An alternative process and one which has much to recommend it, is development with the following hydro-chinon developer:
Hydrochinon 3 g.
Sodium sulphite, dry 7.5 g Potassium carbonate 2 g.
Potassium bromide 1 g.
Water to 100 ccm.
After washing for ten minutes, the plate should be immersed in a one per cent solution of mercuric chloride and dried. This gives the most brilliant colors.
The colors are only seen when the pictures are examined at a certain angle, and then are very pale, due to the reflection of white light from the surface of the film. To destroy this a glass prism of low angle, 10 to 12 degrees, is cemented to the surface with Canada balsam. These prisms can be obtained from any optical house at a reasonable price, as accuracy of surface and angle are not important. The Canada balsam used must be diluted with benzol or xylol, not chloroform, as the latter in time bleaches the image. Before sealing with the prism, a very pretty experiment can be performed. Examine the picture at the best angle, and it is better to support it in one position and shift your position until this is attained. Then, with a glass tube, blow on the film and the colors will be seen to change as the gelatine between the laminae swells. All colors become more red, while as the moisture dries out they change back to blue. It is thus possible to slightly alter the colors of a heliochrome, either by warming or damping, and as soon as the correct colors are seen it should be cemented up. The prism should be clipped on to the picture and then it should be placed in a warm place to dry. When dry, the excess balsam should be scraped off, the edges bound up with black paper, and the back of the glass bearing the picture painted with black varnish. These pictures are best seen when side light is prevented from reaching their surfaces, and a viewing box can easily be made as shown in Fig. 22. In this ABCD is the box, in one end of which is supported the heliochrome, while at the other end is a peep hole E through which the picture can be seen. M is a small piece of mirror, adjustable on a hinge, which throws the light through the aperture KL cut in the top of the box. For exhibition on a screen with a lantern, the principle of the aphen-gescope or opaque object lantern projector must be used, as shown in Fig. 23, in which A represents the arc, C the condensers, and A' the image of the arc. The helio-chrome is placed in the plane HH', so that the beam of light just covers the picture, and L is the projection lens, which should be placed as far from EH' as this is from the point A', so that the image of the arc falls in the objective; the picture will then be shown on the screen SS'.