The authority in Ellenby's voice made Madson look around startledly. "Give me your pipe," Ellenby repeated firmly, holding out his hand.
Madson fished it out of the pocket of the trousers he was about to wash and handed it over. Ellenby knocked out the soggy tobacco, swished it in the water a few times, and began to soap the inside of the bowl.
Madson started to object, but, "You'd be washing it anyway," Ellenby assured him. "Now look here, Madson, I'm going to blow a bubble and I want you to watch, I want you to observe Nature for all you're worth. If poets and physicists have one thing in common it's that they're both supposed to be able to observe. Accurately."
He took a breath. "Now see, I'm going to hold the pipe mouth down and let the bubble hang from it, but with one side of the bowl tipped up a bit, so that the strain on the bubble's skin will be greatest on that side."
He blew a big bubble, held the pipe with one hand and pointed with a finger of the other. "There's the place to watch now. There!" The bubble burst.
"What was that?" Madson asked in a new voice. "It really was black for an instant, dull like soot."
"A bubble bursts because its skin gets thinner and thinner," Ellenby said. "When it gets thin enough it shows colors, as interference eliminates different wavelengths. With yellow eliminated it shows violet, and so on. But finally, just for a moment at the place where it's going to break, the skin becomes only one molecule thick. Such a mono-molecular layer absorbs all light, hence shows as dead black."
"Everything's got a black lining, eh?"
"Black can be beautiful. Here, I'll do it again."
Madson put his hand on Ellenby's shoulder to steady himself. They were standing hip-deep in water, their bodies still flecked with suds. Their heads were inches from the new bubble. As it burst a voice floated down to them.