"New?"
"Somewhat. I've found out how to make the mirror field in a plate of metal, instead of a block. Come on to the lab, and I'll show you."
"What's the advantage? Oh—weight saved, and silver metal saved."
"A lot more than that, Mac. Watch."
At the laboratory, the new apparatus looked immensely lighter and simpler than the old. The atostor, the ionizer, and the twin ion-projectors were as before, great, rigid, metal structures that would maintain the meeting point of the ions with inflexible exactitude under any acceleration strains. But now, instead of the heavy silver block in which a mirror was figured, the mirror consisted of a polished silver plate, parabolic to be sure, but little more than a half-inch in thickness. It was mounted in a framework of complex, stout metal braces.
Kendall started the ion-flame at low intensity, so the UV beam was little more than a spotlight.
"You missed the point, Mac. Now—watch that tungsten-beryllium plate. I'll hold the power steady. It's an eighteen-inch beam—and now the energy is just sufficient to heat that tungsten plate to bright red. But—"
Kendall turned over a small rheostat control—and abruptly the eighteen-inch diameter spot on the tungsten-beryllium plate began contracting; it contracted till it was a blazing, sparkling spot of molten incandescence less than an inch across!
"That's the advantage of focus. At this distance of a few hundred feet with a small beam I can do that. With a twenty-foot beam, I can get a two-foot spot at a distance of nearly ten miles! That means that the receiving end will have the pleasure of handling one hundred times the energy concentration. That would punch a hole through most anything. All you have to do is focus it. The trouble being, if it's out of focus the advantage is more than lost. So if there's any question about getting the focus, we'll get along without it."