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.