=Printing and Mounting Mosaics.=—With an ordinary set of overlapping negatives the first step toward producing a map is to _scale_ the negatives. For this purpose one should be selected which by comparison with a map shows no distortion, and which is on the desired scale, or is known to have been made at the average altitude of flight. A sketch map of the territory should then be drawn, on this scale, based on available maps. This sketch is preferably made on a large ground glass illuminated from behind (Fig. 141). On this all the negatives should be laid, and their proper relative positions sought. When this is done it is evident at once whether all the territory has been covered, and whether there are any superfluous negatives. Each negative should then be examined as to its scale and distortion. If it can be made to fit the scale by simple enlargement or reduction, a line can be drawn on one edge of a length indicating its scale. This line will later be used as a guide in the enlarging camera. If the picture is badly distorted it must either be replaced by another negative, or if rectifying apparatus is available, it must be set aside for the making of a rectified print. [Illustration: FIG. 141.—Scaling negatives for mosaic map-making.] The next step is to make prints from the negatives, which may be done either by contact, or, necessarily if differences of scale must be compensated, in the enlarging camera. If prints to an exact scale are required the shrinkage of the paper must be determined and allowed for. The prints must all show the same tone, and must be uniform from edge to edge. If the focal-plane shutter is not uniform in its travel, as is frequently the case, this means that the print must be “dodged,” or exposed more at one edge than the other, by locally shielding the plate and paper during exposure. A case of the step-like effect caused by uneven shutter action is shown in Fig. 140. The effect due to uneven shutter action is of course absent with a between-the-lens shutter, which constitutes a strong argument in favor of that type for use in mapping cameras. [Illustration: FIG. 142.—Arranging prints for a mosaic map.]