The farther we keep two systems apart, the better and the purer both will be, although the effect is the main object, and all other considerations of less consequence. The more we glaze in water-colour, the more subtle will be our work; the less we glaze in oil, the more satisfactory and firmer will be the result. Yet, obey no arbitrary laws; do whatever brings the conclusion quickest, and it must be legitimate. If your knife does better than your brush, use it; if your fingers, use them. Scratch, scrape, rub, cut, polish, shave, do whatever you think will succeed, and if it does not, try some other experiment until it does come up to your wishes. If the colour you have put on is not the one you need, put its complementary tint beside it and change its character, or dash it over or about it. Have only your object firmly fixed in your mind, and scruple at nothing in the way of experiment. The more methods you employ, the nearer you will be to the ever-changing method of nature. I have not touched upon half my points, and only _touched_ those I have gone over. I could go into details about the manner of mixing up the colours, and how to treat some of the special effects of nature, but that I have already done.[10] I am tempted to draw a sunset, or a storm, or a calm, with all their different objects in the background, middle distance, and foreground, dissect them into paint as I pass down, tell you what is sitting in the flank of one cloud and the total change of shadow in the next, also why it is so, and must be so according to this rule of three. I should like also to show you the same sportive three nestling in the sun-kissed breast of the same air-billow; quivering in the blue-grey haze of the far distance; floating on the glittering ocean, or diving amongst the deep reflections of the lake; peeping out of the froth and the heavy curl of the advancing wave; dashed into pieces on the dulsy sands; darting about the broad masses of the middle distance; playing at hide-and-seek amongst the branches, leaves, and weeds and rocks of the foreground. The three are everywhere, and infinite in their sport--two ever subordinate and obedient, advising, enhancing, leading; one ever acting the monarch, yet ever being dethroned and supplanted by one of the two counsellors, to join the other in obedience and then conspiracy, to leap up, take the reins, sink down vanquished, and all this for ever and for ever. And on Perspective; for although Gainsborough has said that the painter's eye is the best guide, we must have rules here, if only that we may forget them discreetly.