At the surface of all bodies rays of light are promiscuously reflected, or transmitted.
But if the next surface be very near to it, the rays of one colour chiefly are reflected, and the rest transmitted, and these places occur alternately for rays of each of the colours in passing from the thinnest to the thickest parts of the medium; so that several series, or orders, of colours will be visible on the surface of the same thin transparent body. On this principle coloured rings appear between a plane and a convex lens, in a little oil on the surface of water, and in bubbles made with soap and water.
When rays of light pass near to any body, so as to come within the sphere of its attraction and repulsion, an inflection takes place; all the kinds of rays being bent towards, or from, the body, and these powers affecting some rays more than others, they are by this means also separated from each other, so that coloured streaks appear both within the shadow, and the outside of it, the red rays being inflected at the greatest distance from the body.
Part of the light which enters bodies is retained within them, and proceeds no farther; but so loosely in some kinds of bodies, that a small degree of heat is sufficient to expel it again, so as to make the body visible in the dark: but the more heat is applied, the sooner is all the light expelled. This is a strong argument for the materiality of light. Bolognian phosphorus is a substance which has this property; but a composition made by Mr. Canton, of calcined oyster-shells and sulphur, in a much greater degree. However, white paper, and most substances, except the metals, are possessed of this property in a small degree.
Some bodies, especially phosphorus, and animal substances tending to putrefaction, emit light without being sensibly hot.
The colours of vegetables, and likewise their taste and smell, depend upon light. It is also by means of light falling on the leaves and other green parts of plants, that they emit dephlogisticated air, which preserves the atmosphere fit for respiration.
It is light that imparts colour to the skins of men, by means of the fluid immediately under them. This is the cause of tanning, of the copper colour of the North Americans, and the black of the Negroes. Light also gives colour to several other substances, especially the solutions of mercury in acids.