Unfortunately not all field photographs are good, so an author must select from his collection those which will make the best half tones. In making this selection he should of course consider, first, the scientific value of the photograph, and next, its pictorial or artistic quality, which, though of secondary importance, should nevertheless be kept in mind. A feature worthy of illustration deserves good pictorial expression; if it is of superior scientific interest it should not be represented by an inferior photograph. Fortunately, a good, accurate drawing may be made from a poor photograph, and a photographic view that has only minor defects can be successfully retouched. Photographs that need much retouching should generally be larger than publication size, for the effects of retouching--brush marks, etc.--will be softened by reduction. Photographs that need only slight retouching need not be larger than publication size. A photograph can rarely be satisfactorily enlarged in reproduction unless it is sharp in detail and requires no retouching. [Illustration: U. S. GEOLOGICAL SURVEY PREPARATION OF ILLUSTRATIONS. PLATE III. LITHOLOGIC SYMBOLS USED IN STRUCTURE AND COLUMNAR SECTIONS TO REPRESENT DIFFERENT KINDS OF ROCK] Unmounted prints are always preferable for use in making illustrations. A group that is to form a single plate should be placed in an envelope bearing the number of the plate and its title, and each print of the group should bear a corresponding number, written in pencil on its back. The envelope will protect the prints and keep them together, and the numbers will identify them. Red ink should not be used to mark photographs, as it is likely to penetrate the coating or even the fiber of the paper, so that it can not be erased. If a print is of doubtful quality two copies of it should be submitted--one glazed, the other having a dead finish or "mat" surface, which is generally preferable if the print must be considerably retouched. The best prints for use as illustrations are those made on "regular" or "special" semimat velox and glossy haloid papers. The author should indicate prints that may be grouped together according to their relation geographically or by subject. Generally two half tones will be combined on a page, and the list of illustrations should be prepared accordingly. With slight trimming and reduction, three photographs measuring 3-1/4 by 5-1/2 inches may be made up one above the other to form a full-page octavo plate. Four photographs in which the longer dimensions represent vertical distances may sometimes be used if they are placed sidewise on the page, with side titles.