The Hound of the Baskervilles

"I can well remember driving up to his house in the evening some three weeks before the fatal event. He chanced to be at his hall door. I had descended from my gig and was standing infront of him, when I saw his eyes fix themselves over my shoulder and stare past me with an expression of the most dreadful horror. I whisked round and had just time to catch a glimpse of something which I took to be a large black calf passing at the head of the drive. So excited and alarmed was he that I was compelled to go down to the spot where the animal had been and look around for it. It was gone, however, and the incident appeared to make the worst impression upon his mind. I stayed with him all the evening, and it was on that occasion, to explain the emotion which he had shown, that he confided to my keeping that narrative which I read to you when first I came. I mention this small episode because it assumes some importance in view of the tragedy which followed, but I was convinced at the time that the matter was entirely trivial and that his excitement had no justification."