ediate knowledge, indeed, of their fundamental
properties than sight. What makes the eye so vastly superior to the
organ of touch as an instrument of perception, is first of all the range
of its action, taking in simultaneously a large number of impressions
from objects at a distance as well as near; and secondly, though this
may seem paradoxical, the fact that it gives us so much indirectly, that
is, by way of association and suggestion. This is the interesting side
of visual perception, that, owing to the vast complex of distinguishable
sensations of light and colour of various qualities and intensities,
together with the muscular sensations attending the varying positions of
the organ, the eye is able to recognize at any instant a whole external
world with its fundamental properties and relations. The ear comes next
to the eye in this respect, but only after a long interval, since its
sensations (even in the case of musical combinations) do not
simultaneously order themselves in an indefinitely large group of
distinguishable elements, and since even the comparatively few
sensations which it is capable of simultaneously receiving,
being altogether passive--that is to say, having no muscular
accompaniments--impart but little and vague information respecting the
external order. It is plain, then, that in the study of illusion, where
the indirectly known elements are the thing to be considered, the eye,
and after this the ear, will mostly engage our attention.[14]

So much it seemed needful to say about the mechanism of perception, in
order to understand the slight disturbances of this mechanism that
manifest themselves in sense-illusion. It may be added that our study of
these illusions will help still further to elucidate the exact nature of
perception. Normal mental life, as a whole, at once illustrates, and is
illustrated by, abnormal. And while we need a rough provisional theory
of accurate perception in order to explain illusory perception at all,
the investigation of this latter cannot fail to verify and even render
more complete the theory which it thus temporarily adopts.

_Illusions of Perception._

With this brief psychological analysis of perception to help us, let us
now pass to the consideration of the errors incident to the process,
with a view to classify them according to their psychological nature and
origin.

And here there naturally arises the question, How shall we define an
illusion of percept