Salts nature of, 310. Salts of Silver, their preparation and properties, 12; their Photographic action, 18; theory of their reduction by developer, 30; directions for obtaining metal from, 372. Sel d'or, toning process by, its theory, 134; its practical details, 267; its advantages, 271; gives permanent prints, 167. Sensitiveness, term explained, 92; conditions favourable to, 97. Sensitiveness of Collodion film, causes influencing, 92; superior sensitiveness partially explained, 74; preservation of sensitive film, 288. Sensitive paper, theory of preparation of, 22, 122; its darkening by light described, 123; preparation of Albuminized paper, 241; of plain paper, 245; of Ammonio-Nitrate paper, 246, 258; of paper for Negative processes, 259; causes which affect the sensitiveness of Positive paper, 123; which alter the colour of the image, 126; spots and markings on, 285; a large excess of Nitrate of Silver essential, 124; the paper should not be kept too long, 130, 286. Serum of Milk, preparation of, 262, 355; used in Negative printing process, 262. Shadbolt, Mr., his Honey keeping process, 289; employs artificial light in Micro-Photography, 237. Silver, properties of, 362; estimation of, in Nitrate Baths, 371; recovery from waste solutions, 372; reduction from Chloride, 374; stains, removal of, 377. Size, mode of removing, from paper Positives, 255. Solar spectrum, 47, 61. Soluble paper. _See_ Pyroxyline. Specific gravity of liquids, mode of finding, 375. Spherical aberration, 56. Spirits of Wine, preparation and properties, 330; not always sufficiently strong for Collodion, 84; mode of rectifying, 196; sometimes contaminated with, fusel oil, 96. Spots on Collodion plates, 279; on paper Positives, 285; on prints obtained by development, 266; on preserved Collodion plates, 293. Stains, Silver, removal of, 377. Stereoscope, invention of, 67; theory of, 68; Wheatstone's, 69; Brewster's, 70. Stereoscopic Photographs, rules for taking, 71; practical details of, 232. Stops, theory of use of, 57; simple mode of making, 228; position of the stop often important, 230. Strength of acids, tables of, 378. Subchloride of Silver, its preparation and properties, 15; decomposed by fixing agents, 141. Suboxide of Silver, its properties, 18; forms compounds with organic matters. Citric Acid, Albumen, etc., 21. Substitution, laws of, explained, 78. Sulphate of Iron. _See_ Protosulphate of Iron. Sulphate of Quinine, absorption of chemical rays by, 65. Sulphuric Acid, table of strength of, 378. Sutton, Mr., theory of Sel d'or toning process, 134; practical details of, 267; Negative printing process, 262; preparation of Serum of Milk for, 355. Symbols, use of, 318. Syruped Collodion film, 289.