162 Mr. A. M. Mayer on the Phenomena 



The rotator is so made that the time of a rotation of the disk 

 can be held uniform and also accurately measured. On 

 gradually narrowing the opening in the slit, the duration of 

 vision of the blue and grey square was reduced to yeVo °^ a 

 second, when the blue half of the square could not be dis- 

 tinguished from the grey half. 



When the screen was illuminated by a more orange light, 

 obtained by placing a sheet of orange gelatine between the 

 lamp and the screen, the grey and blue were not distinguish- 

 able when the duration of vision was 13 \) of a second. 



In these experiments I only got 11*6 flashes of light from 

 the screen in a second, while the number of flashes required 

 to banish all flickering from the screen was accurately mea- 

 sured as Jg- of a second with the illumination of the lamp 

 alone, and ^ of a second when the lamp-light traversed the 

 orange gelatine film. These experiments show, what is w r ell 

 know r n, viz., that as the intensity of the light diminishes the 

 duration of no perceptible change in the intensity of the resi- 

 dual impression increases. I found that the light from a white 

 cloud, seen, near noon, through a north window on a clear day 

 lasts only ^ 8 - of a second before the residual effect diminishes. 



A grey ring w 7 as placed on an emerald-green ground facing 

 the light from a bright white cloud. When viewed through 

 the opening in the rotator the ring appeared pink on the 

 green ground till the duration of the vision of the ring was 

 reduced to g^o °f a second, when the ring ceased to be 

 visible on the emerald-green ground ; but so long as it was 

 visible the contrast-colour was visible. I found that 43 flashes 

 per second were necessary to render the vision of the pink 

 ring and green ground steady, without the least flickering, 

 while only 11*6 flashes per second were obtained in determin- 

 ing the g^go °f a secon d as the time when contrast- colour 

 ceased to be visible. The fact that the contrast-colour of the 

 ring remained till the ring could not be distinguished from 

 the green ground gave the opinion that I ceased to obtain 

 contrast-colour because the narrow slit used gave too little 

 light to the eye. On doubling the width of the slit and 

 doubling the velocity of rotation, the contrast-colour of the 

 ring reappeared, though the duration of vision was the same 

 as in the former experiment. 



Experiments on simultaneous Contrast- Colours produced by 

 the flash of the Electric Discharge. — The foregoing experiments 

 having proved insufficient to form any opinion as to the time 

 of vision necessary to perceive simultaneous contrast-colour, I 

 made the following experiments with the light obtained by 

 the discharge of a Holtz induction-machine. 



