Nothing happened, however, until the end of the hour during which he received office calls.  As I glanced down the street I was glad that I had taken an inconspicuous post, for I could see Miss Langdale approaching.  She was not in her nurse’s uniform, but seemed to be off duty for an hour or two, and I must confess she was a striking figure, even in that neighborhood which was noted for its pretty and daintily gowned girls.  Almost before I knew it she had entered the English-basement entrance of Doctor Aitken’s.

I thought rapidly.  What could be the purpose of her visit?  Above all, how was I, on the outside, to find out?  I walked down past the house.  But that did no good.  In a quandary, I stopped.  Hesitation would get me nothing.  Suddenly an idea flashed through my mind.  I turned in and rang the bell.

“It’s past the doctor’s office hours,” informed a servant who opened the door.  “He sees no one after hours.”

“But,” I lied, “I have an appointment.  Don’t disturb him.  I can wait.”

The waiting-room was empty, I had seen, and I was determined to get in at any cost.  Reluctantly the servant admitted me.

For several moments I sat quietly alone, fearful that the doctor might open the double doors of his office and discover me.  But nothing happened and I grew bolder.  Carefully I tiptoed to the door.  It was of solid oak and practically impervious to sound.  The doors fitted closely, too.  Still, by applying my ear, I could make out the sound of voices on the other side.  I strained my ears both to catch a word now and then and to be sure that I might hear the approach of anybody outside.

Was Aitken suspiciously interested in the pretty nurse—­or was she suspiciously interested in him?

Suddenly their voices became a trifle more distinct.  “Then you think Doctor Wardlaw has it, too?” I heard her ask.  I did not catch the exact reply, but it was in the affirmative.

They were approaching the door.  In a moment it would be opened.  I waited to hear no more, but seized my hat and dashed for the entrance from the street just in time to escape observation.  Miss Langdale came out shortly, the doctor accompanying her to the door, and I followed her back to the Forum.

What I had heard only added to the puzzle.  Why her anxiety to know whether Wardlaw himself was affected?  Why Aitken’s solicitude in asserting that he was?  Were they working together, or were they really opposed?  Which might be using the other?

My queries still unanswered, I returned to Aitken’s and waited about some time, but nothing happened, and finally I went on to our own apartment.

It was very late when Craig came in, but I was still awake and waiting for him.  Before I could ask him a question he was drawing from me what I had observed, listening attentively.  Evidently he considered it of great importance, though no remark of his betrayed what interpretation he put on the episode.