The function of the brain, which may be regarded as the bulbous root of a plant whose branches grow downwards, is twofold: to affect, and to be affected. In its active and positive condition it affects the whole of the vital and muscular processes in the body, finding expression in vital action. In its passive and negative state it is affected by impressions coming to it in different ways through the sense organs, resulting in nervous and mental action. These two functions are interdependent. It is the latter or afferent function with which we are now concerned. The range of our sense-perceptions puts us momentarily in relations with the material world, or rather, with a certain portion of it. For we by no means sense all that is sensible, and, as I have already indicated, our sense impressions are often delusive. The gamut of our senses is very limited, and also very imperfect both as to extent and quality. Science is continually bringing new instruments into our service, some to aid the senses, others to correct them. The microscope, the microphone, the refracting lens are instances. It used to be said with great certainty that you cannot see through a brick wall, but by means of X-rays and a fluorescent screen it is now possible to do so. I have seen my own heart beating as its image was thrown on the screen by the Rontgen rays. Many insects, birds and animals have keener perceptions in some respects than man. Animalculae and microbic life, themselves microscopic, have their own order of sense-organs related to a world of life beyond our ken. These observations serve to emphasise the great limitation of our senses, and also to enforce the fact that Nature does not cease to exist where we cease to perceive her. The recognition of this fact has been so thoroughly appreciated by thoughtful people as to have opened up the question as to what these human limitations may mean and to what degree they may extend. We know what they mean well enough: the history of human development is the sequel to natural evolution, and this development could never have had place apart from the hunger of the mind and the consequent breaking down of sense limitations by human invention. As to the extent of our limitations it has been suggested that just as there are states of matter so fine as to be beyond the range of vision, so there may be others so coarse as to be below the sense of touch. We cannot, however, assert anything with certainty, seeing that proof must always require that a thing must be brought within our range of perception before we can admit it as fact. The future has many more wonderful revelations in store for us, no doubt. But there is really nothing more wonderful than human faculty which discovers these things in Nature, things that have always been in existence but until now have been outside our range of perception. The ultra-solid world may exist.