The stories in this collection, inasmuch as they constitute a somewhat new departure in this class of literature, require a few words of introduction. The primary function of all fiction is to furnish entertainment to the reader, and this fact has not been lost sight of. But the interest of so-called "detective" fiction is, I believe, greatly enhanced by a careful adherence to the probable, and a strict avoidance of physical impossibilities; and, in accordance with this belief, I have been scrupulous in confining myself to authentic facts and practicable methods. The stories have, for the most part, a medico-legal motive, and the methods of solution described in them are similar to those employed in actual practice by medical jurists. The stories illustrate, in fact, the application to the detection of crime of the ordinary methods of scientific research. I may add that the experiments described have in all cases been performed by me, and that the micro-photographs are, of course, from the actual specimens. I take this opportunity of thanking those of my friends who have in various ways assisted me, and especially the friend to whom I have dedicated this book; by whom I have been relieved of the very considerable labour of making the micro-photographs, and greatly assisted in procuring and preparing specimens. I must also thank Messrs. Pearson for kindly allowing me the use of Mr. H. M. Brock's admirable and sympathetic drawings, and the artist himself for the care with which he has maintained strict fidelity to the text.