No part should be identical with another, and nothing should be symmetrical (idea of variety), but there should be a certain balance of lines and tones, conveying an impression of stability. The principal lines of the composition, with the exception of the verticals, should not be parallel to one another nor to the frame, but should oppose one another (opposition). Balance is obtained, as a rule, by means of an element of secondary interest in the picture (which may be no more than a patch of tone), this secondary feature being placed, for example, at a point diagonally opposed to the principal subject.' It should be noted that a subject which is attractive solely because of its colouring will give very disappointing results in monochrome photography. Some idea of the appearance which a photograph of a subject will present on an ordinary emulsion and on an orthochromatic emulsion can be obtained by examining it through a blue (§ 207) or a green glass (§ 210) respectively.
315. The mistake in composition which is most frequently made is as follows : Having selected a subject (which is easy to the practised eye, able to choose from the attractive subjects which are to be found everywhere), and set up the camera in the place considered most favour able, a move is made to a different view-point, generally much nearer the subject, under the idea that the picture is too small and would not fill the area of the plate to be used. A subject
is attractive from a certain view-point because, relative to that point, the lines of the scene are grouped harmoniously, but the attractivenc ss is generally lost if the view-point is moved either nearer or farther away. Having once selected the view-point, it is better to choose from among the lenses at hand one which will allow the plate to be most completely filled by the bearing in mind the fact that any unwanted parts may be excluded later. If the picture is considered too small, it is advisable to enlarge it, or to have an enlargement made from it if the necessary apparatus is not available. (See Chapter V for the rudiments of photographic perspective and the effect of the distance between view-point and subject.) Having chosen the subject and the view-point, which necessarily fix the angle of the picture and the position of the camera, the limits of the subject are best adjusted by decentring 2 the lens in the direction and to the extent considered necessary (§ 155).