The extent to which the development should be carried depends upon the kind of Print desired. By pushing the action of the Gallic Acid, a dark picture not much altered by the fixing Bath will be produced. But a better result as regards colour and gradation of tone will be obtained by removing the Print from the developing solution whilst in the light red stage, and toning it subsequently by means of Gold; in which case it will correspond both in appearance and properties to a Positive obtained by the direct action of light (see the remarks at page 167). When it is intended to follow the latter plan, the action of the developer must be stopped at a point when the proof appears lighter than it is to remain; since the Sel d'or Bath adds a little to the intensity, and the image becomes somewhat more vigorous on drying. Wash the Prints in cold water in order to extract all the Gallic Acid. Then tone with _Sel d'or_ in the manner described in the next Section, and fix in the usual way. The whites will with care be kept pure; or with only a faint yellow tinge, which is not objectionable. Upon comparing the developed Prints with others obtained by the direct action of light upon the same sensitive paper, it is evident that the advantage is _slightly_ on the side of the latter; but the difference is so small that it would be overlooked in printing large subjects, for which the Negative Process is more especially adapted. The _colour_ of both kinds of Positives is the same, or perhaps a shade darker in the developed proofs, which are usually of a violet-purple tone, but sometimes of a dark chocolate-brown. _A developing process with Serum of Milk._--The use of "whey" as a vehicle for Chloride of Silver has something the same effect as that produced by adding a Citrate. This may be traced to the presence of the Milk Sugar and of a portion of uncoagulated Caseine left in the Serum. The only difficulty in the process is to coagulate the milk in such a way as to separate the greater part but not the whole of the Caseine. Milk which has become sour, or to which an acid has been added, is not considered so good for the purpose as that which has been treated with rennet; and even when rennet is used it must be of the best quality or its action will be imperfect. The serum must filter clear through, blotting-paper; but it should not run very rapidly, or in all probability the whole of the Caseine has been separated, and the fluid contains little besides sugar. The whey which is left after cheese-making, commonly answers the purpose, if clarified by beating it up with the white of an egg and subsequently boiling and filtering. Globules of oil must be separated as far as possible, or they will produce a greasiness of the paper.[50] [50] See the Vocabulary, Part III,, Art. "Milk," for further particulars.