or in a different key, or even
if it should stop altogether, it would "wake us up" with a start. The
change in the stimulus must not be too gradual if it is to be
effective, it must have a certain degree of suddenness. It may be a
change in intensity, a becoming suddenly stronger or weaker; or it may
be change in quality, as in tone, or {246} color, or odor; or it may
be a change in position, a movement in space. When one who is holding
our arm gives it a sudden squeeze to attract our attention, that is a
change of intensity; when we step from the bank into the water, the
sudden change from warmth to cold, that gets our attention without
fail, is a change of quality; and something crawling on the skin
attracts attention by virtue of its motion. Anything moving in the
field of view is also an unfailing stimulus to attention.

_Strength_, or high intensity of a stimulus, is another important
factor of advantage. Other things being equal, a strong stimulus will
attract attention before a weak one. A loud noise has the advantage
over a low murmur, and a bright flash of light over a faint twinkle.

In the case of visible objects, size has about the same effect as
intensity. The large features of the landscape are noticed before the
little details. The advertiser uses large type, and pays for big space
in the newspaper, in the effort to attract the attention of the
reader.

[Footnote: Often he pays more than the space is worth; at least
doubling the size of his "ad" will not, on the whole, double the
amount of attention he gets, or the number of readers whose
attention he will catch. The "attention value" of an advertisement
has been found by Strong to increase, not as fast as the increase in
space, but about as the square root of the space occupied.]

Another similar factor is _repetition_. Cover a billboard with several
copies of the same picture, and it attracts more attention than a
single one of the pictures would. Repeat a "motive" in the decoration
of a building, and it is more likely to be noticed. Repeat a cry or
call several times, and after a while it may be noticed, though not at
first. The "summation of stimuli" has much the same effect as
increasing the intensity of a single stimulus.

If, however, a stimulus is repeated or continued for a long time, it
will probably cease to hold attention, because of its {247} monotony,
or, in other words, because it lacks the element of change.

_Striking q