165. Now, these are the first terms of all well-constructed bas-relief. The mass you have to treat consists of a piece of stone which, however you afterwards carve it, can but, at its most projecting point, reach the level of the external plane surface out of which it was mapped, and defined by a depression round it; that depression being at first a mere trench, then a moat of a certain width, of which the outer sloping bank is in contact, as a limiting geometrical line, with the laterally salient portions of sculpture. This, I repeat, is the primal construction of good bas-relief, implying, first, perfect protection to its surface from any transverse blow, and a geometrically limited space to be occupied by the design, into which it shall pleasantly (and as you shall ultimately see, ingeniously,) contract itself: implying, secondly, a determined depth of projection, which it shall rarely reach, and never exceed: and implying, finally, the production of the whole piece with the least possible labor of chisel and loss of stone. 166. And these, which are the first, are very nearly the last constructive laws of sculpture. You will be surprised to find how much they include, and how much of minor propriety in treatment their observance involves. In a very interesting essay on the architecture of the Parthenon, by the Professor of Architecture of the Ecole Polytechnique, M. Emile Boutmy, you will find it noticed that the Greeks do not usually weaken, by carving, the constructive masses of their building; but put their chief sculpture in the empty spaces between the triglyphs, or beneath the roof. This is true; but in so doing, they merely build their panel instead of carving it; they accept, no less than the Goths, the laws of recess and limitation, as being vital to the safety and dignity of their design; and their noblest recumbent statues are, constructively, the fillings of the acute extremity of a panel in the form of an obtusely summited triangle. 167. In gradual descent from that severest type, you will find that an immense quantity of sculpture of all times and styles may be generally embraced under the notion of a mass hewn out of, or, at least, placed in, a panel or recess, deepening, it may be, into a niche; the sculpture being always designed with reference to its position in such recess: and, therefore, to the effect of the building out of which the recess is hewn. But, for the sake of simplifying our inquiry, I will at first suppose no surrounding protective ledge to exist, and that the area of stone we have to deal with is simply a flat slab, extant from a flat surface depressed all round it.