"_Symmetry of form_, though implied in geometrical regularity, is not identical with it, and requires a separate consideration. The beauty of forms geometrically symmetrical, in contradistinction from those deficient in the correspondence of opposite halves, depends upon two similar series of actions in different pairs or muscles. For example, the survey of an ovate leaf, or indeed that of almost any vegetable leaf--so numerous are the provisions for our gratification--requires for its opposite halves two series of muscular actions, the different parts of the one corresponding with those of the other in duration, intensity, and order of succession. The gratification in this case results from the harmony of muscular sensations individually pleasurable. The agreeableness of this harmony may depend upon a principle more psychological than that of the agreeableness of its elementary sensations. Yet the former is to a certain extent susceptible of a physiological generalization. This harmony would probably have been impaired by any considerable inequality in the distances between the points of insertion of the recti muscles, or in the strength of the antagonists. It is a curious coincidence, that in both these respects, these muscles are more nearly symmetrical than any others in the human body. Physiology, then, explains, not only the agreeableness of the elementary sensations, which give rise to the perception of beauty in regular curves, but unfolds the provisions for two similar series of such sensations, not only in figures simply regular, but in those which are simply symmetrical, and in those which are both symmetrical and regular. The principles of muscular action explain the agreeableness of a rapid succession of varied actions equally distributed among the muscles, and the structure of the optical apparatus explains why the curvature and regularity of an object require such actions in vision. Again, we discover in the symmetrical structure and arrangement of the ocular muscles, a provision for two similar series of pleasurable sensations in the survey of a symmetrical figure, in whatever position it may be placed, provided it retains its symmetry with respect to some visual plane. The coincidence between the location of the ocular muscles diametrically opposite, on the one hand, and our propensity to compare the opposite halves of bodies, and the pleasure afforded by their similarity on the other hand, is curious, and to a certain extent affords a physiological explanation of the beauty of symmetrical forms. "The same principles which apply to the beauty of form of inanimate objects are applicable to the paths described by them in _motion_. The intrinsic beauty of their motions is exclusively referrible to sensations in the ocular muscles of the observer, while the gracefulness of human motions is referrible in part to these, and in part to sensations in other muscles.