Chapter V. contains a sketch of the _Optics_ of Photography--the decomposition of white Light into its elementary rays, the Photographic properties of the different colours, the refraction of Light, and construction of Lenses. In the last Section of the same Chapter will be found a short sketch of the history and use of the Stereoscope. Chapter VI. embraces a more minute description of the sensitive Photographic processes upon Collodion. In it is explained the chemistry of Pyroxyline, with its solution in Alcoholized Ether, or _Collodion_; also the Photographic properties of Iodide of Silver upon Collodion, with the causes which affect its sensitiveness to Light, and the action of the developing solutions in bringing out the image. Chapter VII. continues the same subject, describing the classification of Collodion Photographs as Positives and Negatives, with the distinctive peculiarities of each. Chapter VIII. contains the theory of the production of Positive Photographs upon paper. In this Chapter will be found an explanation of the somewhat complex chemical changes involved in printing Positives, with the precautions which are required to ensure the permanency of the proofs. Chapter IX. is supplementary to the others, and a brief notice of it will suffice. It explains the theory of the Photographic processes of Daguerre and Talbot; especially noticing those points in which they may be contrasted with Photography upon Collodion, but omitting all description of manipulatory details, which if included would extend the Work beyond its proposed limits. The title of the second principal division of the Work, viz. "The practice of Photography upon Collodion," explains itself. Attention however may be invited to the fifth Chapter, in which a classification is given of the principal imperfections in Photographs, with short directions for their removal; and to Chapter VI., which describes the preservation of the sensitiveness of Collodion plates and the mode of operating upon films of Albumenized Collodion. In Part III. will be found, in addition to a statement of the laws of chemical combination, etc., a list of Photographic chemicals, alphabetically arranged, including their preparation and properties as far as required for their employment in the Art.