he 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,
of course, be greater when there is little or no attention directed to
the impression, that is to say, no co-operating central reaction. Thus
it happens that slight stimuli go overlooked, and here illusion may have
its starting-point. The most familiar example of such slight errors is
that of movement. When we are looking at objects, our ocular muscles are
apt to execute very slight movements which escape our notice. Hence we
tend, under certain circumstances, to carry over the retinal result of
the movement, that is to say, the impression produced by a shifting of
the parts of the retinal image to new nervous elements, to the object
itself, and so to transform a "subjective" into an "objective" movement.
In a very interesting work on apparent or illusory movements, Professor
Hoppe has fully investigated the facts of such slight movements, and
endeavoured to specify their causes.[17]

Again, even when the stimulus is sufficient to produce a conscious
impression, the degree of the feeling may not represent the degree of
the stimulus. To take a very inconspicuous case, it is found by Fechner
that a given increase of force in the stimulus produces a less amount of
difference in the resulting sensations when the original stimulus is a
powerful one than when it is a feeble one. It follows from this, that
differences in the degree of our sensations do not exactly correspond to
objective differences. For example, we tend to magnify the differences
of light among objects, all of which are feebly illuminated, that is to
say, to see them much more removed from one another in point of
brightness than when they are more strongly illuminated. Helmholtz
rel