6 Mayer — Phenomena of Simultaneous Contrast Color, 



the grating is equally spaced, the superposed surfaces appear 

 white when compared with the white of the screen Wh. 

 Without the screen Wh the eye has no term of comparison 

 and may take a yellowish white for white. The illumination 

 of the screen Wh should be made equal that of the superposed 

 images of the grating. 



When this experiment is carefully made, with the proper 

 illumination, the effect is surprising when, in favorable condi- 

 tions, on rotating the prism you see the grating actually 

 obliterated with no bands visible, but only a uniform white 

 surface. The grating with spaces of 3 mm gave the best results : 

 In experiments with the screen, hg. 1, formed of the card- 

 board ring and the translucent disk, it is easy, by altering the 

 relative intensities of the daylight and lamplight and by 

 changing the distance of the lamp from the screen, to produce 

 great changes in the saturation of the contrast-colors. Indeed 

 I have sometimes thus obtained, towards nightfall, a cyan-blue 

 so saturated in hue that little or no white was required in the 

 disk of the rotator to match the color on the ring. In the 

 same manner similar changes in color have been observed in 

 the hue of the ring illuminated by the lamp. 



On illuminating one side of the screen by daylight and the 

 other by the white light of a Welsbach incandescent lamp, and 



then placing colored glasses or 

 films of colored gelatine between 

 the screen and the Welsbach lamp 

 effects of contrast are obtained 

 which are so readily imagined by 

 the physicist as to require no 

 description ; but they are worth 

 viewing. 



Increase of the purity and 

 luminosity of the contrast colors 

 by a rotating screen. — Make a 

 disk of white cardboard of 27 cms 

 in diameter, with eight sectors 

 cut out of it, and cover it with 

 translucent paper so that the 

 paper extends beyond the disk 

 and makes a border 3 cms wide. A circle of 8 cms in diameter is 

 left in the center of the disk and a ring of l^ cm on its border. 

 This disk is shown in fig. 8, but with only six sectors. It 

 should have eight. It is placed between the lamp and the 

 window, and rotated with a velocity of one revolution in about 

 \\ seconds, while it is viewed on the side facing the window. 

 The blue sectors thus take successively the places just before 

 occupied by the sectors of orange-yellow and the effect is to 



