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.

_Exceptional Relation of Stimulus to Organ._

A transition from the preceding to the following class of illusions is
to be met with in those errors which arise from a very exceptional
relation between the stimulus and the organ of sense. Such a state of
things is naturally interpreted by help of more common and familiar
relations, and so error arises.

For example, we may grossly misinterpret the intensity of a stimulus
under certain circumstances. Thus, when a man crunches a biscuit, he has
an uncomfortable feeling that the noise as of all the structures of his
head being violently smashed is the same to other ears, and he may even
act on his illusory perception, by keeping at a respectful distance from
all observers. And even though he be a physiologist, and knows that the
force of sensation in this case is due to the propagation of vibrations
to the auditory centre by other channels than the usual one of the ear,
the deeply organized impulse to measure the strength of an external
stimulus by the intensity of the sensation asserts its force.

Again, if we turn to the process of perceptional construction properly
so called, the reference of the sensation to a material object lying in
a certain direction, etc., we find a similar transitional form of
illusion. The most interesting case of this in visual perception is that
of a disturbance or displacement of the organ by external force. For
example, an illusory sense of direction arises by the simple action of
closing one eye, say the left, and pressing the othe