PART IV.

GENERAL CHARACTERISTICS. 688. We have hitherto, in a manner forcibly, kept phenomena asunder, which, partly from their nature, partly in accordance with our mental habits, have, as it were, constantly sought to be reunited. We have exhibited them in three divisions. We have considered colours, first, as transient, the result of an action and re-action in the eye itself; next, as passing effects of colourless, light-transmitting, transparent, or opaque mediums on light; especially on the luminous image; lastly, we arrived at the point where we could securely pronounce them as permanent, and actually inherent in bodies. 689. In following this order we have as far as possible endeavoured to define, to separate, and to class the appearances. But now that we need no longer be apprehensive of mixing or confounding them, we may proceed, first, to state the general nature of these appearances considered abstractedly, as an independent circle of facts, and, in the next place, to show how this particular circle is connected with other classes of analogous phenomena in nature. THE FACILITY WITH WHICH COLOUR APPEARS. 690. We have observed that colour under many conditions appears very easily. The susceptibility of the eye with regard to light, the constant re-action of the retina against it, produce instantaneously a slight iridescence. Every subdued light may be considered as coloured, nay, we ought to call any light coloured, inasmuch as it is seen. Colourless light, colourless surfaces, are, in some sort, abstract ideas; in actual experience we can hardly be said to be aware of them.--Note Z. 691. If light impinges on a colourless body, is reflected from it or passes through it, colour immediately appears; but it is necessary here to remember what has been so often urged by us, namely, that the leading conditions of refraction, reflection, &c., are not of themselves sufficient to produce the appearance. Sometimes, it is true, light acts with these merely as light, but oftener as a defined, circumscribed appearance, as a luminous image. The semi-opacity of the medium is often a necessary condition; while half, and double shadows, are required for many coloured appearances. In all cases, however, colour appears instantaneously. We find, again, that by means of pressure, breathing heat (432, 471), by various kinds of motion and alteration on smooth clean surfaces (461), as well as on colourless fluids (470), colour is immediately produced. 692. The slightest change has only to take place in the component parts of bodies, whether by immixture with other particles or other such effects, and colour either makes its appearance or becomes changed. THE FORCE OF COLOUR. 693. The physical colours, and especially those of the prism, were formerly called "_colores emphatici_," on account of their extraordinary beauty and force. Strictly speaking, however, a high degree of effect may be ascribed to all appearances of colour, assuming that they are exhibited under the purest and most perfect conditions. 694. The dark nature of colour, its full rich quality, is what produces the grave, and at the same time fascinating impression we sometimes experience, and as colour is to be considered a condition of light, so it cannot dispense with light as the co-operating cause of its appearance, as its basis or ground; as a power thus displaying and manifesting colour. THE DEFINITE NATURE OF COLOUR. 695. The existence and the relatively definite character of colour are one and the same thing. Light displays itself and the face of nature, as it were, with a general indifference, informing us as to surrounding objects perhaps devoid of interest or importance; but colour is at all times specific, characteristic, significant. 696. Considered in a general point of view, colour is determined towards one of two sides. It thus presents a contrast which we call a polarity, and which we may fitly designate by the expressions _plus_ and _minus_. _Plus. Minus_. Yellow. Blue. Action. Negation.[1] Light. Shadow. Brightness. Darkness. Force. Weakness. Warmth. Coldness. Proximity. Distance. Repulsion Attraction. Affinity with acids. Affinity with alkalis. COMBINATION OF THE TWO PRINCIPLES. 697. If these specific, contrasted principles are combined, the respective qualities do not therefore destroy each other: for if in this intermixture the ingredients are so perfectly balanced that neither is to be distinctly recognised, the union again acquires a specific character; it appears as a quality by itself in which we no longer think of combination. This union we call green. 698. Thus, if two opposite phenomena springing from the same source do not destroy each other when combined, but in their union present a third appreciable and pleasing appearance, this result at once indicates their harmonious relation. The more perfect result yet remains to be adverted to. AUGMENTATION TO RED. 699. Blue and yellow do not admit of increased intensity without presently exhibiting a new appearance in addition to their own. Each colour, in its lightest state, is a dark; if condensed it must become darker, but this effect no sooner takes place than the hue assumes an appearance which we designate by the word reddish. 700. This appearance still increases, so that when the highest degree of intensity is attained it predominates over the original hue. A powerful impression of light leaves the sensation of red on the retina. In the prismatic yellow-red which springs directly from the yellow, we hardly recognise the yellow. 701. This deepening takes place again by means of colourless semi-transparent mediums, and here we see the effect in its utmost purity and extent. Transparent fluids, coloured with any given hues, in a series of glass-vessels, exhibit it very strikingly. The augmentation is unremittingly rapid and constant; it is universal, and obtains in physiological as well as in physical and chemical colours. JUNCTION OF THE TWO AUGMENTED EXTREMES. 702. As the extremes of the simple contrast produce a beautiful and agreeable appearance by their union, so the deepened extremes on being united, will present a still more fascinating colour; indeed, it might naturally be expected that we should here find the acme of the whole phenomenon. COMPLETENESS THE RESULT OF VARIETY. 703. And such is the fact, for pure red appears; a colour to which, from its excellence, we have appropriated the term "purpur."[2] 704. There are various modes in which pure red may appear. By bringing together the violet edge and yellow-red border in prismatic experiments, by continued augmentation in chemical operations, and by the organic contrast in physiological effects. 705. As a pigment it cannot be produced by intermixture or union, but only by arresting the hue in substances chemically acted on, at the high culminating point. Hence the painter is justified in assuming that there are _three_ primitive colours from which he combines all the others. The natural philosopher, on the other hand, assumes only _two_ elementary colours, from which he, in like manner, developes and combines the rest. COMPLETENESS THE RESULT OF VARIETY IN COLOUR. 706. The various appearances of colour arrested in their different degrees, and seen in juxtaposition, produce a whole. This totality is harmony to the eye. 707. The chromatic circle has been gradually presented to us; the various relations of its progression are apparent to us. Two pure original principles in contrast, are the foundation of the whole; an augmentation manifests itself by means of which both approach a third state; hence there exists on both sides a lowest and highest, a simplest and most qualified state. Again, two combinations present themselves; first that of the simple primitive contrasts, then that of the deepened contrasts. HARMONY OF THE COMPLETE STATE. 708. The whole ingredients of the chromatic scale, seen in juxtaposition, produce an harmonious impression on the eye. The difference between the physical contrast and harmonious opposition in all its extent should not be overlooked. The first resides in the pure restricted original dualism, considered in its antagonizing elements; the other results from the fully developed effects of the complete state. 709. Every single opposition in order to be harmonious must comprehend the whole. The physiological experiments are sufficiently convincing on this point. A development of all the possible contrasts of the chromatic scale will be shortly given.[3] FACILITY WITH WHICH COLOUR MAY BE MADE TO TEND EITHER TO THE PLUS OR MINUS SIDE. 710. We have already had occasion to take notice of the mutability of colour in considering its so-called augmentation and progressive variations round the whole circle; but the hues even pass and repass from one side to the other, rapidly and of necessity. 711. Physiological colours are different in appearance as they happen to fall on a dark or on a light ground. In physical colours the combination of the objective and subjective experiments is very remarkable. The epoptical colours, it appears, are contrasted according as the light shines through or upon them. To what extent the chemical colours may be changed by fire and alkalis, has been sufficiently shown in its proper place. EVANESCENCE OF COLOUR. 712. All that has been adverted to as subsequent to the rapid excitation and definition of colour, immixture, augmentation, combination, separation, not forgetting the law of compensatory harmony, all takes place with the greatest rapidity and facility; but with equal quickness colour again altogether disappears. 713. The physiological appearances are in no wise to be arrested; the physical last only as long as the external condition lasts; even the chemical colours have great mutability, they may be made to pass and repass from one side to the other by means of opposite re-agents, and may even be annihilated altogether. PERMANENCE OF COLOUR. 714. The chemical colours afford evidence of very great duration. Colours fixed in glass by fusion, and by nature in gems, defy all time and re-action. 715. The art of dyeing again fixes colour very powerfully. The hues of pigments which might otherwise be easily rendered mutable by re-agents, may be communicated to substances in the greatest permanency by means of mordants. [1] Wirkung, Beraubung; the last would be more literally rendered _privation_. The author has already frequently made use of the terms _active_ and _passive_ as equivalent to _plus_ and _minus_.--T. [2] Wherever this word occurs incidentally it is translated _pure red_, the English word _purple_ being generally employed to denote a colour similar to violet.--T. [3] No diagram or table of this kind was ever given by the author.--T.