parts of a single exposition. The crudest defect in illustration is a neglect of this fundamental requirement, yet it is a fault which has wide prevalence in certain forms of book-making to-day. A fixed amount of illustration per chapter, or hundred pages, is supposed to be expected by the reader, and an illustrator is engaged to furnish the required number of pictures. In the class of books where this custom prevails—for example, in light fiction—the quality of the demand for illustration falls to its lowest point. There is practically no situation the understanding of which requires visual exemplification. The story itself is commonly little more than a succession of pictures, each relatively simple and having a completely obvious relation to its neighbors. It is partly, at least, because the reader's demand is so far from exacting that such slight consideration is given to the really illustrative character of pictures in works of this class.
When the primary function of the picture has thus lapsed, the reason for its introduction is to be sought elsewhere; for value of some kind it must possess if its introduction is not to be regarded as a sheer misconception of the reader's desire. This reason is to be found, it need hardly be said, in the mere decorative function of the picture. It is a trivial motive, which also ignores a fundamental canon of esthetics, yet one which has a distinct psychological value. It neglects the requirements of esthetics, for if the making of a book be treated as a work of art, everything which appears between its covers should be instrumental to the development of the central conception for which the work stands. No picture, from this point of view, is admissible which does not help—in the strictest sense, which is not indispensable—to make the meaning clear. But if the principle of function be neglected, a multitude of pictures may be introduced in a merely illustrative, as opposed to explanatory, way. One might, for example, insert the picture of a pen or inkstand each time the article was mentioned, though an acquaintance with these things on the part of the reader is fairly to be assumed. Such illustration of course has its place wherever the objects in question are unfamiliar and the reader is liable to construct in imagination a wrong representation of them.
Further, if the principle of unity be ignored the illustrations which are introduced may be chosen in virtue of any element of desirability which they possess. Value of this kind has a wholly indefinite range. It may be merely quantitative: so many pictures to so many pages of text; no bunching of illustrations in one part of the work while another is left bare; and the like. But such rules refer only to the distribution of pictures, not to their introduction itself. If illustration be not invariably an elucidation of the text, as it clearly is not, it must satisfy some other human need, or pictures would not then be found in books. One such motive has already been mentioned: the decorative sense. The picture satisfies an elementary esthetic demand. It is introduced as the
