gives the peculiarity to the case thus wrongly interpreted has been
referred to the organism. In the illusions to which we now pass, it will
be referred to the environment. At the same time, it is plain that there
is no very sharp distinction between the two classes. Thus, the visual
illusion produced by pressing the eyeball might be regarded not only as
the result of the organic law of the "specific energy" of the nerves,
but, with almost equal appropriateness, as the consequence of an
exceptional state of things in the environment, namely, the pressure of
a body on the retina. As I have already observed, the classification
here adopted is to be viewed simply as a rough expedient for securing
something like a systematic review of the phenomena.




CHAPTER V.

ILLUSIONS OF PERCEPTION--_continued_.

A. _Passive Illusions (b) as determined by the Environment._


In the following groups of illusion we may look away from nervous
processes and organic disturbances, regarding the effect of any external
stimulus as characteristic, that is, as clearly marked off from the
effects of other stimuli, and as constant for the same stimulus. The
source of the illusion will be looked for in something exceptional in
the external circumstances, whereby one object or condition of an object
imitates the effect of another object or condition, to which, owing to a
large preponderance of experience, we at once refer it.