Having glued on the interior face of a plate-holder (the slide being drawn), in the place of a sensitive plate, a cone made of strong cardboard, superpose on it an unexposed film which has been cut to the form of the development of the cone (as shown in Fig. 77). The film is secured by means of two or three pins. Having focused on a point of the subject in a middle plane, the ground glass is afterwards drawn back a distance equal to half the height of the cone, taking care not to derange either the subject or the objective. To obtain a sharp image a very small diaphragm must necessarily be used, but with a rapid plate and good light that is of little moment. The camera should be placed in the dark room, the lens being inserted in a hole in the partition just its size, and the subject in the adjoining apartment opposite the lens—this because the cone will not allow the plate-holder to be closed by the slide.
Fig. 76 shows the arrangement of the camera and holder. The exposure made, the film is developed, as usual. The negative gives a print deformed as shown in Fig. 76. The original, if not grotesque appearance of the head disappears when the print is rolled into a conical form and the observer places his eye in the prolongation of the axis of the cone. Fig. 78 shows the head as seen under these conditions.
[MAKING DIRECT POSITIVES IN THE CAMERA.]
Prepare a saturated solution in water of the crystals of thiosinamine, and add from two to eight minims of it to an ordinary pyro or eikonogen developer. Expose rather less than usual. The effect of this addition to the developing agent is an entire reversal of the image, a positive instead of a negative being obtained. Ammonia will assist the reversal. Colonel Waterhouse, the discoverer of this process, recommends in some cases the plates being subjected to a bath of 5 per cent nitric acid and 3 per cent potassium bichromate before exposure, followed by a thorough washing.
[INSTANTANEOUS PHOTOGRAPHY.]
In the very earliest days of photography this term was applied to what would now be considered very slow work indeed. We now usually apply this term when the exposure does not exceed one second. In some cases this only amounts to the one-thousandth part of a second. This exceedingly brief exposure is usually given to the plate by means of a suitably constructed shutter.