“There’s breakfast upstairs,” said Kemp, speaking as easily as possible, and he was delighted to find his strange guest rose willingly. Kemp led the way up the narrow staircase to the belvedere. “Before we can do anything else,” said Kemp, “I must understand a little more about this invisibility of yours.” He had sat down, after one nervous glance out of the window, with the air of a man who has talking to do. His doubts of the sanity of the entire business flashed and vanished again as he looked across to where Griffin sat at the breakfast-table—a headless, handless dressing-gown, wiping unseen lips on a miraculously held serviette. “It’s simple enough—and credible enough,” said Griffin, putting the serviette aside and leaning the invisible head on an invisible hand. “No doubt, to you, but—” Kemp laughed. “Well, yes; to me it seemed wonderful at first, no doubt. But now, great God! ... But we will do great things yet! I came on the stuff first at Chesilstowe.” “Chesilstowe?” “I went there after I left London. You know I dropped medicine and took up physics? No; well, I did. _Light_ fascinated me.” “Ah!” “Optical density! The whole subject is a network of riddles—a network with solutions glimmering elusively through. And being but two-and-twenty and full of enthusiasm, I said, ‘I will devote my life to this. This is worth while.’ You know what fools we are at two-and-twenty?” “Fools then or fools now,” said Kemp. “As though knowing could be any satisfaction to a man! “But I went to work—like a slave. And I had hardly worked and thought about the matter six months before light came through one of the meshes suddenly—blindingly! I found a general principle of pigments and refraction—a formula, a geometrical expression involving four dimensions. Fools, common men, even common mathematicians, do not know anything of what some general expression may mean to the student of molecular physics. In the books—the books that tramp has hidden—there are marvels, miracles! But this was not a method, it was an idea, that might lead to a method by which it would be possible, without changing any other property of matter—except, in some instances colours—to lower the refractive index of a substance, solid or liquid, to that of air—so far as all practical purposes are concerned.” “Phew!” said Kemp. “That’s odd! But still I don’t see quite ... I can understand that thereby you could spoil a valuable stone, but personal invisibility is a far cry.”