Various shades of grey placed next each other in gradation will exhibit at their edges, either blue and violet only, or red and yellow only, according as the darker grey is placed over or under.
A series of such shades of grey placed horizontally next each other will be coloured conformably to the same law according as the whole series is relieved, on a black or white ground above or below.
The observer may see the phenomena exhibited by the prism at one glance, by enlarging the plate intended to illustrate this section.[1]
It is of great importance duly to examine and consider another experiment in which a grey object is placed partly on a black and partly on a white surface, so that the line of division passes vertically through the object.