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 experience and association, have become the
signs of differences of depth in space. Among these are the degree of
indistinctness of the impression, the apparent or retinal magnitude (if