The developer is nothing but hot water, and it is preferable to work at as low a temperature as possible. A dish should be filled with water at 380 C. (100° F.), and it is advisable to have a kettle or saucepan with water at a higher temperature, which can be added if the temperature in the dish sinks. After about five minutes the temperature of the water in the dish should be taken with a thermometer, and if it has cooled down too much, which it may do if a thick stoneware dish be used, add more hot water. Immerse the print, on its temporary support, in the hot water, and if any air bubbles adhere, break them with the fingers. If the print has been properly exposed, it will be seen in about two or three minutes that some of the pigmented gelatine will begin to ooze out from under the paper. Then one edge of the paper should be gently lifted with the finger nail and the paper pulled off with a gentle steady pull; this paper can be thrown away, as it has done its work. There will be no sign of a picture; the face of the celluloid will be covered with a smeary mess, but if the celluloid be gently moved about, to and fro and up and down, the soluble gelatine will dissolve and the picture gradually make its appearance.

An alternative method of working is, after removal of the paper, to slip a sheet of glass under the celluloid, fasten it at two sides by means of metal clips, and immerse it face down, supporting it by two glass or metal blocks at each end, when the soluble gelatine will sink away from the face of the print by its own weight. Another method is to place one edge of the glass bearing the celluloid face downwards in the side of the dish and holding the other in the fingers move the celluloid gently up and down. Or, if a large dish be used, the glass and the print may be supported by one hand at an angle of about 30 degrees from the horizontal, and the hot water splashed on it with the other hand. It is easy to see whether development is proceeding as it should do, and provided that the exposure is correct and the temperature of the water kept up, in about five minutes the print should be finished. It may then be placed face up in a dish of cold water, or a very gentle stream of cold water allowed to run over it from the tap, so as to remove any trace of soluble gelatine that may adhere. It should then be immersed in a 5 per cent solution of ordinary alum for fifteen minutes and then put to wash in cold and frequently changed water, or gently running water may be used, and twenty minutes is long enough for this. It should then be hung up to dry.

The developing dish should now be well cleaned out and the second and third prints developed in precisely the same way as the first. If, as soon as the first print is placed in the cold water, the second is developed, and then the third dealt with in the same way, all three prints can be finished and in the alum bath well within an hour. One caution must not be omitted, and that is that the print, until it has dried, is very easily damaged; therefore, placing all three prints in the same dish of cold water or alum solution should be avoided, otherwise some rough edge may dig a piece out of the tender gelatine.

When the prints are dry we can proceed to transfer them to their final support in superposition. Specially prepared final supports are obtainable commercially in various tints, thicknesses and surfaces, but only white should be used, any tint naturally affecting the color of the final picture. If the operator is desirous of making his own final support, a sheet of bromide or developing paper should be fixed, well washed, and immersed in 10 per cent solution of formaldehyde for ten minutes and hung up to dry without washing. At first rough papers should not be used, as they are much more difficult to handle than the smooth kinds.

The three prints on their celluloid supports can be temporarily superimposed, when dry, to see whether anything like satisfactory color-rendering has been obtained. If it is sufficiently pleasing, the surface of each print should be gently rubbed over with a pad soaked in benzol to remove any possible traces of wax and again hung up to dry. As the benzol is very volatile, this will only take a few minutes. Meanwhile immerse a sheet of the final support, which should always be larger than the print, in a dish of cold water and allow it to soak for at least half an hour (an hour is not too long); this is to give the paper fibers a chance to fully expand.

A cement is required to make the prints stick together; a plain 6 per cent solution of gelatine may be used, or the following may be made up and will keep well in a corked bottle:

Soft gelatine 10 g.

Glacial acetic acid 10 ccm.

Distilled water 480 ccm.

Allow to soak for about half an hour and then melt in a water bath and add:

Methyl or denatured alcohol 500 ccm.

Phenol (carbolic acid) 10 ccm.

This cement must be melted by heat each time before using and should be applied rather thinly with a broad flat brush.

The yellow print should be immersed in cold water for ten minutes, then brought into contact with the soaked final paper, lifted out together with this, thoroughly squeegeed, and hung up to dry. When quite dry the temporary support of celluloid may be stripped off, leaving the print on the paper. This print should now be pinned to a board by the corners, and the blue print on its support, which may have the corners cut off to allow room for the pins of the yellow print, should be placed on top and the two shifted about until accurate register is obtained, and two more pins driven through the two supports; the top support bearing the blue impression should then be lifted up and the yellow print quickly painted with the cement, which must not be too hot, and the blue print then lowered into place. Possibly some little shift may be necessary, but it should not be; anyway, one can examine the register with a magnifying glass. When satisfactory, the two should be hung up to dry and the celluloid stripped as before. The third, the red impression, is superimposed in the same way. It should be noted that the pins referred to above are the glass headed push-pins, and if these be driven with steady straight pressure through the supports, there is very little tendency for the prints to shift.