Another advantage of considering objects thus merely as shells composed of lines, is, that by these means we obtain the true and full idea of what is call'd the _out-lines_ of a figure, which has been confin'd within too narrow limits, by taking it only from drawings on paper; for in the example of the sphere given above, every one of the imaginary circular threads has a right to be consider'd as an out-line of the sphere, as well as those which divide the half, that is seen, from that which is not seen; and if the eye be supposed to move regularly round it, these threads will each of them as regularly succeed one another in the office of out-lines, (in the narrow and limited sense of the word:) and the instant any one of these threads, during this motion of the eye, comes into sight on one side, its opposite thread is lost, and disappears on the other. He who will thus take the pains of acquiring perfect ideas of the distances, bearings, and oppositions of several material points and lines in the surfaces of even the most irregular figures, will gradually arrive at the knack of recalling them into his mind when the objects themselves are not before him: and they will be as strong and perfect as those of the most plain and regular forms, such as cubes and spheres; and will be of infinite service to those who invent and draw from fancy, as well as enable those to be more correct who draw from the life. In this manner, therefore, I would desire the reader to assist his imagination as much as possible, in considering every object, as if his eye were placed within it. As straight lines are easily conceiv'd, the difficulty of following this method in the most simple and regular forms will be less than may be first imagined; and its use in the more compounded will be greater: as will be more fully shewn when we come to speak of composition. But as [Fig. 2 L p. I] may be of singular use to young designers in the study of the human form, the most complex and beautiful of all, in shewing them a mechanical way of gaining the opposite points in its surface, which never can be seen in one and the same view; it will be proper to explain the design of it in this place, as it may at the same time add some weight to what has been already said. [Illustration: Fig. 2]