198 



TRANSACTIONS OF THE 



[mar. 1, 



placed on the axis of the machine, were screened from all side 

 illumination by black screens suitably adjusted. The observer 

 was placed at a distance of about one meter in front of the i-ota- 

 tion machine and so seated that his head was just below the 

 level of the disks. 



The total luminosit3^ of the window during observations was 

 between one thousand and two thousand candle power. The 

 variation in luminosity of the more refrangible colors with re- 

 spect to the less refrangible ones, due to a variation in illumina- 

 tion, an effect described by Purkinje,* was found to be inappre- 

 ciable so long as the illumination did not vary bej'ond the above 

 limits. 



The Phenomenon of Flickering. 



If a white disk and a black disk or two gray disks differing by 

 more than about 40 per cent, in luminosity are combined pair- 

 wise, as in Fig, I. C, and are rotated, the following phenomena 

 may be observed under good daylight illumination : 



I. 



With a rate of rotation between ten and twenty-five revolu- 

 tions per second, the compound disk seems to be divided into a 

 number of rotating sectors alternately light and dark. The ap- 

 parent number of these sectors increases with the rate ot rota- 

 tion, and the phenomenon is rather trying on the eyes. 



If the two disks are of different colors, as well as of different 

 luminosities, the rotating sectors will differ alternately in color 

 as well as in luminosity'. 



* Zur Physiologie der Sinne. Vol.11. Page 109. 



