It was supposed that nervous impulse, which, must necessarily form the
basis of sensation, was beyond any conceivable power of visual scrutiny.
But Dr. Bose showed that this impulse is actually attended by change of
form, and is, therefore capable of direct observation. He also showed
that the disturbance, instead of being single, is of two different
kinds--_viz._, one of expansion (positive) and the other of contraction
(negative)--and that, when the stimulus is feeble, the positive is
transmitted, and, when the stimulus is stronger, both positive and
negative are transmitted, but the negative, however, being more intense,
masks the positive. He identified the wave of expansion travelling along
the nerve with the tendency to pleasure, and the wave of contraction,
with the tendency to pain. It thus appears that all pain contains an
element of pleasure, and that pleasure, if carried too far becomes
pain--that "the tone of our sensation is determined by the intensity of
nervous excitation that reaches the central perceiving organ."