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 objective 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. In a picture the eyes can only focus at one distance (the
distance the eye is from the plane of the picture when you are looking
at it), and this is one of the chief causes of the perennial difficulty
in painting backgrounds. In nature they are out of focus when one is
looking at an object, but in a painting the background is necessarily on
the same focal plane as the object. Numerous are the devices resorted to
by painters to overcome this difficulty, but they do not concern us
here.

The fact that we have two flat pictures on our two retinas to help us,
and that we can focus at different planes, would not suffice to account
for our knowledge of the solidity and shape of the objective world, were
these senses not associated with another sense all important in ideas of
form, #the sense of touch#.