Our senses, notwithstanding twenty-four centuries of study, observations, and researches, are far from being an exhausted subject. Take, for example, the ear. A celebrated natural philosopher, Wollaston, occupied himself with it; and immediately we learn, that with an equal sensibility as regards the low notes a certain individual can hear the highest tones, whilst another cannot hear them at all; and it becomes proved that certain men, with perfectly sound organs, never heard the cricket in the chimney-corner, yet did not doubt but that bats occasionally utter a piercing cry; and attention being once awakened to these singular results, observers have found the most extraordinary differences of sensibility between their right ear and their left ear, &c. Our vision offers phenomena not less curious, and an infinitely vaster field of research. Experience has proved, for example, that some people are absolutely blind to certain colours, as red, and enjoy perfect vision relatively to yellow, to green, and to blue. If the Newtonian theory of emission be true, we must irrevocably admit that a ray ceases to be light as soon as we diminish its velocity by one ten thousandth part. Thence flow those natural conjectures, which are well worthy of experimental examination: all men do not see by the same rays; decided differences may exist in this respect in the same individual during various nervous states; it is possible that the calorific rays, the dark rays of one person, may be the luminous rays of another person, and reciprocally; the calorific rays traverse some substances freely, which are therefore called diathermal, these substances, thus far, had been called opaque, because they transmit no ray commonly called luminous; now the words opaque and diathermal have no absolute meaning. The diathermals allow those rays to pass through which constitute the light of one man; and they stop those which constitute the light of another man. Perhaps in this way the key of many phenomena might be found, that till now have remained without any plausible explanation. Nothing, in the marvels of somnambulism, raised more doubts than an oft-repeated assertion, relative to the power which certain persons are said to possess in a state of crisis, of deciphering a letter at a distance with the foot, the nape of the neck, or the stomach. The word _impossible_ in this instance seemed quite legitimate. Still, I do not doubt but some rigid minds would withhold it after having reflected on the ingenious experiments by which Moser produces, also at a distance, very distinct images of all sorts of objects, on all sorts of bodies, and in the most complete darkness. When we call to mind in what immense proportion electric or magnetic actions increase by motion, we shall be less inclined to deride the rapid actions of magnetizers.