Carton and I followed, fascinated by the minuteness of his investigation and knowledge.

“You see,” he explained, “when a voice or a passage of music sounds or is sung before a phonograph, its modulations received upon the diaphragm are written by the needle point upon the surface of the cylinder or disc in a series of fine waving or zig-zag lines of infinitely varying depth and breadth.

“Close familiarity with such records for about forty years has taught Mr. Edison the precise meaning of each slightest variation in the lines.  I have taken up and elaborated his idea.  By examining them under the microscope one can analyze each tone with mathematical accuracy and can almost hear it—­just as a musician reading the score of a song can almost hear the notes.”

“Wonderful,” ejaculated Carton.  “And you mean to say that in that way you can actually identify a voice?”

Kennedy nodded.  “By examining the records in the laboratory, looking them over under a microscope—­yes.  I can count the overtones, say, in a singing voice, and it is on the overtones that the richness depends.  I can recognize a voice—­ mathematically.  In short,” Craig concluded enthusiastically, “it is what you might call the Bertillon measurement, the finger-print, the portrait parle of the human voice!”

Incredible as it seemed, we were forced to believe, for there on the table lay the graphic evidence which he had just so painstakingly interpreted.

“Who was it?” asked Carton breathlessly.

Kennedy picked up another microphotograph.  “That is the record I took of one of the calls I made—­merely for the purpose of obtaining samples of voices to compare with this of the impersonator.  The two agree in every essential detail and none of the others could be confounded by an expert who studied them.  Your ‘wolf’ was your old friend Kahn!”

“Fighting back at me by his usual underhand methods,” exclaimed Carton in profound disgust.

“Or else trying himself to get control of the Black Book,” added Kennedy.  “If you will stop to think a moment, his shafts have been levelled quite as much at discrediting Langhorne as yourself.  He might hope to kill two birds with one stone—­and incidentally save himself.”

“You mean that he wants to lay a foundation now for questioning the accuracy of the Black Book if it ever comes to light?”

“Perhaps,” assented Kennedy carefully.

“Surely we should take some steps to protect ourselves from his impostures,” hastened Carton.

“I have no objections to your calling him up and telling him that we know what he is up to and can trace it to him—­provided you don’t tell him how we did it—­yet.”

Carton had seized the telephone and was hastily calling every place in which Kahn was likely to be.  He was not at either of his offices, nor at Farrell’s, but at each place successively Carton left a message which told the story and which he could hardly fail to receive soon.