on the light-adapted eye. These facts do not offer
any serious logical obstacle to the formulation of
statements of intensity relation, as all such are sim-
ply required to specify that the relation holds only
for a uniform condition of the sense-organ and
nervous connections; but they introduce serious
practical difficulties, because it is not always possi-
ble to ascertain whether the condition of an organ
is uniform during any given period of experimenta-
tion.

The difficulty of the discovery of definite inten-
sity relationships of sensation and stimulus is in-
creased by the difficulty of estimation. Direct meas-
urement of sensation-intensities is impossible. We
can only compare one sensation with another, and
determine which of the two is more intense. And
this determination is strictly relative. The apparent
intensity of a sensation is affected by other sensa-
tions, aside from any change in the actual intensity
of the sensation. For instance: a candle burning
near a coal-oil lamp seems dimmer than the same
candle burning beside the flame of a minute gas-
jet; yet, if the experiment is performed in moder-
ate daylight, the candle flame is practically as
bright sensationally in the one case as in the other.
When the time factor enters, and sensations pres-

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