Yet, when I weighed the circumstances as a whole calmly and coolly, I saw that if the unknown person to whom the Professor had signalled on that fateful night could be found a very great point would be gained towards the solution of the problem. The pulling up and down of the drawing-room blind was, no doubt, in order to inform some person waiting without of his journey north. Was that person who received the signal afterwards the assassin? Yet the fact that the crime was committed behind locked doors, that both the victim and the assassin had to pass within a few feet of where Miss Ethelwynn was seated, and that into the unfortunate Professor's face some terribly corrosive fluid had been dashed, formed a problem which held me mystified. There was something uncanny in the whole inexplicable affair. I now realised for the first time how complete was the mystery of the Professor's death, even apart from the other facts of his signals and his journey north. Kirk, this dealer in secrets, admittedly posed as a friend of the family. Greer trusted him. To him Ethelwynn had fled for assistance at the first suspicion of anything being wrong. Therefore would it not have been easier for him than for anyone else to enter the house in secret and kill the man who had stolen from him that mysterious secret? Yet, try how I would, I was unable to rid myself of the grave conviction that my new acquaintance was cognisant of more than he had told me. He was naturally a reserved man, it was true; yet there was an air of cosmopolitanism about him which spoke mutely of the adventurer. His refusal to allow a doctor to see the Professor's daughter was nothing short of culpable. Had Antonio, that sly, crafty Italian, to whom I had taken such instinctive dislike, summoned a doctor at once, it was quite possible that the poor girl's life might have been saved. But why had she returned to the house in a manner so secret? Why had she crept into the dining-room and removed her hat? It would almost seem as though she had returned for good, for if she had intended to go back to her aunt's she would not have taken off her hat and laid it aside. And why had she done so in the dining-room, of all places? Why had she not ascended to her own room? And why, most of all, had she not summoned Antonio? Was it because of fear of him? Kirk and Antonio were friends. That I had detected from the very first. The Italian was polite, urbane, servile, yet I saw that the bow was only a shallow make-believe. Alone together, the pair would, no doubt, stand upon an equal footing. The reason she had returned home was mysterious enough, yet the greater problem was the reason why she also had been struck down and the same corrosive liquid flung into her fair countenance.