Since the publication of the previous editions of this Manual, two changes of importance have occurred ; the nomenclature and atomic weights of chemical compounds have to a great extent been altered, and the collodion process has, especially in England, been largely supplanted by gelatine emulsion.It has, however, seemed judicious to the Editor of this edition neither to entirely discard the old nomenclature in favor of the new, nor to assume the collodion process to be defunct. Hence a work, intended to be of every-day use to photographers, would have its value impaired by the exclusive use of terms as yet imperfectly understood.
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