We have now come to the end of our discourse upon the theories of odour, and it must be confessed that we are still very much in the dark as to the nature of the odorous, and as to the manner in which it excites the olfactory organ to activity. Still more mysterious, however, is the process by which the physical quality of odour becomes the sensation of the mind we call smell. The transmutation of a physical quality into a sensation is indeed the great mystery of all our senses. Olfaction is not the only one before which we throw up our hands, and this in spite of the detailed and voluminous information which modern physiology, neurology, and psychology place at our disposal, perhaps less in spite of this information than because of it, seeing that the further our knowledge extends the wider seems the unknown realm beyond. Our science is an ever-expanding sphere, no doubt, but it is expanding into the infinite. How is it that the rhythmic vibration of matter becomes what we call “sound,” or the rhythmic vibration of the ether “light”? How does the physical pass into and become part of the psychic? According to recent teaching, the physical can be followed as such from the sensory end-organ itself as far as the first synapse, or junction with the neurone. But there something happens; ... then it reappears in a new guise, vibration becomes sensation, the physical psychic, the objective subjective, the real ideal, the dead alive! In that brief tumble of time what a miraculous transformation! Modern science has cleared up much of the mystery of the objective world, and although it may be far from the end of its search, although, indeed, the search, one must think, can never entirely elucidate the dense obscurity that envelops us on every side, dark as a starless night around a candle, yet we already know this much, that the real world is very different from the world depicted for us by our senses. Only a little imagination is needed to convey us out of the magic circle into which we have been born, and what a strange universe do we then find ourselves in! Entangled in a meshwork of space-time and permeated by whirling maelstroms of varied and innumerable oscillations, we lose all hold on reality in the very act of grasping it. But although we do possess some sort of vague notion as to the constitution of the outer universe, before the inner we stand ignorant and speechless. Regarded as a machine, the brain, it is true, like the world without, is reluctantly yielding up its secrets one by one. We are learning how it works as a chemical factory, as a physical power-house, so that already we can surmise that here also we have probably to deal with a multiplicity of vibrations, of exquisitely minute transformations of energy, of involved intercommunications, of deft though intricate associations, of rapid yet permanent recordings and registrations.