Any of the relief processes already described for prints may be used here, and for this work the best film to use is motion picture positive film, which can be usually obtained to order. This is on celluloid only 0.005 inch thick and has no back coating; the emulsion is excellent for positive making, though rather faster than the usual run of transparency plates.

As regards the dyes for staining up, one has a fairly wide range, as acid dyes are the best. The following baths were suggested by Lumiere, and their only disadvantage is that the dyeing is very slow. On the other hand, it must be recognized that the most brilliant results are obtained with slow dyeing, that is, in weak solutions; strong solutions and rapid dyeing always give flat pictures.

For The Red

Erythrosin 1.5 g.

Water 1000 ccm.

Time of dyeing from one to five hours. Rinse with water and immerse for five minutes in five per cent solution of cupric sulphate.

For The Yellow

Chrysophenin (aurophenin) 2 g.

Water 1000 ccm.

Dissolve by heat and add:

Alcohol 200 ccm.

The great disadvantage of this dye is that it must be used in the form of the ammonium salt, and forms insoluble muddy brownish colors with calcium salts; therefore the relief must be immersed in a 1 per cent solution of hydrochloric acid for five minutes and then washed in distilled water, before staining; it is as well, therefore, not to use this.

For The Blue

Pure diamine blue FF 2 g.

Fish glue 12 ccm.

Water 1000 ccm.

Dyeing takes from two to ten hours. The glue causes a slower and more even dyeing. This picture should be immersed in the copper bath after rinsing, but this treatment is not necessary for the yellow.

Von Hubl recommended the following, which do not take so long as the previous baths.

For The Red

Erythrosin 0.25 g.

Alcohol 100 ccm.

Water to 1000 ccm.

For The Yellow

Naphthol yellow S 0.5 g.

Alcohol 100 ccm.

Glacial acetic acid 5 ccm.

Chrome alum, saturated solution 50 ccm.

Water to 1000 ccm.

For The Blue

Bluish fast green 0.1 g.

Alcohol 100 ccm.

Glacial acetic acid 5 ccm.

Water to 1000 ccm.

Other dyes as suggested for prints may be used; for the red, rhodamine B, or better still xylene red B, are good, for the blue, patent blue V, and for the yellow, mikado yellow.

One of the best methods of working is to use acid and basic dyes, as this gives one a much greater range and power over the colors and it is extraordinary what deep colors can be obtained. This method of working is based on the fact that the acid and basic dyes mutually precipitate one another, so that it is possible to faintly stain up with an acid dye and then by treatment with a basic dye to obtain a very deep result, or the order of the dyes may be reversed. This method was originally suggested by J. H. Powrie, and elaborated by A. E. Bawtree, whose choice of dyes is followed. For blue, soluble blue 5 per cent, oxalic acid 5 per cent; for peacock blue, which gives the best colors, soluble blue 5 per cent, naphthol green 1 per cent, malachite green 5 per cent. For yellow, aurantia 2 per cent, auramin 0.3 per cent. For red, rose Bengal 10 per cent, auramin 0.1 per cent. The method of using these is to immerse the picture in the first named dye for a short time, then wash and immerse in the second; for instance for yellow immerse in the aurantia first, then in the auramin. This alternation of the baths can be repeated, with intermediate slight washings, till sufficient depth is obtained; this in fact may be likened to a species of dye intensification.

The pinatype process gives very brilliant transparencies, but it is a little trouble. Glass should be coated with 5 per cent solution of hard gelatine containing 2 per cent of ammonium bichromate, and 30 ccm should be allowed for every 100 square inches. Weak daylight or artificial light may be used, but the plates must be dried in the dark. They will keep for about fourteen days, but it is better to coat the glass with plain gelatine solution and sensitize as wanted. Transparencies must be used to print from, and it is advisable to print from the minus red one first, that is, from the transparency taken through the green filter. It is then washed with water until free from bichromate, or to save time it may be immersed in a 2 per cent solution of sodium bisulphite, and washed and dyed up. It must now be dried, placed on a leveled stand, coated again with the bichromated gelatine and dried. The minus blue transparency, taken through the red filter, should be placed over the film and registration secured by the aid of a magnifier; the two plates are kept from shifting by two strong metal clips, or a printing frame can be used. The exposure is about twice as long as for the red impression, because some of the bichromate penetrates into the red image, so that the sensitiveness is reduced, and an increase of the bichromate is not advisable. After printing, the plate is washed or treated with the bisulphite, and dyed up. The yellow impression is obtained in the same way; but it is easier to make the yellow picture on another glass and reverse it, so that it can be used as the cover glass for the red-plus-blue image.

In all these processes, the blue constituent may be obtained by the aid of a silver image, and this toned with the cyanotype mixture as already described. A combination of processes can also be used, excellent results being obtained by making a blue-toned picture and transferring the red and yellow to it by the imbibition method. Enough has probably been said to lead the beginner in the right road to making successful lantern slides, for after all a slide or transparency is nothing more than a print with glass as a support instead of paper. Attention to details and extreme cleanliness in working are essential, and above all things absolute freedom from dust, as this shows up most distinctly when projected on a large scale on the screen.