168. A _flat_ slab, observe. The flatness of surface is essential to the problem of bas-relief. The lateral limit of the panel may, or may not, be required; but the vertical limit of surface _must_ be expressed; and the art of bas-relief is to give the effect of true form on that condition. For observe, if nothing more were needed than to make first a cast of a solid form, then cut it in half, and apply the half of it to the flat surface;--if, for instance, to carve a bas-relief of an apple, all I had to do was to cut my sculpture of the whole apple in half, and pin it to the wall, any ordinarily trained sculptor, or even a mechanical workman, could produce bas-relief; but the business is to carve a _round_ thing out of a _flat_ thing; to carve an apple out of a biscuit!--to conquer, as a subtle Florentine has here conquered,[33] his marble, so as not only to get motion into what is most rigidly fixed, but to get boundlessness into what is most narrowly bounded; and carve Madonna and Child, rolling clouds, flying angels, and space of heavenly air behind all, out of a film of stone not the third of an inch thick where it is thickest. 169. Carried, however, to such a degree of subtlety as this, and with so ambitious and extravagant aim, bas-relief becomes a tour-de-force; and, you know, I have just told you all tours-de-force are wrong. The true law of bas-relief is to begin with a depth of incision proportioned justly to the distance of the observer and the character of the subject, and out of that rationally determined depth, neither increased for ostentation of effect, nor diminished for ostentation of skill, to do the utmost that will be easily visible to an observer, supposing him to give an average human amount of attention, but not to peer into, or critically scrutinize, the work.