What chiefly concerns us here is the fact that the pictures on our retinas are flat, of two dimensions, the same as the canvas on which we paint. If you examine these visual pictures without any prejudice, as one may with a camera obscura, you will see that they are composed of masses of colour in infinite variety and complexity, of different shapes and gradations, and with many varieties of edges; giving to the eye the illusion of nature with actual depths and distances, although one knows all the time that it is a flat table on which one is looking.
Seeing then that our eyes have only flat pictures containing two-dimension information about the 40objective world, from whence is this knowledge of distance and the solidity of things? How do we see the third dimension, the depth and thickness, by means of flat pictures of two dimensions?
The power to judge distance is due principally to our possessing two eyes situated in slightly different positions, from which we get two views of objects, and also to the power possessed by the eyes of focussing at different distances, others being out of focus for the time being.