Thus his heads exhibit the aim at beauty and dignity belonging to the earlier period, only combined with more truth of nature.  His draperies unite its pure taste and softness of folds with greater breadth; the realistic principle being apparent in that greater attention to detail which a delicate indication of the material necessitates.  Nude figures are studied from nature with the utmost fidelity; undraped portions are also given with much truth, especially the hands; only the feet remain feeble.  That, however, which is almost the principal quality of his art, is the hitherto unprecedented power, depth, transparency, and harmony of his colouring.  To attain this he availed himself of a mode of painting in oil which he and his brother had perfected.  Oil painting, it is true, had long been in use, but only in a very undeveloped form, and for inferior purposes.  According to the most recent and thorough investigations, the improvement introduced by the Van Eycks, and which they doubtless only very gradually worked out, were the following.  First, they removed the chief impediment which had hitherto obstructed the application of oil-paint to pictures properly so called.  For, in order to accelerate the slow drying of the oil colours, it had been necessary to add a varnish to them, which consisted of oil boiled with a resin.  Owing to the dark colour of this varnish, in which amber, or more frequently sandarac, was used, this plan, from its darkening effect on most colours, had hitherto proved unsuccessful.  The Van Eycks, however, succeeded in preparing so colourless a varnish that they could apply it without disadvantage, to all colours.  In painting a picture they proceeded on the following system.  The outline was drawn on a gesso ground, so strongly sized that no oil could penetrate the surface.  The under painting was then executed in a generally warm brownish glazing colour, and so thinly that the light ground was clearly seen through it.  They then laid on the local colours, thinner in the lights, and, from the quantity of vehicle used, more thickly in the shadows; in the latter availing themselves often of the under painting as a foil.  In all other parts they so nicely preserved the balance between the solid and the glazing colours as to attain that union of body and transparency which is their great excellence.  Finally, in the use of the brush they obtained that perfect freedom which the new vehicle permitted; either leaving the touch of the brush distinct, or fusing the touches tenderly together, as the object before them required.  Of all the works which are now attributed to Hubert, but one is genuine and historically authenticated.  This noble work is certified by an inscription.  It is a large altar picture, consisting of two rows of separate panels, once in the Cathedral of St. Bavon at Ghent.  It was painted, as before remarked, for Jodocus Vydts, Seigneur of Pamele, and Burgomaster of Ghent, and his wife Elizabeth,