Iceland moss, its use in Positive printing, 128; formula for preparing paper with, 245. Imperfections in Collodion Negatives, 282; in Positives, 284; in paper Positives, 285. Intensity, explanation of term, 92; mode of increasing in Negatives, 99, 114; effect of Acetate of Silver upon, 116; of Nitrite of Silver upon, 102; mode of diminishing, in glass Positives, 109, 110; conditions affecting intensity in paper Positives, 123; in developed paper Positives, 266. Invisible images, theory of formation of, 34; development of, 36; experiments illustrating, 25. Iodate, how formed in Collodion film, 94; produces insensitiveness, 198. Iodide of Ammonium, preparation of, 198; not fitted for iodizing Collodion required to be kept long, 210. Iodide of Iron, an accelerator to Collodion, 116. Iodide of Potassium and Silver, properties of, 42; mode of iodizing Calotype papers by, 177. Iodide of Potassium, tests of purity of, 197; extent of solubility in Alcohol, 351; dissolves Iodide of Silver, 42. Iodide of Silver, its preparation and properties, 16; unaffected by direct action of light, 19; highly sensitive to invisible image, 24; hypothesis of formation of latent image on, 34; possibility of its reduction by Pyrogallic Acid shown, 33; excess of Nitrate of Silver essential to its blackening by developer, 36; diagrams of chemical spectrum on, 61, 64; fixing agents for, 42; its solubility in the Nitrate Bath, 86; retards the action of Hypo fixing and toning Bath, 136; superior permanency of developed prints on, 167; details of Negative printing process on, 263. Iodine, in Collodion, diminishes sensitiveness, 94; forms Nitric Acid and Iodate in the Bath, 94; often useful in Positive Collodion, 110; in Negative Collodion, if fogging occurs, 105; mode of removing from Collodion, 86. Iodized Collodion. See Collodion. Kaolin, properties of, 335; used to decolorize Nitrate Baths, 91; importance of purifying it before use, 245. Landscape Photography, with preserved Collodion plates, 288. Latent image. See Invisible image. Laws of substitution explained, 78. Le Grey, M,, his toning process with Chloride of Gold, 132; his waxed paper Negative process, 178. Lenses, various forms of, 51; foci of, 52; formation of images by, 53; use of stops, 58; portrait, 59; chemical foci of, 60; chromatic aberration of, 54; spherical aberration of, 56; simple directions for using lenses, 227; for finding chemical focus, 229. Light; its action upon Silver Salts, 19; experiments illustrating, 21; formation of invisible images by, 24; its alternating action upon Daguerreotype plate, 39; its compound nature, 46; photographic action of coloured light, 60; refraction of light, 49. Llewellyn, Mr., his Oxymel process, 291.