74, GORHAM, ON THE 
subject, would seem to balance each other by their reciprocal 
action. A necessary condition for the production of the 
contrasted colours is, that the part of the retina in which the 
new colour is to be excited shall be in a state of comparative 
repose; hence the small object itself must be gray. A 
second condition is, that the colour of the surrounding sur- 
face shall be very bright, that is, it shall contain much white 
lieht:? * 
The required conditions are fulfilled very exactly by rota- 
tion in the following experiment. Take a disc composed of 
equal parts (half-discs) of white and red, and rotate; during 
rotation drop down upon the spindle the black double semi- 
zone (fig. 9), which will quickly revolve with the same 
velocity as that of the colour-top itself; now gently breathe 
upon this black zone, and when one of its rings appears of a 
deep red colour the other ring will present a greenish tint ; 
the greenish hue which is thus evoked, and which is the 
complementary of the red, is rendered visible by being thus 
thrown directly upon a contiguous gray surface. Analogous 
results take place with every colour by arranging them in 
the above order ; the illustrations are generally chosen, how- 
ever, from green and red, as the contrasts are more palpable 
to the eye, and assist in educating the uninitiated in the 
perception of such delicate phenomena. 
4. Contrast of Colour. 
If the eye sees at the same time two contiguous colours, 
they will both appear modified from their contiguity. ““When 
it is asserted of the phenomena of simultaneous contrast,” 
says Chevreul, “that one colour placed beside another re- 
ceives a modification from it, it must not be forgotten that 
this manner of speaking does not mean that the two colours, 
or rather the two material objects that present them to us, 
have a mutual action, either physical or chemical; it is 
really only applied to the modification that takes place before 
us, when we perceive the simultaneous impression of these 
two colours, and which reciprocally excite each other in the 
retina of the eye.” 
The modifications of contiguous colours result from the 
addition to each of them of the complementaries of the other. 
If, for example, two narrow strips of red and orange paper 
are placed side by side, in contact at their edges, and gazed 
* ©Hand-book of Physiology,’ by Kirkes and Paget. Second edition, 
pp. 552, 553, 
