Leopoldo Nobili

“ When a particular colour is ill-assorted with another, the eye is offended, as the ear is hurt by a discord. If we pass from one of these colours to the other through the intermediate tints, the first feeling will be changed into an agreeable sensation. Our scale, I repeat it, produces the same agreeable impression upon all, and it is to the inimitable beauty of its colours, and the manner in which they melt into each other, that this effect is due. ”

“ Let any colour whatsoever be exposed to some given degree of light and let the eyes be kept steadily fixed on it for some time: if the eyes be afterwards closed we have the impression of a different colour, which though it is never the same for one tint that it is for another, is always the same for the same tint. These colours, in some measure the offspring of the real colours, are called imaginary by philosophers, and by others they are named fantastic or accidental. ”

“ Depth, or intensity, and brightness are very different qualities. No one indeed confounds the intensity of a fine red with the brightness of a fine yellow. In the scale of the latter quality the white occupies the first place. A bright tint may be considered as a mixture, in which there is a little colour with a great quantity of white light; and, vice versâ, a strong or deep colour, as a mixture of much colour with a little white light. Painters therefore when they want to give brightness to their colours add white, but when they want to increase their intensity they add a different colour. ”

“ The origin of the violet colour given to steel to prevent it from rusting is the same. The layer however which produces this tint in the steel does not perhaps consist solely of oxygen, as it does when the metals are pure. Steel is a carburet of iron, and the oxygen of the air in being precipitated on this compound, becoming combined with the carbon in some manner or other might form the layer in question. At all events the layer does not change its nature; it is always electro-negative, and secures the metal from oxidation as effectually as the layers applied by the electro-chemical process. ”

“ I will not deny that the aptitude of the retina to cause an imaginary to arise from a real colour is of some account in the effect produced by colour. I am even inclined to believe that colours attentively observed are, by this tendency of the organ, associated with a sentiment and endowed with an expression which they could not otherwise possess. This however would be a species of melody and not of harmony.
Harmony is an instantaneous effect produced on the mind by several colours united altogether independent of the development of imaginary colours. ”

“ However I will say with a great philosopher, that "there is in Newton's rings no yellow that has green for its complement: the colour transmitted is invariably the blue; and this fact accords with the construction given by Newton for the composition of colours. ”

“ 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. ”

“ For if some of the rays destined to be transmitted are absorbed by the very substance of the gold, how can all the other rays, which are destined to be reflected in the interior of the same substance, escape undiminished? If the phænomenon be incomplete in respect to transmission, it will be equally so in respect to reflexion, and the tint formed will be necessarily different from that produced by the ordinary thin plates, which are so transparent as to arrest no species of rays whatsoever. ”

“ All who observe the seven colours of the spectrum will instantly perceive that they differ greatly in brightness. The clearest of them is the yellow. Fraunhofer, who has analysed the spectrum with so much care, assigns to that colour the highest degree of brightness.
The tints of our scale as well as the natural colours are far from being pure: they are all composed of several others. Hence arises a law of brightness different from that of the prismatic colours. The clearest on our scale are,
The most obscure tints are Nos. 10, 11, and 12, in which the violet and blue predominate. ”

“ In the next moment the chemical combination is effected, which takes place whenever the action of the heat is sufficiently prolonged. If the plate of copper be removed from the red-hot iron as soon as the first indication of a change of colour is perceived at any point, the process of coloration will then go on more slowly, the copper will not be oxidized, and the oxygen, which would produce this effect under a more prolonged action of the heat, now covers the plate with a film, which adheres to it like a varnish, and by its transparency produces the usual colours. ”

“ Leonardo da Vinci promised to give a table showing the colours which harmonize with one another and those which do not [15] : but he did not fulfill the promise, and no other painter that I know has pointed out precise rules for the harmony of colours. Several have observed merely that red combined with green has an agreeable effect; Newton apprises us that orange agrees with indigo; and Virgil was perhaps of the same opinion when he put this verse into the mouth of his Naiad,
Mollia luteola pingit vaccinia caltha. ”

“ Yellow approaches more nearly to the nature of red than to that of azure, and is consequently rather warm than cold. Pure green cannot be said to be cold or warm: it inclines however to the former or to the latter accordingly as it is combined with blue or with yellow.
Cheerfulness and Gloominess.
Cheerfulness is not to be confounded with beauty, nor gloominess with monotony: they are more distinct sensations and seem to belong, the first to the lower colours of the spectrum, such as the red, orange, &c., and the second to the superior colours, such as the violet, indigo, &c. ”

“ 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. ”

“ If bodies where composed of thin layers such as those which form the chromatic scale and Newton's rings, their colours would change with every change of incidence, conformably to the law which we have just indicated. In nature the number of those colours that change is but small in comparison with those that remain fixed. ”

“ Let us return, for an instant, to the experiment of the coloured rings developed on a surface of platina by means of the electro-chemical apparatus described in the beginning of this Memoir. The platina surface belongs to the positive pole of the pile, and the electro-negative elements of the solution (which in the present case are the oxygen of water and the acid of acetate of lead) are deposited at this pole. I will not undertake to say by what species of affinity or force it is that these elements are attracted to each other and spread out into thin films on the platina. ”

“ The violet rings are the narrowest and nearest; the red are the widest and most distant; the rings of the intermediate colours are of intermediate dimensions and at intermediate distances. When the incident light is white, the series of homogeneous rings are formed simultaneously and overlap each other; all the colours are intermixed in different proportions, and none stands isolated. It is to these combinations that we are to attribute the tints of the thin layers which we are about to analyse on our scale. ”

“ Those tints which contain the element of red are by painters called warm, and those in which the element of azure abounds are termed cold.
Red is the strongest and the most vivid colour: it is the colour of fire and of blood, and it warms and inflames all the tints into which it is introduced. ”

“ In this case, as well as in others of a similar kind, the colour that we observe is not given out by one continuous surface, such as that of a single plate: it is produced by a multitude of threads or fibres, so nicely overlapping one another that they seem to form a perfect plane, although they are really but a vast number of distinct minute surfaces, the position and thickness of which it would be necessary to know in order to apply the general law to them with any prospect of success. The phænomenon possesses all the characteristics of that produced by thin plates ”

“ An exclamation or shout of joy consists of notes ascending from the grave to the acute; a cry proceeding from grief or pain consists, on the contrary, of notes descending from the acute to the grave. It is not more singular than true, although it has never before been remarked, that the same notes sung or executed on an instrument will produce in the ascending scale a very different effect from that which they produce in the descending scale. ”

“ 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. ”

“ The combinations that may be obtained in this way are almost innumerable, and, it will be said, need well be so, in order to match the variety which nature exhibits. Such is our opinion too; but we shall not attempt to conceal the difficulty presented by the fact, that several of the natural colours, especially those of the metallic substances, have but a very slight resemblance to the colours of thin plates, among which it were vain to seek, for example, either the yellow of gold or the red of copper. ”

“ Nothing can be more beautiful than the varying colours: when we call them varying it is unnecessary to say why they please. Painters, we know, in order to give a finish to their productions, overlay them with certain tints. The colours of the painting appear through the tint, are mingled but not confounded with it, and thus are produced a variety and vividness unattainable by any other means. ”

“ It is to arrange these homogeneous tints in their natural order, so that they may form a scale or gamut which I shall henceforth designate by the epithet chromatic.
Science never consults its interests so truly as when it aims at some useful object connected with the arts. ”

“ But this is not all: the superficial colours of which we speak are changeable, and belong evidently to the same class as those produced by thin plates. Now the pure metals, as we have already seen, are, from their opacity, incapable of this species of coloration. Can they acquire that capacity in their first degree of oxidation by becoming suddenly transparent in consequence of their union with a small quantity of oxygen? The hypothesis far exceeds the bounds of probability, and the phænomenon requires to be otherwise explained. ”

“ Some of those changes are produced even by day if the colours of the scale are viewed through a green crystal, and others if they are viewed through yellow or azure crystals. Artificial light is without doubt differently constituted from that of the sun: it contains probably a scanty mixture of red rays with an abundance of the yellow, green, and azure. ”

“ This is a novel state for oxygen and the acids, and is distinguished from their ordinary combination by the three following peculiarities: 1st, The metal retains, beneath the deposited layer, its natural brilliancy; 2nd, this layer produces the phænomenon of the coloured rings in all its beauty; 3rd, instead of oxidizing the metal, these electro-negative elements contribute to secure it against oxidation in every part to which they are applied [12] . ”

“ The colour of the thinnest plate is placed first, and the others follow in the order of the progressively increasing thickness of the plates [3] . In this arrangement I cannot be mistaken, because the layers or thin plates which produce the several colours are all applied by the same electro-chemical process. The pile, the solution, the distances remain exactly the same. There is nothing variable but the duration of the action, which in respect to the layer No. 1. is very short, a little longer in respect to the second, and increases progressively from the lowest to the highest number. ”

“ Artists, it is true, being generally unacquainted with physical theories, will find it difficult to follow me in my inquiries. My labour, however will not be altogether useless to them, if, as I intended, I have succeeded in treating certain parts of the subject in a manner likely to bring them within the reach of every understanding. ”

“ It was necessary, I suppose, that the atmosphere should be composed of the most subtile particles, in order that they might remain suspended in space; the earth did not require to be of so delicate a texture. ”

“ 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 most gloomy tints on the scale are, according to the generally received opinion, those of Nos. 10, 11 and 12, in which the higher colours of the spectrum abound. These colours, it cannot be denied, are also the least bright, and this quality may well be the cause of the gloominess which is felt in viewing them. ”