358 Mr. SM/ord Bidwell [March 5, 



conjunction with the much brighter yellow and orange of the primary 

 being merely an illusory effect of contrast. [This contrast effect was 

 illustrated by a lantern slide.] It seems likely, then, that the effect 

 which has been spoken of as recurrent vision, is due principally, if 

 not entirely, to an action of the violet nerve fibres. It need hardly 

 be pointed out that it represents only a transient phase of the well 

 known positive after-image, and it had even been observed in a vague 

 and uncertain sort of way long before the date of Prof. Young's 

 experiment. Helmholtz, for example, mentions the case of o positive 

 after-image which seemed to disappear and then to brighten up 

 again ; but he goes on to explain that the seeming disappearance was 

 illusory. 



M. Charpentier, of Nancy, whose name I have already mentioned, 

 was the first to notice and record a remarkable phenomenon which, 

 in some form or other, must present itself many times daily to 



every person who is not blind, but 

 which, until about six years ago, had 

 been absolutely and universally ignored. 

 The law which is associated with Char- 

 pentier's name is this : — When darkness 

 is followed by light, the stimulus which 

 the retina at first receives, and which 

 causes the sensation of luminosity, is 

 succeeded by a brief period of in- 

 sensibility, resulting in the sensation of 

 momentary darkness. It appears that 

 the dark period begins about -^q^ second 

 after the light has first been admitted 

 Fig. 3. to the eye, and lasts for about an equal 



time. The whole alternation from light 

 to darkness and back again to light is performed so rapidly, that 

 except under certain conditions, which, however, occur frequently 

 enough, it cannot be detected. 



The apparatus which Charpentier employed for demonstrating 

 and measuring the duration of this effect is very simple. It consists 

 of a blackened disc with a white sector mounted upon an axis. When 

 the disc is illuminated by sunlight and turned rather slowly, there 

 appears upon the white sector close behind its leading edge a narrow 

 but well-defined dark band (See Fig. 3). The portion of the retina 

 which is apparently occupied at any moment by the dark band is 

 that upon which the light reflected by the leading edge of the white 

 sector has fallen -J-^ second previously. 



But no special apparatus is required to show the dark reaction ; 

 it is, as I have said, an exceedingly common phenomenon. In Fig. 4 

 an attempt has been made to illustrate what any one may see if he 

 simply moves his hand between his eyes and the sky or any strongly 

 illuminated white surface. The hand appears to be followed by a 

 dark outline separated from it by a bright interval. The same kind 



