Beyond the Vanishing Point - Ray Cummings

When George Randolph first caught sight of Orena, he was astounded by its gleaming perfection. Here were hills and valleys, lakes and streams, glowing with the light of the most precious of metals. And, more astonishing than that, it was a world of miniature perfection—an infinitely tiny universe within a golden atom!

But for Randolph it was also a world aglow with danger. Somewhere in its tiny vastness were the friends he had to rescue. Captives of a madman, they had been reduced to native Orena size; to return to Earth they needed the growth capsules Randolph was bringing them. It was up to Randolph to find them—and quickly—for the longer they stayed tiny, the closer they came to passing BEYOND THE VANISHING POINT! - Summary by Raymond King Cummings