Thus, if the reader will look at the drawing of the box-like solid (Fig.

3, p.

79), he will find that, after a trial or two, he succeeds in seeing it as a _concave_ figure representing the coyer and two sides of a box as looked at from within.[49] Many of my readers, probably, share in my power of variously interpreting the relative position of bands or stripes on fabrics such as wall-papers, according to wish.

I find that it is possible to view now this stripe or set of stripes as standing out in relief upon the others as a ground, now these others as advancing out of the first as a background.

The difficulty of selecting either interpretation at will becomes greater, of course, in those cases where there is a powerful suggestion of some particular local arrangement, as, for example, the case of patterns much brighter than the ground, and especially of such as represent known objects, as flowers.