Dr Roylott was a clever and dangerous man who knew India well. The idea of using a poison which could not possibly be discovered by any chemical test was the kind of idea that he would have. The speed with which such a poison would work would also, from his point of view, be an advantage. It would be a sharp-eyed policeman, indeed, who would see the two little dark marks which would show where the poisonous fangs of the snake had bitten. Then I thought of the whistle. Of course Dr Roylott must call back the snake before the morning. He had trained it, probably with the milk which we saw, to return to him when he whistled for it to come. He would put the snake through this ventilator late at night, being sure that it would come down the rope and land on the bed. It might or might not bite the person in the bed. Perhaps she might escape every night for a week, but sooner or later she must be bitten.

"I had come to these conclusions before I entered his room. An inspection of his chair showed me that he had been in the habit of standing on it, which of course would be necessary for him to reach the ventilator. The sight of the safe, the saucer of milk, and the loop which he used to hold the snake were enough to finally clear away any doubts which I still had. The metallic clang heard by Miss Stoner was obviously caused by her stepfather quickly closing the door of his safe upon the terrible creature inside. Once I had made up my mind, you know what I did to prove I was right. When I heard the snake hiss, as I have no doubt that you did also, I quickly lit the light and attacked it."