would lead to partial irrigation, and
therefore to suppression of action in some parts of the brain, and
that this is really the case seems to be proved by the absence of
common sense during dreams.

A convenient distinction is made between hallucinations and illusions.
Hallucinations are defined as appearances wholly due to fancy;
illusions, as fanciful perceptions of objects actually seen. There
is also a hybrid case which depends on fanciful visions fancifully
perceived. The problems we have to consider are, on the one hand,
those connected with "_induced_" vision, and, on the other hand,
those connected with the interpretation of vision, whether the
vision be _direct_ or _induced_.

It is probable that much of what passes for hallucination proper
belongs in reality to the hybrid case, being an illusive
interpretation of some induced visual cloud or blur. I spoke of the
ever-varying patterns in the optical field; these, under some slight
functional change, may become more consciously present, and be
interpreted into fantasmal appearances. Many cases could be adduced
to support this view.

I will begin with illusions. What is the process by which they are
established? There is no simpler way of understanding it than by
trying, as children often do, to see "faces in the fire," and to
carefully watch the way in which they are first caught. Let us call
to mind at the same time the experience of past illnesses, when the
listless gaze wandered over the patterns on the wall-paper and the
shadows of the bed-curtains, and slowly evoked the appearances of
faces and figures that were not easily laid again. The process of
making the faces is so rapid in health that it is difficult to
analyse it without the recollection of what took place more slowly
when we were weakened by illness. The first essential element in
their construction is, I believe, the smallness of the area covered
by the glance at any instant, so that the eye has to travel over a
long track before it has visited every part of the object towards
which the attention is directed generally. It is as with a plough,
that must travel many miles before the whole of a small field can be
tilled, but with this important difference--the plough travels
methodically up and down in parallel furrows; the eye wanders in
devious curves, with abrupt bends, and the direction of its course
at any instant depends on four causes: (1) on the easiest sequence
of muscular motion, spe