The art of painting, properly so called, consists in laying on the least possible colour that will produce the required result, and this measurement, in all the ultimate, that is to say, the principal, operations of colouring, is so delicate that not one human hand in a million has the required lightness. The final touch of any painter properly so named, of Correggio--Titian--Turner--or Reynolds--would be always quite invisible to any one watching the progress of the work, the films of hue being laid thinner than the depths of the grooves in mother-of-pearl. The work may be swift, apparently careless, nay, to the painter himself almost unconscious. Great painters are so organized that they do their best work without effort: but analyze the touches afterwards, and you will find the structure and depth of the colour laid mathematically demonstrable to be of literally infinite fineness, the last touches passing away at their edges by untraceable gradation. The very essence of a master's work may thus be removed by a picture- cleaner in ten minutes. Observe, however, this thinness exists only in portions of the ultimate touches, for which the preparation may often have been made with solid colours, commonly, and literally, called "dead colouring," but even that is always subtle if a master lays it--subtle at least in drawing, if simple in hue; and farther, observe that the refinement of work consists not in laying absolutely _little_ colour, but in always laying precisely the right quantity. To lay on little needs indeed the rare lightness of hand; but to lay much,--yet not one atom _too_ much, and obtain subtlety, not by withholding strength, but by precision of pause,--that is the master's final sign-manual--power, knowledge, and tenderness all united. A great deal of colour may often be wanted; perhaps quite a mass of it, such as shall project from the canvas; but the real painter lays this mass of its required thickness and shape with as much precision as if it were a bud of a flower which he had to touch into blossom; one of Turner's loaded fragments of white cloud is modelled and gradated in an instant, as if it alone were the subject of the picture, when the same quantity of colour, under another hand, would be a lifeless lump. The following extract from a letter in the _Literary Gazette_ of 13th November, 1858, which I was obliged to write to defend a questioned expression respecting Turner's subtlety of hand from a charge of hyperbole, contains some interesting and conclusive evidence on the point, though it refers to pencil and chalk drawing only:--