that would infallibly convict him of this murder.”  He stroked his moustache thoughtfully.  “And to think, too,” he went on with a somewhat sheepish air, “we should have had those photographs there in our power all those days and nights, and have let them in the end slip like that through our fingers!  To think he should have found it out sooner than we!  To think that an amateur like the murderer should have outwitted us!”

“But how do you know,” I cried, “there was ever more than one photograph?  How do you know this wasn’t the only negative?”

“Because,” the Inspector answered quickly, pointing to a figure in the corner of the proof, “do you see that six?  Well, that tells the tale.  Each plate of the series was numbered so in the apparatus.  Number six could only fall into focus after numbers one, and two, and three, and four, and five, had first been photographed.  We’ve only got the last—­and least useful for our purpose.  There must have been five earlier ones, showing every stage of the crime, if only we’d known it.”

I was worked up now to a strange pitch of excitement.

“And how did this one come into your possession?” I asked, all breathless.  “If you managed to lay your hands on one, why not on all six of them?”

The Inspector drew a long breath.

“Ah, that’s the trouble!” he replied, still gazing at me hard.  “You see, it was this way.  As soon as we found the camera was missing, we came to the conclusion the murderer must have returned to The Grange to fetch it.  But it was a large and heavy box, and the only one of its kind as yet manufactured; so, to carry it away in his hands would no doubt have led to instant detection.  I concluded, therefore, the man would take off the box entire, so as to prevent the danger of removing the plates on the spot; and as soon as he reached a place of safety in the shrubbery, he’d fling away the camera, either destroying the incriminating negatives then and there or carrying them off with him.  The details of the invention had already been explained to me by your father’s instrument-maker, who set up the clockwork for him from his own designs; and I knew that the removal of the plates from the box was a delicate, and to some extent a difficult, operation.  So I felt sure they could only have been taken out in a place of comparative safety, not far from the house; and I searched the shrubbery carefully, to find the camera.”

“And you found it at last?” I asked, unable to restrain my agitation.

“I found it at last,” he answered, “near the far end of the grounds, just flung into the deep grass, behind a clump of lilacs.  The camera was there intact, but five plates were missing.  The sixth, from which the positive you hold in your hand was taken, had got jammed in the mechanism in the effort to remove it.  Evidently the murderer had tried to take out the plates in a very great hurry and with trembling hands, as was not unnatural.