FIG. 27.—“L” type camera showing open negative magazines and shutter mechanism.] _U. S. Air Service variable-aperture shutter._ This shutter is incorporated in the American deRam and in other late American cameras (Fig. 28). Its characteristic feature is the introduction of an idler, whose distance from the main curtain roller can be varied. Tapes whereby the following curtain is attached to the spring roller pass over this idler, and by changing its position the aperture or distance between the two curtain elements is altered over a large range. Tests of this shutter are shown in Fig. 30. A speed of 1/50 second is provided for by a slit width of five centimeters, and the highest speed is fixed only by the practical limit of approach of the jaws. Experiment shows great uniformity of rate of travel to be attainable by combining careful choice of spring length and tension with good workmanship in the mechanical features. Variable-aperture fixed-tension shutters have a definite advantage over the variable-tension type in that they can utilize for all speeds that tension which gives uniform action. The capping feature of this shutter is provided in the American deRam by flaps, in the automatic film camera by an auxiliary curtain. The shutter is removable in the deRam, but built into the other camera. [Illustration: FIG. 28.—Variable aperture curtain developed in U. S. Air Service, and used in American deRam, and “K” type automatic film cameras.] _The Klopcic_ variable-tension, variable-aperture, self-capping shutter is an example of an attempt to meet all shutter requirements with an entirely self-contained mechanism. It is shown diagrammatically in Fig. 29. Tapes _G__{1}, _G__{2} are used to connect the following curtain _B_ directly to the spring roller _T_, at a fixed distance, while the leading curtain, _A_, may be slid along the tapes by small friction buckles, _C__{1}, _C__{2}, auxiliary springs _R__{1}, _R__{2} serving to keep it taut in any position. When the shutter is being set the buckles are arrested against stops while the winding-up continues for what is to be the following half of the curtain in exposing. When released the curtain moves across with an aperture fixed by the point of setting of the buckle stops. At the end of the travel the buckles are arrested by other stops, while the following portion of the curtain continues its travel to the end. On re-winding, therefore, the aperture is closed. Variable tension as well as variable aperture is provided, although little used. In the French cameras a lens flap is also inserted behind the lens, but this is not needed if the self-capping feature functions properly. On the hand cameras this flap is said to be necessary in order to prevent a curious kind of accident: if the camera is held on the knee, pointing upward, an image of the sun may be formed on the curtain and burn a hole through it. [Illustration: