A singular anomaly however requires notice, viz. that if the plate be prepared with the mixed vapours of Bromine and Iodine, in place of Iodine alone, then the yellow light cannot be made to develope the image. In fact, the same coloured ray which continues the action of white light upon a surface of _Iodide_ of Silver, actually _destroys_ it, and restores the particles to their original condition, with a surface of _Bromo_-Iodide of Silver. These facts, although not of great practical importance, are interesting in illustration of the delicate and complex nature of the chemical changes produced by light. _The Strengthening of the Daguerreotype Image by means of Hyposulphite of Gold._--The use of the Hyposulphite of Gold to whiten the Daguerreotype image, and render it more lasting and indestructible, was introduced by M. Fizeau, subsequent to the original discovery of the process. After removal of the unaltered Iodide of Silver by means of Hyposulphite of Soda, the plate is placed upon a levelling stand and covered with a solution of Hyposulphite of Gold, containing about one part of the salt dissolved in 500 parts of water. The flame of a spirit-lamp is then applied until the liquid begins to boil. Shortly a change is seen to take place in the appearance of the image; it becomes whiter than before, and acquires great force. This fact seems to prove conclusively that metallic Mercury enters into its composition, since a surface of Silver--such, for instance, as that of the Collodion image--is _darkened_ by Hyposulphite of Gold. The difference in the action of the gilding solution upon the image and the pure Silver surrounding it illustrates the same fact. This Silver, which appears of a dark colour, and forms the shadows of the image, is rendered still darker; a very delicate crust of metallic Gold _gradually_ forming upon it, whereas with the image the whitening effect is immediate and striking. SECTION II. _Theory of the Talbotype and Albumen Processes._ _The Talbotype or Calotype._--This process, as practiced by many at the present time, is almost identical with that originally described by Mr. Fox Talbot. The object is to obtain an even and finely divided layer of Iodide of Silver upon the surface of a sheet of paper; the particles of the Iodide being left in contact with an excess of Nitrate of Silver, and usually with a small proportion of Gallic Acid, to heighten, still further, the sensibility to light. The English papers sized with Gelatine are commonly used for the Calotype process; they retain the film more perfectly at the surface, and the Gelatine in all probability assists in forming the image. With a foreign starch-paper, unless it be re-sized with some organic substance, the solutions sink in too deeply, and the picture is wanting in clearness and definition.