Blue prints may be made by brushing over any fairly pure paper with equal quantities of (a) citrate of iron and ammonia 1 part, water 4 parts; and (b) potassium ferricyauide 1 part, water i parts; these are printed in usual way. Or the first solution may be used alone, and the second solution applied as a developer after exposure. A blue-green image on a brownish ground is produced, but the brown washes away in clean water, leaving the image fixed. It is advisable, however, to give the prints a citric acid bath, 1 in 40. Paper for this ferro-prussiate process as it is called, may be obtained ready lor use of any photographic dealer in packets each containing twenty-five hair-].late pieces. Blue pictures may be made by the carbon process, which is the most satisfactory and permanent process to employ; the tissue (or sensitive paper) may be obtained in any desired colour. The paper, which appears to be almost black, is exposed as usual, but does not print a visible image. it may be timed by an actinometer or by another negative of the same density printing on P.O.P. It is next squeegeed into close contact with a paper coated with insoluble gelatine ami placed in iiot water.

The parts unaffected by light dissolve away after the top paper has been stripped off, leaving the image in pigmented gelatine on a white or other ground. The print then merely requires immersion in alum, and slight washing to remove the bichromate.