VISION 129 



discs on the one hand and of the black and white discs on the other 

 may then be adjusted till the whole surface becomes a uniform grey. 

 The quantities of each colour may then be read off by means of a 

 scale graduated in degrees and expressed in the form of an equation, 

 as in the following instance. 



156Y + 156B + 48G = 106W + 254Bk. 



Take overlapping discs of red and blue; by adjusting the relative 

 amounts of the two, and rotate, all shades of reddish-blue, blue- 

 purple, red-purple, bluish-red can be obtained. 



25. Colour wheel. Contrast. Rotate a disc having 

 alternate rings, one ring consisting of green and the 

 other of black in one half of the circle and white in the 

 remaining half. The white and black instead of fusing 

 into grey will appear pink. In order to avoid after- 

 sensations, the disc should only be uncovered and ob- 

 served, when it has attained its full rate of rotation. 



26. Take now a disc of alternate rings of green and white, and 

 view when it is rotating rapidly, the pink colour will be much less 

 distinct than in the previous experiment. 



27. Binocular antagonism. Set up the stereoscope, 

 and into the frame place two squares of cardboard on 

 which have been drawn designs. The designs should be 

 dissimilar and should be of different colours; e.g. on one 

 draw in red a square divided into a series of small 

 squares, on the other draw in green a circle divided into 

 segments by a number of diameters. On first looking at 

 the designs through the instrument, both will be seen at 

 one and the same time lying superposed on one another. 



Very shortly one of them disappears, the other re- 

 maining visible. 



A few moments later the images reverse, there being 

 an intermediate stage during which both are visible. 



