The idea that stereoscopy, the name for stereo vision, is just due to the vision in the two eyes. While certainly true, is not quite as simple as just being totally explained on that basis. And that was pretty obvious from time on to the present day. And one of the confounds that Wheatstone himself described and made an observation from the outset of work on stereopsis was a little bit confusing. Maybe not a little bit confusing, was quite confusing. Was that if you just switch the views of the right and left eye, you might imagine that all of stereoscopic depth is changed. And this was done by something that Wheatstone called his pseudoscope. And the pseudoscope that he made is diagrammed here. And what he did was very clever. He switched the views of the left and the right eye by using two prisms in this position. And the left eye in this situation. Now get's what the right eye would have normally seen. And the right eye gets what the left eye would have normally seen. Just by this prismatic inversion. So here's the normal view of this cube. And here's the pseudoscopic view of the cube. And you can see that the left right eye views are reversed. Now if stereo were just the result of the differences between right eye and left eye views, well you'd expect the whole stereo world to be reversed. And to see everything in this fashion. Instead of in the normal fashion. But that's not what was seen. And it's a little difficult to describe the outcome of looking through a pseudoscope. We've made one a few years ago just to kind of experience the phenomenon that Wheatstone described. What he described was that ambiguous objects, let's say you're looking at the cast of a medal and maybe you know what bas relief is. Bas relief, let me write that down. So you know what I'm talking about. Bas relief is the appearance of the cast metal, the face of the hero or whatever it might be coming up at you. When you look through the pseudoscope at a bas relief medal that could be seen switching back and forth between bar relief and intaglio. Intaglio is just the name for the opposite that the cast would be appearing to go into the metals that are coming out toward you. So ambiguous objects like that, like the bottom of a cup. Many examples that he described in his writings on this between 1830 and the 1850s, were indeed seeing in the way you might expect, having confused the right and left eye view. But for the most part, this is certainly true from my experience looking through a pseudoscope the world looks pretty normal. Well what does that mean? It probably just means that stereoscopic vision and the monocular views of vision, which of course are perfectly present when you look at the world with two eyes. They're just in addition to the stereoscopic information you have or, better put, the stereoscopic information is In addition to the major contribution of all the monocular cues that we talked about. What you see is a combination of those two things and it's not just a switch generated by the pseudoscope. So as in so many of these things, it's more complicated than you might imagine. But it was Wheatstone who put the foundation in place for basically understanding stereoscopic vision as the different eyes views just from the geometry of their position across the face, the angle at which they look at near by objects. And of course when they look at distant objects, the stereoscopic effect goes down and you have to see it with bigger and bigger objects because the eyes are more and more parallel. And that should be obvious looking at a diagram such as the ones that we use to show that the two eye views are different, like this. This is remember our greeting distance, 30 centimeters so as you go further and further distant in the objects that you are looking at they have to be bigger and bigger separated by bigger and bigger distances in real world space to generate some sensations during a scalpic depth at a distance. But both things are contributing all the time to the distance. And depth combination, that tells you how far away things are in the real world.
