tion originates is endowed with specific sensory action, so the irritation of that part can alone produce impressions of vision; but this irritation may be imparted to it by other parts of the brain, or by other nerves. It has been already stated that irritations of the brain produced by narcotics are transmitted only by proximity to the terminal extremity of the optic nerve. At the same time, it may be that the irritation proceeds from another nerve, from a nerve of touch or of hearing, and, penetrating to the brain, affects it so strongly as to send on the concussion to the optic centre. It is this that takes place when, after having listened to disagreeable sounds, you are seized with certain sensations in the nerves of touch, for instance, in those of the teeth; or, having gazed into the bright light, you become aware of a tickling sensation in the nose, causing you to sneeze. In a word, it is here a question of so-called sympathy, to be explained by transmission of the irritation from one nerve to the other.
The disposition to such sympathetic sensation is increased by a general irritability of the nervous system; while, in a calmer state of the nerves, the excitations run more regularly in the paths directly affected by the originating causes. In this manner, those indirectly provoked visual impressions which preponderate in circumstances of sickness and disease are augmented.
In these indirect visual sensations, as in the direct excitation of the mechanism of the visual nerve, only subjective sight has been treated of, cut off from from every relation with the outer world. We are quite ready to attach credit to the fact that, at exhibitions of somnambulism, when the natural irritability of nervously-disposed individuals is heightened, subjective visual impressions are produced in an unusual degree. Should, however, any connection with surrounding objects be founded on these results, or any transmission of the specific sensory action into other parts, as, for instance, transmitting to the skin of the abdomen the power of producing objective visual perceptions, such as are necessary for reading, these assurances are to be ranged in the same category of physiological blunders as the Münchausen hunting-story.
By what means, then, does the mechanism of the visual nerve, which we have hitherto regarded merely as the instrument of subjective sight, become a practical bridge between our ideas and the outer world, and a medium of the true and accredited operation of the senses? I answer, by a normal relation to a definite irritation proceeding from an object. This, which we might call the adequate sensory irritation, is light.
Let us consider the general relation between light and the organ of sight. Unable to discover with certainty the nature of light, it is explained in physics as being the undulating motion of an elastic body called ether, diffused throughout the universe. According to this, the irritation by light represents the shock of the undulations of ether on
