Apart from the relief given by shading in painting, there is an important mechanical method of improving the illusion, though this can only be occasionally adopted. The figures in any well painted picture will appear to stand out in high relief if we lose sight of the frame and other surroundings which distinctly inform us that the work is a flat surface. This is why a painting invariably seems to improve if seen through a tube of such diameter that the frame is excluded from the vision. Advantage of this fact has been many times taken in the exhibition of single pictures, when, by the exclusion of the frame, the concealment of the edges of the work by curtain arrangements, and the concentration of all the available light upon the canvas, such perfect relief has been obtained that observers have been sometimes unable to distinguish the art from the life. It was the effect of the surroundings of a picture upon the sight, that led to a practice in design resulting in the exclusion of these surroundings to some extent when the eye is directed towards the centre of the work where the principal figure is commonly stationed. This practice is to avoid accessories as far as possible near the figure, and to provide considerable open space above it, and also at the sides when the composition allows, so that the observing eye has not of necessity to range close to the frame of the picture. In a good design of this kind the central figure or figures come out in strong relief, the attraction of the work being consequently much enhanced. Obviously the painted figures should be of life size, or nearly so, for the illusion of relief to be strikingly marked, and the conditions necessarily prevent the adoption of the scheme in a design of many figures. It is most successful with a single figure, and has been carried out with two figures, but never with more than two except in a few pictures of great size. [Illustration: PLATE 21 (See page 240) The Litta Madonna, by Lionardo da Vinci (_Hermitage_)] The number of artists who have taken advantage of this mechanical device is not large, but it includes some of the first masters. The plan may be used in both exterior and interior scenes. In the former the figures must be thrown against the sky, and it is a distinct advantage if there be no trees or other objects on either side of the figures, which also stand out above the horizon, though this is immaterial if the figure be set in a confined space, as an arch, or between the columns of a loggia, and the foliage is not seen through this space.