Moreover, the out-line of a globe is but a circle on the paper; yet, according to the manner of filling up the space within it, with this shade, it may be made to appear either flat, globular, or concave, in any of its positions with the eye; and as each manner of filling up the circle for those purposes must be very different, it evidently shews the necessity of distinguishing this shade into as many species or kinds, as there are classes or species of lines, with which they may have a correspondence.
In doing which, it will be found, that, by their correspondency with, and conformity to objects, either composed of straight, curved, waving, or serpentine lines, they of course take such appearances of variety as are adequate to the variety made by those lines; and by this conformity of shades we have the same ideas of any of the objects composed of the above lines in their front aspects, as we have of them by their profiles; which otherwise could not be without feeling them.
Now instead of giving engraved examples of each species of shade, as I have done of lines, I have found that they may be more satisfactorily pointed out and described by having recourse to the life.
But in order to the better and more precisely fixing upon what may be there seen, as the distinct species, of which all the shades of the retiring kind in nature partake, in some degree or other, the following scheme is offered, and intended as an additional means of making such simple impressions in the mind, as may be thought adequate to the four species of lines described in Chapter VII. Wherein we are to suppose imperceptible degrees of shade gradating from one figure to another.
The first species to be represented by, 1, 2, 3, 4, 5. the second by, 5, 4, 3, 2, 1, 2, 3, 4, 5. and the third by, 5, 4, 3, 2, 1, 2, 3, 4, 5, 4, 3, 2, 1, 2, 3, 4, 5. gradating from the dots underneath, repeated either way.
As the first species varies or gradates but one way, it is therefore least ornamental, and equal only to straight lines.
The second gradating contrary ways, doubling the others variety, is consequently twice as pleasing, and thereby equal to curved lines.
The third species gradating doubly contrary ways, is thereby still more pleasing in proportion to that quadruple variety which makes it become capable of conveying to the mind an equivalent in shade, which expresses the beauty of the waving line, when it cannot be seen as a line.