The measurement of the absolute threshold, though perhaps not as im-
portant for the development of psychology as Fechner's insights into
difference thresholds, has led to many significant advances in under-
standing sensory systems. Before considering in detail the various psy-
chophysical methods for measuring DL's and RL's, let us consider
examples of how measuring absolute thresholds has facilitated our un-
derstanding of vision, audition, touch, and olfaction.

The eye is an extremely light-sensitive instrument capable of responding
to almost unbelievably small amounts of light energy. However, a simple
answer cannot be given to the question: How sensitive is the eye to light?
The absolute sensitivity of the eye cannot be gauged by a single threshold
value, since the minimum amount of light necessary for vision has been
found to depend on the conditions of stimulation. Therefore, the absolute
sensitivity of the visual system is best understood by examining the
functional relationships between the absolute threshold and the conditions
that determine its value.

The value of the absolute threshold depends upon previous stimula-
tion. Exposing the eye to intense light greatly decreases the absolute
sensitivity of the eye. Sensitivity is recovered gradually if the eye is
subsequently kept in darkness. Nearly complete recovery of sensitivity
occurs after about one hour in the dark. The dark adaptation curve is
traced out by measuring an observer's absolute threshold periodically

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