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 varj'ing 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 advan- 

 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. 

 Everj' 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 phaenomenon 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 elementarj' groups the form of plates or laminae ? 

 It does not appear so ; it seems rather, on the principles of crj'stallo- 

 graphy, Avhich 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 ab the side or face on which the incident rays fall. 

 In passing from the perpendicular incidence o m to the oblique inci- 

 dences p m, g m, it is evident that, allowance being made for the effect 



