The Adventurers looked down over the rocky decline that separated them from the stronghold of the Cabal. A wisp of pale smoke drifted up from a round ventilation hole, marking the spot.
“I think I see a path,” Francis Hebert said. “See there?”
Max shielded his eyes. “Dammit, I can’t tell whether that’s concave or convex. This place is crazier than chopsticks for a snake.”
Larry Niven And Steve Barnes, The Barsoom Project, 1989.
Concave means ‘having an outline or surface curved like the interior of a circle or sphere’. Convex means ‘having an outline or surface curved like the exterior of a circle or sphere’. [The New Fowler’s Modern English Usage, Revised 3rd ed.]
The spoon’s inner surface is concave — that is, it is hollow like a cave. (That’s a good way to remember the distinction between con cave and con vex.) When you look into the spoon, you’ll notice that the top part is shaped so that it reflects its light slightly downward, like a mirror held high. At the same time, the bottom part is shaped so that it reflects its light slightly upward, like a mirror on the floor. These “high” and “low” reflectors give you a stand-on-the-head image, exactly as the above-your-head and on-the-floor mirrors did in the preceding explanation.
Robert Wolke, What Einstein Told His Barber : More Scientific Answers to Everyday Questions, Random House, 2000.
The Owl of Saint George was at its undulating peak. The psychedelic lights exploded against the walls and ceiling in rhythmic crescendos; bodies were concave and convex, none seemingly upright, all swaying, writhing violently.
Robert Ludlum, The Cry of the Halidon, 1974.