POSITIVES IN COLOURS There have been many methods of obtaining positives in colours. E. J. Wall, in American Photography, has summarised them as (i) the so-called diazotype processes (fully described under the heading " Diazotype "), in which the action of light on diazo compounds is utilised by the formation of dyes from the compounds thus formed or decomposed by light. (2) That class in which a silver image is obtained in the ordinary way, and then converted into a salt which acts as a mordant for the dye. The second class was first described by Georges Richard in Comptes Rendus, 1896, and ten years later Traube, of Munich, patented a pro cess, known as " diachrome," which is the application of Richard's principle. Traube converts a silver image into silver iodide by immersion in— Iodine . . . 8y grs. 20 g.
Potassium iodide . 218 „ ,, Distilled water to . to oz. 1,00o ccs.
and then after washing immerses the positive in a solution of a basic dye which immediately precipitates or forms a lake with the iodide and gives a coloured image. Dissolve the iodide in a fifth of the water, add the iodine, stir till dissolved, and add the remainder of the water. The silver iodide is of course opaque, but this may be dissolved out by potassium cyanide or a " hypo " bath containing tannin or tartar emetic, the action of these substances being to prevent the washing out of the dye. After
fixation the positive is merely washed, and the image consists of a perfectly transparent dye and is of extraordinary brilliancy and trans parency in the shadows. Basic dyes must be used, as so far the only acid dyes which give satis factory results are those of the triphenylmethane series to which the cosines belong. The following dyes are therefore available : acridine orange, chrysoidine, rhodamine 6 G, rhodamine B, xylene red B, methyl and crystal violet, victoria -pure blue B, and all the other victoria blues, methylene blues, methylene green, brilliant, emerald, diamond, and victoria greens, and all the cosine group of dyes which comprises cosine, erythrosine, rose bengal, phloxine, uranine, etc. Compound tints are best obtained by successive baths of different dyes, as mixtures are apt to stain unequally. Frequently, too, good effects may be obtained by first staining up with a basic dye and then applying an acid dye ; and if the combinations are suitably chosen, very intense colours may be obtained in this way, as the basic acts as a mordant for the acid dye.
Traube's process has been modified by many workers, Tauleigne and Namias being notable among them.