If the directions given by the makers of the paper are followed, pure chemicals used, and separate toning and fixing baths, there is little danger of P.O.P. prints fading, It is perhaps in the fixing and washing of the prints that errors are likely to be made. The fixing bath, which must not be in an acid condition, should be at the normal temperature and sufficiently strong; if either of these points is neglected fading of prints may result. The bath should be made with warm water, as there is considerable loss of heat in dissolving hypo, and when the temperature is low the bath does its work too slowly When the prints are put in the hypo the unaltered' silver is changed into silver thiosulphate, which is insoluble, and then into a double thiosulphate of silver and sodium, which is soluble. Unless the bath is strong enough to form the double thiosulphate, stains and fading may result. The proper strength for P.O.P is hypo 3oz., water 20 oz. For albumen prints use a 10-percent, solution of hypo. The prints must be kept moving while they are in the fixing bath. It is important that alter fixing is completed every trace of hypo should be removed from the print.

For this purpose a mechanical washer may be used; this keeps the prints moving round the washer whilst the hypo sinks to the bottom and is syphoned off. Or the prints may be transferred by hand backwards and forwards between two dishes alternately filled with clean water. After about forty minutes thorough washing the prints should be free from hypo. A test, however, should be applied. Put a small quantity of starch into a test-tube and add a few drops of a solution of iodine, thus forming blue iodide of starch. Pour half of this blue iodide into another test-tube, and, lifting one of the prints from the washing water, hold it by one corner and allow the last few drops of the drainings from it to fall into one of the test-tubes If any hypo is present in the drainings it will turn the blue solution white. Compare the colour of the solutions in the tubes by holding them side by side against a sheet of white paper.