He pointed to the palette lying on his work-table.  Different shades of black pigments were brushed on it.  One, in particular, I could hardly see.  It gave my eyes a blurring sensation, and I rubbed them and looked again.

“That,” he said impressively, “is the blackest black you or any mortal man ever looked upon.  But just you wait, and I’ll have a black so black that no mortal man will be able to look upon it—­and see it!”

On the other hand, I used to find Paul Tichlorne plunged as deeply into the study of light polarization, diffraction, and interference, single and double refraction, and all manner of strange organic compounds.

“Transparency:  a state or quality of body which permits all rays of light to pass through,” he defined for me.  “That is what I am seeking.  Lloyd blunders up against the shadow with his perfect opaqueness.  But I escape it.  A transparent body casts no shadow; neither does it reflect light-waves—­that is, the perfectly transparent does not.  So, avoiding high lights, not only will such a body cast no shadow, but, since it reflects no light, it will also be invisible.”

We were standing by the window at another time.  Paul was engaged in polishing a number of lenses, which were ranged along the sill.  Suddenly, after a pause in the conversation, he said, “Oh!  I’ve dropped a lens.  Stick your head out, old man, and see where it went to.”

Out I started to thrust my head, but a sharp blow on the forehead caused me to recoil.  I rubbed my bruised brow and gazed with reproachful inquiry at Paul, who was laughing in gleeful, boyish fashion.

“Well?” he said.

“Well?” I echoed.

“Why don’t you investigate?” he demanded.  And investigate I did.  Before thrusting out my head, my senses, automatically active, had told me there was nothing there, that nothing intervened between me and out-of-doors, that the aperture of the window opening was utterly empty.  I stretched forth my hand and felt a hard object, smooth and cool and flat, which my touch, out of its experience, told me to be glass.  I looked again, but could see positively nothing.

“White quartzose sand,” Paul rattled off, “sodic carbonate, slaked lime, cutlet, manganese peroxide—­there you have it, the finest French plate glass, made by the great St. Gobain Company, who made the finest plate glass in the world, and this is the finest piece they ever made.  It cost a king’s ransom.  But look at it!  You can’t see it.  You don’t know it’s there till you run your head against it.

“Eh, old boy!  That’s merely an object-lesson—­certain elements, in themselves opaque, yet so compounded as to give a resultant body which is transparent.  But that is a matter of inorganic chemistry, you say.  Very true.  But I dare to assert, standing here on my two feet, that in the organic I can duplicate whatever occurs in the inorganic.

“Here!” He held a test-tube between me and the light, and I noted the cloudy or muddy liquid it contained.  He emptied the contents of another test-tube into it, and almost instantly it became clear and sparkling.