Of course, the sheet does not connect two points, not reversing the curve, covering one of them. But who told us about the type of surface on which to solve the problem? And if this is not a plane, and, say, a bagel or a car tire – all turns out easily. A man too accustomed to two-dimensional world. Our "voobrazitelnaya" intuition here never fails us. But when it comes to spatial representations, it begins to limp. The height of the house estimate is much more difficult than its length or width.
And tell how far away the plane or a cloud, an untrained person can not even approximately. Third coordinate – not what the Fourth! – We have yet to master and master. The reason here is not psychological, but a purely physiological. The thing is the device of our eyes. When we look at the remote object, the specific muscles bend the lens of the eye – the natural lens to change its focal length and allow us to see the object clearly. If the muscles are tired, we have to start points and change the focal length artificially. Focusing the camera – perfect analogy to this process, which Physiology is called accommodation.
And yet in each eye there is a group of six muscles that turn it so that the direction of gaze right and left eyes intersect at one point. This is called convergence. This creates a binocular effect – we see the world volume. Stereoscope, in which regard the "bulging" image was built on the same principle.