vibration which will arouse a sensation of tone
is about twelve meters, and the shortest about
one-half of a millimeter. These magnitudes are
thresholds.

If we consider ether-vibrations of sufficient energy
falling on a normal retina, commencing with waves
too long to be visible and steadily decreasing in
length, we can conceive that when a certain wave
length is reached a sensation of light will occur; it
will "enter the mind" or "enter consciousness" at
that point. This point at which the sensation
"steps in" is accordingly dubbed the threshold.
In the cases of sound and light there are two thresh-
olds; you can approach the limits of sensibility
from either direction; and the same may be true of
smell. There may be gases which are odorless be-
cause their molecular weights are too great.

These thresholds are sometimes--and incorrectly
--called qualitative. There is no such thing as a
qualitative threshold. Clearness and accuracy can
be attained by referring to the auditory wave-fre-
quency thresholds, the visual wave-length thresh-
olds, etc.

The second sort of thresholds occur in the series
of intensities of stimulations, and this sort of thresh-
old is always meant by the term when not expressly

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