The U. S. Air Service method of mounting is to provide the lens barrel with a long thread, which screws into a flange that in turn is mounted on a platform in the camera cone, by means of thumb-screws. The lens is focussed by screwing in and out, and then clamped by a screw through the side, bearing on the thread. The whole mount may be quickly removed by loosening the thumb-screws, and once focussed in one cone, can be transferred to another similar, machine-made cone without change of focus. Fig. 18 shows a 20 inch lens mounted in this manner. The photograph shows as well the ring on the front of the lens by means of which circular color filters may be held in place. This ring screws down on the filter, and the catch is dropped into the nearest vertical groove to the tight position. [Illustration: FIG. 18.—50 centimeter F/6 lens in U. S. standard mount, showing color filter retaining ring and catch.] A somewhat different and better method of tightening the lens in the flange, when focussed, has been adopted in the English lens mount, which is in general similar to the American. The threaded part of the flange is split by a slot cut parallel to the flange base, and a screw is run into the flange from the front, through the split portion. By tightening this screw, which is always accessible, the split part of the flange is squeezed together, thus rigidly holding the lens barrel. CHAPTER V THE SHUTTER =Permissible Exposure in Airplane Photography.=—A definite limitation to the length of exposure in airplane cameras is set by the motion of the plane. If we represent the speed of the plane by _S_, the altitude of the plane by _A_, and the focal length of the lens by _F_, we obtain at once from the diagram (Fig. 19), that _s_, the rate of movement of the image on the plate, is given by the relation, _s_ _F_ ——— = ——— _S_ _A_ If we call the permissible movement _d_, then the permissible exposure time, _t_, is given by the relation— _d_ _Ad_ _t_ = ——— = ———— _s_ _FS_ As a representative numerical case, expressing all quantities in centimeters and in centimeters per second, let _F_ = 50, _S_ = 20,000,000/3600 (200 kilometers per hour), and _A_ = 300,000, then 50 × 20,000,000 _s_ = ——————————————— = .9 centimeters 300,000 × 3600 If we take for the permissible undetectable movement, .01 centimeter, which is, as has been shown, a reasonable figure for lens defining power, we have, then, that the _longest permissible exposure is .011 second_—in round numbers, one-hundredth.