And whenever you look at any piece of sculpture, determine first to which of these classes it belongs; and then consider how the sculptor has treated it with reference to the necessary structure--that reference, remember, being partly to the mechanical conditions of the material, partly to the means of light and shade at his command. [Illustration: FIG. 9.] 177. To take a single instance. You know, for these many years, I have been telling our architects, with all the force of voice I had in me, that they could design nothing until they could carve natural forms rightly. Many imagined that work was easy; but judge for yourselves whether it be or not. In Plate XII., I have drawn, with approximate accuracy, a cluster of Phillyrea leaves as they grow, Now, if we wanted to cut them in bas-relief, the first thing we should have to consider would be the position of their outline on the marble;--here it is, as far down as the spring of the leaves. But do you suppose that is what an ordinary sculptor could either lay for his first sketch, or contemplate as a limit to be worked down to? Then consider how the interlacing and springing of the leaves can be expressed within this outline. It must be done by leaving such projection in the marble as will take the light in the same proportion as the drawing does;--and a Florentine workman could do it, for close sight, without driving one incision deeper, or raising a single surface higher, than the eighth of an inch. Indeed, no sculptor of the finest time would design such a complex cluster of leaves as this, except for bronze or iron work; they would take simpler contours for marble; but the laws of treatment would, under these conditions, remain just as strict: and you may, perhaps, believe me now when I tell you that, in any piece of fine structural sculpture by the great masters, there is more subtlety and noble obedience to lovely laws than could be explained to you if I took twenty lectures to do it in, instead of one. [Illustration: XIII. GREEK FLAT RELIEF, AND SCULPTURE BY EDGED INCISION.]