t may be that they are both moving in
contrary directions, or that one only is moving, or finally, that both
are moving in the same direction, the one faster than the other.
Experience and habit here again suggest the interpretation which is most
easy, and not unfrequently produce illusion. Thus, when we watch clouds
scudding over the face of the moon, the latter seems moving rather than
the former, and the illusion only disappears when we fix the eye on the
moon and recognize that it is really stationary. The probable reason of
this is, as Wundt suggests, that experience has made it far easier for
us to think of small objects like the moon moving rapidly, than of large
masses like the clouds.[36]

The perception of distance, still more than that of direction, is liable
to be illusory. Indeed, the visual recognition of distance, together
with that of solidity, has been the great region for the study of "the
deceptions of the senses." Without treating the subject fully here, I
shall try to describe briefly the nature and source of these
illusions.[37]

Confining ourselves first of all to near objects, we know that the
smaller differences of distance in these cases are, if the eyes are at
rest, perceived by means of the dissimilar pictures projected on the two
retinas; or if they move, by this means, together with the muscular
feelings that accompany different degrees of convergence of the two
eyes. This was demonstrated by the famous experiments of Wheatstone.
Thus, by means of the now familiar stereoscope, he was able to produce a
perfect illusion of relief. The stereoscope may be said to introduce an
exceptional state of things into the spectator's environment. It
imitates, by means of two flat drawings, the dissimilar retinal pictures
projected by a single solid receding object, and the lenses through
which the eyes look are so constructed as to compel them to converge as
though looking on a single object. And so powerful is the tendency to
interpret this impression as one of solidity, that even though we are
aware of the presence of the stereoscopic apparatus, we cannot help
seeing the two drawings as a single solid object.

In the case of more remote objects, there is no dissimilarity of the
retinal pictures or feelings of convergence to assist the eye in
determining distance. Here its judgment, which now becomes more of a
process of _conscious_ inference, is determined by a number of
circumstances which, through