ponse only
that it happens to be incorrect. If the present stimulus has something
in {453} common with the stimulus which has in the past aroused a
certain perception, we may make the same response now as we did
before--especially, of course, if the present mantel set favors this
response.

[Illustration: Fig. 67.--The Ladd-Franklin illusion of monocular
perspective. Close one eye, and hold the book so that the other eye is
at the common center from which the lines radiate; this center is
about 5 inches from the figure. Hold the book horizontally, and just a
little below the eye.]

A good instance of this type is the "proofreader's illusion", so
called, perhaps, because the professional proofreader is less subjcet
to it than any one else. The one most subject to it is the author of a
book, for whom it is almost impossible to find every misspelled word
and other typographical error in reading the proof. Almost every book
comes out with a few such errors, in spite of having been scanned
repeatedly by several people. A couple of misprints have purposely
been left in the last few lines for the reader's benefit. If the word
as printed has enough resemblance to the right word, it arouses the
same percept and enables the reader to get the sense and pass on
satisfied. {454} Before we began to pore over books and pictures, the
lines that we saw usually were the outlines of solid objects, and now
it requires only a bare diagram of lines to arouse in us the
perception of a solid object seen in perspective. An outline drawing,
like those of the cube and staircase used to illustrate ambiguous
perspective, is more readily seen as a solid object than as a flat
figure.

[Illustration: Fig. 68.--Aristotle's illusion.]

Another illusion of this general type dates away back to Aristotle.
Cross two fingers, perhaps best the second and third, and touch a
marble with the crossed part of both fingers, and it seems to be two
marbles; or, you can use the side of your pencil as the stimulus. In
the customary position of the fingers, the stimuli thus received would
mean two objects.

A much more modern illusion of the same general type is afforded by
the moving pictures. The pictures do not actually show an object in
motion; they simply show the object in a series of motionless
positions, caught by instantaneous photography. The projector shows
the series of snap-shots in rapid succession, and conceals them by a
shutter while they