1911.] 



Simultaneous Colour Contrast. 



551 



images appear monochromatic in themselves, but different in hue and 

 luminosity. This is particularly noticeable in the blue-green region of the 

 spectrum, as this is one of the portions of the spectrum in which the 

 monochromatic divisions possess the fewest wave-lengths. The images are, 

 however, larger, because of the increased separation of the wave-lengths in 

 this region. When viewed with the double-image prism, a monochromatic 

 division shows two monochromatic images side by side, and exactly similar 

 in every respect when the intensity of both is similar, but, if the intensity of 

 one be greater than that of the other, one will appear definitely blue and the 

 other definitely green. I am inclined to think that Prof. "Watson's results* 

 are due to this cause, especially in association with stray light of different, 

 wave-lengths. 



This method is one which enables us to study very accurately the effects of 

 simultaneous contrast. A monochromatic region can be viewed with the 

 double-image prism, so that it appears as two images with a small space 

 between. One of the shutters of the spectrometer can then be moved, 

 so that the two images increase in size and just touch. It will be noticed 

 that the effect of contrast is most apparent at the edges — for instance, if 

 a yellow region be observed, one edge appears green and the other adjacent 

 edge appears orange. The whole of the image appears to be altered, that is 

 to say, the image at which the orange edge is seen appears to be more yellow 

 throughout, and the green one more green throughout. Each appears as if 

 it were moved further from the other in the spectral range. We can, 

 however, make the same wave-length appear as different colours in the 

 following way : — If a monochromatic region be isolated, for instance, yellow, 

 no difference being detected, the whole of the wave-lengths occupying this 

 region appear yellow. If, however, we take the wave-length occupying the 

 central position of the region and move the shutter on the green side until 

 it occupies this central position, we can then move the shutter on the red 

 side until a fresh monochromatic region is observed. This appears absolutely 

 uniform in colour, but the colour appears orange-yellow instead of yellow, 

 including that portion that was previously seen as yellow. We can now 

 move the shutter on the red side till it occupies the centre of the first- 

 mentioned yellow region, and then, extending the shutter on the green side, 

 form a fresh monochromatic region. The colour will appear absolutely 

 uniform as before, but it has now changed, and the whole has become 

 greenish-yellow. 



The contrast colour is most developed when the surface on which it is 

 seen is small and situated on a large surface of very pale colour which it 

 * ' Eoy. Soc. Proc.,' B, 1911, p. 118. 



2 S 2 



