After-images following brief Retinal Excitation, 2G9 



Apparatus. 



The only apparatus employed was a rotating disk of which one- 

 half of the surface was black and the rest white, a sector of 45° 

 being cut out at the junction of the black and white portions. The 

 disk, which was 8 inches (20 cm.) in diameter, was made of tin- 

 plate ; a piece of thick brass wire was fixed along the arc bounding 

 the open sector to restore the balance. Half the disk was covered 

 with black velvet, for no pigment among several that were tried 

 appeared sufficiently black in a strong light ; the remaining portion 

 of the surface was coated with white paper. The disk w r as turned 

 in such a direction that the open sector was preceded by black and 

 followed by white. It was usually illuminated by an electric lamp 

 of 25-candle power with a ground glass bulb, the intensity of the 

 illumination being regulated by varying the distance of the lamp ; 

 sometimes sunlight or diffused daylight was employed. The objects 

 under examination were placed on the other side of the disk and 

 viewed intermittently through the open sector. 



Experiment I. 



A card with black letters printed upon it was placed behind the 

 disk, which was illuminated in front by the lamp at a distance of 

 2 feet (60 cm.). When the disk made about six turns per second, the 

 black letters appeared to be bright red ; this was of course so far 

 merely a repetition of the old observation. The lamp was then 

 gradually brought nearer to the disk, and as it approached the 

 aspect of the letters underwent a change. At first they exhibited a 

 peculiar shimmer, independently described by a great number of 

 persons as either a " silvery," "lustrous," or "metallic " appearance; 

 it was plainly due to flashes across the letters of some tint much 

 lighter than the red, by which they were still mainly characterised. 

 When the distance of the lamp was diminished to about 4 inches 

 (10 cm.), all trace of the red had disappeared, and the letters 

 assumed a luminous greenish-blue tint. 



Experiment II. 



A card was substituted upon which were printed words and figures 

 in both black and red. It was found possible, by suitably adjusting 

 the intensity of the light and the speed of rotation, to make such a 

 perfect match between the letters which were actually red and 

 those which only appeared so, that an unpractised observer could 

 not distinguish between them. When the lamp was placed near 

 the disk, all the letters, both black and red, appeared greenish-blue. 

 The greenish-blue tint of the actually red letters was, moreover, more 



