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." MEMORY IMAGE AND ITS REVIVAL 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. DEATH-STRUGGLE AND MEMORY REVIVAL