They say cowardice is infectious; but then argument is, on the other hand, a great emboldener; and so when each had said his say, my mother made them a speech. She would not, she declared, lose money that belonged to her fatherless boy; “If none of the rest of you dare,” she said, “Jim and I dare. Back we will go, the way we came, and small thanks to you big, hulking, chicken– hearted men. We’ll have that chest open, if we die for it. And I’ll thank you for that bag, Mrs. Crossley, to bring back our lawful money in.”

Of course I said I would go with my mother, and of course they all cried out at our foolhardiness, but even then not a man would go along with us. All they would do was to give me a loaded pistol lest we were attacked, and to promise to have horses ready saddled in case we were pursued on our return, while one lad was to ride forward to the doctor’s in search of armed assistance.

My heart was beating finely when we two set forth in the cold night upon this dangerous venture. A full moon was beginning to rise and peered redly through the upper edges of the fog, and this increased our haste, for it was plain, before we came forth again, that all would be as bright as day, and our departure exposed to the eyes of any watchers. We slipped along the hedges, noiseless and swift, nor nor did we see or hear anything to increase our terrors, till, to our relief, the door of the Admiral Benbow had closed behind us.

I slipped the bolt at once, and we stood and panted for a moment in the dark, alone in the house with the dead captain’s body. Then my mother got a candle in the bar, and holding each other’s hands, we advanced into the parlour. He lay as we had left him, on his back, with his eyes open and one arm stretched out.

“Draw down the blind, Jim,” whispered my mother; “they might come and watch outside. And now,” said she when I had done so, “we have to get the key off THAT; and who’s to touch it, I should like to know!” and she gave a kind of sob as she said the words.

I went down on my knees at once. On the floor close to his hand there was a little round of paper, blackened on the one side. I could not doubt that this was the BLACK SPOT; and taking it up, I found written on the other side, in a very good, clear hand, this short message: “You have till ten tonight.”

“He had till ten, Mother,” said I; and just as I said it, our old clock began striking. This sudden noise startled us shockingly; but the news was good, for it was only six.

“Now, Jim,” she said, “that key.”

I felt in his pockets, one after another. A few small coins, a thimble, and some thread and big needles, a piece of pigtail tobacco bitten away at the end, his gully with the crooked handle, a pocket compass, and a tinder box were all that they contained, and I began to despair.

“Perhaps it’s round his neck,” suggested my mother.

Overcoming a strong repugnance, I tore open his shirt at the neck, and there, sure enough, hanging to a bit of tarry string, which I cut with his own gully, we found the key. At this triumph we were filled with hope and hurried upstairs without delay to the little room where he had slept so long and where his box had stood since the day of his arrival.

“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 would not be so brilliant, not so clearly visible, as a diamond box, because there would be less refraction and reflection. See that? From certain points of view you would see quite clearly through it. Some kinds of glass would be more visible than others, a box of flint glass would be brighter than a box of ordinary window glass. A box of very thin common glass would be hard to see in a bad light, because it would absorb hardly any light and refract and reflect very little. And if you put a sheet of common white glass in water, still more if you put it in some denser liquid than water, it would vanish almost altogether, because light passing from water to glass is only slightly refracted or reflected or indeed affected in any way. It is almost as invisible as a jet of coal gas or hydrogen is in air. And for precisely the same reason!”

“Yes,” said Kemp, “that is pretty plain sailing.”

“And here is another fact you will know to be true. If a sheet of glass is smashed, Kemp, and beaten into a powder, it becomes much more visible while it is in the air; it becomes at last an opaque white powder. This is because the powdering multiplies the surfaces of the glass at which refraction and reflection occur. In the sheet of glass there are only two surfaces; in the powder the light is reflected or refracted by each grain it passes through, and very little gets right through the powder. But if the white powdered glass is put into water, it forthwith vanishes. The powdered glass and water have much the same refractive index; that is, the light undergoes very little refraction or reflection in passing from one to the other.