In the 1970s, Bill Brand was a young experimental filmmaker. The kind of filmmaker who made cat videos that did not get millions of views.

We’re in Brand’s New York studio today, watching a reel from his student days on a jerry-rigged projector. “This is a film called Zip Tone Cat Tune, where I used positive and negative images of a cat, seen through a grid of dots,” he says. Onscreen, a kitten grooms herself under an overlay of strobing Technicolor dots.

“I make usually fairly short, fairly abstract films,” Brand says. “They don’t usually tell a story, they’re more related to painting or sculpture or poetry or something like that you might say.”

Not many people saw Zip Tone Cat Tune, or Brand’s other short films from the ‘70s: Demolition of a Wall, Circles of Confusion, or Touch Tone Phone Film. After a while, Bill wondered what it would be like to have a larger audience. And an idea came to him on the Chicago L train.

“Riding the train, I think anyone has had the experience of looking out the window and feeling like this is very cinematic. This is like looking at a movie. And so I just started imagining what would happen if you had frames that work like a flipbook that you could see out the window,” says Brand.

Brand’s idea was to paint a 300-foot-long filmstrip inside the New York City subway. His subway movie would work like conventional film in reverse. Instead of sitting in a theatre while filmstrips pass through a projector, the filmstrip would be stationary. You would pass by it.

Brand called his hypothetical subway movie Masstransiscope. He got funding and permission to make it happen. The problem was, he didn’t actually know how to construct it. And figuring it out meant going back to basics, to lessons he learned about film watching, of all things, Woody Woodpecker.

“When I was a child, there was a program,” he says. “The Walter Lantz Woody Woodpecker show would always begin with Walter Lantz himself. You’d see him at the drawing board, and he’d explain how animation worked, and I learned that it was a series of individual frames.”

“That became very fascinating to me,” Brand says. So he started drawing frames onto a strip of paper, hoping to make an animation. “I would pull the strip by my eye and it didn’t make an animation, it just would become blurry. That was a puzzle to me.”

Bill’s animation was missing a crucial element: The shutter.

Imagine sitting in a movie theatre—one that plays actual film reels. When the projector’s shutter is up, you see the frame of film. When it’s down—nothing. In fact, half the time you’re sitting in a movie theater, the screen is completely dark.

That intermittency—the “on and off” flicker between image and shutter—you need that to create the illusion of motion. But how do you create a shutter in the New York City subway?

“The way it works in the Masstransiscope is that you actually view the painting through a wall of slits,” Brand says. “So the painting sits inside a box that I created and put on the platform. In the front of the box, every 15 inches is about a half-inch wide slit. So as you pass by, you’re looking through that slit.”

You might ask why subway riders don’t notice a flicker. It has to do with how our visual system registers images.

“When you look at something, or when light passes through your eye, it creates these chemical changes and it persists for a period of time. And we all know that if you’ve had your picture taken with a flash bulb, you see this ball of light for a fairly long period of time that’s quite annoying. But actually that process is happening all the time, consistently, and if it didn’t, we probably wouldn’t see at all,” Brand explains.

You could think of each Masstransiscope frame like a camera bulb’s flash. It makes an impression on the retina that takes time to fade away. Not a long time, but long enough that if those flashes happen quickly enough, we won’t notice the gaps between them. In fact, we’ll swear we’ve been looking at one continuous image the entire time.

That was the theory. To test it, Brand experimented with a scale model in his studio. What he didn’t do was a test run on the actual subway. So when Masstransiscope opened to the public in September 1980, Bill Brand saw his movie for the very first time, along with everyone else.

Masstransiscope worked. And it still works today.

On a recent Sunday, I meet Mildred Vasquez, a Brooklynite and first-time Masstransiscope viewer. She and her husband watch out the Q train window.

“There it goes look, look, look! Look!” she exclaims. “It’s like a person turning into a diamond, oh! Opening up! Ah! Look squares! Now it’s a spiral, now it’s a spiral. Rocketship! Look it took off. There it goes. That was fantastic!”

Geometric shapes morph into people, then landscapes. There’s a rocket ship—or squid, depending on who you ask. Squares shuffle into what might be a deck of cards. A swirling tendril reaches out from the ground. It’s abstract, non-representational, and playful­—just like Brand’s avant-garde films. Except the whole show lasts 25 seconds.

Masstransiscope is mass entertainment. But Brand says catching sight of it in a crowded subway car can feel oddly intimate.

“The experience of viewing it is very private. It’s not announced, you have to be looking to see it. So, it’s very public, but it’s everybody’s secret,” he says.

And if you haven’t seen Masstransiscope already, well, now it’s yours too.