ust be nearly over. These are scarcely errors of sense, but they are
errors of perception.

Where we tend to err in one certain direction from the truth, as in
the examples just cited, psychology speaks of a "constant error", and
evidently the knowledge of such constant errors is of importance
wherever the facts are of importance. In a court of law, a witness
often has to testify regarding the length of time occupied by some
event, and a knowledge of the constant errors in time perception would
therefore be of considerable legal importance. They would need to be
worked out in considerable detail, since they differ according to the
desires and attitude of the witness at the time of the event.

Besides constant errors, there are accidental or variable errors, due
to slight momentary causes. Both constant and variable errors can be
illustrated by a series of shots at a target. The variable error is
illustrated by the scatter of {448} the hits, and the constant error
by the excess of hits above the bull's-eye, or below, or to the right
or left. The constant error can be corrected, once you know what it
is; if results show that you tend to shoot too high, you can
deliberately aim lower. But the variability of any performance cannot
be eliminated except by long practice, and not altogether even then.

[Illustration: Fig. 66.--Constant error and scatter in hitting at a
target. The little circle was the target, but the center of the actual
distribution of the attempts lies at the cross, which was drawn in
afterwards. The constant error could be stated by saying that the
center of distribution was so far from the target, and in such and
such a direction. The scattering of the attempts can be measured
also.]

Experimental psychology has taken great pains in measuring the
accuracy of different sorts of perception. How small a difference in
length can be perceived by the eye, how small a difference of weight
by the hand--these are sample problems in this line.

For example, to measure the fineness with which weights can be
perceived when "hefted" in the hand, you take two objects that are
alike in size and appearance but differing slightly in weight, and
endeavor to decide which is the heavier just by lifting them. You try
repeatedly and keep track of the number of errors, using this number
as a measure of the accuracy of perception. Now, if one weight were
twice as heavy as the other (one, for example, weighing 100 grams