nd and brushing against the overcoat
as the person walked; this series occupied not over five seconds. On
touching an object in the dark, you may feel it as one thing and
another till some response is aroused that fits the known situation
and so satisfies you. Such trial and error perception can be observed
very frequently if one is on the watch for {425} psychological
curiosities; and it justifies the distinction between sensation and
perception, since the sensation remains virtually unchanged while
perception changes.

Another sort of shifting perception is seen in looking steadily at the
"ambiguous figures" which were considered in the chapter on attention,
the cube, staircase, and others; and the "dot figures" belong here as
well. [Footnote: See p. 252.] In these cases the stimulus arouses two
or more different perceptions, alternately, while the sensation
remains almost or quite unchanged.

Perception and Image

The experiment with ambiguous figures also gives an answer to the
question whether perception consists in the addition of recalled
memory images to the sensations aroused by the present stimulus. If
that were so, you should, when you see the upper side of the flight of
stairs, see them as wooden stairs or stone stairs, as carpeted or
varnished, with shadows on them such as appear on a real flight of
stairs, with a railing, or with some other addition of a similar
nature; and, when the appearance changes to that of the under side of
a flight of stairs, the colors, shadows, etc., should change as well.
The usual report is that no such addition can be detected, and that
the subject sees no filling-in of the picture, but simply the bare
lines--only that they seem at one moment to be the bare outline of the
upper side, and at another moment an equally bare outline of the lower
side, of a flight of stairs.

So again, when you "hear the street car", you do not ordinarily, to
judge from the reports of people who have been asked, get any visual
or kinesthetic image of the car, but you simply know the car is there.
You will quite {426} possibly get some such image, if you _dwell_ on
the fact of the car's being there, just as some persons, in talking to
a friend over the telephone, have a visual image of the friend. There
is no reason why such images should not be aroused, but the question
is whether they are essential to perception of the fact, and whether
they occur before or after the fact is perceived. Ofte