PREFACE. T HIS little work has been written to serve the needs of the practical photographer, who has no time for experimental work, or for the sifting of scientific books to find working instructions, which have passed the test of time and have proved to be thoroughly practical. No others have been quoted. Whilst the book chiefly deals with the application of Collodion Emulsion to Process Work, it is also hoped that it may- be an introduction to other branches of applied photography, where Collodion Emulsion is not as well known as it deserves to be. The Author, having been closely connected with Collodion Emulsion and its re-introduction into this country during the last six years, has had the opportunity of collecting information which, when now published in this book, may materially assist beginners over their initial difficulties. The credit of this re-introduction of Collodion Emulsion must rest with the firm of Penrose and Co., who, with self-sacrificing tenacity, have estab lished Collodion Emulsion in this country, in face of very poor financial prospects. I wish to acknowledge my indebtedness to Baron Von Hiibl for his permission to quote extensively from his works on this subject, and to Mr. William Gamble for the revision of the manuscript. London, August, 1905. HENRY O. KLEIN.