“And—­what is it, sir?” asked Chettle eagerly, eyeing the photo as if it were some fearful living curiosity.  “What, Mr. Allerdyke?”

“Why, it’s a photograph of my photograph!” almost shouted Allerdyke, with a thump of his big hand on the table.  “That’s the truth.  This has been reproduced from mine, d’ye see?  Look here—­happen you don’t know much about photography, but you’ll follow me—­I always use a certain sort of printing-out paper; I’ve stuck to one particular sort for years—­all the photos in that album are done on that particular sort.  The four prints I made of James’s last photo were done on that paper.  Now then—­this photo, this print that you found in Lydenberg’s watch, is not done on that paper—­it’s a totally different paper.  Therefore—­this is a reproduction!  It is not my original print at all—­it’s been copied from it.  See?”

Chettle, who had followed all this with concentrated attention, nodded his head several times.

“Clever—­clever—­clever!” he said with undisguised admiration.  “Clever, indeed!  That’s a smart bit of work, sir.  I see—­I understand!  Bless my soul!  And what do you gather from that, Mr. Allerdyke?”

“This!” answered Allerdyke.  “Just now, Mrs. Marlow said to me, speaking of her photo—­the fourth print, you know—­’I misplaced it some time ago,’ she said, ’and couldn’t lay hands on it, but I came across it accidentally this morning.’  Now then, Chettle, here’s the thing—­somebody took that fourth print from Mrs. Marlow, reproduced it—­and that—­that print which you found in Lydenberg’s watch is the reproduction!”

“So that,” began Chettle suggestively, “so that—­”

“So that the thing now is to find who it is that made the reproduction,” said Allerdyke.  “When we’ve found him—­or her—­I reckon we shall have found the man who’s at the heart of all this.  Leave that to me!  Keep this a dead secret until I tell you to speak—­we shall have to tell all this, and a bonny sight more, to your bosses at headquarters—­off you go to Hull, and do what you have to do, and I’ll get on with my work here.  I said I didn’t know whether this discovery makes things thicker or clearer, but, by George, it’s a step forward anyway!”

Chettle put the reproduction back into the case of the watch and bestowed it safely in his pocket.

“One step forward’s a good deal in a case like this, Mr. Allerdyke,” he said.  “What are you going to do about the next step, now?”

“Try to find out who made that reproduction,” replied Allerdyke bluntly.  “No easy job, either!  The ground’s continually shifting and changing under one’s very feet.  But I don’t mind telling you my present theory—­somebody’s got information of that jewel deal from Fullaway’s office, somebody who had access to his papers, somebody who managed to steal that photo of mine from Mrs. Marlow for a few days or until they could reproduce it.  What I want to find now is—­an idea of that somebody.  And—­I’ll get it!—­I’ll move heaven and earth to get it!  But—­other matters.  You say your folks at the Yard are going to follow up that Perrigo woman’s clue?  They think it important, then?”