The existence of grace seems to depend more upon the character of mental, than of corporeal beauty. All its motions seem to indicate, and to be regulated by the utmost delicacy of sentiment! I have placed it between the highest sentiment of the human mind, sublimity, that no rules can teach, and the highest sentiment that rules can teach, exact beauty, the two extremes of the vrai réel, and the vrai idéal. Grace seems as it were to hang between the influence of both; the irregular sublime giving character and relief to the negative and determined qualities of beauty; and beauty, i. e. truth confining within due bounds the eccentric qualities of sublimity, forming both to sight and in idea, orderly variety, the waving line, neither straight nor crooked. The waving line is the symbol, or memento, as I may say of grace, wherever it is seen, in whatever object animate or inanimate; and may be justly stiled the line of taste or grace.
Frances Reynolds, An Enquiry Concerning the Principles of Taste, and of the Origin of Our Ideas of Beauty (1785), p. 14. 'Grace' here means aesthetic gracefulness. The 'line of grace', which is a particular version of what is more commonly known as the Line of Beauty, is usually associated with William Hogarth, The Analysis of Beauty (1753):
Reynolds is right that this is only a sort of "symbol, or memento" of a larger point, although it seems like the Line itself is the only thing most people picked up from Hogarth. In any case, this is Hogarth's explanation of the serpentine line as the line of grace:
It is to be observed, that straight lines vary only in length, and therefore are least ornamental.
That curved lines as they can be varied in their degrees of curvature as well as in their lengths, begin on that account to be ornamental.
That straight and curv'd lines join'd, being a compound line, vary more than curves alone, and so become somewhat more ornamental.
That the waving line, or line of beauty, varying still more, being composed of two curves contrasted, becomes still more ornamental and pleasing, insomuch that the hand takes a lively movement in making it with pen or pencil.
And that the serpentine line, by its waving and winding at the same time different ways, leads the eye in a pleasing manner along the continuity of its variety, if I may be allowed the expression; and which by its twisting so many different ways, may be said to inclose (tho' but a single line) varied contents; and therefore all its variety cannot be express'd on paper by one continued line, without the assistance of the imagination, or the help of a figure; see where that sort of proportion'd, winding line, which will hereafter be call'd the precise serpentine line, or line of grace, is represented by a fine wire, properly twisted round the elegant and varied figure of a cone.
Due to the fact that she situates grace between sublimity and beauty, Reynolds is adapting this idea in a slightly different direction.
