Elementary bodies, table of, 306; combination of, 307. Engravings, mode of copying, 231; often yield dark-coloured prints, 255. Equivalent proportions, 320. Ether, properties of, 339; purification of, for Photography, 195; must be kept in a dark place, 196; should not be distilled from residues of old Collodion, 96. Experiments, illustrating action of Light upon Chloride of Silver, 21; illustrating formation and development of invisible images, 25; illustrating photographic action of coloured light, 62. Exposure in the Camera, rules for Positives, 221; for Negatives, 225; for preserved Collodion plates, 292; for microscopic photographs, 238; effects of under and over-exposing, 35; exposure required in Calotype process, 177; in waxed paper, 180; in Albumen negative process, 181; in dry Collodion process, 301; in Taupenot's process, 297. Fading of Positives, explained at length, 160; Author's researches on, 153. Film, sensitive. _See_ Collodion film. Filters, mode of cutting, 376. Fixing, theory of, 41; of paper prints explained, 128; solution for fixing glass Positives and Negatives, 212; manipulatory details of fixing, 225; fixing paper Positives with Ammonia, 271. Fixing and toning Bath, its preparation, 250; conditions which favour or retard its action, 135; certain states of the Bath injurious to the proofs, 136; importance of keeping it in an active condition, 168; must not be employed immediately after mixing, 251; must not be allowed to become acid by constant use, 168; theory of the gradual change of properties it undergoes, 156. Foci, actinic and luminous, 60; actinic, mode of finding, 229; variation between them in microscopic objectives, 237. Focussing the object, 220. Fogging, theory of, 103; mode of detecting causes of, 276. Formulæ for solutions required in Collodion process, 201; for papers used in Positive printing, 241; want of correspondence between, 257.