“The most obvious method is that of keeping the prisoner in such a state of debility as would confine him to his bed.  But such debility could be produced by only starvation, unsuitable food, or chronic poisoning.  Of these alternatives, poisoning is much more exact, more calculable in its effect and more under control.  The probabilities, then, were in favour of chronic poisoning.

“Having reached this stage, I recalled a singular case which Jervis had mentioned to me and which seemed to illustrate this method.  On our return home I asked him for further particulars, and he then gave me a very detailed description of the patient and the circumstances.  The upshot was rather startling.  I had looked on his case as merely illustrative, and wished to study it for the sake of the suggestions that it might offer.  But when I had heard his account, I began to suspect that there was something more than mere parallelism of method.  It began to look as if his patient, Mr. Graves, might actually be Jeffrey Blackmore.

“The coincidences were remarkable.  The general appearance of the patient tallied completely with Mr. Stephen’s description of his uncle Jeffrey.  The patient had a tremulous iris in his right eye and had clearly suffered from dislocation of the crystalline lens.  But from Mr. Stephen’s account of his uncle’s sudden loss of sight in the right eye after a fall, I judged that Jeffrey had also suffered from dislocation of the lens and therefore had a tremulous iris in the right eye.  The patient, Graves, evidently had defective vision in his left eye, as proved by the marks made behind his ears by the hooked side-bars of his spectacles; for it is only on spectacles that are intended for constant use that we find hooked side-bars.  But Jeffrey had defective vision in his left eye and wore spectacles constantly.  Lastly, the patient Graves was suffering from chronic morphine poisoning, and morphine was found in the body of Jeffrey.

“Once more, it appeared to me that there were too many coincidences.

“The question as to whether Graves and Jeffrey were identical admitted of fairly easy disproof; for if Graves was still alive, he could not be Jeffrey.  It was an important question and I resolved to test it without delay.  That night, Jervis and I plotted out the chart, and on the following morning we located the house.  But it was empty and to let.  The birds had flown, and we failed to discover whither they had gone.

“However, we entered the house and explored.  I have told you about the massive bolts and fastenings that we found on the bedroom doors and window, showing that the room had been used as a prison.  I have told you of the objects that we picked out of the dust-heap under the grate.  Of the obvious suggestion offered by the Japanese brush and the bottle of ‘spirit gum’ or cement, I need not speak now; but I must trouble you with some details concerning the broken spectacles.  For here we had come upon the crucial fact to which, as I have said, all sound inductive reasoning brings one sooner or later.