The Venetians went further than Leonardo in this view and practice; and he seems to allude to them when he criticises certain painters, who, in aiming at clearness and fulness of colour, neglected what, in his eyes, was of superior importance, namely, gradation and force of chiaro-scuro.[5] That increase of colour supposes increase of darkness, as so often stated by Goethe, may be granted without difficulty. To what extent, on the other hand, increase of darkness, or rather diminution of light, is accompanied by increase of colour, is a question which has been variously answered by various schools. Examples of the total negation of the principle are not wanting, nor are they confined to the infancy of the art. Instances, again, of the opposite tendency are frequent in Venetian and early Flemish pictures resembling the augmenting richness of gems or of stained glass:[6] indeed, it is not impossible that the increase of colour in shade, which is so remarkable in the pictures alluded to, may have been originally suggested by the rich and fascinating effect of stained glass; and the Venetians, in this as in many other respects, may have improved on a hint borrowed from the early German painters, many of whom painted on glass.[7] At all events, the principle of still increasing in colour in certain hues seems to have been adopted in Flanders and in Venice at an early period;[8] while Giorgione, in carrying the style to the most daring extent, still recommended it by corresponding grandeur of treatment in other respects. The same general tendency, except that the technical methods are less transparent, is, however, very striking in some of the painters of the school of Umbria, the instructors or early companions of Raphael.[9] The influence of these examples, as well as that of Fra Bartolommeo, in Florence, is distinctly to be traced in the works of the great artist just named, but neither is so marked as the effect of his emulation of a Venetian painter at a later period. The glowing colour, sometimes bordering on exaggeration, which Raphael adopted in Rome, is undoubtedly to be attributed to the rivalry of Sebastian del Piombo. This painter, the best of Giorgione's imitators, arrived in Rome, invited by Agostini Chigi, in 1511, and the most powerful of Raphael's frescoes, the Heliodorus and Mass of Bolsena, as well as some portraits in the same style, were painted in the two following years. In the hands of some of Raphael's scholars, again, this extreme warmth was occasionally carried to excess, particularly by Pierino del Vaga, with whom it often degenerated into redness. The representative of the glowing manner in Florence was Fra Bartolommeo, and, in the same quality, considered abstractedly, some painters of the school of Ferrara were second to none.