The details of the picture should be fully out, and the dark parts should have a bronzed appearance. Care is required to prevent the blue becoming less intense, and therefore the white lines not showing up so much. A print too much exposed appears weak, but the same occurs with too little exposure. The ferricy.inide used should be as pure as possible. It is affected by air and light, which may change it into ferrocyanide. The first forms a blue precipitate, and the second a white. Crystals of ferricyauide should therefore be rinsed before use to rid them of the changed outside covering. The first washing water should be acidulated with hydrochloric or citric acid. This tends to improve and retain the brightness of the image, by dissolving out the remaining iron, and preventing the deposition of a white precipitate over the blue. It is very desirable that the paper should not in any case be washed for a lengthy period.