Perfect cleanliness is essential. The salting or iodizing solution and the Aceto-Nitrate Bath must be filtered clear, as the effect of small suspended particles in producing spots is more seen when the image is brought out by a developer. It will be necessary to be far more careful in excluding white light than in the ordinary process; and when Iodide of Silver is used, all the precautions required in the case of Collodion Negatives must be taken. Observe particularly that the dishes are kept clean, or the Gallo-Nitrate of Silver will be rapidly discoloured (read the remarks at page 179). Stereoscopic Negatives and small portraits are not successfully printed by development; since it is difficult to obtain the most elaborate definition, and there is a slight tendency to yellowness in the white parts. Positives may be developed upon Albumen paper, but the Gallic Acid is apt to discolour the lights. In printing by development upon Chloride of Silver, the theory of the subject must be particularly studied. When the weather is cold and the light bad, the development of the image proceeds slowly, the Gallic Acid Bath remains clear, and good half-tones are obtained; but under opposite conditions, the developer may become turbid and the shadows be lost by excessive deposit of Silver. This _over-development_ will be remedied by printing the Negative in a more feeble light (near to the open window of a room), and by adding Acetic Acid to the developer, about 5 or 10 minims to the ounce, so as to bring out the image more slowly. The intensity of action is thus lessened, and if the picture be not under-exposed, the half-tones will be good. Observe also when preparing papers with Citrate, that if too much Carbonate of Soda be added in neutralizing the Citric Acid, Carbonate of Silver will be deposited in the paper, the effect of which is to remove by degrees the acidity of the Nitrate Bath, and to produce over-development and excessive sensibility to light. The colour of the proofs when taken from the Gallic Acid should be _light red_; the gradation of tone not being usually so perfect when the development is carried into the second or black stage. It is not recommended to prepare too large a stock of the salted papers, as they will probably be liable to mouldiness and decomposition unless kept perfectly dry. SECTION III. _The Sel d'or Process for toning Positives._ This process is somewhat more troublesome than the plan of fixing and toning in one solution, but possesses advantages which will presently be enumerated. The description may be divided into the preparation of the toning Bath, and the manipulatory details. THE PREPARATION OF THE TONING BATH. Take of Chloride of Gold 1 grain. Pure Hyposulphite of Soda 3 grains. Hydrochloric Acid 4 minims. Water, distilled or common 4 fluid ounces.