Diderot, in the essay on painting translated by Goethe, expressed himself as follows on this head: "He who once has truly felt and secured the apparition of flesh-colour is far on his way to perfect victory. Thousands of painters have died without such a feeling, and many thousands more will die without doing so." In so far as the material is concerned, by means of which this untransparent vitality of flesh is reproduced, the first medium to declare its suitability for such an effect was the oil-pigment. Work in mosaics is of all the least fitting to present us such a composite effect. Its permanency is no doubt a recommendation, but inasmuch as it can only express colour gradations through variously coloured glass cubes or stones placed in juxtaposition, it is wholly unable to reproduce the intermingling flow of one unified presentment of many colours. Fresco and tempera painting carry us considerably further in this direction. Yet in the case of fresco-painting the colours are put on the wet plaster with too great rapidity, so that, on the one hand, the greatest facility and sureness of brushwork is an essential, and, on the other, the work has to be carried out with broad adjacent strokes, which on account of their drying so rapidly do not admit of a fine degree of finish[301]. The same kind of difficulty meets us in the case of tempera-painting, a process[302] which no doubt admits of great lucidity of expression[303] and beautiful contrasts of light and shadow, yet for all that, by reason of the fact that its medium dries so quickly, is less adapted to the fusion and elaboration of its effects, and necessitates an articulate surface made up of definite strokes of the brush. The oil pigment, on the contrary, not only permits of the most tender and subtle melting together and elaborate fusion of colour effect, so that transitions are so imperceptible we cannot say where one colour begins and where it leaves off, but it is, where its component elements are properly fused and the execution of it is as it should be, itself remarkable for a luminous quality like that of precious stones, and it can, by virtue of its distinctions between opaque or transparent colours[304], reproduce in a far higher degree than tempera painting the translucency of different layers of colour.