10 Mayer — Phenomena of Simultaneous Contrast Color, 



of no perceptible change in the intensity of the residual im- 

 pression increases. I found that the light from a white cloud, 

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

 only Jg- of a second before the residual effect diminishes. 



A gray ring was 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-gVo of a second, when the ring ceased to be visible on the 

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

 trast color 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 determining the -g-gVo °f a 

 second as the time when contrast-color ceased to be visible. The 

 fact that the contrast color of the ring remained till the ring 

 could not be distinguished from the green ground gave the 

 opinion that I ceased to obtain contrast-color because the nar- 

 row slit used gave too little light to the eye. On doubling the 

 width of the slit and doubling the velocity of rotation the 

 contrast-color of the ring reappeared, though the duration of 

 vision was the same as in the former experiment. 



Experiments on simultaneous contrast-colors 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-color I made 

 the following experiments with the light obtained by the dis- 

 charge of a Holtz induction machine. 



The gray ring was placed on the emerald green ground near 

 a Holtz machine, which gave a very bright flash of 8 cms long. 

 The condensing surface on the two jars connected with the 

 electrodes of this machine equalled 135 square centimeters. 

 Professor O. N". Rood* measured the duration of the flash of 

 Leyden jars of 738 and 71 square centimeters of surface, 

 charged by an inductorium. The durations of the discharges 

 of these jars were respectively ■j,-roi,uo o" second and YT.Wir.innr 

 of a second. From these measures 1 infer that the duration of 

 the flash of the Holtz machine did not exceed the T ¥ilJ -^o7oo"o 

 of a second. With short striking distances between the elec- 

 trodes the flash is formed of several separated acts, as shown 

 by Henry,f Feddersen,;}; Rood,§ and Mayer.] In the case of 

 the discharge of a large inductorium the writer (|) has shown 

 that when the striking distance between brass ball electrodes 

 is only one millimeter, with a Leyden jar of 242 sq cms of sur- 



* This Journal, Sept.. 1871 + Proc. Amer. Phil. Soc , 184 2. 



i Pogg. Ann., vol. cxvi, p. 132. § This Journal, Oct., 18^2. 



I This Journal, Dec, 1874. 



