How old the magic lantern is, no one knows! The Egyptians had something corresponding to it, although the illuminant was sunlight introduced into the darkness of the temples for projection purposes through small circular diaphragms, perhaps fitted with lenses. Roger Bacon is credited with the magic lantern amongst his other ingenious inventions. At any rate, for exhibition purposes the projection lantern is at least two hundred and fifty years old.
Just at present the lantern slide is threatened with two serious rivals. Public entertainers are now expected to use the more agile and exciting cinematograph film. In the smaller lecture-hall the mirroscope will show on the screen, with good illumination and definition, images from ordinary prints and postcards, thus obviating the necessity of making any transparency. But those who take the trouble to make lantern slides of their work, will be well repaid by the greater clearness, brilliancy, and delicate light effects which they are certain to render from good negatives.
Unless only a very small portion of a plate is required, the additional work involved in making slides by reduction is very slight, and is likely to give the best and most artistic results. Special cameras are made for the purpose; but they are usually mere fixed-focus boxes, made to reduce a particular size of plate to lantern size. The negative may be put in a frame against the window and photographed through the camera on to the lantern plate. But the apparatus which we employ ourselves is so easily constructed, and will adapt itself so well to give exact reduction of different-sized plates, that most readers will probably prefer a method of the kind.
Take a long, planed deal board about 1 in. thick and of the same width as the camera. Screw a bevel on each side, so that the camera may slide evenly up and down the plank as required for focussing. The length of the board will depend on the focal length of the lens. For the ordinary 5 to 6 in. lens 3 ft. will be ample. At the opposite end to the camera an upright square of wood is fixed as an easel, in which there is an aperture cut, of centre corresponding to height of lens, and of size to take the largest negative that will be required to be reduced, say half-plate, with carriers for 5 x 4 and quarter-plate negatives.
On the focussing screen of the camera draw in pencil, exactly corresponding with the lantern-plate carrier which will go in the dark slide, a square 3 1/4 x 3 1/4 in. Place the negative in position on the easel, upside down, with film facing the camera, and focus. Allowance must be made for the black mask and border; so that the actual size of image should not exceed 2§ in. A small stop is advisable for exposure, in order to secure great sharpness. Remember that your little pictures, less than 3 in. in size, may have to appear on the screen enlarged up to twenty feet.
When using daylight for the exposure, care must be taken that features in the landscape do not intrude their shadow and cause uneven illumination. We always incline the board at an angle towards the sky. Choose a north light for preference; at all events avoid the neighbourhood of strong sunlight.
If daylight is not available, magnesium wire, or any artificial light behind a condenser, may be tried. We know of many workers who make their lantern slides in the ordinary gaslight enlarger. Traces of uneven illumination will possibly make themselves apparent unless the slide is much over-exposed or under-developed. Ground glass between the light and the condenser ought to remove this difficulty.
The lantern plates for reduction purposes are coated with an emulsion similar to that on bromide papers; for contact slides, gaslight plates (coated with the same emulsion as gaslight papers) are most satisfactory. Each may be developed in just the same way as the corresponding papers; and the gaslight slides will give a variety of tints according to the time of exposure and strength of developer.
Pyro, metol-quinol, glycin, amidol, or rodinal are all most suitable for lantern plates, but with the first of these care must be taken not to over-develop. The plate may generally remain until the shadows are visible on the glass side; the exact density will be learnt by practice. Beginners usually make their slides too thin, when they must be intensified by the bichromate method, which will also tone them a rich brown colour. If they are too dense, reduce them in the following:
Iron Perchloride.......15 gr.
Citric Acid........20 „
The best test of the density of a lantern slide after fixing is to hold it about a foot from the wall of an ordinary room fairly lighted. The pattern of the wall paper ought to be discernible through the darkest shadows. The high lights must not be veiled, and yet very little of the plate ought to be clear glass.