(6) After your negatives are dry, the next step is to make a lantern transparency from them. The method is very simple. A 3 1/4 by 3 1/4 gelatino-bromide plate is exposed behind the negative to the light of a good gas-lamp for 3-30 seconds, as the density of the negative may require (6 seconds is the rule for a good negative). Develop with ferrous oxalate, to which a little chloride of ammonium may be added (3 drops of 10 per cent, solution to 1 oz. of developer): fix, wash well, immerse in alum solution made slightly acid with hydrochloric acid, wash dry, and mount behind a piece of good clean glass. (H. P. H.)
All the manipulations can be carried on in the evening, with much greater rapidity, as well as economy, than any other method of producing pictures. A perfect lantern slide must possess two qualifications, viz., absolutely clear glass in the high lights, and, when held up to the window with a ground glass or other suitable background, full and distinct details in the shadows. For contact printing (which is the method largely practised), the following should be provided: - One glass pan (4 by 5) for developing; two glass pans (5 by 7); as these will hold two plates each at a time, they will often be found useful when fixing and clearing. It frequently happens that the first plate is not entirely fixed by the time the second is ready to be placed in the' hypo.; hence a large tray is quite essential. One deep printing frame. A student or other kerosene lamp, with a porcelain shade, is the best for making the exposure, and a developing lantern yielding plenty of diffused orange-coloured light is essential for the dark room. In the dark room the negative is now placed in the printing frame, and the box of sensitive plates is opened; one is then laid upon the face of the selected portion of the negative most suitable for a slide.
Next hold the frame up in front of the orange-coloured lantern, to obtain the correct adjustment of the slide with reference to the picture, and carefully keep the plate in position while laying the frame down to put in the pressure board. It is now ready for exposure, which should be made with the frame fixed at a distance of about 12 in. from the lamp. Considerable latitude is allowable in the duration of the exposure, provided the developer is made to correspond to it. A long exposure, 15-40 seconds (according to the density of the negative), with a dilute developer is the most suitable, yielding warm brownish tones with fine detail in the shadows.
After the exposure is made, the plate is developed in the dark room, with the following solutions, prepared after Carbutt's formula:
No. 1. - Iron.
Water ... 4 oz.
Sulphate of iron . 480 gr.
Dissolve, filter and add Sulphuric acid . 5 drops or minims
No. 2. - Oxalate.
Water ... 8 oz.
Sulphite of soda
(crystals) . 120 gr. Citric acid . 15 gr.
Dissolve and add
Citric acid . 50 gr. Bromide of potassium 10 „ And filter.
No. 3. - Fixing Bath. Hyposulphite of soda 1 oz. Water ... 5 oz.
The developer should be mixed as follows:-
No, 2 (Oxalate potash solution) . 4 oz. To which add
No. 1 (Iron solution). 1 oz. And then dilute with
Water . 1 to 3 oz.
The development will be slow, and should be continued until the details are fully out, and the image distinctly seen on the back of the plate. After development, the plate is well washed in" running water; and placed in the fixing bath; another thorough washing is necessary, then it is immersed for 30 seconds in the following :-
Alum ... 160 gr.
Water ... 5 oz.
Sulphuric acid . 2 dr.
Again wash the plate thoroughly, and finish by holding it under the tap, while passing over the face of it a broad camel-hair brush, which will remove any adhering particles of sediment. The plate is then placed on a negative rack, and should be thoroughly dry before being mounted.
Abandoned negatives, with the film cleared off by boiling in water, can be utilised for the protecting glasses by being cut up to the proper size. Care should be taken to use glass free from spots or bubbles. (J. E. Brush.)
(d) The advantages of the carbon process are twofold: - (1) Absolute control over the tone of the transparency; (2) purity of the high lights. To ensure this latter, however, one precaution is necessary, namely, that the room where the tissue is dried must not be warmed by gas or lamps, unless means are provided for carrying off the products of combustion. If they are present in the air, an insoluble ski a is formed on the tissue, and the high lights are consequently degraded. An actinometer is necessary as a guide to exposure, and in most carbon printing works one is usually employed by the printers, but not always.
Woodburytype is especially suitable for lantern slides. The relief is simply a carbon print; but the tissue, instead of being highly charged with colour, is only lightly tinted, the object being to obtain as great a thickness as possible in the shadows, so as to facilitate the printing operations afterwards. Skill is here required - not only in the preparation of the tissue, but in drying the developed relief. The advantages of this process are that, once a satisfactory relief is obtained, any number of printing moulds can be secured from it by pressure in the hydraulic press, from each of which numbers of prints can be obtained of the exact tone and depth desired. It has the further advantage of allowing of a considerable amount of retouching. For instance: if the negative be full of pinholes, these produce little raised points on the relief, which can be cut down. On the other hand, if there be any black spots on the negative, these form raised ones on the leaden mould, which can also be cut down. These advantages are shared by stannotype. All spots are touched out fully on the negative, and from this a positive is made, in which they show as white spots.