n" to the
unanalyzed sensory datum that gives a knowledge of the point
stimulated.

In handling an object, as also in walking and many other movements,
the cutaneous and kinesthetic senses are stimulated together, and
between them furnish data for the perception of many spatial facts,
such as the shape of an object examined by the hand. The spherical
shape is certainly better perceived by this combination of tactile and
kinesthetic {441} sensations than by vision, and the same is probably
true of many similar spatial facts. That is, when we see a round ball,
the visual stimulus is a substitute for the tactile and cutaneous
stimuli that originally had most to do with arousing this perception.

In part by this route of the substitute stimulus, the sense of vision
comes to arouse almost all sorts of spatial perceptions. Of itself,
the retina has "local sign" since we can tell where in the field of
view a seen object is, i.e., in what direction it is from us. This
visual perception of location is so much more exact than the cutaneous
or kinesthetic that it cannot possibly be derived from them; and the
same is true of the visual perception of difference in length, which
is one of the most accurate forms of perception. The retina must of
itself afford very complete stimuli for the perception of location and
size, as far as these are confined to the two dimensions, up-down and
right-left. But, when you stop to think, it seems impossible that the
retina should afford any data for perceiving distance in the
front-back dimension.

The retina is a screen, and the stimulus that it gets from the world
outside is like a picture cast upon a screen. The picture has the
right-left and up-down dimensions, but no front-back dimension. How,
then, does it come about, as it certainly does, that we perceive by
aid of the eye the distance of objects from us, and the solidity and
relief of objects? This problem in visual perception has received much
attention and been carried to a satisfactory solution.

Consider, first, what stimuli indicative of distance and relief could
affect a single motionless eye. The picture on the retina could then
be duplicated by a painter on canvas, and the signs of distance
available would be the same in the two cases. The painter uses
foreshortening, making a man in the picture small in proportion to his
distance away; {442} and in the same way, when any familiar object
casts a small picture on the retina, we