 Arch, ale physiol. norm, et path., Paris, 1892, Ser. 5, tome iv. p. 541 ; and 1896, 

 tome viii. p. 677. 



3 Sitzungsb. d. k. Alcad. d. JVissensch., Wien, 1864, Bd. xlix. Abth. 2, S. 128. 



4 Arch. f. d. ges. Physiol., Bonn, 1868, Bd. i. S. 375. 



