The chief use of the Eustachian tube is to allow a free interchange of air between the ear and the throat, and it is very important that its use in this respect should be understood. Persons who go down in diving bells soon begin to feel a great pressure in the ears, and, if the depth is great, the feeling becomes extremely painful. This arises from the fact that in the diving bell the pressure of the air is very much increased, in order to balance the weight of the water above; and thus it presses with great force upon the membrane of the drum, which, if the Eustachian tube has been kept closed, has only the ordinary uncompressed air on the inner side to sustain it. It is therefore forced inward and put upon the stretch, and might be even broken. Many cases, indeed, have occurred of injury to the ear, producing permanent deafness, from descents in diving bells, undertaken by persons ignorant of the way in which the ear is made; though the simple precaution of frequent swallowing suffices to ward off all mischief. For, if the Eustachian tube is thus opened, again and again, as the pressure of the outside air increases, the same compressed air that exists outside passes also into the inside of the drum, and the membrane is equally pressed upon from both sides by the air, and so is free from strain. The same precaution is necessary in ascending lofty mountains.-- DR. JAMES HINTON. THE COLORED CURTAIN IN THE EYE (p. 238).--This ring-like curtain in the eye, of gray, green, bluish-green, brown, and other colors, is one among the very many remarkable contrivances of the organic world. The eye can not bear the entrance of too much light, and the colored curtain so regulates its own movements as to serve this requirement. The dark circular aperture in the center, known as the pupil, is consequently forever altering in size; on a bright, sunshiny day, out in the open, it may be only the size of a pin's head, but at night, when there is no light stronger than starlight, it is even bigger than a pea. The eye curtain is fixed at its outer edge, leaving the inner edge to contract or expand, which it does automatically and quite independent of the will, ever preserving its circular outline. Its movements may be watched in a variety of ways, some of which we shall describe.