I need but to refer to the well-known devices of linear and aerial perspective, by which this result is secured.[40] The value of these means of producing illusion at the command of the painter, may be illustrated by the following fact, which I borrow from Helmholtz.

If you place two pieces of cardboard which correspond to portions of one form at the sides and in front of a third piece, in the way represented above, so as just to allow the eye to follow the contour of this last, and then look at this arrangement from a point at some little distance with one eye, you easily suppose that it stands in front of the side pieces.

The explanation of the illusion is that this particular arrangement powerfully suggests that the outline of the whole figure, of which the two side pieces are parts, is broken by an intervening object.

Owing to the force of these and other suggestions, it is easy for the spectator, when attending to the background of a landscape painting, to give himself up for a moment to the pleasant delusion that he is looking at an actual receding scene.
In connection with pictorial delusion, I may refer to the well-known fact, that the eye in a portrait seems to follow the spectator, or that a gun, with its muzzle pointing straight outwards, appears to turn as the spectator moves.[41] These tricks of art have puzzled many people, yet their effect is easily understood, and has been very clearly explained by Sir D.Brewster, in the work already referred to (letter v.).

They depend on the fact that a painting, being a flat projection only and not a solid, continues to present the front view of an object which it represents wherever the spectator happens to stand.