SOME CURIOSITIES OF VISION. 



201 



Fig 



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 fibers. It need hardly be pointed out that it represents 

 only a transient phase of the well-known positive afterimage, and it 

 had even been observed in a vague and uncertain sort of way long 

 before the date of Professor Young's experiment. Helmholtz, for 

 example, mentions the case of a 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. Cbarpentier, 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 pen tier'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 insensibility, resulting in the sensation of momentary darkness. 

 It appears that the dark period begins about one-sixtieth of a second 

 after the light has first been admitted to the eye, and lasts for about 

 an equal time. The whole alteration from light to darkness and back 

 again to light is performed so rapidly that except 

 under certain conditions, which, however, occur fre- 

 quently enough, it can not be detected. 



The apparatus which Charpeutier employed for 

 demonstrating and measuring the duration of this 

 effect is very simple. It consists of a blackened disk 

 with a white sector mounted upon an axis. When 

 the disk 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 

 one-sixtieth of a second previously. 



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

 as I have said, an exceedingly common phenomenon. In figure 4 an 

 attempt has been made to illustrate what anyone may see if he simply 

 moves his hands between his eyes and the sky or any strongly illumi- 

 nated white surface. The hand appears to be followed by a dark out- 

 line separated from it by a bright interval. The same kind of thing 



Fig. 4. 



