Earlier this year, I had the pleasure of serving as one of several judges for the Nikon Small World contest. Our task was to sit in a dimly lit room and try to rank the hundreds of entries—images taken by professional and amateur scientists around the world using visible-light microscopes.

Some were easy: The rules of the contest, which Nikon has run since 1974, forbid images obtained with nonlight microscopes such as electron-based instruments. Any of those that slipped past the contest's initial screens could be discarded.

But most of the choices were very difficult. Even for someone red-green color-blind like me, the beauty of many of these images was, you might say, blindingly obvious. There were lots of diatoms—tiny single-celled algae—sometimes painstakingly arranged to look like common objects. There were also lots of insects, and some brain scans. But there were also rocks and other man-made items that had never been "alive."

In this slide show, we present this year's 20 winning pictures, along with captions describing what you're seeing and how the image was obtained.

Slide Show: View Top 20 Winning Photos from the Contest