“Why, then, the normal pupils of the eyes?  Simply because the criminal put a little atropine, or belladonna, with the morphine.  My tests show absolutely the presence of atropine, Dr. Hanson,” said Craig, bowing to the physician.

“The best evidence, however, is yet to come.  A second box of six capsules, all intact, was discovered yesterday in the possession of Henry Vandam.  I have analysed the capsules.  One contains no quinine at all—­it is all morphine and atropine.  It is, without doubt, precisely similar to the capsule which killed Mrs. Vandam.  Another night or so, and Henry Vandam would have died the same death.”

The old man groaned.  Two such exposures had shaken him.  He looked from one of us to another as if not knowing in whom he could trust.  But Kennedy hurried on to his next point.

“Who was it that gave the prescription to Mrs. Vandam originally?  She is dead and cannot tell.  The others won’t tell, for the person who gave her that prescription was the person who later substituted the fatal capsule in place of the harmless.  The original prescription is here.  I have been able to discover from it nothing at all by examining the handwriting.  Nor does the texture of the paper indicate anything to me.  But the ink—­ah, the ink.

“Most inks seem very similar, I suppose, but to a person who has made a study of the chemical composition of ink they are very different.  Ink is composed of iron tannate, which on exposure to air gives the black of writing.  The original pigment—­say blue or blue-black ink—­is placed in the ink, to make the writing visible at first, and gradually fades, giving place to the black of the tannate which is formed.  The dyestuffs employed in the commercial inks of to-day vary in colour from pale greenish blue to indigo and deep violet.  No two give identical reactions—­at all events not when mixed with the iron tannate to form the pigment in writing.

“It is owing to the difference in these provisional colouring matters that it is possible to distinguish between writing written with different kinds of ink.  I was able easily to obtain samples of the inks used by the Vandams, by Mrs. Popper, by Mr. Farrington, and by the druggist.  I have compared the writing of the original prescription with a colour scale of my own construction, and I have made chemical tests.  The druggist’s ink conforms exactly to the writing on the two pill-boxes, but not to the prescription.  One of the other three inks conforms by test absolutely to the ink in that prescription signed ‘Dr. C. W. H.’ as a blind.  In a moment my chain of evidence against the owner of that bottle of ink will be complete.”

I could not help but think of the two pendulums on the shelf behind the curtain, but Craig said nothing for a moment to indicate that he referred to that apparatus.  We sat dazed.  Farrington seemed nervous and ill at ease.  Mrs. Popper, who had not recovered from the hysterical condition of her exposure, with difficulty controlled her emotion.  Vandam was crushed.