In the curves we call flowing and graceful, we have, on the contrary, a more natural and rhythmical set of movements in the optic muscles; and certain points in the various gyrations make rhymes and assonances, as it were, to the eye that reaches them. We find ourselves at every turn reawakening, with a variation, the sense of the previous position. It is easy to understand by analogy with the superficially observed conditions of pleasure, that such rhythms and harmonies should be delightful. The deeper question of the physical basis of pleasure we have not intended to discuss. Suffice it that measure, in quantity, in intensity, and in time, must involve that physiological process, whatever it may be, the consciousness of which is pleasure. _Symmetry._ Sec. 22. An important exemplification of these physiological principles is found in the charm of symmetry. When for any reason the eye is to be habitually directed to a single point, as to the opening of a gate or window, to an altar, a throne, a stage, or a fireplace, there will be violence and distraction caused by the tendency to look aside in the recurring necessity of looking forward, if the object is not so arranged that the tensions of eye are balanced, and the centre of gravity of vision lies in the point which one is obliged to keep in sight. In all such objects we therefore require bilateral symmetry. The necessity of vertical symmetry is not felt because the eyes and head do not so readily survey objects from top to bottom as from side to side. The inequality of the upper and lower parts does not generate the same tendency to motion, the same restlessness, as does the inequality of the right and left sides of an object in front of us. The comfort and economy that comes from muscular balance in the eye, is therefore in some cases the source of the value of symmetry.[5]