considered, enclosed in a common boundary, there are various ways in which their walls can be made to meet internally, three by three, at equal angles; and these differences will entail differences also in the curvature of the walls, and consequently in the shape of the cells. The question is somewhat complex; it has been dealt with by Plateau, and treated mathôÙeôÙmatôÙiôÙcally by M. Van Rees7.
If within our boundary we have three cells all meeting {375} internally,