ings 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."

"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 get a brilliant
appearance of flashing reflections and translucencies--a sort of
skeleton of light. A glass box wou