Anyone who is interested in the collection, study, and
preservation of our photographic heritage is likely to
encounter problems in dating and categorizing specimens. For
example, are unknown pictures 'collotypes', 'calotypes', or
'kallitypes'? The information may be needed to date them, to
determine their market value, or simply to put correct labels
on them for display.
The literature on historical photography is voluminous, and it can be a tedious task to sort through chronological descriptions in narrative history books in search of a description that seems to match a picture in question. There are several very useful flow-chart guides, for example Coe & Haworth-Booth , Gill , Reilly  and Rempel . But such references generally do not attempt to include all known types of pictures with details arranged for identification. Taft  remarks " Anyone who finds the profusion of types bewildering should at least be grateful to the author for not mentioning all the types that flourished during the first quarter century of photography." Unfortunately a reader may not be grateful to find that the description of a particular picture is one of those omitted for convenience.
The number of major and minor variations produced in only sixty years seems nearly endless, and some simplification in classification is necessary in a manageable identification system. This book attempts to improve on the degree of complete-ness of many previous histories without becoming encumbered with trivial variations.
Beaumont Newhall has remarked on the nomenclature of early photographs that ... "the list of types is imposing and an industrious researcher could easily turn up fifty or more." This is a fair estimate: this volume includes about one hundred names, but many are synonyms. There has been much confusion over names, definitions, and inventors. The work by Vogt-O'Connor and Pearce-Moses [109, 110] on the development of a thesaurus of photographic terms is a valuable clarification. It has been incorporated in The Art and Architecture Thesaurus, reference . In addition, an interesting history of the nomenclature is found in reference .
One question is whether to count processes that were invented, patented, named, and published, but never became commercial realities. For historical reasons they have been included, at the same time noting that museums and collectors are not likely to find specimens. Or will they? Maybe historical accounts over-looked something, and somewhere there is an attic trunk...
The subject of this book is necessarily technological. 19th century inventors made the best use of their contemporary science that they could: some photographic pioneers were physicians, possibly because of their knowledge of chemistry. Numerous college professors and at least one noted astronomer made lasting contributions. And of course there were many self-taught amateurs. But innovation in early photography demanded technical familiarity and discipline, and a book on the subject will not do the reader a favor by side-stepping the fact.
Most of us think we know what a photograph looks like. Be warned, however, to take nothing for granted in nineteenth century photography. Some processes were highly praised because they produced pictures that looked as little like 'photographs' as possible. Why? To please patrons who preferred the artistic appearance of paintings. Others were photomechanical reproductions that "to the untrained eye are indistinguishable from actual photographs". But what is an actual photograph?
Defining a photograph is not without difficulty. Silver content cannot be a criterion; it would eliminate gum bichromate, platinum prints, cyanotypes, uranium prints, and dye images. "Emulsion-coated paper" as a criterion would exclude platinum prints and the salt prints of Fox Talbot. The photomechanical prints of Woodbury were comprised of gelatin on paper and might be considered emulsions. 'Primary images' would exclude multiple prints from such classics as the negatives of Ansel Adams, and other derivatives.
Gernsheim  describes photography as implying a permanent picture made by means of a camera. Some would argue that pictures in newspapers fit this limited description. The first permanent image of the Frenchman Nicephore Niepce, discovered by Gernsheim and generally regarded as the oldest surviving photograph, was made by the action of focused rays of light on a coating of bitumen. It was the result of an effort to find a better process for reproducing pictures in ink.
The definition of a photograph used in this book is "a permanent picture made by means of a camera and originally comprised of photosensitive materials on any substrate” which eliminates the medium of printers' ink and photomechanical reproductions. However, a survey of the subject of photomechanical reproduction is included in this book to clarify the recognition of certain types of reproductions that closely resemble photographs, such as Woodburytypes and carbon prints.
Early photographic inventors, starting with Louis J. M. Daguerre, liked the idea of combining their names with the suffix "-type", or else adopting poetic prefixes such as "calo-" (from "kalo", Greek for beautiful). Fox Talbot (*William Henry Fox Talbot is frequently referred to in the literature as 'Fox Talbot'; Fox was an old and distinguished English family name. The cover title of Talbot's book "The Pencil of Nature by H. Fox Talbot" implies his own preference.) later changed "calotype" to "Talbotype" in his own honor, thereby bequeathing posterity two names for the same process. Calotypes are also often called salt prints, adding to the confusion. Batchen  provides some fascinating sidelights on the origins of photographic names.
Webster's Unabridged Dictionary defines "type" as, among other things, "... a figure, image, form, or representation of something to come." The use of the appendage "-type", largely a nineteenth century usage, was thus appropriately applied to photography; one wonders whether "something to come" could presage the latent image concept.
During research on this book, a discouraging amount of disagreement between 'authorities' was encountered. To professional historians this observation will not be a revelation, but to a mere student of history it was dismaying. This is the reason the Bibliography includes a considerable number of historical references. I am under no illusion that discrepancies in dates, process details, and attributions have all been eradicated, but a serious effort was made to do so.
Nineteenth century photography was an arena of promoters, inventions both serendipitous and inspired, ferocious litigation, fleeting fame, imperfectly understood science, and rapid obsolescence. Are we so different today? Early photographers performed heroic feats of endurance to get their pictures, and they sickened and died from toxic chemicals in an age when people legally took opium for tooth ache. Their surviving pictures record humdrum life, great beauty, and momentous history, and surely are worth our best efforts to recognize and conserve this time machine to the past.