"Well," he rejoined, "you've hit the nail on the head,--that's just what it is, but you are entirely wrong in your assumption that the thumb-mark can have no value as evidence. Do you not know that there are no two thumbs in the world which are capable of making indistinguishable marks?" I was not aware of this. "How do you know," I asked, "that this mark was made by the assassin? It seems to me there can hardly be a doubt that one of the painters, while priming the sill, accidentally pressed his thumb against the glass. His hands would naturally have been painty, and this impression would as naturally have resulted."
"What you say," replied Maitland, "is very good, so far as it goes. My reasons for believing this thumb-mark was made by the assassin are easily understood. First: there was another impression of a thumb in the moist paint of the sill directly under that upon the glass. Both marks were made by the same thumb and, in the lower one, the microscope revealed minute traces of gravel dust, not elsewhere discernible upon the sill. The thumb carried the dust there, and was the thumb of the hand pressed into the gravel,--the hand of which I have a cast. You see how this shows how the thumb came to have paint upon it when pressed upon the glass. Second: the two men engaged in priming the house, James Cogan and Charles Rice, were the only persons save the assassin known to have been upon that side of the house the day of the murder. "Here," he said, carefully removing two strips of glass from a box, "are the thumb-marks of Cogan and Rice made with the same paint. You see that neither of these men could, by any possibility, have made the mark upon the glass. So there you are. But we are missing the question before us. What line of procedure can you suggest, Doc? I'm all at sea."
"We must find someone," I said, "who could have had a motive. This someone ought to have a particularly good reason for concealing his footprints, and is evidently lame besides. I can't for the life of me see anything else we have to go by, unless it be the long nail of the little finger, and I don't see how that is going to help us find the assassin--unless we can find out why it was worn long. If we knew that it might assist us. As I have already suggested, a Chinaman might have a long nail on the little finger, but he would also have the other nails long, wouldn't he? Furthermore, he might use the boards to conceal the prints of his telltale foot-gear; but why should he not have put on shoes of the ordinary type? If he had time to prepare the boards,--the whole affair shows premeditation, --clearly he had time to change his boots. The Chinese are usually small, and this might easily account for the smallness of the hand as shown by your cast. These are the pros and cons of the only clue that suggests itself to me. By the way, Maitland, it's a shame we did not try, before it was too late, to track this fellow down with a dog."