Another important point in the projection or viewing of screen-plates is that the color elements are adjusted to give the best results for a mean daylight, and the color rendering is not correct for any other light; although we are in the habit of considering the projection arc and lime-light as white, they are distinctly yellow. To obtain the best possible results, the lantern must be provided with a color filter, or each slide bound up with a stained gelatine plate. The latter method is the one which should be adopted if the screen-plates are to be shown in alternation with black and white slides, while if color slides are to be shown alone, then only one filter need be prepared and this should be placed close to the condenser. This correction filter can be made with rose Bengal and patent blue, and as similar filters are required, as will be seen later on, for visual examination of the pictures, it will simplify matters to give the methods of making these now.

In the first place, three stock solutions should be prepared:

A. Gelatine 7 % solution.

B. Patent blue 1: 1000 solution.

C. Rose Bengal 1: 1000 solution.

For use with an arc the following quantities of the above solutions are required for one square meter of filter surface:

Solution A 300 ccm.

Solution B 30 ccm.

Solution C 30 ccm.

Water 225 ccm.

For oil, gas, or the old carbon filament electric lights, use:

Solution A 300 ccm.

Solution B 37.5 ccm.

Solution C 23 ccm.

Water 225 ccm.

For incandescent gas and Mazda lamps use:

Solution A 300 ccm.

Solution B 23 ccm.

Solution C 37.5 ccm.

Water 225 ccm.

In order to save the trouble of coating with gelatine solution, some fixed-out dry plates may be immersed in the dye solution mixed in the above ratios, using water instead of the gelatine solution, for about five minutes, then rinsed and the screen-plates examined through them against the particular lights with which they are to be used. One can soon tell whether the plates require deeper staining or reducing in color; the actual tint required is quite a weak one. The idea of using these tinted filters is to cut down the excess of red and green, the patent blue cutting down the former and the rose Bengal the latter. Other dyes with similar absorptions may be used; but in all cases the filters must be dry before the pictures are examined, as with many dyes the tints change considerably in drying.

To increase the brilliancy of the pictures it has been suggested that aluminum screens be used, and they certainly are an improvement for narrow rooms and halls; but in theatres of greater width the audience at the sides has a less satisfactory view than when an ordinary screen is used. For home exhibitions, ordinary Bristol cardboard forms an excellent screen, or, if the room is long enough to permit of the lantern being placed behind the screen, a sheet of ground glass or white sheeting or pure white, not blue-white, tracing cloth stretched over a frame may be used.

For viewing in the hand a viewing-frame has become very general. This is nothing more than a mirror placed horizontally, with a wooden or metal frame, at an angle of 45 degrees, with an aperture for the picture. Side wings of cloth, card or black material prevent side light from reaching the mirror, which is placed flat on the table, the picture being viewed in the mirror. Naturally the picture is placed upside down and inside out, that is, with its picture side away from the mirror. In all cases the pictures should be protected by a cover glass and bound up like an ordinary slide. For showing at exhibitions, it is better to arrange them in frames, so that no white light can creep round the edges, and use the long line-o-lite electric lamps, placing these at the top and bottom of the rows of pictures, with asbestos or metal backing painted white, arranged so that no light, except that passing through the pictures, can reach the eyes. This arrangement enables them to be shown at night and also makes one entirely independent of daylight, which is sometimes difficult to reflect so as to give even illumination. To prevent access of daylight to the front of the pictures, battens may be run out over the top of the frames and black cloth nailed to them, the extent to which the battens should project being determined by the position of the windows of the gallery. Side wings at the end of each row, and within it if it is very long, are also needed.

Printing From Screen-Plates

Having obtained a color picture by means of a screen-plate, one naturally wants to print from it, and we can divide this subject into two distinct parts; first, the production of another screen-plate, and secondly, the production of paper prints.

As regards the first, we may dismiss the separate method in very few words, as it merely entails the making of other black and white transparencies and binding them up with the viewing screens. But in the combined processes, the reproduction on another plate involves some little difficulties.

At the outset it must be recognized that an absolutely faithful reproduction is out of the question. The second picture must always be contaminated with black, that is, the colors are of lowered luminosity. On the other hand the results are not displeasing, and if the original is not at hand to be compared with, no one is any the wiser. The most satisfactory method of reproducing an autochrome by contact is that devised by the Lumieres, and if carefully followed the failures are few; the necessary apparatus can be easily knocked up, even of cardboard. A rectangular light-tight box A, B, C, D, (Fig. 20), 40 cm (16 in.) in length is required, which should be painted dead black inside, to prevent reflections. At one end is cut an aperture, in which is placed a special compensating filter E, the composition of which has been given in connection with flash-light work. This must be arranged so that no white light can creep around the edges. At the other end is placed a plate-holder, a printing frame or some similar arrangement, which also must be light-tight. In this is placed the picture to be reproduced, with its glass toward the inside of the box, and behind the picture is placed the autochrome plate, with its glass in contact with the film of the picture; behind is placed the usual black card. G is a block of wood, or other support, preferably weighted at the bottom, so that it will not tip over, and into the top is screwed an ordinary brass electric binding screw. S is a spiral of iron wire; G must be so placed that the spiral of wire is opposite the center of the filter, and about 5 cm from it. The spiral should be formed by coiling iron wire round a cylindrical rod of 3 or 4 mm diameter, as closely as possible, and then slightly pulling it out until the coils of the spiral are separated by about 1 cm. This is merely the holder for the magnesium ribbon that is used as the illuminant. Copper or brass wire must not be used for the spiral.