mode of backing up, 226; spots and markings on, 284; mode of printing Positives on Collodion, 272. Positives, enlarged, mode of printing, 272. Practice of Collodion process, 183. Preservative processes for Collodion plates, 289. Printing, Photographic, theory of, 120; practical details of, 240. Prism, refraction of light by, 51; diagram of formation of spectrum by, 47; explained, 54. Prismatic spectrum, 47, 61. Protonitrate of Iron, preparation of, 206; a feeble developer when free from excess of Sulphate of Iron, 98; theory of its mode of action, and rules for its use, 112; cannot be prepared in quantity by adding Nitrate of Potash to Sulphate of Iron, 314; sometimes requires the addition of Nitrate of Silver, 206. Protosulphate of Iron, its preparation and properties, 29; its characteristics as a developer for Collodion Positives, 111; not well adapted for developing Collodion Negatives, 117; preparation of the solution for Positives, 205; mode of applying it to the plate, 221; to remove iron stains on glass, 215. Pyrogallic Acid, its preparation and properties, 28; solution for developing glass Positives, 205; for Negatives, 211; cannot be used without Acetic Acid, 105; less adapted for developing paper pictures, 178; requires addition of Nitric Acid when used for Positives, 111; superior to Sulphate of Iron for developing Negatives, 117, 144; mode of obviating the brown discoloration of developing solutions, 212. Pyroxyline, its nature and properties, 75; preparation of, by Mr. Hadow's formula, 186; by a rule-of-thumb mixture of the acids, 188; by the Oil of Vitriol and Nitre process, 190; details of immersing, washing, and drying, 191; the glutinous variety produced by cold acids, 83; recapitulation of the effects of varying the strength of the acid mixture, 193; spontaneous decomposition of Pyroxyline, 166. Reduction of metallic oxides by developers, theory of, 26; of Silver salts by developers, theory of, 30; practical details of reducing Silver compounds to metallic state, 372.