Pictorial backgrounds have usually been painted on the same principle as a landscape picture, and one of the earliest things the painter has to determine is, where he shall represent that line where the sky and earth appear to meet--technically, the _horizontal line_. This settled, all the lines, not vertical or horizontal in the picture, below this are made to appear to rise up to it, and those above descend, and if all these are in due proportion the perspective is correct, no matter whether this governing line is assumed to be in the upper, lower, or middle part of the picture. A painter can suppose this imaginary line to be at any height he pleases in his picture, and paint accordingly. In photography it is invariable, and is always on a level with the lens of the camera. To illustrate the relation of the horizontal line to the human figure, when a pictorial background is to be introduced, let us imagine that we are taking a portrait out-of-doors, with a free and open country behind the person standing for his carte-de-visite. The camera and the model are, as a matter of course, on the same level. Now focus the subject and observe the linear construction of the landscape background of nature. See how all the lines of the objects below the level of the lens run up to it, and the lines of the objects above run down to it. Right across the lens is the horizontal line, and the centre is the point of sight, where all the lines will appear to converge. Suppose the lens to be on a level with the face of the subject, the horizontal line of the picture produced on the ground glass will be as near as possible as high as the eyes of the subject. Trees and hills in the distance will be above, and the whole picture will be in harmony. This applies to interior views as well, but the ocular demonstration is not so conclusive, for the converging lines will be cut or stopped by the perpendicular wall forming the background. Nevertheless, all the converging lines that are visible will be seen to be on their way to the point of sight. Whether a natural background consisted of an interior, or comprised both--such as a portion of the wall of a room and a peep through a window on one side of the figure--the conditions would be exactly the same. All the lines above the lens must come down, and all that are below must go up. The following diagrams will illustrate this principle still more clearly. [Illustration: Fig. 1.] [Illustration: Fig. 2.]