“How do you manage to do that? It must be very difficult there.” “I shall photograph those,” he answered. “The whole system of taking finger-marks is extraordinarily fascinating,” she said; “I don’t see how you do it.” “Oh, it’s not difficult when once you know how.” “Like so many things, I suppose,” she remarked, “but it must require great dexterity and special training.” “I don’t know,” he answered: insensibly his tone had become that of the expert, good-humouredly answering the questions of a layman; and he was not altogether sorry to have an opportunity to show this clever amateur the science that lay beneath his methods. “Of course,” he went on, “I have taken prints for years, and practice makes perfect, they say; but I don’t know that it is specially difficult.” “How do you set about it in the ordinary way?” she asked. “Well, it depends of course what they are on,” he replied rather sententiously. “And you can do it in several ways.” “Yes, but ordinarily.” “Oh, well, if they were on paper, for example, and you were to sprinkle a little lamp-black over the place, you would see all there was: if on something dark, well, there are several things one can use; fine flour does as well as anything.” “And it shows up the marks?” “If there are any; but you have to dust it off very lightly. It’s the grease on the hand, you see, miss, that makes the mark and you want something light that will stick on the place when the rest is brushed off. Of course you can touch plenty of things and not leave a mark, anything rough for instance; in moments of great agitation, however, the human hand becomes moist, which facilitates the work of discovery.” “I see; it’s very interesting. Well, thank you. I hope I haven’t wasted your time. Oh,” a little smile played round her lips, “I know I haven’t done that.” He was slightly nettled. “And how do you know?” he asked. “Because—well, it sounds rude but I don’t mean it to be—because you are wasting it for yourself out there. The man who made those marks is not the man who killed Sir Roger.” She nodded pleasantly and left him to stare after her with open mouth and profoundly puzzled eyes, as she went slowly upstairs to luncheon.