It was not until the evening of the following day that Harley rang me up, and: 

“I want you to come round at once,” he said urgently.  “The Deepbrow case is developing along lines which I confess I had anticipated, but which are dramatic nevertheless.”

Knowing that Harley did not lightly make such an assertion, I put aside the work upon which I was engaged and hurried around to Chancery Lane.  I found my friend, pipe in mouth, walking up and down his smoke-laden study in a state which I knew to betoken suppressed excitement, and: 

“Did Wessex find your photographer?” I asked on entering.

“Yes,” he replied.  “A first-class man, as I had anticipated.  As I had further anticipated he did a number of copies of the picture for the foreign gentleman—­about fifty, in fact!”

“Fifty!”

“Yes!  Does the significance of that fact strike you?” asked Harley, a queer smile stealing across his tanned, clean-shaven face.

“It is an extraordinary thing for even an ardent admirer to have so many reproductions done of the same picture!”

“It is!  I will show you now what I found trodden into one of the footprints where the struggle took place beside the car.”

Harley produced a piece of thick silk twine.

“What is it?”

“It is a link, Knox—­a link to seek which I really went down to Deepbrow.”  He stared at me quizzically, but my answering look must have been a blank one.  “It is part of the tassel of one of those red cloth caps commonly called in England, a fez!”

He continued to stare at me and I to stare at the piece of silk; then: 

“What is the next move?” I demanded.  “Your new clue rather bewilders me.”

“The next move,” he said, “is to retire to the adjoining room and make ourselves look as much like a couple of Oriental commercial travellers as our correctly British appearance will allow!”

“What!” I cried.

“That’s it!” laughed Harley.  “I have a perpetual tan, and I think I can give you a temporary one which I keep in a bottle for the purpose.”

Twenty minutes later, then, having quitted Harley’s chambers by a back way opening into one of those old-world courts which abound in this part of the metropolis, two quietly attired Eastern gentlemen got into a cab at the corner of Chancery Lane and proceeded in the direction of Limehouse.

There are haunts in many parts of London whose very existence is unsuspected by all but the few; haunts unvisited by the tourist and even unknown to the copy-hunting pressman.  Into a quiet thoroughfare not three minutes’ walk from the busy life of West India Dock Road, Harley led the way.  Before a door sandwiched in between the entrance to a Greek tobacconist’s establishment and a boarded shop-front, he paused and turned to me.

“Whatever you see or hear,” he cautioned, “express no surprise.  Above all, show no curiosity.”