Ben

“To paint after nature is to transfer three-dimensional corporeality to a two-dimensional surface. This you can do if you are in good health and not colorblind. Oil paint, canvas, and brush are material and tools. It is possible by expedient distribution of oil paint on canvas to copy natural impressions; under favorable conditions you can do it so accurately that the picture cannot be distinguished from the model. You start, let us say, with a white canvas primed for oil painting and sketch in with charcoal the most discernible lines of the natural form you have chosen. Only the first line may be drawn more or less arbitrarily, all the others must form with the first the angle prescribed by the natural model. By constant comparison of the sketch with the model, the lines can be so adjusted that the lines of the sketch will correspond to those of the model. Lines are now drawn by feeling, the accuracy of the feeling is checked and measured by comparison of the estimated angle of the line with the perpendicular in nature and in the sketch. Then, according to the apparent proportions between the parts of the model, you sketch in the proportions between parts on the canvas, preferably by means of broken lines delimiting these parts. The size of the first part is arbitrary, unless your plan is to represent a part, such as the head, in 'life size.' In that case you measure with a compass an imaginary line running parallel to a plane on the natural object conceived as a plane on the picture, and use this measurement in representing the first part. You adjust all the remaining parts to the first through feeling, according to the corresponding parts of the model, and check your feeling by measurement; to do this, you place the picture so far away form you that the first part appears as large in the painting as the model, and then you compare. In order to check a given proportion, you hold out the handle of your paintbrush at arm's length towards this proportion in such a way that the end of the thumbnail on the handle coincides with the other end of the proportion. If then you hold the paintbrush out towards the picture, again at arm's length, you can, by the measurement thus obtained, determine with photographic accuracy whether your feeling has deceived you. If the sketch is correct, you fill in the parts of the picture with color, according to nature. The most expedient method is to begin with a clearly recognizable color of large area, perhaps with a somewhat broken blue. You estimate the degree of matness and break the luminosity with a complimentary color, ultramarine, for example, with light ochre. By addition of white you can make the color light, by addition of black dark. All this can be learned. The best way of checking for accuracy is to place the picture directly beside the projected picture surface in nature, return to your old place and compare the color in your picture with the natural color. By breaking those tones that are too bright and adding those that are still lacking, you will achieve a color tonality as close as possible to that in nature. If one tone is correct, you can put the picture back in its place and adjust the other colors to the first by feeling. You can check your feeling by comparing every tone directly with nature, after setting the picture back beside the model. If you have patience and adjust all large and small lines, all forms and color tones according to nature, you will have an exact reproduction of nature. This can be learned. This can be taught. And in addition, you can avoid making too many mistakes in 'feeling' by studying nature itself through anatomy and perspective and your medium through color theory. That is academy.”
― Kurt Schwitters, The Dada Painters and Poets: An Anthology