This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
Distance, however, is a thing which the eye certainly learns to appreciate by experience. A child, from its entrance into the world, no doubt sees objects as things outside itself; but it learns only by practice the distances of different objects.
This might be gathered from watching the movements of infants; but it has been more distinctly demonstrated by observations made on persons born blind, who have gained sight after some years, by a surgical operation. Such persons see every thing at first as if close at hand, and, from not understanding the effects of distance, form most erroneous ideas of the sizes of objects; and they handle things when they look at them, so as to compare the results of vision with those of common sensation. The effects of distance on the eyes, which experience teaches us to translate, are of various descriptions. (1) The distance of the object looked at determines the degree of convergence of the eyes; (2) it determines the focus; (3) it affects the intensity of light and shade, and the colour, producing what is called perspective of colour; (4) it diminishes the apparent size of objects, producing linear perspective; so that when from custom or otherwise the size of an object is known, its distance is estimated. That the convergence of the eyes is of some use in enabling us to appreciate the exact distance of near objects, is illustrated by the difficulty which one has in threading a needle when one eye is shut. But by a little practice that difficulty is overcome, which shows that the use of two eyes is not essential to judging distances.