According to the paper, and also according to the developer employed, the quantity of bromide recommended by the different makers ranges from slightly less than 2 gr. to 25 gr. per 20 Oz. of working developer (o-2 grm. to 3 grin. per litre). A series of solutions may be prepared, containing in 20 OZ.
gr.-3 gr.-7 gr.-13 gr. and 25 gr.
of potassium bromide.
A small quantity of solution of each strength only need be prepared, and for facilitating the correct measuring of these small quantities of bromide, it may be used in the form of a i per cent solution. This rriav be measured from a dropping tube, after having ascertained how many drops represent, say, one dram.
A strip of the paper to be tested should be exposed to light with black paper covering half its width, the other half being uncovered. The exposure should be sufficient for the most intense black that the paper will give on develop ment. The strip should then be cut into five pieces, which should be marked on the back and developed, each in a solution with a different proportion of bromide. The time necessary for obtaining a dense black should be noted and the action of tile developer allowed to continue until either staining or veiling appears. The smallest proportion of bromide which will allow a reasonable time to elapse between full develop ment of the black and degradation of the whites may be regarded as the correct quantity for that paper. It will be necessary to fix the test pieces, so that they may be examined in a good light and the quality of the black correctly judged. A greenish black indicates that the pro portion of bromide has probably been too great. 56o. Stopping Development with an Acid Bath. On account of the great speed of develop ment of positive papers, there is a risk of development continuing beyond the desired point, and even continuing irregularly, if the print be merely rinsed in plain water between development and fixing. Omission of rinsing, i.e. the transfer of the print direct from the developer to an acid fixing bath, is not to be recommended ; the accumulation of developer in the fixing bath would soon discolour it to such a degree that the prints would lose their purity.
Most frequently the action of the developer is stopped by immersing the prints in an acid bath, where they can remain witlitnit detriment for a long time. This avoids the necessity of
having to handle the developer and the fixing bath alternately ; in this case, omission to rinse the hands would involve a risk of staining the prints.
This stop bath may be simply a weak solution of sodium bisulphite, or of any acid, or an acidified solution of alum. This latter possesses the advantage of becoming turbid as soon as the acid is saturated by the successive quantities of developer carried into it by the prints, and thus affords an automatic indication that the bath must be either re-acidified or renewed.
This acid bath may be— Chrome alum . iSo gr. (20 grin.) Acetic acid, glacial . iSo min. (20 c.c.) Water, to make 20 OZ. (1,000 C.C.) Some papers have a tendency to acquire a very slight greenish tint after treatment with chrome alum ; in that case, chrome alum may be replaced by ordinary alum. The acetic acid may also be replaced by any other acid which is less costly, and, particularly, by a smallei quantity of hydrochloric acid.
56i. Handling of Positive Sensitive Papers in the Tropics. The precautions previously sug gested for negative films and plates (§ 391.) apply equally to papers. The preliminary soak ing should be in a solution of sodium sulphate, or, if spirit is not costly, in water to which about 20 per cent of its volume of methylated spirit has been added ; and sodium sulphate should be added to the developer. In order to avoid the use of liquid acids which involve difficulty in transport, or costly solid acids such as tartaric or citric, the stop bath may be acidified with either boric acid or sodium. bisulphate.
(b) BROMIDE PAPERS' 562. Non-actinic Light. Bromide papers are of sufficiently low sensitiveness to allow of their manipulation in yellow, greenish-yellow, or orange light. The use of red light is not only useless but distinctly undesirable, as it renders it very difficult to judge the contrast and quality of the image correctly during development. With regard to the choice of the methods of lighting, the testing of non-actinic screens, and their preparation, reference should be made to §§ 248 to 251, where these matters have already been dealt with.