104 M.NOBILI ON COLOURS, AND ON A NEW CHROMATIC SCALE 
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 yarying 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 adyan- 
tages, 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 spe- 
cified 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 inci- 
dence 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 phenomenon must therefore be explained in a dif- 
ferent, 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 elementary groups the form of plates or laminz ? 
It does not appear so; it seems rather, on the principles of crystallo- 
graphy, 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 a6 the side or face on which the incident rays fall. 
In passing from the perpendicular incidence 0 m to the oblique inci- 
dences pm, gm, it is evident that, allowance being made for the effect 
a ry 
