Something more remains of Luca della Robbia; something more of a history, of outward changes and fortunes, is expressed through his work. I suppose nothing brings the real air of a Tuscan town so vividly to mind as those pieces of pale blue and white earthenware, by which he is best known, like fragments of the milky sky itself, fallen into the cool streets, and breaking into the darkened churches. And no work is less imitable: like Tuscan wine, it loses its savour when moved from its birthplace, from the crumbling walls where it was first placed. Part of the charm of this work, its grace and purity and finish of expression, is common to all the Tuscan sculptors of the fifteenth century; for Luca was first of all a worker in marble, and his works in terra cotta only transfer to a different material the principles of his sculpture. These Tuscan sculptors of the fifteenth century worked for the most part in low relief, giving even to their monumental effigies something of its depression of surface, getting into them by this means a pathetic suggestion of the wasting and etherealisation of death. They are haters of all heaviness and emphasis, of strongly-opposed light and shade, and seek their means of delineation among those last refinements of shadow, which are almost invisible except in a strong [65] light, and which the finest pencil can hardly follow. The whole essence of their work is expression, the passing of a smile over the face of a child, the ripple of the air on a still day over the curtain of a window ajar. What is the precise value of this system of sculpture, this low relief? Luca della Robbia, and the other sculptors of the school to which he belongs, have before them the universal problem of their art; and this system of low relief is the means by which they meet and overcome the special limitation of sculpture. That limitation results from the material and other necessary conditions of all sculptured work, and consists in the tendency of such work to a hard realism, a one-sided presentment of mere form, that solid material frame which only motion can relieve, a thing of heavy shadows, and an individuality of expression pushed to caricature. Against this tendency to the hard presentment of mere form trying vainly to compete with the reality of nature itself, all noble sculpture constantly struggles; each great system of sculpture resisting it in its own way, etherealising, spiritualising, relieving its stiffness, its heaviness, and death. The use of colour in sculpture is but an unskilful contrivance to effect, by borrowing from another art, what the nobler sculpture effects by strictly appropriate means. To get not colour, but the equivalent of colour; to secure the expression and the play of life; to [66] expand the too firmly fixed individuality of pure, unrelieved, uncoloured form:--this is the problem which the three great styles in sculpture have solved in three different ways.