mode of preparing, 247; best applied to the paper by brush or rod, 248; Oxide of Silver in Nitrate of Ammonia, a useful substitute for it, 249. Ammonio-Nitrate paper, formula for, 246; a more simple formula, but less sensitive than the last, 258. Atomic theory explained, 322. Bath for fixing and toning Positives. _See_ Fixing and toning Bath. Bichloride of Mercury, whitening action on glass Positives explained, 113; solution for, 207; used to intensify Negatives, 118; bleaches paper prints, 151; should not be added to paste used in mounting prints, 164; removes Silver stains, 377. Binocular vision, phenomena of, explained, 66. Blackening Negatives, 37, 117. Black tones, mode of obtaining, in paper Positives, 168, 246. Bromide of Silver, its preparation and properties, 17; its superior sensibility to coloured light, 63; less acted on by white light than Chloride, 19; less sensitive to invisible image than Iodide, 25; employment in Collodion, 101; found useful in Photographing by artificial light, 66; diagram of chemical spectrum on, 64. Bromo-Iodide of Silver, 173. Brushes, mode of applying Silver solutions by, 248. Calotype process, theory of, 176. Camera, its first invention, 7; theory of its construction, 54; mode of testing accuracy of, 229; cause of the image being inverted, 53; the term "flatness of field" explained, 54; best position of the Camera for portraits, 220; for architectural subjects, 231; a funnel-shaped tube placed in front of the lens, 229; stereoscopic Camera, 234; microscope Camera, 236. Causes of failure in Collodion process, 276. Chemical affinity, illustrations of, 312. Chemical elements, 306. Chemical focus, directions for finding, 229; shorter than visual in non achromatic lenses, 60; longer than visual in microscopic objectives, 237; varies slightly with the nature of the light, 238. Chemical spectrum, 61. Chemicals, Photographic, Vocabulary of, 327. Chloride of Silver, its preparation and properties, 14; more sensitive to white light than Bromide or Iodide, 19; less sensitive to invisible image, 24; its blackening by light explained, 20, 141; accelerated by excess of Nitrate, 19; by organic matter, 20, 142; experiments illustrating darkening of papers prepared with, 21; simple explanation of the mode of preparing sensitive papers with, 22; agents which dissolve it, 42; mode of reducing it to metallic state. 374. Chloride of Gold, its preparation and properties, 342; action of Ammonia upon it, 343; use of an alkaline solution of, for toning, 132, 271; compounds formed on adding it to Hyposulphite of Soda, 133; mode of preparing the fixing and toning Bath with, 250; the Sel d'or Bath with, 267. Chromatic aberration, 54. Citric Acid, forms a red compound with Suboxide of Silver, 21, 338;