I don't have the object here, but when going through a collection of donated camera equipment at the Photographic Historical Society, I came across a viewfinder for some camera (German?) that was meant to "project" a frame on your vision. It was meant to be used when keeping both eyes open. The rest of the guys in the group thought it was pretty neat. It didn't work for me because I do not really have binocular vision, I'm wall-eyed, and while each eye works pretty well, they don't work together.