row the shifting of attention
into clearer relief. Look fixedly at a single letter written on a
blank sheet of paper, and notice how one part after another of the
letter stands out; notice also that attention does not stick
absolutely to the letter, since thoughts obtrude themselves at
intervals.

[Illustration:

O O O
O O
O O O
O O
O O O

Fig. 42.--A dot figure, from Sanford. Look steadily at it.]

Or, make a "dot figure", composed of six or eight or more dots
arranged either regularly or irregularly, and look steadily at the
collection. Probably you will find that the dots seem to fall into
figures and groups, and that the grouping changes frequently.
Objectively, of course, the dots are grouped in one way as much as
another, so that any particular grouping is your own doing. The
objective stimulus, in other words, is capable of arousing several
grouping reactions on your part, and does arouse different reactions
one after another

Shifting also appears in looking at an {253} "ambiguous figure", drawn
so as to represent equally well a solid object in either of two
different positions. The transparent cube, showing near and far edges
alike, is a good example. Look steadily at such a drawing, and the
cube will appear to shift its position from time to time. Numerous
such figures can be constructed; the most celebrated is the ambiguous
staircase. Look steadily at it, and suddenly you see the under side of
a flight of stairs, instead of the upper; and if you keep on looking
steadily, it shifts back and forth between these two positions.

[Illustration: Fig. 43.--The ambiguous cube figure.]

[Illustration: Fig. 44.--The ambiguous staircase figure.]

A still more striking case of shifting goes by the name of "binocular
rivalry", and occurs when colors or figures that we cannot combine
into a single picture are presented, {254} one to one eye, and the
other to the corresponding part of the other retina. Hold red glass
close in front of one eye and blue before the other, and look through
both at once towards a bright background, and you will see red part of
the time and blue part of the time, the two alternating as in the case
of ambiguous figures.

[Illustration: Fig. 45.--Another ambiguous figure, which can be seen
in three ways.]

The stereoscope is a great convenience in applying inconsistent
stimuli to the two eyes, and by aid of this instrument a great variety
of e