sensations which arise from a stimulation of the peripheral regions of
the nerve may be called illusions rather than hallucinations. Or, if
they must be called hallucinations, they may be distinguished as
"peripheral" from those "central" hallucinations which arise through an
internal automatic excitation of the sensory centre. It is plain from
this that the region of subjective sensation is an ambiguous region,
where illusion and hallucination mix and become confused. To this point
I shall have occasion to return by-and-by.

I have now probably said enough respecting the illusions that arise
through the fact of there being fixed limits to our sensibility. The
_rationale_ of these illusions is that whenever the limit is reached, we
tend to ignore it and to interpret the impression in the customary way.


_Variations of Sensibility._

We will now pass to a number of illusions which depend on something
variable in the condition of our sensibility, or some more or less
exceptional organic circumstance. These variations may be momentary and
transient or comparatively permanent. The illusion arises in each case
from our ignoring the variation, and treating a given sensation under
all circumstances as answering to one objective cause.

First of all, the variation of organic state may affect our mental
representation of the strength of the stimulus or external cause. Here
the fluctuation may be a temporary or a permanent one. The first case is
illustrated in the familiar example of taking a room to be brighter
than it is when emerging from a dark one. Another striking example is
that of our sense of the temperature of objects, which is known to be