In writing the following story, the author has had in view no purpose other than that of affording entertainment to such readers as are interested in problems of crime and their solutions; and the story itself differs in no respect from others of its class, excepting in that an effort has been made to keep within the probabilities of ordinary life, both in the characters and in the incidents.  
Nevertheless it may happen that the book may serve a useful purpose in drawing attention to certain popular misapprehensions on the subject of finger-prints and their evidential value; misapprehensions the extent of which may be judged when we learn from the newspapers that several Continental commercial houses have actually substituted finger-prints for signed initials.
 
The facts and figures contained in Mr. Singleton's evidence, including the very liberal estimate of the population of the globe, are, of course, taken from Mr. Galton's great and important work on finger-prints; to which the reader who is interested in the subject is referred for much curious and valuable information.
 
In conclusion, the author desires to express his thanks to his friend Mr. Bernard E. Bishop for the assistance rendered to him in certain photographic experiments, and to those officers of the Central Criminal Court who very kindly furnished him with details of the procedure in criminal trials.