These are given rather with the idea of making the subject complete than for their practical value, as in nearly all cases the images are too opaque to be of any value. The iron toning process, however, which gives a blue, has been frequently used as the foundation image for the other colors, and the yellow mercury image may be used in the same way.

The best way of obtaining the blue-toned image, which actually gives a very satisfactory foundation for the other images, is by a modification of the bath given for vanadium, using ferric oxalate instead. As commercial ferric oxalate is rarely suitable for the work, the following method should be adopted for making it: add 520 g of ammonium iron alum to 200 ccm distilled water, and heat until dissolved. Then cool, add 200 ccm strong ammonia, and allow to stand for fifteen minutes with an occasional stir. Now transfer to a tall 1000 ccm graduate, and rinse the first vessel out with repeated lots of distilled water and add to the graduate till full. Allow to stand until the precipitate has settled somewhat, then syphon off the supernatant liquid, fill up the graduate again with distilled water, shake well, again allow to settle, and again syphon off the liquid; repeat this operation until the water no longer smells of ammonia. Then allow the graduate to stand until the precipitate and the water together measure not more than 850 ccm. Add 215 g pure oxalic acid, stir well, and allow to stand for a short time. The precipitate will gradually dissolve and form a bright green solution; when all the red ferric hydroxide has dissolved, filter the solution and add distilled water to make the bulk up to 1000 ccm. This will be a 20 per cent solution of ferric oxalate with an excess of 1.2 per cent of oxalic acid. The addition of the oxalic acid must be made by artificial light, and the solution must be kept in the dark. The toning bath is made up exactly like the vanadium bath, merely substituting the iron for the former.

The blue image given by this bath is known generally as a cyanotype or Prussian blue image, and it may be used as the foundation for the red and yellow images, as any silver image, whether on paper or on glass, is toned blue by it. It is thus possible to make the print from the minus-blue negative, on bromide or development paper, or on a transparency plate, and superimpose the other images.

The yellow-toned image is obtained by treating the black silver image with mercuric-potassium iodide, which is made as follows:

Mercuric chloride 11 g.

Distilled water 500 ccm.

Heat until dissolved, and add the following solution:

Potassium iodide 27 g.

Distilled water 500 ccm.

Shake the mixture well and filter. The print or transparency is immersed in this solution for fifteen minutes and then thoroughly washed. The image turns brown, and only after washing does the yellow color appear. Another variation of the above is the following:

Mercuric chloride 50 g.

Distilled water 900 ccm.

When dissolved, add slowly with constant stirring:

Potassium iodide 2.5 g.

Distilled water 100 ccm.

In this the image turns yellow at once.

There is no satisfactory method of obtaining a good red image by direct toning. There are several methods of obtaining colored images by the use of lead salts, but these are so opaque that the resulting pictures are so dirty and muddy as to be valueless. The blue toning process described above is satisfactory for both transparencies and prints; but the yellow toning process can only be used to make the foundation print, as it is too opaque for lantern slides.