164 Mr. A. M. Mayer on the Phenomena 



slit in the shutter of a darkened room, which slit was covered 

 by a piece of cobalt glass. 



A square of thin green glass, 4 centim. by 6 centim,, was 

 placed on a piece of thin silvered glass, 4 centim. by 12 

 centim., so that the edges of the green glass and of the 

 silvered glass coincided. This arrangement gave a surface, 

 half of silvered mirror, half of green glass. This apparatus 

 was so placed that the electrodes of the machine and the flash 

 were reflected from it to the eye. The room was dark. At the 

 moment of the flash its reflexion appeared as in fig. 12. On 



Fi tf .12 



the mirror the line of the discharge was white ; W in the 

 figure. The continuation of this line on the surface of the 

 green glass appeared red (R in figure), though really white. 

 In front of and parallel to this line was a green line, G, pro- 

 duced by the light of the flash reflected from the surface of 

 the silvered mirror, and having traversed twice the thickness 

 of the green glass. In this experiment we obtain contrast 

 colours in the source of light itself. 



The explanation of the phenomena of simultaneous Contrast- 

 Colours^ generally given in works on chromatics, is that they 

 are due to " error of judgment," to "deception of judgment," 

 or to " fluctuation of judgment." The reasoning given may 

 be convincing if all the conditions really exist which the 

 writers assume to exist in their explanations of these phe- 

 nomena. One of these conditions is that a judgment can be 

 formed in the minute interval of time only necessary in which 

 to perceive contrast-colours. In the experiments just de- 

 scribed with the electric flash we have apparently instantaneous 

 perception of the contrast-colours in the grey ring placed on 

 the green and ultramarine grounds, in the candle and electric 

 fight shadows and even in the very flash itself when this is 

 seen reflected from the top surface of a green glass and from 

 the mirror on which the green glass rests. 



Many careful experiments made by me and others, using 

 as chronometer three pairs of forks of the octave of UT 6 , 

 giving respectively 10, 12, and 15 beats per second, showed 

 that certainly the interval between the flash and the per- 



