Next some shade like a small fire-screen must be placed between the blaze and the camera, in order to protect the sensitized plate from the full glare of the firelight. Now of course the photograph is not actually secured by the coal flame illumination, which would not be bright enough to give proper exposure, so recourse is had to dropping some material into the fire which will burn rapidly with a bright white flame. Magnesium powder is generally used for this purpose.
Supposing the group to have been arranged and the camera firmly in position, the person (B, in [Fig. 3]) seated next the grate should hold a tablespoonful of saltpeter and also a square inch or so of sheet zinc. Then, all being so far ready, let the outside member of the group (marked A in [Fig. 3]) open the camera shutter and slip back to his seat, whilst the flashlight operator drops the saltpeter and zinc successively among the glowing coals. The flame of dazzling brilliancy which results records the sitters’ figures on the plate, so that directly it is over, the person (A) may again visit the camera and close the shutter. His movements will not be noticeable, since they are made before and after the flashlight.
The operation of development may be proceeded with at once and should go fairly easily, but flashlight exposures are difficult to estimate accurately, and therefore, although a square inch of zinc has sufficed for a small group with stop and an extra rapid plate, this amount may have to be increased if the group be large or if other conditions be changed.
One last hint as to behavior of the sitters. Let them sit as naturally and quietly as possible, but be advised to blink their eyes as much as the bright light prompts them rather than keep them staring wide open, when their faces must wear a most inane expression in the finished photo.
Experiments B.—“Photo-Chemical”
Salts of silver form the basis of most modern photographic processes. Thus in order to perform chemical experiments of a photographic nature, some solution of silver must be available, the nitrate salt being usually employed.
Fig. 4.—Silver solution and precipitate.
It is best procured at the druggist’s in solution or as crystals, in which latter case it must be dissolved for use in clean rain or distilled water. The solution need be only weak, but must be kept in a dark bottle screened from daylight. Chemical test-tubes, if they can be obtained, will be found best for the experiments.
(1) Prepare a weak solution of table salt, and add it drop by drop to a little of the silver nitrate in a test tube (or wine-glass as a makeshift). A white sediment is precipitated, which, by shading part of the tube with a band of paper and exposing to daylight, may be shown to be sensitive to light, inasmuch as the unscreened part will rapidly turn purple. This precipitate consists of silver chloride, which, in combination with unaltered nitrate, forms the essential ingredient of printing paper. In [Fig. 4], A is Solution; B, Precipitate; C, Band of Paper.