2. Changes can be made in half-tone plates only by re-etching certain parts to make them lighter and by burnishing certain parts to make them darker. If the proof shows a general loss of detail the fault may lie either in the proving of the cut or in the reproduction. If it is in the reproduction it can not be remedied without reengraving. A slight loss of detail may be expected in all half tones, especially in those that are smaller than the copy submitted. 3. Minor changes can be made in photolithographs and chromolithographs, but changes can not be made twice in one place without danger of affecting the printing. It is customary to approve all lithographic proofs subject to the corrections indicated, the printed edition being examined and compared, but if the changes are numerous and radical second proofs may be required. Second combined proofs of chromolithographs are very expensive. (See p. 89.) PROOF READING ILLUSTRATIONS. An author should examine the proofs of his illustrations closely and should compare them carefully with the original drawings. A mere cursory examination may fail to detect errors that have not been caught by the regular proof reader. Every correction desired should be clearly indicated with pen and ink in the body of the proof and inclosed in a loop from which a line should be carried to a marginal note or comment, but if the time available is short a pencil may be used. In correcting type matter or lettering (such as that in a geologic legend or explanation) the ordinary proof reader's marks should be used. The author or the person examining the proofs should initial each one at the place indicated by a rubber stamp. Proofs should be held only long enough to examine them properly and to compare them with the original illustrations, for a time limit is fixed in each contract for engraving, and if the author holds proofs beyond a reasonable time he causes a delay in the fulfillment of the contract. As the illustrations for many reports contain important data that will be discussed in the text, proofs of illustrations can not be supplied to any applicant without consent from the Director's office. GENERAL CONSIDERATIONS. The following requirements are essential to obtain good original illustrations: 1. The material selected should be pertinent and expressive; it should have the qualities essential to good illustrations.