This infinity, or endless variation of hue, tint, and relation, of which the tertiaries are susceptible, gives a boundless license to the revelry of taste, in which the genius of the pencil may display the most captivating harmonies of colouring, and the most chaste and delicate expressions; too subtle to be defined, too intricate to be easily understood, and often too exquisite to be felt by the untutored eye. Nature always melodizes by imperceptible gradations, while she harmonizes by distinct contrasts. At different seasons we have blossoms of all hues, variously subordinated; and when the time of flowers may be considered past, as if she had no further use for her fine colours, or were willing to display her ultimate skill and refinement, Nature lavishes the contents of her palette, not disorderly, but in multiplied relations, over all vegetal creation, in those rich and beautiful accordances of broken and finishing colours with which autumn is decorated ere the year decays and sinks into olive darkness. As a rule, no colour exists in nature without gradation, which is to colours what curvature is to lines. The difference in mere beauty between a gradated and ungradated colour may be seen by laying an even tint of rose-colour on paper, and putting a rose leaf beside it. The victorious beauty of the rose, as compared with other flowers, depends wholly on the delicacy and quantity of its colour gradations, all other flowers being either less rich in gradation, not having so many folds of leaf; or less tender, being patched and veined instead of flushed. It is not enough, however, that colour should be gradated in painting by being made simply paler or darker at one place than another. Generally, colour changes as it diminishes, and is not only darker at one spot, but also purer at one spot than elsewhere; although it does not follow that either the darkest or the lightest spot should be the purest. Very often the two gradations more or less cross each other, one passing in one direction from paleness to darkness, another in another direction from purity to dulness; but there will almost always be both of them, however reconciled. Hence, every piece of blue, say, laid on should be quite pure only at some given spot, from which it must be gradated into blue less pure--greyish blue, or greenish blue, or purplish blue--over all the rest of the space it occupies. In Turner's largest oil pictures, there is not one spot of colour as large as a grain of wheat ungradated; and it will be found in practice that brilliancy of hue, vigour of light, and even the aspect of transparency in shade, are essentially dependent on this character alone; hardness, coldness, and opacity, resulting far more from equality of colour than from nature of colour. Given some mud off a city crossing, some ochre out of a gravel pit, a little whitening, and some coal-dust, and a luminous picture might be painted, if time were allowed to gradate the mud, and subdue the dust.