Determining the correct enlargement for each negative of a mosaic is perhaps the most important problem in the use of the enlarging camera for aerial work. The correct setting of the camera may be found by either of two methods: the negative may be previously scaled and marked with a line on its edge, which must be projected to a definite size; or the true location of several points in the picture as obtained from an accurate map may be marked on the enlarging camera easel according to the desired scale, and the negative image projected to coincide with these. In either case, if an exact scale is desired, allowance must be made for paper shrinkage, a matter which must be determined by previous experiment. =Rectifying.=—Negatives taken when the plane is not flying level will be distorted (Figs. 134 and 135). Contact prints from these will not fit into a mosaic, and no mere enlargement or reduction will make them available. It is necessary with these negatives to resort to a _rectifying camera_. This is an enlarging camera built so that the negative and print easel may be inclined about vertical and horizontal axes, thereby purposely introducing a distortion sufficient to offset the distortion of the negative. Thus, if the bottom of the printing surface is moved away from the lens, that part of the picture will be enlarged; if moved toward the lens, reduced. For small rectifications the common practice is to tilt the printing surface alone, a method that is practical as long as this tilting does not affect the focus so much as to require prohibitive stopping down of the lens. For great distortions, such as that inherent in the principle of the Bagley camera, it is necessary to tilt both negative and print in order to preserve an approximate focus, a given portion of the negative moving toward the lens as the corresponding portion of the print is moved away. Both schemes for rectification are shown diagrammatically in Fig. 122. =Developing and Drying Prints.=—The developing of prints follows closely that of cut or roll film, and so need not be treated separately. The drying of emulsions on paper is more easily accomplished than the drying of emulsions on glass, for two reasons: the emulsions on paper are much more thinly coated, and there is diffusion of moisture into the atmosphere from front and back of the printing medium. In the field a common method has been to soak the prints in water-free alcohol and then burn off the alcohol, thus securing a dry print within two or three minutes after the conclusion of washing. A later method very generally employed is to cover wooden frames three or four feet above the ground with chicken wire or muslin, and on these lay the prints after soaking them in alcohol. Below the frames currents of warm air rise from pans of burning alcohol, previously used to soak the prints and now useless as alcohol because of their high water content. [Illustration: