Fig. 72.—A bas-relief lighted from the left.
Fig. 73.—An intaglio lighted from the left.
In [Fig. 74] the upper object is a disk and the lower is a sphere. In a [Fig. 74] the lighting is due to a source of light of rather small physical dimensions directly above the objects. The same objects illuminated by means of highly diffused light (that is, light from many directions and of uniform intensity) appear as in b. Both objects now appear as disks. It is obvious that under appropriate lighting a disk might be taken for a sphere and vice versa, depending upon which dominates the judgment or upon the formulation of the attendant assumptions. Incidentally an appearance quite similar to that of a, [Fig. 74] is obtained when the light-source is near the observer; that is, when it lies near the line of sight.
Fig. 75.—A concave hemispherical cup on the left and a convex hemisphere on the right
lighted by a light-source of large angle such as a window.
Somewhat similar to the confusion of intaglio with bas-relief is the confusion of the two hemispherical objects illustrated in [Fig. 75]. The one on the left is concave toward the observer. In other words, both could be hemispherical shells—one a mold for the other. Under the lighting which existed when the original photographs were made they could both be taken for hemispheres. The lighting was due to a large light-source at the left, but if the object on the left is assumed (incorrectly) to be a hemisphere convex toward the observer or a sphere, it must be considered to be lighted from the right, which is also an incorrect assumption. Obviously, if the direction of the dominant light is clear to the observer, he is not likely to make the error in judgment. Incidentally the object on the right might be assumed to be a sphere because a sphere is more commonly encountered than a hemisphere.