outline of the shadow is still drawn on the plane, but being interrupted by the surface B at C, the shadow runs up that plane till it meets the rays 1, 2, which define the shadow on plane B. This is an important point, but is quite explained by the figure.
Fig. 268.
Although we have said that the rays pass over the top of the object casting the shadow, in the case of an archway or similar figure they pass underneath it; but the same principle holds good, that is, we draw lines from the guiding points in the arch, 1, 2, 3, &c., at the same angle of 45° to meet the traces of those rays on the plane of shade, and so get the shadow of the archway, as here shown.
Fig. 269.