The general purpose of this book was well explained in the preface to its first edition, and that preface is, therefore, reprinted on the following pages. The various editions which have followed have seen a gradual enlargement of the book, with greater accuracy of detail; but its purpose and general scope have not been changed.A longer period (seven years) has elapsed since the fifth edition than occurred between any other successive two. This has not been so much because of the lack of new material on the subject as because of difhcvdty in organizing that material in the best form for presentation. There is a smaller increase than usual in the number of new dves described in this edition; but this is offset bv the addition of histochemical reagents which are not dyes in themselves, but develop color when properly applied to tissue containing the compounds which the reagents are intended to demonstrate.