XXXIII.
EPOPTICAL COLOURS.
429.
We have hitherto had to do with colours which appear with vivacity, but which immediately vanish again when certain conditions cease. We have now to become acquainted with others, which it is true are still to be considered as transient, but which, under certain circumstances, become so fixed that, even after the conditions which first occasioned their appearance cease, they still remain, and thus constitute the link between the physical and the chemical colours.
430.
They appear from various causes on the surface of a colourless body, originally, without communication, die or immersion (βαφή); and