Washington—As the microscope gained favor in the mid-17th century, Dutch naturalist Antonie van Leeuwenhoek peered into the world of bacteria and blood cells. "I then most always saw, with great wonder," he wrote of a bit of plaque scraped from his teeth, "that in the said matter there were many very little living animalcules, very prettily a-moving." Three and a half centuries later, confocal microscopes bring to the viewer three-dimensional images of a cell's most intricate structures.