In attempting to impart knowledge on any subject, it is not sufficient that the writer should himself be acquainted with that which he professes to teach. Even supposing such to be the case, yet much of the success of his effort must depend upon the manner in which the information is conveyed; for as, on the one hand, a system of extreme brevity always fails of its object, so, on the other, a mere compilation of facts imperfectly explained tends only to confuse the reader.

A middle course between these extremes is perhaps the best to adopt; that is, to make selection of certain fundamental points, and to explain them with some minuteness, leaving others of less importance to be dealt with in a more summary manner, or to be altogether omitted.

But independently of observations of this kind, which apply to educational instruction in general, it may be remarked, that there are sometimes difficulties of a more formidable description to be overcome. For instance, in treating of any science, such as that of Photography, which may be said to be comparatively new and unexplored, there is great danger of erroneously attributing effects to their wrong causes! Perhaps none but he who has himself worked in the laboratory can estimate this point in its proper light. In an experiment where the quantities of material acted upon are infinitesimally small, and the chemical changes involved of a most refined and subtle description, it is soon discovered that the slightest variation in the usual conditions will suffice to alter the result.

Nevertheless Photography is truly a science, governed by fixed laws; and hence, as our knowledge increases, we may fairly hope that uncertainty will cease, and the same precision at length be attained as that with which chemical operations are usually performed.

The intention of the author in writing this work, is to impart a thorough knowledge of what may be termed the "First Principles of Photography," that the amateur may arm himself with a theoretical acquaintance with the subject before proceeding to the practice of it. To assist this object, care will be taken to avoid needless complexity in the formulÆ, and all ingredients will be omitted which are not proved to be of service.

The impurities of chemicals will be pointed out as far as possible, and special directions given for their removal.

Amongst the variety of Photographic processes devised, those only will be selected which are correct on theoretical grounds, and are found in practice to succeed.

As the work is addressed to one supposed to be unacquainted both with Chemistry and Photography, pains will be taken to avoid the employment of all technical terms of which an explanation has not previously been given.

A SKETCH OF THE MAIN DIVISIONS TO BE ADOPTED, WITH THE PRINCIPAL SUBJECT-MATTER OF EACH.

The title given to the Work is "A Manual of Photographic Chemistry," and it is proposed to include in it a familiar explanation of the nature of the various chemical agents employed in the Art of Photography, with the rationale of the manner in which they are thought to act.

The division adopted is threefold:—

Part I. enters minutely into the theory of Photographic processes; Part II. treats of the practice of Photography upon Collodion; Part III. embraces a simple statement of the main laws of Chemistry, with the principal properties of the various substances, elementary or compound, which are employed by Photographers.

Part I., or "the Science of Photography," includes a full description of the chemical action of Light upon the Salts of Silver, with its application to artistic purposes; all mention of manipulatory details, and of quantities of ingredients, being, as a rule, omitted.

In this division of the Work will be found nine Chapters, the contents of which are as follows:—

Chapter I. is a sketch of the history of Photography, intended to convey a general notion of the origin and progress of the Art, without dwelling on minute particulars.

Chapter II. describes the Chemistry of the Salts of Silver employed by Photographers; their preparation and properties; the phenomena of the action of Light upon them, with experiments illustrating it.

Chapter III. leads us on to the formation of an invisible image upon a sensitive surface, with the development or bringing out to view of the same by means of chemical re-agents. This point, being of elementary importance, is described carefully;—the reduction of metallic oxides, the properties of the bodies employed to reduce, and the hypotheses which have been entertained on the nature of the Light's action, are all minutely explained.

Chapter IV. treats of the fixing of Photographic impressions, in order to render them indestructible by diffused light.

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.

The reader will at once gather from this sketch of the contents of the volume before him, that whilst the general theory of every Photographic process is described, with the preparation and properties of the chemicals employed, minute directions in the minor points of manipulation are restricted to Photography upon Collodion, that branch of the Art being the one to which the time and attention of the author have been especially directed. Collodion is allowed by all to be the best vehicle for the sensitive Silver Salts which is at present known, and successful results can be obtained with a very small expenditure of time and trouble, if the solutions employed in the process are prepared in a state of purity.