To select a second era sufficiently marked by addition or revolution of principle, is difficult. To the age of Phidias, the art continued certainly to improve, but very slowly, being left far in the rear by Sculpture. The genius of this consummate master, who indeed had originally commenced his career as a painter, extended to all the arts; and, under such an instructer, his brother Penænus, very highly distinguished himself, though vanquished in a contest for the public prize, then instituted at Delphos and Corinth. From the middle of the fifth century, then, a decided movement commences in the history of painting,--a preparation for something still greater. The influence extended among the able contemporaries of the great sculptor. Polygnotus of Thasos then first succeeded, to borrow a phrase, 'in the expression of undescribed being,' and whose pictures Pliny admired six hundred years afterwards. Improvement was carried forward for half a century by Mycon, famous in horses; Pauson, his rival; Dionysius of Colophon, praised by Ælian for minute accuracy; Aglaophon, bold and energetic; Colotes, sculptor and painter; Evenor, father of Parrhasius; and finally, greatest of all, Apollodorus the Athenian, who invented or perfected the knowledge of light and shade. With this artist, the precursor and contemporary of Zeuxis, and whose discovery may be placed about the commencement of the fourth century B. C., may be terminated the second era. The propriety of this division will more obviously appear, when it is considered that to this period, not only was the art deficient in the most powerful of its means, the magic of chiar' oscura, but also in its instruments. The ancient paintings, as late as the age of Phidias, were executed with the _cestrum_, a species of pliant _stylus_, similar to that used in writing. This is the diagraphic, or linear method, and seems to have resembled our chalk and crayon, or perhaps more closely our pen and reed drawing. The process, however, can be explained only by conjecture. The tablet, primed in white, was laid over with a varnish of resin mixed with wax, and usually incorporated with a dark-reddish coloring matter. Upon this the subject was traced, and the lights worked in with the cestrum of various fineness. At what precise period this imperfect instrument was superseded by the pencil, or if the effects of the two were combined, is unknown. But the invention must have been made after the death of Polygnotus, and prior to the ninetythird Olympiad, a period of twenty or thirty years, when Apollodorus is known to have handled the pencil with great effect. It is not unlikely, therefore, that this artist either was the inventor or the improver of this tool, whose mastery so decidedly ministered to his reputation.