the same general field, as of taste or vision, it is evident that there is a still more
fundamental difference existing between the various fields. One can, for
example, compare red with blue or sweet with sour, and tell which quality he
prefers. But let him try to compare red with sweet, or blue with sour, and the
quality-difference is so profound that there seems to be no basis for comparison.
Differences in intensity of sensation are familiar to every person who prefers two
lumps of sugar rather than one lump in his coffee; the sweet is of the same
quality in either case, but differs in intensity. In every field of sensation, the
intensity may proceed from the smallest amount to the greatest amount
discernible. In general, the intensity of the sensation depends on the intensity of
the stimulus, though the condition of the sense-organ as regards fatigue or
adaptation to the stimulus has its effect. It is obvious that a stimulus may be too
weak to produce any sensation; as, for example, a few grains of sugar in a cup of
coffee or a few drops of lemon in a quart of water could not be detected. It is
also true that the intensity of the stimulus may be so great that an increase in
intensity produces no effect on the sensation; as, for example, the addition of
sugar to a solution of saccharine would not noticeably increase its sweetness.
The lowest and highest intensity points of sensation are called the lower and
upper limen, or threshold, respectively.
By extensity is meant the space-differences of sensations. The touch of the point
of a toothpick on the skin has a different space quality from the touch of the flat
end of a pencil. Low tones seem to have more volume than high tones. Some
pains feel sharp and others dull and diffuse. The warmth felt from spreading the
palms of the hands out to the fire has a "bigness" not felt from heating one
solitary finger. The extensity of a sensation depends on the number of nerve
endings stimulated.
The duration of a sensation refers to the time it lasts. This must not be confused
with the duration of the stimulus, which may be either longer or shorter than the
duration of the sensation. Every sensation must exist for some space of time,
long or short, or it would have no part in consciousness.