Manu Prakash seeks to bring a minuscule world to the masses. Illustration by Christian Gralingen

Antoni van Leeuwenhoek wrote a letter to the Royal Society of London, in 1683, announcing the discovery of something extraordinary in his mouth. He was a haberdasher by trade, in the Dutch city of Delft, but he was known for his enthusiastic work with microscopes, which he made himself. By modern standards, Leeuwenhoek’s devices were rudimentary, and fickle in their operation. They were nearly flat, consisting of a tiny magnifying glass sandwiched between metal plates, with an adjustable spit to hold the sample being viewed. But they could be effective, particularly when an unsqueamish eye was at the peephole. Leeuwenhoek had already examined eels’ blood, dogs’ sperm, and the bile of elderly rabbits, among other substances. Now he had turned his attention to dental plaque.

Leeuwenhoek had an intensive routine of oral prophylaxis, which involved rubbing salt on his teeth each morning and buffing his molars with a cloth after meals. Nevertheless, he wrote, the plaque lay “thick as if ’twere batter.” He scraped some off, mixed it with rainwater, deposited a droplet on one of his microscopes, and held it up to the light. The sample was teeming with “many very little living animalcules, very prettily a-moving.” When he reproduced the experiment with the plaque of an old man, he found even wilder specimens, which “bent their body into curves.” Leeuwenhoek had revealed a world that few of his contemporaries were willing to believe existed. As he lamented to another microscopist in 1680, “I suffer many contradictions, and oft-times hear it said that I do but tell fairy-tales about the little animals.”

In September, a biophysicist named Manu Prakash examined some of his own plaque, at high magnification, in honor of the anniversary of Leeuwenhoek’s letter. Prakash, who is thirty-five, is slightly built, with curly brown hair, a beard, and a birthmark like a child’s thumbprint over the bridge of his nose. He doesn’t floss, and perhaps for that reason he found that his plaque contained spirochetes, bacteria that bend their bodies into curves when they move—what Leeuwenhoek observed in the old man. Prakash has his own laboratory in Stanford University’s bioengineering department, and he is best known for having invented a microscope, which was inspired by Leeuwenhoek’s. He has a passion for what he calls the “microcosmos,” meaning all things infinitesimal. “It’s not good enough to read about it,” he told me. “You have to experience it.”

One major difference between the two microscopes is that Prakash’s is made almost entirely from a sheet of paper. He calls it the Foldscope, and it comes in a kit. (Mine arrived in a nine-by-twelve-inch envelope.) The paper is printed with botanical illustrations and perforated with several shapes, which can be punched out and, with a series of origami-style folds, woven together into a single unit. The end result is about the size of a bookmark. The lens—a speck of plastic, situated in the center—provides a hundred and forty times magnification. The kit includes a second lens, of higher magnification, and a set of stick-on magnets, which can be used to attach the Foldscope to a smartphone, allowing for easy recording of a sample with the phone’s camera. I put my kit together in fifteen minutes, and when I popped the lens into place it was with the satisfaction of spreading the wings of a paper crane.

The Foldscope performs most of the functions of a high-school lab microscope, but its parts cost less than a dollar. Last year, with a grant from Gordon Moore’s philanthropic foundation (Moore co-founded Intel), Prakash and some of his graduate students launched an experiment in mass microscopy, mailing fifty thousand free Foldscopes to people in more than a hundred and thirty countries, who had volunteered to test the devices. At the same time, they created Foldscope Explore, a Web site where recipients of the kits can share photos, videos, and commentary. A plant pathologist in Rwanda uses the Foldscope to study fungi afflicting banana crops. Maasai children in Tanzania examine bovine dung for parasites. An entomologist in the Peruvian Amazon has happened upon an unidentified species of mite. One man catalogues pollen; another tracks his dog’s menstrual cycle.

With my Foldscope, I looked at peach flesh, pinkie cuticle, Himalayan sea salt, and grime from a subway pole. (The last of these resembled a Klimt painting stripped of color.) Prakash likes to abet this sort of observation, and he engages in it himself, contributing to Foldscope Explore frequently, even though he has seventy thousand unopened e-mails. A video of his teeth scrapings is there, as are photos of “the gazillion little things” that sprayed from his mouth one night during a coughing fit.

One of Prakash’s interests is biomimicry—understanding how and why certain organisms work so well, and using that knowledge to build new tools. “Plants, insects, tiny bugs under the sink, bacteria, day after day, accomplish things that no scientist anywhere in the world knows how to do,” he has said. Among insects alone, about nine hundred thousand species have been named, but millions more remain to be identified and described. The Foldscope increases Prakash’s reach. “I now have eyes and ears around the world looking at small things,” he told me.

Prakash’s hope is that those eyes and ears will make discoveries of their own. He and his chief collaborator on the project, Jim Cybulski, plan to make the Foldscope available for purchase by the summer. Prakash is particularly keen on getting kits to people who live without electricity or modern sanitation, and who have likely never observed the microcosmos directly. In October, India committed to rolling out a countrywide Foldscope program. Prakash is travelling there to demonstrate the instrument to teachers, students, health-care workers, and forest rangers. (It isn’t yet clear how the Foldscope will help the rangers, who are mainly concerned with the survival of the one-horned rhinoceros.) “There’s a very deep connection between science education and global health,” Prakash told me. “Unless you get people curious about the small-scale world, it’s very hard to change mind-sets about diseases.”

The idea for the Foldscope crystallized when Prakash was in Thailand, in 2011. “I found myself at a field station that had a really expensive microscope,” he said. “Everyone was afraid of it. It was worth five times the salary of the person trying to operate it. It just made no sense, out there in the jungle.” Three years later, with a prototype Foldscope in hand, he and Cybulski, who was then his student, went to Nigeria to conduct studies at a malaria research center in Lagos. One day, they drove north from the city to find a school. The students had just finished classes for the day, but Prakash persuaded them to stay so that he could show them the Foldscope. They caught a mosquito that was feeding on one of the children and mounted it on a paper slide, which they inserted into the Foldscope. Prakash passed it to the boy, who raised it to his eye and looked through the lens, using a small L.E.D. (also included in the kit) as his light source. “For the first time, he realized this was his blood, and this little proboscis is how it feeds on his blood,” Prakash said. “To make that connection—that literally this is where disease passes on, with this blood, his blood—was an absolutely astounding moment.” The exercise had its intended effect. The boy said, “I really should sleep under a bed net.”