"How did they solve it?" Dona asked. "Somebody invented a shield," said Kim, as drily as before. "It was a force-field. It has the property of a magnetic field on a conductor with a current in it, except that it acts on mass as such. A current-carrying conductor in a magnetic field tends to move at right angles both to the current and the field. This force-field acts as if mass were an electric charge. "Anything having mass, entering the field, tries to move sidewise. The faster it moves, the stronger the sidewise impulse. Neutrons, gamma particles, met rays and even electrons have mass. So has light. Everything moving that hits the shielding field moves sidewise to its original course. Radiation from the sun isn't reflected, at right angles. "So, with the shield up, men can stay on the planet when it is less than three diameters from its sun. No heat reaches it. No neutrons. No radiations at all. It doesn't heat up. And that's the answer. For three months in every four-year revolution, they have to keep the shield up all the time. For three months more, they keep it up intermittently, flashing it on for fractions of a second at a time, just enough to temper the amount of heat they get. "They live on great platforms of uranium glass, domed in. When they go out mining they wear shielded space-suits and work in shielded machines. The whole trick was worked out about five hundred years ago, they say, and the last space-line went out of existence, because they could use a matter-transmitter for all but six of our months of that planet's year." "And did you find out how it's done?" asked Dona. "Hardly," said Kim. "The planet belongs to the king of Pharos Three. Even five hundred years ago the governments of all the planets were quite tight corporations. Naturally Pharos wouldn't let the secret get out. There are other planets so close to their primaries that they're radioactive. If the secret were to be disclosed there'd be competition. There'd be other plutonium mines in operation. So he's managed to keep it to himself. But we've got to find out the trick." There was silence. Kim began to check over the pictures the cameras had taken and developed. He shook his head. Then he stared at a photograph which showed the blue-white dwarf itself. His face looked suddenly very drawn and tired. "Kim," said Dona presently. "It's stupid of me, but I don't see how you're going to learn the secret." Kim put the picture on the enlarger, for examination in a greater size.