learned in the same way. The hands must actually touch
and handle the object, experiencing its hardness or smoothness, the way
this curve and that angle feels, the amount of muscular energy it takes
to pass the hand over this surface and along that line, the eye taking
note all the while, before the eye can tell at a glance that yonder
object is a sphere and that this surface is two feet on the edge.

3. THE PERCEPTION OF SPACE

Many have been the philosophical controversies over the nature of space
and our perception of it. The psychologists have even quarreled
concerning whether we possess an _innate_ sense of space, or whether it
is a product of experience and training. Fortunately, for our present
purpose we shall not need to concern ourselves with either of these
controversies. For our discussion we may accept space for what common
sense understands it to be. As to our sense of space, whatever of this
we may possess at birth, it certainly has to be developed by use and
experience to become of practical value. In the perception of space we
must come to perceive _distance_, _direction_, _size_, and _form_. As a
matter of fact, however, size is but so much distance, and form is but
so much distance in this, that, or the other direction.

THE PERCEIVING OF DISTANCE.--Unquestionably the eye comes to be our
chief dependence in determining distance. Yet the muscle and joint
senses give us our earliest knowledge of distance. The babe reaches for
the moon simply because the eye does not tell it that the moon is out of
reach. Only as the child reaches for its playthings, creeps or walks
after them, and in a thousand ways uses its muscles and joints in
measuring distance, does the perception of distance become dependable.

At the same time the eye is slowly developing its power of judging
distance. But not for several years does visual perception of distance
become in any degree accurate. The eye's perception of distance depends
in part on the sensations arising from the muscles controlling the eye,
probably in part from the adjustment of the lens, and in part from the
retinal image. If one tries to look at the tip of his nose he easily
feels the muscle strain caused by the required angle of adjustment. We
come unconsciously to associate distance with the muscle sensations
arising from the different angles of vision. The part played by the
retinal image in judging distance is easily understood in looking at two
trees, on