connected with the different senses.

It may be said once for all that in thus marking off from one another
certain groups of illusion, I am not unmindful of the fact that these
divisions answer to no very sharp natural distinctions. In fact, it will
be found that one class gradually passes into the other, and that the
different characteristics here separated often combine in a most
perplexing way. All that is claimed for this classification is that it
is a convenient mode of mapping out the subject.




CHAPTER IV.

ILLUSIONS OF PERCEPTION--_continued_.

A. _Passive Illusions (a) as determined by the Organism._


In dealing with the illusions which are related to certain peculiarities
in the nervous organism and the laws of sensibility, I shall commence
with those which are connected with certain limits of sensibility.


_Limits of Sensibility._

To begin with, it is known that the sensation does not always answer to
the external stimulus in its degree or intensity. Thus, a certain amount
of stimulation is necessary before any sensation arises. And this will,