Many operators omit this operation altogether, but it can be recommended, for not only does it give good protection to the tender film but it also increases the transparency of the picture. An excellent formula is:
Gum dammar 20 g.
Manila copal, powdered 50 g.
Carbon tetrachloride 1000 ccm.
The mixture should be well shaken or stirred up occasionally during twenty-four hours. Then the bottle should be placed in a water bath and the temperature raised until the solution boils; this should be continued for five minutes and the solution filtered while still hot. This varnish dries rapidly and gives a colorless film. Alcoholic, ethereal or celluloid varnishes must not be used. It is not actually necessary to warm the plate before varnishing, but it is preferable to do so, as it drives the last trace of moisture from the gelatine.
The extreme slowness of the combined screen-plates and the consequent long exposures in the light of an ordinary studio naturally led to experiments with flashlight mixtures, with excellent results as regards color rendering. The formulas for these and the necessary compensating filters have already been given. It is advisable to use a diffusing screen made of some such material as thin white linen in front of the flash lamp; this screen should be of goodly size, about four and one-half feet high and three feet wide, and placed about eighteen inches from the lamp. A suitable arrangement of the lamp and sitter is shown in Fig. 19, in which C is the camera, R a white reflector, F the background, M the sitter, B the white diffusing screen, and L the lamp, which preferably should be of the type in which the powder is spread out 111 a long tray. The quantity of powder is dependent on the distance between the lamp and the sitter, the size of the plate used, and the aperture of the lens. For a 3 1/4 x 4 1/4 or 4 x 5 plate, with the sitter about six feet from the lamp and a lens working at f: 4.5, about eight grams should be used. The lamp should be rather higher than the head of the sitter.
For flower and still life work, magnesium ribbon may be used with good effect, and if it be burnt behind an opaque screen so as to shield the lens, it can be much nearer the object and less used. It may be of use to point out that three pieces of ribbon, each five inches long, do not give the same light as one piece fifteen inches long. This is probably due to the long glowing ash of the longer piece. For instance, setting the action of four inches of ribbon as one, that of an eight-inch piece was 2.5. To the photomicrographer the screen-plate process is invaluable, particularly in petrological work and crystallography with polarized light, and probably the separate method will commend itself, as from the original negative any number of slides or prints may be made. It is impossible to give any useful hints as to the exposure, as this will depend on the light, the numerical aperture of the condenser and the objective, and the degree of magnification. As to the filters to be used, possibly enough on this subject will be found in the following pages.
Unfortunately perfect immunity from failures cannot be insured, and these are the more serious because one is limited in remedial processes, as the color balance is so apt to be upset. Possibly the easiest method of dealing with the subject will be to divide it up into the different practical steps.
Exposure failures: General want of sharpness may be due to not focusing through the filter, or to the ground glass not being reversed, or to the plane of the latter not being coincident with that of the sensitive surface. This latter defect can be remedied in future work by measuring the thickness of the screen-plate and obtaining a ground glass of the correct thickness, or, as already suggested, using a cleaned-off plate for this; the average thickness of the autochrome plate is from 1.2 to 1.8 mm.
In the case of the separate process, general or local want of sharpness may be due to insufficient contact between the taking screen and the sensitive plate, which may probably generally be ascribed to want of flatness in the latter.
If the plate after the first development is more or less black over the whole surface and shows no color, it has been placed the wrong way round in the holder, that is, with the sensitive side, and not the glass, toward the lens. To avoid the occurrence of such an accident, it is as well after filling the plate holders to draw up the sliding shutters an inch or so, naturally working at a safe distance from the lamp; the reflection of the light will at once show whether the glass is in the correct position; also, it looks black, whereas the sensitive surface looks white.
The picture may be very dense, with marked want of detail both in the high-lights and shadows. This is caused by gross underexposure and insufficient primary development. There is practically no remedy, though the results can be somewhat improved by reduction, but this is a delicate matter, as the thickness of the gelatine is so little that the action of a reducer may be too violent. Probably the best reducer is made by adding five parts of the reversing solution to 100 parts of water. The reduction should be carried on in white light, and close to a water supply, so that the moment enough action has been obtained the plate can be rapidly rinsed and the action arrested. If the plate is rather too dense in the shadows only, reduction may be resorted to and intensification omitted. If the plate is too heavy everywhere and yet shows full detail, the primary development was a little too short or the temperature of the solution was allowed to drop.