Such were the artists, who, according to the most corresponding data, formed the style of that second period, which fixed the end and established the limits of art, on whose firm basis arose the luxuriant fabric of the third or the period of refinement, which added grace and polish to the forms it could not surpass; amenity or truth to the tones it could not invigorate; magic and imperceptible transition to the abrupt division of masses; gave depth and roundness to composition; at the breast of Nature herself caught the passions as they rose, and familiarized expression: The period of Apelles, Protogenes, Aristides, Euphranor, Pausias, the pupils of Pamphilus and his master Eupompus, whose authority obtained what had not been granted to his great predecessor and countryman Polycletus, the new establishment of the school of Sicyon.[19] The leading principle of Eupompus may be traced in the advice which he gave to Lysippus (as preserved by Pliny), whom, when consulted on a standard of imitation, he directed to the contemplation of human variety in the multitude of the characters that were passing by, with the axiom, 'that Nature herself was to be imitated, not an artist.' Excellence, said Eupompus, is thy aim, such excellence as that of Phidias and Polycletus; but it is not obtained by the servile imitation of works, however perfect, without mounting to the principle which raised them to that height; that principle apply to thy purpose, there fix thy aim. He who, with the same freedom of access to Nature as another man, contents himself to approach her only through his medium, has resigned his birth-right and originality together; his master's manner will be his style. If Phidias and Polycletus have discovered the substance and established the permanent principle of the human frame, they have not exhausted the variety of human appearances and human character; if they have abstracted the forms of majesty and those of beauty, Nature, compared with their works, will point out a grace that has been left for thee; if they have pre-occupied man as he _is_, be thine to give him that air with which he actually _appears_.[20]