Among the works of Polygnotus, as mentioned by Pliny, are his paintings in the Temple at Delphi, in the Propylaea of the Acropolis, in the Temple of Theseus, and in the Temple of the Dioscuri at Athens. He painted in a truly religious spirit, and upon symmetrical principles, with great grandeur and freedom, resembling Michael Angelo more than any other modern artist. The use of oil was unknown to the ancients. The artists painted upon wood, clay, plaster, stone, parchment, but not upon canvas, which was not used till the time of Nero. They painted upon tablets or panels, and not upon the walls,--the panels being afterward framed and encased in the walls. The stylus, or cestrum, used in drawing and for spreading the wax colors was pointed on one end and flat on the other, and generally made of metal. Wax was prepared by purifying and bleaching, and then mixed with colors. When painting was practised in watercolors, glue was used with the white of an egg or with gums; but wax and resins were also worked with water, with certain preparations. This latter mode was called encaustic, and was, according to Plutarch, the most durable of all methods. It was not generally adopted till the time of Alexander the Great. Wax was a most essential ingredient, since it prevented the colors from cracking. Encaustic painting was practised both with the cestrum and the pencil, and the colors were also burned in. Fresco, or water-color, on fresh plaster, was used for coloring walls, which were divided into compartments or panels. The composition of the stucco, and the method of preparing the walls for painting, is described by the ancient writers: "They first covered the walls with a layer of ordinary plaster, over which, when dry, were successively added three other layers of a finer quality, mixed with sand. Above these were placed three layers of a composition of chalk and marble-dust, the upper one being laid on before the under one was dry; by which process the different layers were so bound together that the whole mass formed one beautiful and solid slab, resembling marble, and was capable of being detached from the wall and transported in a wooden frame to any distance. The colors were applied when the composition was still wet. The fresco wall, when painted, was covered with an encaustic varnish, both to heighten the color and to preserve it from the effects of the sun or the weather; but this process required so much care, and was attended with so much expense, that it was used only in the better houses and palaces." The later discoveries at Pompeii show the same correctness of design in painting as in sculpture, and also considerable perfection in coloring. The great artists of Greece--Phidias and Euphranor, Zeuxis and Protogenes, Polygnotus and Lysippus--were both sculptors and painters, like Michael Angelo; and the ancient writers praise the paintings of these great artists as much as their sculpture.