The first adjuvant used for developing bluish tones in any developer seems to have been quinine hydrochloride (K. Kieser, 1928). Very numerous substances, of which many are not easily obtainable commercially, have been sug gested for this purpose (A. Steigmann, 1931; A. Seyewetz, 1932, 1935; G. Schwarz, 1936). One of the best seems to be in con centrations of 1/2,000 to x/5,000. Besides organic substances, a mineral substance, sodium fluor ide, at a concentration of 0-4 per cent has also been suggested for developing bluish tones (I.-G. Farbenindustrie, 1934).
The slightest traces of hyposulphite in the developer usually prevent bluish tones, even if the adjuvant tending to give these tones is increased in quantity.
573. Method of Development. Development of gaslight papers is far too rapid to allow of the use of the Watkins method, recommended for bromide prints.
The beginner, or the occasional printer, to whom some rule is necessary for testing the time of exposure, should develop for a constant time. With the metol-hydroquinone developer given above, the normal time of development for most papers of this type will he from 20 to 40 seconds at a temperature of 65° F., or from 30 to 6o seconds at 55° F. (B. T. J. Glover, 1924). 30 seconds may be selected as a constant time of development at average temperatures, this time being increased to 6o seconds if the tem perature falls below 55° F. Under these condi
tions it is very easy to determine systematically the correct exposure 553).
The instructions issued by the manufacturers almost always specify the normal time of development in the solutions which they recom mend, and their instructions should be followed.
At too low- a temperature it is difficult to obtain vigorous images ; a developer which is too warm tends to produce fog, either a grey veiling or yellow stain. A solution containing insufficient carbonate, or a developer too dilute or exhausted by previous use, will produce white patches with irregular outlines, suggesting that the paper has not been uniformly coated with emulsion. This is especially the case if the paper has been kept under such conditions that the emulsion has absorbed an appreciable quantity of moisture.
A developer which is too warm will often give images of a bluish-black, suggesting an insufficient proportion of bromide in the solu tion ; the whites are also stained or veiled.
An acid stop bath is necessary to stop the action of the developer at the desired moment. In the absence of an acid bath between develop ment and fixing, an acid fixing solution must be used, and the print kept on the move in it, so as to avoid development continuing in par ticular parts.