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.