i6o. Cameras for " While-you-wait " Photo graphy. Brief reference may be made to the cameras (using ferrotype plates or post cards) with which is incorporated a box in which developing and fixing are done, the operator introducing his arms in sleeves of opaque material fixed to the camera, and secured round his arms by elastic bands. Others, for the pro duction of real post cards, are fitted with a copying bench of simple construction, which folds down along the tripod while the photo graph is being taken, and is then erected in correct position to hold the still wet paper negative when copying it on another post card as a positive.
We may further mention various types of automatic or semi-automatic machines usually operated by the insertion of a coin. These com prise a cabin with a fixed seat for the sitter and containing an arrangement for artificial illumina tion, a camera recording successively several pictures on a strip of sensitive paper and usually also a device for developing, reversing, and dry ing, or for copying the developed and fixed image by projection on another strip and then processing this positive strip.
161. Testing of Cameras. Newly acquired cameras need testing, and such examination is also necessary from time to time with apparatus in service. The following points need attention— Absence of play after the various parts have been set in position ; parallelism of the front and back, tested with a square ; the two parts require to be perpendicular to the long sides of the baseboard agreement in register between the focussing screen and the dark slides ; ab sence of reflections from the inside surfaces, which must be covered with a dead matt var nish ; and absolute light-tightness of the camera and slides.
In order to check the register between the focussing screen and the dark slides, place a glass plate in the dark slide and fit the latter to the camera. Unscrew the lens and introduce a rod fitted with a sliding cross-piece that can be clamped. The tip of the rod is pressed against the surface of the plate, and the cross-piece is pushed against the lens flange and clamped at that point of its travel. The dark slide is then replaced by the focussing screen, and all that is necessary is to see that the tip of the rod touches the surface of the screen when the cross-piece is again placed against the flange. When the focussing screen and dark slides are fitted to the camera in the same manner, it suffices to see that they have the same register.
To do this, use a thick wooden rule of perfectly flat surfaces, through the middle of which a screw has been placed so that it can be screwed in or out at will. The rule is placed across the front of the frame of the focussing screen and the screw is turned until its point just touches the ground surface. Plates are now put in the dark slides, and the rule is placed across the latter. if the register is correct, the point of the screw will touch the glass surface without pressing the plate back on the springs that hold it against the stops.
To see whether the camera is light-tight, take it into a dark or dimly-lit room and put an electric lamp inside it, first through the aperture in the lens board, a dark slide being in position, and then through the back of the camera, the lens being in place and capped. The opening through which the lamp has been introduced must be closed with the focussing cloth, and a careful inspection then made to see if any rays from the lamp can be detected escaping at the folds and corners of the bellows or at the various joints.
The light-tightness of the slides can be tested only by photographic tests, viz, by loading them with plates (or, more cheaply, with bromide paper backed by a piece of card or a glass plate) and then exposing them for a considerable time to light (preferably sunlight) in all possible positions. The slide is then placed in position on the camera, the lens capped, the shutter withdrawn, and the camera left exposed to full light. Plates or papers subjected to the above treatment should not show more general fog than material taken directly from the packet and developed at the same time, nor should they show local fog. Of course, the number of the slide will have been marked on each plate or sheet of paper, so that the defective slide may be identified and the fault located.
These tests must always be supplemented, and may even be replaced, by practical tests with the camera under normal conditions of use. As a rule, no new and untried instrument should be used for any work which is of special or cannot be repeated. It is especially inadvis able to start on any trip, still more so on any long journey, with a camera which has not been thoroughly tested,