Though it is of vital importance to every composition that there be variety of Lines, little can be said on the subject in addition to what has been advanced in relation to parts, that is, to shape and quantity; both having a common origin. By a line in Composition is meant something very different from the geometrical definition. Originally, it was no doubt used as a metaphor; but the needs of Art have long since converted this, and many other words of like application, (as _tone_, &c.,) into technical terms. _Line_ thus signifies the course or medium through which the eye is led from one part of the picture to another. The indication of this course is various and multiform, appertaining equally to shape, to color, and to light and dark; in a word, to whatever attracts and keeps the eye in motion. For the regulation of these lines there is no rule absolute, except that they vary and unite; nor is the last strictly necessary, it being sufficient if they so terminate that the transition from one to another is made naturally, and without effort, by the imagination. Nor can any laws be laid down as to their peculiar character: this must depend on the nature of the subject. In the wild and stormy scenes of Salvator Rosa, they break upon us as with the angular flash of lightning; the eye is dashed up one precipice only to be dashed down another; then, suddenly hurried to the sky, it shoots up, almost in a direct line, to some sharp-edged rock; whence pitched, as it were, into a sea of clouds, bellying with circles, it partakes their motion, and seems to reel, to roll, and to plunge with them into the depths of air.