FIG. 29.—Mechanism of Klopcic variable aperture self-capping shutter.] The performance of the French shutter in respect to uniformity has already been shown in Fig. 24. It leaves very much to be desired. Besides non-uniformity of action during its travel it exhibits another common defect of variable-tension shutters, namely, the curtain must be released several times after a change of tension before the new speed is established (Fig. 30, tensions 5 and 5´). [Illustration: FIG. 30.—Performances of various shutters used on aerial cameras. Speeds expressed in reciprocals of fractional parts of one second.] The French shutter as made for the deMaria cameras is a removable unit. The small size (13 × 18 cm.) sets by the straight pull of a projecting pin, the larger (18 × 24 cm.) by winding up a milled head. The former is the more convenient motion for an aerial camera. Care must be taken with either type that the motion of setting is not stopped when the first resistance is encountered; this occurs when the tape buckles strike their stop and the slit begins to open. CHAPTER VI PLATE-HOLDERS AND MAGAZINES In the earlier days of airplane photography the ordinary plate-holder or double dark slide was used to some extent, but it is ill-suited to the purpose because of the considerable time and attention required for its operation. It has nevertheless the merit of adding little to the length of the camera, and it works in any position. For these reasons it has remained in occasional use for the taking of oblique views with long focus cameras in a cramped fuselage.