Dr. Bose next pointed out that there remains, for every response, a
certain residual effect. A substance, which has responded to a given
stimulus, retains, as an after-effect, a 'latent impression' of that
stimulus and this 'latent impression' is capable of subsequent revival
by bringing about the original condition of excitation. The impress made
by the action of stimulus, though it remains latent and invisible, can
be revived by the impact of a fresh excitatory impulse.
Experimenting with a metallic _leaf_, Dr. Bose demonstrated the revival
of a latent impression under the action of diffused stimulus. The
investigation by Dr. Bose on the after-effects of stimulus has thrown
some light on the obscure phenomenon, of 'memory.' It appears that, when
there is a mental revival of past experience, the diffuse impulse of the
'will' acts on the sensory surface, which contains the latent impression
and re-awakens the image which appears to have faded out. Memory is
concerned, thus, with the after-effect of an impression induced by a
stimulus. It differs from ordinary sensation in the fact that the
stimulus which evokes the response, instead of being external and
objective, is merely psychic and subjective.
Dr. Bose has, by experimental devises, shown the possibility of tracing
'memory-impression' backwards even in inorganic matter, such latent
impression being capable of subsequent revival. An investigation of the
after-effects of stimulus, on living tissues would open out the great
problem of the influence of past events on our present condition.