This solution may be prepared by the same formula as that given for direct Positives at page 203, acidifying the solution with Acetic Acid in preference to Nitric Acid. THE DEVELOPING SOLUTION. Pyrogallic Acid 1 grain. Acetic Acid (glacial) 10 to 20 minims, or Beaufoy's Acetic Acid fort. 1 fluid drachm. Alcohol 10 minims. Distilled Water 1 fluid ounce. In place of Distilled Water, pure Rain-Water may be used (see Part III., Art. "Water"). The quantity of Acetic Acid required will vary with the strength of the Acid and the temperature of the atmosphere. An excess enables the manipulator to cover the plate more easily before the action begins, but when the picture is taken in a dull light, is apt to give a bluish, inky hue to the image. In cold weather, use less of the Acetic and twice the quantity of Pyrogallic Acid. With Collodion prepared from Spirits nearly anhydrous, and iodized with Iodide of Cadmium, the full quantity of Acetic Acid will be required, as there is sometimes a little difficulty in making the developer flow up to the edge of the film. If the image cannot be rendered sufficiently black, two or three minims of the Nitrate Bath solution may be added to each drachm towards the end of the development. If the solution be kept for some time after its first preparation, it becomes brown and discoloured. In this state it will still develope the image, but is less likely to give a clear and vigorous picture. A solution of Pyrogallic Acid in Acetic Acid will keep for many weeks, and may be diluted down when required for use. The following is a good formula:-- Pyrogallic acid 12 grains. Beaufoy's Acetic acid 1 fluid ounce. To one drachm add seven drachms of water. THE FIXING LIQUID. Cyanide of Potassium 2 to 12 to 20 grains. Water 1 fluid ounce. or, Hyposulphite of Soda 1/2 ounce. Water 1 fluid ounce. For remarks on the Cyanide of Potassium Fixing Bath, see the last Section, page 207. CHAPTER III. MANIPULATIONS OF THE COLLODION PROCESS. These may be classed under five heads:--Cleaning the Plates.--Coating with Iodide of Silver.--Exposure in the Camera.--Developing the image.--Fixing the image.--In addition to this, the present Chapter will include in separate Sections directions for the choice and management of lenses, for copying engravings, manuscripts, etc., and for taking stereoscopic and microscopic photographs. CLEANING THE GLASS PLATES. Care should be taken in selecting glass for use in Photography. The ordinary window-glass is inferior, having scratches upon the surface, each of which may cause an irregular action of the developing fluid; and the squares are seldom flat, so that they are apt to be broken in compression during the printing process.