158 Mr. A. M. Mayer on the Phenomena 



The translucent paper of the grating G and of the screen 

 Y appear orange-yellow. The bands of white cardboard of 

 the grating appear cyan-blue. The translucent paper of the 

 screen Wh is white. 



Sit a little to one side of the grating so as not to intercept 

 the light from the window, and look at the grating through 

 the calc-spar prism. Kotate it till the blue bands of the 

 grating are superposed on the orange bands, when, if the sur- 

 face of 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 super- 

 posed images of the grating. 



When this experiment is carefully made, with the proper 

 illumination, the effect is surprising when, in favourable con- 

 ditions, 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 milllim. gave the best 

 results. 



In experiments with the screen (fig. 1) , formed of the card- 

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

 relative intensities of the daylight and lamp-light and by 

 changing the distance of the lamp from the screen, to pro- 

 duce great changes in the saturation of the contrast-colours. 

 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 colour on the 

 ring. In the same manner similar changes in colour 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 coloured glasses or films of coloured 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- 

 Colours by a Rotating Screen. — Make a disk of white card- 

 board of 27 centim. 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 centim. wide. 

 A circle of 8 centim. in diameter is left in the centre of the 

 disk and a ring of 1^ centim. 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 1^ second, 



