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444 HIGH-SPEED PHOTOGRAPHY November
8-mm cameras, those now in manufacture have a prism which is the same length as that of the 16-mm camera so that by substituting the aperture plate with one whose slits are twice as long as provided, a full-width 16-mm picture is obtained, with an 8-mm frame height. This will prove of advantage in making ballistic studies and detonation-rate studies as well as burning of fuel in cylinders. It will be necessary for the customer to provide his own aperture plate for this change.
In the case of the Fastax camera, where the film is mounted on a sprocket when operated, the lens is focused at the plane of the aerial image which is approximately 6*/2 mils above the surface of the sprocket. There are four holes in the sprocket. By rotating the sprocket by hand until one of the holes is lined up with the rotating prism, when the prism faces are perpendicular to the optical axis, the image formed by the objective lens is picked up on a ground glass on the door of the camera. This image is projected on to the ground glass by first having passed through a right-angle prism, through a 32-mm microscope objective, through another prism, and on to the ground glass. There is a little trap door in the housing which should be at right angles to the door of the camera as pictures are being taken. If there is a bright light back of the camera and the trap is open, the sprocket holes are exposed on the film.
It is possible by using a 3/8-inch punch to view the subject up until it is ready to be taken by punching a hole in the middle of the frame of the film. This hole is lined up with the hole in the sprocket and focusing is done normally. This hole should be punched about from 12 to 18 inches from the end of the film so that the camera can be loaded normally and be ready for use.
The Fastax camera is also equipped with a V4-watt argon-lamp timer. The l/rwatt argon lamps can be actuated directly from 120volt, 60-cycle alternating current which will lay down 120 pips per second, or it can be operated from an oscillator which will drive the lamp with a square-top wave. In order to increase the intensity of the lamp to provide an image which is stronger, the voltage may be increased to 135 to 150 root-mean-square volts. In the event that the x/4-watt argon lamps (AR-3) are broken, the replacement lamp should be burned on 60-cycle, 120 -volt alternating current for 24 hours in order to age them, otherwise there may be a lack of stabilization between the firing voltages of the two electrodes. If a square-wave oscillator is used it is not necessary to do this since only one electrode