FACILITY WITH WHICH COLOUR MAY BE MADE TO TEND EITHER TO THE PLUS OR MINUS SIDE.
710.
We have already had occasion to take notice of the mutability of colour in considering its so-called augmentation and progressive variations round the whole circle; but the hues even pass and repass from one side to the other, rapidly and of necessity.
711.
Physiological colours are different in appearance as they happen to fall on a dark or on a light ground. In physical colours the combination of the objective and subjective experiments is very remarkable. The epoptical colours, it appears, are contrasted according as the light shines through or upon them. To what extent the chemical colours may be changed by fire and alkalis, has