Colours may be arranged into seven classes, each of which depends upon different principles.
1. Transparent coloured fluids—transparent coloured gems—transparent coloured glasses—coloured powders—and the colours of the leaves and flowers of plants.
2. Oxidations on metals—colours of Labrador feldspar—colours of precious and hydrophanous opal, and other opalescences—the colours of the feathers of birds, of the wings of insects, and of the scales of fishes.
3. Superficial colours, as those of mother-of-pearl and striated surfaces.
4. Opalescences and colours in composite crystals having double refraction.
5. Colours from the absorption of common and polarized light by doubly refracting crystals.
6. Colours at the surfaces of media of different dispersive powers.
7. Colours at the surface of media in which the reflecting forces extend to different distances, or follow different laws.
The first two of these classes are the most important. The Newtonian theory appears to be strictly applicable to the phenomena of the second class; but those of the first class cannot, we conceive, be referred to the same cause.
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