All day I turned these facts over in my mind, endeavouring to hit upon some theory which could reconcile them all, and to find that line of least resistance which my poor friend had declared to be the starting-point of every investigation. I confess that I made little progress. In the evening I strolled across the Park, and found myself about six o’clock at the Oxford Street end of Park Lane. A group of loafers upon the pavements, all staring up at a particular window, directed me to the house which I had come to see. A tall, thin man with coloured glasses, whom I strongly suspected of being a plain-clothes detective, was pointing out some theory of his own, while the others crowded round to listen to what he said. I got as near him as I could, but his observations seemed to me to be absurd, so I withdrew again in some disgust. As I did so I struck against an elderly, deformed man, who had been behind me, and I knocked down several books which he was carrying. I remember that as I picked them up, I observed the title of one of them, The Origin of Tree Worship, and it struck me that the fellow must be some poor bibliophile, who, either as a trade or as a hobby, was a collector of obscure volumes. I endeavoured to apologize for the accident, but it was evident that these books which I had so unfortunately maltreated were very precious objects in the eyes of their owner. With a snarl of contempt he turned upon his heel, and I saw his curved back and white side-whiskers disappear among the throng.

My observations of No. 427 Park Lane did little to clear up the problem in which I was interested. The house was separated from the street by a low wall and railing, the whole not more than five feet high. It was perfectly easy, therefore, for anyone to get into the garden, but the window was entirely inaccessible, since there was no waterpipe or anything which could help the most active man to climb it. More puzzled than ever, I retraced my steps to Kensington. I had not been in my study five minutes when the maid entered to say that a person desired to see me. To my astonishment it was none other than my strange old book collector, his sharp, wizened face peering out from a frame of white hair, and his precious volumes, a dozen of them at least, wedged under his right arm.

“You’re surprised to see me, sir,” said he, in a strange, croaking voice.

I acknowledged that I was.

“Well, I’ve a conscience, sir, and when I chanced to see you go into this house, as I came hobbling after you, I thought to myself, I’ll just step in and see that kind gentleman, and tell him that if I was a bit gruff in my manner there was not any harm meant, and that I am much obliged to him for picking up my books.”

“You make too much of a trifle,” said I. “May I ask how you knew who I was?”

“Well, sir, if it isn’t too great a liberty, I am a neighbour of yours, for you’ll find my little bookshop at the corner of Church Street, and very happy to see you, I am sure. Maybe you collect yourself, sir. Here’s British Birds, and Catullus, and The Holy War -a bargain, every one of them. With five volumes you could just fill that gap on that second shelf. It looks untidy, does it not, sir?”

I moved my head to look at the cabinet behind me. When I turned again, Sherlock Holmes was standing smiling at me across my study table. I rose to my feet, stared at him for some seconds in utter amazement, and then it appears that I must have fainted for the first and the last time in my life. Certainly a gray mist swirled before my eyes, and when it cleared I found my collar-ends undone and the tingling after-taste of brandy upon my lips. Holmes was bending over my chair, his flask in his hand.