In general it may be safe to assert that it is a defect for anything in a picture to be capable of being likened to another. Nothing is so charming as when things have their own quality, and are like nothing but themselves: always remembering that of the many qualities of which one object may be composed or partake, such only will be most prominent which are forced into notice from existing comparisons. It may be observed, however, that in description things can only be presented to the mind's eye by resemblances, and, in this case, when the object is to exalt the particular thing, exaggeration is allowable and necessary. Thus cheeks are like roses, clouds like gold, flesh like snow and vermilion, &c. In imitative art, where these substances are addressed to the actual eye, they require to be distinguished from each other. Still, the various modes in which nature may be rendered [according as the letter or the spirit is most aimed at, and above all according to the comparison or contrast of the moment], there will always be a resemblance between a painted imitation as an effect, and some general quality in nature independent of, and in addition to, the particular imitation aimed at. The truth of this is admitted by the terms of praise, and still more by those of dispraise, used to characterize pictures. A picture, for instance, is said to be golden, to be silvery, to [p. 378] be gem-like--to be mossy, to be wooly, to be wooden, to be tinny, &c. Now if we consider the laudatory comparisons which relate either to the color or some collateral quality, we find that no quality comprehends such absolute and universal excellence as the gem-like. It comprehends the golden and the silvery, only adding the quality of transparency; the pearly, the sparkling, the velvety, the glittering, the pure, the definite, are all comprehended in the gem-like. What is or may be wanting is the solid which borders on the opaque, the soft which borders on the misty, the flexible or undulating, &c. The qualities which more particularly constitute the gem, and which may be aimed at in painting, are precision of leading forms, and sharpness soon lost in softness which may always insure a sufficient approach to the flexible; this may be translated into precision of touch rather than of general form. The lights are the minimum of the colour, the deepest shades the maximum; reflexions are infinite and bright, but only sparkling in points. The shades are transparent, but all is transparent; and the character of the gem certainly is to be most lucid and clear in the lightest parts. This is Tintoret's system: his deep shades are often opaque and too black, but they are lighted up by sparks of brilliancy which, originally no doubt, gave transparency to the whole mass. Any colour, whether trunk of tree, rock, earth, or what not, may thus partake of the gem. [p. 379] The greatest care should be to make the reflexions sparkling and brilliant, for the lights will take care of themselves. A transparent substance exhibits its own colour, and reflects but little of others. To avoid too much monotony in a drapery, for instance, the hue may be varied ad infinitum if necessary by glazing variously, but still it will present but a series of gems, and not give the idea of an opaque substance reflecting foreign hues. Violent orange, vermilion, and all colours whose light is their maximum are not gem-like, but they give great value to those that are so. White can hardly be gem-like, unless the lights are treated with precision. Its cool shade is generally surrounded with a warm outline, for everything beyond it is probably warmer than itself. This warm outline is agreeable even when coming on blue. Black is most gem-like when glazed on white, so as to have none but internal lights, and if any are on the surface, still precise and definite. Hair has a silken quality of its own which does not partake of the gem; it reflects the light [does not drink it like a jewel], but with as much of its own colour as possible. The purple lights on black hair give it a very opaque look.
The same sort of resemblance in this good sense of the word [that is, a resemblance to something most perfect of its kind] may be aimed at in the choice of colours: various reds, for instance, are beautiful when they resemble the rose, the blood, the flame, the ruby: the colour of wine in a transparent glass is the same as the gem. [The Venetians loved to place it on a silvery white tablecloth.] Yellow, the golden, the gem-like; blue, the sapphire: the last is the most difficult colour to make brilliant, yet Titian does it. [pp. 378-381]