With some effort, I learned the technique myself. I have some of my dad’s images hanging in my own home now, and when people visit, I also invite them to take a second look. Cross your eyes, I say, so that you are looking at the left image with your right eye and the right image with your left eye. Eventually a third picture will appear between them. It will be fuzzy at first, but if you keep your eyes relaxed, it will soon snap into focus. Then you’ll see what my dad was so excited about: a whole world, right there before you. No tricks, just depth.

He shot mostly landscapes and still lifes. The results were never generic, but they often appeared at first to be impersonal, even a little cold. It was the depth that made them meaningful. He especially liked to shoot into rooms or buildings from the outside, because the sharp change in the lighting heightened the stereo effect, just as chiaroscuro adds depth to Renaissance paintings. He thought a lot about parallax. If he moved the camera farther between exposures, for instance, he could create what is known among stereo enthusiasts as the model-train effect — real landscapes looked like tiny dioramas. The reason for this was simple: Parallax, the angle of sightlines converging on an object, is how the brain calculates the size of things. If the effective space between your pupils suddenly goes from a few inches to a few feet, you become Gulliver, and your brain says, “This must be Lilliput.”

Free viewing has some real utility — it’s the easiest way to look at the millions of vintage stereo pairs on the internet, for instance — but it is best appreciated as a different way to think. It was, for my dad, a kind of practice, a way to hold two ideas in mind that were, if not opposed, then at least divergent. Seeing in stereo was a way to think in stereo. He expressed himself with parallax, and understanding that expression required people to do something they often resisted. For my dad, the real art was in some sense simply persuading people to try, just for a moment, to see that world as he did — two views converging.

In recent years, he has moved on to other interests. He sold the house I grew up in and gave me all the thousands of regular pictures he shot in the years before he took up stereo photography: faded prints, boxes of unexamined negatives, multiple binders of carefully sorted slides — half a lifetime of work. Those backward-looking images seemed to hold little sentimental value for him. But he kept his stereo prints. They are there in his new home, and he still seems pleased when people take a moment to see them as he sees them.