increase in the
light than if we add one more light when twenty are already burning.

A third great result of this line of study is that different sorts of
perception are very unequal in their fineness and reliability.
Perception of brightness is about the keenest, as under favorable
conditions a difference of one part in one hundred can here be
perceived with very few errors. Visual perception of length of line is
good for about one part in fifty, perception of lifted weight for
about one part in ten, perception of loudness of sound for about one
part in three. But the perception of small differences in the pitch of
musical tones is keener still, only that, not following Weber's law in
the least, it cannot be expressed in the same way. A person with a
good ear for pitch can distinguish with very few errors between two
tones that differ by only one vibration per second, and can perceive
this same absolute difference equally well, whether the total
vibration rate is 200, 400, or 800 vibrations per second.

Illusions

An error of perception is often called an "illusion", though this term
is commonly reserved for errors that are large and curious. When one
who is being awakened by a bell perceives it as a tom-tom, that is an
illusion. An {451} illusion consists in responding to a sensory
stimulus by perceiving something that is not really there. The
stimulus is there, but not the fact which it is taken to indicate.
Illusion is false perception.

The study of illusions is of value, not only as showing how far a
given kind of perception can be trusted, but also as throwing light on
the process of perception. When a process goes wrong, it sometimes
reveals its inner mechanism more clearly than when everything is
running smoothly. Errors of any kind are meat to the psychologist.

Illusions may be classified under several headings according to the
factors that are operative in causing the deception.

1. Illusions due to peculiarities of the sense organs.

Here the stimulus is distorted by the sense organ and so may easily be
taken as the sign of an unreal fact.

Separate the points of a pair of compasses by about three-quarters of
an inch, and draw them across the mouth, one point above it and the
other below; you will get the illusion of the points separating as
they approach the middle of the mouth (where the sensory nerve supply
is greatest), and coming together again as they are drawn to the cheek
at the other