“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.

“You make the glass invisible by putting it into a liquid of nearly the same refractive index; a transparent thing becomes invisible if it is put in any medium of almost the same refractive index.  And if you will consider only a second, you will see also that the powder of glass might be made to vanish in air, if its refractive index could be made the same as that of air; for then there would be no refraction or reflection as the light passed from glass to air.”