"One, because a mirror is really a reflecting pinhole. A mirror just reflects the light that would go through the hole. So if instead of putting square holes here, I put square mirrors, I get a reflection of the images that would go through those holes.
"Two, I also realized that aperture size was just about limitless. Here was a 30-inch by 46-inch mirror. Now that's certainly not a pinhole, but over there on the wall 130-feet across the museum floor, the resolution became so good that you could actually recognize the sun and the clouds and the fog going by.