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