impressions of sight may be corrected by the touch, especially in infancy.
The confirmation of them by one another cannot of course be given by any
one of them. Many intuitions which are inseparable from the act of sense
are really the result of complicated reasonings. The most cursory glance
at objects enables the experienced eye to judge approximately of their
relations and distance, although nothing is impressed upon the retina
except colour, including gradations of light and shade. From these
delicate and almost imperceptible differences we seem chiefly to derive our
ideas of distance and position. By comparison of what is near with what is
distant we learn that the tree, house, river, etc. which are a long way off
are objects of a like nature with those which are seen by us in our
immediate neighbourhood, although the actual impression made on the eye is
very different in one case and in the other. This is a language of 'large
and small letters' (Republic), slightly differing in form and exquisitely
graduated by distance, which we are learning all our life long, and which
we attain in various degrees according to our powers of sight or
observation. There is nor the consideration. The greater or less strain
upon the nerves of the eye or ear is communicated to the mind and silently
informs the judgment. We have also the use not of one eye only, but of
two, which give us a wider range, and help us to discern, by the greater or
less acuteness of the angle which the rays of sight form, the distance of
an object and its relation to other objects. But we are already passing
beyond the limits of our actual knowledge on a subject which has given rise
to many conjectures. More important than the addition of another
conjecture is the observation, whether in the case of sight or of any other
sense, of the great complexity of the causes and the great simplicity of
the effect.

The sympathy of the mind and the ear is no less striking than the sympathy
of the mind and the eye. Do we not seem to perceive instinctively and as