Fig. 1 is a section of the linear construction of a picture, and will show how the lines converge from the point of observation to the point of sight. Artists, in constructing a landscape of an ordinary form, allot to the sky generally about twice the space between the base and horizontal lines. But for portraits and groups, where the figures are of the greatest importance and nearer to the eye, the proportion of sky and earth is reversed, so as to give increased value to the principal figures, by making them apparently larger, and still preserving the proper relation between them and the horizontal line (see fig. 2). This diagram represents the conditions of a full-length carte portrait, where the governing horizontal line is on a level with the camera. If a pictorial background, painted in the usual way, with the horizontal line low in the picture, is now placed behind the sitter, the resulting photograph will be incongruous and offensive. It will be seen, on referring to fig. 2, that all the lines below the horizon must of necessity run up to it, no matter how high the horizontal line may be, for it is impossible to have two horizons in one picture; that is, a visible horizon in the landscape background, and an imaginary one for the figure, with the horizontal line of the background far below the head of the figure, and the head far up in the sky. The head of a human figure can only be seen so far above the horizontal line under certain conditions; such as being elevated above the observer by being mounted on horseback, standing on higher ground, or otherwise placed considerably above the base line, none of which conditions are present in a studio. Whenever the observed and observer are on the same level, as must be the case when a photographer is taking the portrait of a sitter in his studio, the head of the subject could not possibly be seen so high in the sky, if the lens included a natural background instead of a painted one. As, for convenience, the painted background is intended to take the place of a natural one, care should be taken that the linear and aerial perspectives should be as true to nature as possible, and in perfect harmony with the size of the figures. The lens registers, on the prepared plate, the relative proportions of natural objects as faithfully as the retina receives them through the eye, and if we wish to carry out the illusion of pictorial backgrounds correctly, we _must_ have the linear construction of the picture, which is intended to represent nature, as true in every respect as nature is herself.