SOME CURIOSITIES OF VISION. 203 



which is experienced after the extinction of a light is for a small frac- 

 tion of a second more intense than common darkness. 



I believe that the first mention of this dark reaction occurs in the 

 article which I contributed to Nature in 1885, in which it was stated 

 that when the current was cut off from an illuminated vacuum tube 

 "the luminous image was almost instantly replaced by a corresponding 

 image which appeared to be intensely black upon a less dark back- 

 ground," and which was estimated to last from one-fourth to one-half 

 of a second. "Abnormal darkness," it was added, "follows as a reac- 

 tion after the luminosity." 



In the Eoyal Society paper, to which I have before referred, the point 

 is further discussed, and a method is described by which the stage of reac- 

 tion may be easily exhibited and its duration approximately measured, 

 If a translucent disk made of stout drawing paper and having an open 

 sector is caused to rotate slowly in front of a luminous background, a 

 narrow radial dark band like a streak of black paint appears upon the 

 paper very near the edge whieh follows the open sector. From the 

 space covered by this band when the disk 

 was rotating at a known speed, the duration 

 of the dark reaction was estimated to be 

 about one-fiftieth of a second. (The experi- 

 ment was shown and is illustrated in fig. (3.) 

 One more interesting point should be 

 noticed in the train of visual phenomena 

 which attend a period of illumination. The 

 sensation of luminosity which is excited when 

 light first strikes the eye is for about one- 

 sixtieth of a second much more intense than 

 it subsequently becomes. This is shown by 

 the fact that the bright band intervening 



between the leading edge of the white sector of a Oharpentier disk 

 and the dark band appears to be much more strongly illuminated than 

 any other portion of the sector. 



I propose now to say a few words about a curious phenomenon of 

 vision which occupied my attention toward the end of last year. 1 



Eather more than two years ago Mr. 0. E. Benbam brought out a 

 pretty little toy which he called the artificial spectrum top. It consists 

 of a cardboard disk, one half of which is painted black, while on the 

 otber half are drawn four successive groups of concentric black lines 

 at different distances from the center. When the disk rotates rather 

 slowly each group of black lines generally appears to assume a different 

 color, the nature of which depends upon the speed of the rotation and 

 the intensity and quality of the light. Under the best conditions the 

 inner and outer groups of lines become bright red and dark blue; at the 

 same time the intermediate groups also appear tinted, but the hues 



1 Proc. Roy. Soc, Vol. LX, p. 370 (1896). 



