In this lesson, we're going to talk about a related sense of distance. There is a relation between seeing with two eyes and seeing with one eye. Seeing with two eyes, at least if your eyes are at the front of your heads like ours and many other animals, and I'll say more about this in a minute. You get a sense of depth that's basically quite different from your sense of distance, although the two things are related. The reasons for seeing depth is what we're going to go into. And once I explain the reasons for seeing depth, you'll see how different seeing the world with two eyes and why the sense of depth is really fundamentally different. And a sense of distance even though the two are related. So let's go back to the 19th century and talk a little bit about this gentleman, Charles Wheatstone. He was born near the beginning of the 19th century and lived to a good age. And was again like many of the 19th century giants in physiology, in psychology, in physics, interested and contributed to a whole variety of different disciplines. And those of you who know something about electronics will be familiar with the Wheatstone bridge that he invented as a way of measuring electrical resistance. Those of you who may know something about telegraphy will know that he contributed importantly to that field. But in some ways, he's best known for his work on vision and his seminal explanation of why it is that when we look at the world with two eyes we have a special sense of depth. And this sense of depth arises basically from the fact that when we see the world, when we look at the world, and our eyes are places as eyes separated across the face by about 65 millimeters but capable of looking at the same point in space with either the right eye or the left eye. It should be pretty obvious and you can certainly demonstrate this to your self by holding any object at reading distance and closing one eye and the other and convince yourself that we have a very different view, well not completely different, but we have a different view in the left eye and the right eye when we look at something that's relatively near by. As I said reading distance is kind of the best way if you hold a cube or any object that's handy at reading distance, close one eye and look at it. Remember what the imaged looked like and then close the other eye and look at it. You'll see that the two images are not the same for fairly obvious reasons. Here in this diagram the line of sight. And remember that the line of sight is the fovea when it's directed at a point in space like this black dot on the surface of the cube, is making a different projection of the cube entire block dot in both cases is going to the full view. Fine that's the line of sight. But all the other points and we'll talk about this in more detail in a minute all the other points are going to different points on the right and left eye retinas. And that means that we have basically a different perception generated by those differences when we look through the left eye or the right eye independently and convince ourselves that yeah, the views are really majorly different. Now Wheatstone did another thing that probably is familiar to many of you. He invented something called the stereoscope. Let me just write that down. The stereoscope that's familiar today is something you can buy in Toys R Us for a few bucks, and it's basically exactly the same instrument that Wheatstone created in the 1830s to demonstrate the basis of stereopsis. Why it is that we have this special sense of depth when we look at the world with two eyes? What the modern version of the stereoscope entails is a set of images. They might be Bugs Bunny, Spiderman, what have you. Something from Disneyland. And as we rotate this you have a view in one eye of something that's taken with a camera from the perspective of let say the left eye and something that's taken from the perspective of the right eye. And so these two photographic pictures here are basically images of what we would see looking at something with the left eye and the right eye at reading distance. The manufacturers of these devices get a little freebie by increasing the distance between our two eyes which is just 65 Millimeter. They'll put the cameras a little further part to give you this sort of sense of death and when you look through stereoscope you will see that is seeing as being more profoundly and depth for the free dimensional space than you would normally see in just looking at the objects in reality, so now? Why would nature have bothered creating this special sense of depth? What value does it have? Well it does have a lot of value. And it's, I think, particularly obvious when you try to do certain things, and the certain things that you can and can't do with stereopsis. Indicate what animals can and can't do in nature, particularly predatory animals that depend on accurately capturing let's say, a mouse in the case of a raptor or some other small rodent in the case of fox or a cat. Take two pencils that happen to be handy. And with two eyes open, seeing these in stereoscopic depth, you have no trouble touching the points together. You do that very easily. Now close one eye and try to do that. I'm not faking this. I'll really try to touch the points together. And, I missed. Sometimes, you'll miss by a lot, sometimes by a little, but you have much more trouble accurately touching the points together with one eye than with two eyes open, taking advantage of stereoscopic depth. So, again, you can see how that's beneficial to an animal that needs to judge the distance of objects for capturing prey or for whatever other reason. If you're a primate like we are, you need to manipulate things. And non-human primates and primates like us obviously, you take advantage of accurately manipulating things close by and stereopsis is a big help in that. Stereopsis also works at a distance but in a less dramatic way that's differently useful but that's why I say that distance binocularly seen. And distance binocularly seen, they overlap and they're both together. If you look at distant objects, stereo is contributing something to your sense of the distance of those objects, that are let's say 100 meters away. As well as to objects that are, let's say 30 centimeters away at reading distance.
