“I’d like to know what is in that,” remarked Craig, turning it over and over.

He appeared to be considering something, for he rose suddenly, and with a nod of his head to himself, as though settling some qualm of conscience, shoved the letter into his pocket.

A moment later the clerk returned.  “I’ve just had Mr. Whitney on the wire,” he reported.  “I don’t think he’ll be back at least for an hour.”

“Is he at the Prince Edward Albert?” asked Craig.

“I don’t know,” returned the clerk, oblivious to the fact that we must have seen that in order to know the telephone number he must have known whether Mr. Whitney was there or elsewhere.

“I shall come in again,” rejoined Kennedy, as we bowed ourselves out.  Then to me he added, “If he is with Senora de Moche and they are at the Edward Albert, I think I can beat him back with this letter if we hurry.”

A few minutes later, in his laboratory, Kennedy set to work quickly over an X-ray apparatus.  As I watched him, I saw that he had placed the letter in it.

“These are what are known as ‘low tubes,’” he explained.  “They give out ‘soft rays.’”

He continued to work for several minutes, then took the letter out and handed it to me.

“Now, Walter,” he said brusquely, “if you will just hurry back down there to Whitney’s office and replace that letter, I think I will have something that will astonish you—­though whether it will have any bearing on the case remains to be seen.  At least I can postpone seeing Whitney himself for a while.”

I made the trip down again as rapidly as I could.  Whitney was not back when I arrived, but the clerk was there, and I could not very well just leave the letter on the table again.

“Mr. Kennedy would like to know when he can see Mr. Whitney,” I said, on the spur of the moment.  “Can’t you call him up again?”

The clerk, as I had anticipated, went into Whitney’s office to telephone.  Instead of laying the letter on the table, which might have excited suspicion, I stuck it in the letter slot of the door, thinking that perhaps they might imagine that it had caught there when the postman made his rounds.

A moment later the clerk returned.  “Mr. Whitney is on his way down now,” he reported.

I thanked him, and said that Kennedy would call him up when he arrived, congratulating myself on the good luck I had had in returning the letter.

“What is it?” I asked, a few minutes later, when I had rejoined Craig in the laboratory.

He was poring intently over what looked like a negative.

“The possibility of reading the contents of documents inclosed in a sealed envelope,” he replied, still studying the shadowgraph closely, “has already been established by the well-known English scientist, Dr. Hall Edwards.  He has been experimenting with the method of using X-rays recently discovered by a German scientist, by which radiographs of very thin substances, such as a sheet of paper, a leaf, an insect’s body, may be obtained.  These thin substances, through which the rays used formerly to pass without leaving an impression, can now be easily radiographed.”