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.<br><br>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.