violaceous tint of No. 12. The intermediate steps of the transition can- not be discerned, — a decisive proof that the surfaces of the fibres which produce the green when the incidence is perpendicular, are not those which produce it when the incidence is oblique. The transition from No. 32 to No. 12 is so abrupt as to warrant this inference.
At all events the properties of the varying hues presented in nature are sufficiently interesting to be made the subject of a specific inquiry. I am at present engaged in collecting these colours, and hope that naturalists and experimental philosophers will contribute whatever they can toward the execution of a design likely to be attended with advantages, not only to Optics, but to other branches of science.
Unvarying Colours.
Nature presents a multitude of colours corresponding with the colours of the scale; but these are extremely changeable, while the natural colours are altogether unchangeable, except in the particular cases specified in the last paragraph but one. Let us fix our attention for a moment on the green, which is more prevalent than any other colour. Every herb, every leaf, is more or less of this colour. The green tints in the scale, of whatever order they may be, become red when the incidence is oblique: the same colours in herbs and leaves furnish no sign of such a transformation.
We know already, that the changes of tone to which the tints of the thin plates are subject diminish as the density of the plates increases. Were the substance of herbs and leaves much more dense than that of water, it might be said that it is owing to their excessive density that they suffer no perceptible change of tint from obliquity of incidence. But their density is far from being considerable; it is not so great as that of water. The phænomenon must therefore be explained in a different, and, as I think, in the following manner.
In applying the principle of the thin layers or plates to the explanation of the colours of bodies, it is supposed that those bodies are composed of layers analogous to the air and water introduced between Newton's glasses. Bodies are undoubtedly formed of very subtile particles; but have those particles or elementay groups the form of plates or laminae? It does not appear so; it seems rather, on the principles of crystallography, which divides them into cubes, octahedrons, tetrahedrons, &c., that their forms are [polyhedral] solids. This circumstance makes a serious difference, and ought to be attentively examined.
Let us take, for example, the cubical, which is one of the simplest forms. Let us suppose the section of one of these cubes made in the plane of reflexion, and the side or face on which the incident rays fall. In passing from the perpendicular incidence to the oblique incidences , , it is evident that, allowance being made for the effect
