"[W]e must begin by explaining how we have classed the different conditions under which colour is produced. We found three modes... three classes... or rather three exhibitions of them all. ...[W]e considered colours, as far as they... belong to the eye itself, and to depend on an action and re-action of the organ; next... as perceived in, or by means of, colourless mediums; and lastly... as belonging to particular substances. We have denominated the first, physiological, the second, physical, the third, chemical colours. The first are fleeting and not to be arrested; the next are passing, but... for a while enduring; the last may be made permanent for any length of time."