[XXXIII.]
EPOPTICAL COLOURS.
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.
They appear from various causes on the surface of a colourless body, originally, without communication, die or immersion (βαφή); and we now proceed to trace them, from their faintest indication to their most permanent state, through the different conditions of their appearance, which for easier survey we here at once summarily state.
First condition.—The contact of two smooth surfaces of hard transparent bodies.
First case: if masses or plates of glass, or if lenses are pressed against each other.