Glass mirrors can be silvered either on the rear or front surface. If on the rear, both surfaces must be accurately parallel, which means much greater labor and expense than if the front surface can be utilized. The difficulty with front surface mirrors is that the metallic coating is easily tarnished or scratched, especially if silver is used, which is almost imperative, since all the other metals have considerably lower reflecting powers. (Gold might serve both as mirror and color filter, because of its yellow color.) Placing the mirror inside the camera body in part obviates this trouble, but means the use of a special elbow lens cone. In any case the mirror or prism occasions at least a 10 per cent. loss of light. Pictures taken by reflectors of any kind are reversed, and must either be printed in a camera, or on transparent film which may be viewed from the back. The most usual condition for making obliques is to fly very low (300 to 600 meters), with the line of sight of the camera from 12 to 45 degrees from the horizontal. This low altitude necessitates very short exposures, to avoid movement of the image. The picture may be taken either the long or the short way of the plate, depending on the character of the object and the information desired. It is to be noted that successive oblique pictures cannot be mounted to form a continuous panorama—this being possible with obliques only if they are taken from one point, as from a captive balloon. If successive views are made on a straight flight at intervals so as to exactly juxtapose in the foreground, they overlap by a large margin the middle, and a point on the horizon, if that shows, will be in the same position in every picture. Mosaics of obliques could be made only by some system of conical mounting. =Sights for Oblique Photography.=—Any of the sights previously discussed for vertical work, such as the tube sights, are applicable to obliques. They must, however, be suited for mounting at an angle, in a position convenient for the observer. In addition, provision must be made for adjusting the angle so that the lines of sight of camera and finder are parallel. Mounting outside the fuselage is practically the only feasible way, and is less objectionable with oblique than with vertical sights, as oblique sighting does not require the observer to stand up and lean over the edge of the cockpit. Windows in the side of the fuselage, either of celluloid or non-breakable glass, are a great aid to oblique observation. Marks upon the transparent surface can be utilized for the rear points of a sight of which the front point is a single fixed bead or rectangle. CHAPTER XXVIII STEREOSCOPIC AERIAL PHOTOGRAPHY