to say, "That tree is ten rods distant." Form and size are to be 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, one
thirty feet and the other three hundred feet distant. We note that the nearer tree