202 SOME CURIOSITIES OF VISION. 



happens in a more or less marked degree whenever a dark object moves 

 across a bright background, or a bright object across a dark background. 

 In order to see the effect distinctly by Charpentier's original method 

 the illumination must be strong. If, however, the arrangement is 

 slightly varied, so that transmitted instead of reflected light is made 

 use of, comparatively feeble illumination is sufficient. A very effective 

 way is to turn a small metal disk having an open sector of about 60° in 

 front of a sheet of ground or opal glass, behind which is a lamp. By an 

 arrangement of this kind upon a larger scale the effect may easily be 

 rendered visible to an audience. The eyes should not be allowed to 

 follow the disk in its rotation, but should be directed steadily upon the 

 center. 



The acute and educated vision of Oharpentier enabled him, even 

 when working with his black and white disk, to detect the existence, 

 under favorable conditions, of a second and sometimes a third dark 

 band of greatly diminished intensity, though he remarks that the obser- 

 vation is a very difficult one. What is probably the same effect can, 



however, be shown quite easily in a different 

 manner. If a disk with a very narrow radial 

 slit, one-fiftieth of an inch or one-half milli- 

 meter wide, is caused to rotate at the rate of 

 about one turn per second in front of a bright 

 background, such as a sheet of ground glass 

 with a lamp behind it, the moving slit 

 assumes the appearance of a fan-shaped 

 luminous patch, the brightness of which 

 diminishes with the distance from the lead- 

 ing edge. And if the eyes are steadily fixed 

 upon the center of the disk it will be noticed 

 FlG 5 that this bright image is streaked with a 



number of dark radial bands, suggestive of 

 the ribs or sticks of the fan. Bear the circumference as many as four 

 or five such dark streaks can be distinguished without difficulty; 

 toward the center they are less conspicuous, owing to the overlapping 

 of the successive images of the slit. 1 (The effect was demonstrated 

 by means of a rotating disk in the lantern, and is roughly indicated 

 in fig. 5.) 



The dark reaction known as the Oharpentier effect occurs at the 

 beginning of a period of illumination. There is also a dark reaction 

 of a very short duration at the end of a period of illumination. I 

 should explain that, owing to what is called the proper light of the 

 retina, ordinary darkness does not appear absolutely black. Even in a 

 dark room on a dark night, with the eyes carefully covered, there is 

 always some sensation of luminosity which would be sufficient to show 

 up a really black image, if one could be produced. Now the darkness 



1 Proc. Roy. Soc, Vol. LVI, p. 142 (1894). A similar observation was described by 

 Charpentier, Comptes Rendns, January, 189U. 



