PREFACE 



The general purpose of this book was well explained in the pref- 

 ace 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 them- 

 selves, but develop color when properly applied to tissue containing 

 the compounds which the reagents are intended to demonstrate. 



Although reference is given to certain histochemical reagents, 

 the general subject of histochemistry (in spite of its growing im- 

 portance) is treated very sketchily. This book hardly seems the 

 place to consider that field in any detail, especially considering 

 that new text books dealing with the subject alone and written by 

 specialists in the field, are now beginning to appear. Neverthe- 

 less, one of the important changes in this edition, in comparison 

 with the fifth, is in its references to histochemical usage of certain 

 dyes and dye-like reagents. 



Perhaps the greatest change in this edition is the greater stress 

 laid on methods of identification of stains. All previous editions 

 have referred to spectrophotometric measurements and their use 

 in characterizing dyes; but beyond giving the absorption maxima 

 (sometimes not fully verified) under the individual description of the 

 stains, such data were largely omitted. At present, the Biological 

 Stain Commission has much more material on this subject avail- 

 able, from data that it has been collecting during the last five years 

 in its laboratory at the University of Rochester Medical School, 

 and it seems worth while to include some of that material in this 

 book. As a result, this book contains some 20 illustrations in 

 which spectral graphs of nearly all the stains now on the certifi- 

 cation basis are included. In every case these graphs are of typical 

 samples of the stain in question, selected after a comparison of 

 numerous batches of the same dye which have been accumulating 

 in the Commission laboratories for twenty years or more. These 

 curves are very useful to the Stain Commission in the identifi- 

 cation of stain samples submitted for certification^ and it is hoped 

 that their inclusion here may increase the familiarity of users with 

 the optical properties of dj^es and help them to use the stains more 

 intelligently. 



