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.