side.

Under this same general head belong also after-images and contrast
colors, and also double vision whenever for any reason the two eyes
are not accurately converged upon an object. The fact that a vertical
line appears longer than an equal horizontal is supposed to depend
upon some peculiarity of the retina. Aside from the use of this class
of illusions in the detailed study of the different senses, the chief
thing to learn from them is they so seldom are full-fledged illusions,
because they are ignored or allowed for, and not taken as the signs of
facts. An after-image would constitute a genuine illusion if it were
taken for some real {452} thing out there; but as a matter of fact,
though after-images occur very frequently--slight ones practically
every time the eyes are turned--they are ignored to such an extent
that the student of psychology, when he reads about them, often thinks
them to be something unusual and lying outside of his own experience.
The same is true of double images. This all goes to show how strong is
the tendency to disregard mere sensation in the interest of getting
objective facts.

2. Illusions due to preoccupation or mental set.

When an insane person hears the creaking of a rocking-chair as the
voice of some one calling him bad names, it is because he is
preoccupied with suspicion. We might almost call this an
hallucination,[Footnote: See p. 375.] since he is projecting his own
auditory images and taking them for real sensations; it is, at any
rate, an extreme instance of illusion. In a milder form, similar
illusions are often momentarily present in a perfectly normal person,
as when he is searching for a lost object and thinks he sees it
whenever anything remotely similar to the desired object meets his
eyes; or as when the mother, with the baby upstairs very much on her
mind, imagines she hears him crying when the cat yowls or the
next-door neighbors start their phonograph. The ghost-seeing and
burglar-hearing illusions belong here as well. The mental set
facilitates responses that are congruous with itself.

3. Illusions of the response-by-analogy type.

This is probably the commonest source of everyday illusions, and the
same principle, as we have seen, is operative in a host of correct
perceptions. Perceiving the obliquely presented rectangle as a
rectangle is an example of correct perception of this type. Perceiving
the buzzing of a fly as an aeroplane is the same sort of res