Sight is an _active_, hearing a _passive_ sense. Therefore sounds affect our mind in a disturbing and hostile manner, and indeed they do so the more in proportion as the mind is active and developed; they distract all thoughts and instantly destroy the power of thinking. On the other hand, there is no analogous disturbance through the eye, no direct effect of what is seen, _as such_, upon the activity of thought (for naturally we are not speaking here of the influence which the objects looked at have upon the will); but the most varied multitude of things before our eyes admits of entirely unhindered and quiet thought. Therefore the thinking mind lives at peace with the eye, but is always at war with the ear. This opposition of the two senses is also confirmed by the fact that if deaf and dumb persons are cured by galvanism they become deadly pale with terror at the first sounds they hear (Gilbert’s “_Annalen der Physik_,” vol. x. p. 382), while blind persons, on the contrary, who have been operated upon, behold with ecstasy the first light, and unwillingly allow the bandages to be put over their eyes again. All that has been said, however, can be explained from the fact that hearing takes place by means of a mechanical vibration of the nerve of hearing which is at once transmitted to the brain, while seeing, on the other hand, is a real _action_ of the retina which is merely stimulated and called forth by light and its modifications; as I have shown at length in my physiological theory of colours. But this whole opposition stands in direct conflict with that coloured-ether, drum-beating theory which is now everywhere unblushingly served up, and which seeks to degrade the eye’s sensation of light to a mechanical vibration, such as primarily that of hearing actually is, while nothing can be more different than the still, gentle effect of light and the alarm-drum of hearing. If we add to this the remarkable circumstance that although we hear with two ears, the sensibility of which is often very different, yet we never hear a sound double, as we often see things double with our two eyes, we are led to the conjecture that the sensation of hearing does not arise in the labyrinth or in the cochlea, but deep in the brain where the two nerves of hearing meet, and thus the impression becomes simple. But this is where the _pons Varolii_ encloses the _medulla oblongata_, thus at the absolutely lethal spot, by the injury of which every animal is instantly killed, and from which the nerve of hearing has only a short course to the labyrinth, the seat of acoustic vibration. Now it is just because its source is here, in this dangerous place, in which also all movement of the limbs originates, that we start at a sudden noise; which does not occur in the least degree when we suddenly see a light; for example, a flash of lightning. The optic nerve, on the contrary, proceeds from its _thalami_ much further forward