"When the head is erect, in examining a _straight horizontal_ line we employ one of the lateral recti; if the line be vertical we employ the rectus inferior or superior. In either case, but one muscle acts, and that continuously. The muscle is not relieved, and its action is not attended with the maximum amount of pleasurable sensation. When the vision has been extended along the whole line, if we then immediately proceed to examine it in the opposite direction, the opposite rectus must at one exert a force sufficient to overcome the _momentum_ of the eyeball, and then exert a _continuous_ action. Both these circumstances are unfavorable to pleasure. If the line is _oblique_, one lateral together with one inferior or one superior muscle is exerted, and the same principles which have been applied to the single muscles, apply to the muscles acting in pairs. "_The Beauty of Curved Lines._--As from the foregoing analysis of the vision of straight lines in general, it results that they are deficient in the elements of ocular agreeableness, in other words, of beauty; little more need be said of regular and gentle curves, than that the survey of them is not attended with the abovementioned disadvantages. In viewing a regular curve, no muscle of the eyeball acts continuously and uniformly, but enjoys partial relief by remissions, or total relief by intermissions of its action; and the regularity of these remissions and intermissions, as well as the equal distribution of exercise, is promoted by the regularity of the curve. Acting in succession, the muscles afford mutual relief after actions of such short duration and variable intensity, as to afford positive pleasure; and in this _muscular pleasure_ of the eye consists the _beauty of configuration_.