"Well, ordinary polished silver is a mighty poor radiator. Homely example: Try waiting for your coffee to cool if it's in a polished silver pot. Then try it in a tungsten-beryllium pot. No matter how you polish that tungsten-beryllium, the stuff WILL radiate heat. That's why an IP ship is always so blamed cold. You know the passenger ships use polished aluminum outer walls. The big help is, that the tungsten-beryllium will throw off the energy pretty fast, and in a big ship, with a whale of a lot of matter to heat, the Strangers will simply give up the idea." "Yes, but only two ships in the system compare with them in size." "Sorry--but I didn't build the IP fleet, and there are lots of tungsten and beryllium on Earth. Enough anyway." "Will they use that beam on the fort? And can't we use the thing on them?" "They won't and we won't--though we could. A bank of those new million watt tubes--perhaps a hundred of them--and we'd have a pretty effective heater--but an awful waste of power. I've got something better." "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."