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."
"Precisely," said Griffin. "But consider: Visibility depends on the action of the visible bodies on light. Either a body absorbs light, or it reflects or refracts it, or does all these things. If it neither reflects nor refracts nor absorbs light, it cannot of itself be visible. You see an opaque red box, for instance, because the colour absorbs some of the light and reflects the rest, all the red part of the light, to you. If it did not absorb any particular part of the light, but reflected it all, then it would be a shining white box. Silver! A diamond box would neither absorb much of the light nor reflect much from the general surface, but just here and there where the surfaces were favourable the light would be reflected and refracted, so that you would
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