it was his cigarette case which had been found in the carriage。 Then； if Ada Mason and he were working together； and she recognized it when we showed it to her； would she not act precisely as she had done？ At first； taken aback； she quickly evolved a plausible theory that would agree with Mr Kettering's guilt。 Bien entendu； that was not the original idea。 The te de la Roche was to be the scapegoat； though Ada Mason would not make her recognition of him too certain； in case he should be able to prove an alibi。 Now； if you will cast your mind back to that time； you will remember a significant thing that happened。 I suggested to Ada Mason that the man she had seen was not the te de la Roche； but Derek Kettering。 She seemed uncertain at the time； but after I had got back to my hotel you rang me up and told me that she had e to you and said that； on thinking it over； she was now quite convinced that the man in question was Mr Kettering。 I had been expecting something of the kind。 There could be but one explanation of this sudden certainty on her part。 After my leaving your hotel； she had had time to consult with somebody； and had received instructions which she acted upon。 Who had given her these instructions？ Major Knighton。 And there was another very small point； which might mean nothing or might mean a great deal。 In casual conversation Knighton had talked of a jewel robbery in Yorkshire in a house where he was staying。 Perhaps a mere coincidence … perhaps another small link i