"There are more things in heaven and earth, Horatio, 126. Why are some persons short-sighted? Because the rays of light are brought to a focus before they reach the retina, and consequently produce an indistinct picture on the retina. This will be more clearly illustrated by the accompanying engraving. 127. The remote causes of this defect of the eye may be various; as too great density of the humours-too great a convexity either of the cornea or the lens-and from the space between the retina and the lens being too short. Where it depends on too much convexity, as age approaches, it may disappear; but where too great density is the cause, age is apt rather to aggravate than to relieve it. For while in the young the convexity both of the cornea and the lens is greater than in advanced life, the less density of the humours counterbalances it; and again as life advances, the more scanty supply of humours, along with the diminishing convexity, are neutralised by the increasing density; therefore, if these keep pace with each other the eye remains to a very protracted period of life without the necessity for artificial assistance. 128. Why do the eyes sometimes become "bloodshot?" Because when the eye is healthy its blood-vessels are so small that only the colourless portion of the blood finds its way through them. But when the eye is inflamed, the coats of these vessels becoming weaker, they are readily distended by the impulse of the blood against them; they then become enlarged, the red globules find their way into them, and they are rendered red and visible to the naked eye. 129. Why, when the sight is impaired in old age, are persons compelled to wear spectacles, or to hold objects at a distance from them in order to distinguish them perfectly? Because the diminished refractive power of the organ is accompanied by a divergence of the rays of light which tends to confuse and obscure the sight. By holding objects at a distance the divergence becomes diminished, and the same effect is produced by convex glasses. ""Tis greatly wise to walk with our best hours, And ask them what report they've borne to heaven."-Young. 130. It is customary with opticians to place in the hands of a person requiring spectacles a book or newspaper, and by the distance at which the one or the other is instantly held from the sight, the optician is enabled to judge of the degree of the defect of the vision and the kind of glass that is required. 131. Why does the eye lose its lustre in old age and sickness? Because the lustre of the healthy eye is produced by the action of the muscles of the eye-ball maintaining it firmly in its place, pressing it against the fatty cushion on which it rests, and giving to it that tension necessary to render the corner prominent, clear, and full. In old age and sickness the muscles of the eye partake of the general debility of the system, and its functions are thus deranged. 132. Why do some persons squint? Strabismus, or squinting, is caused by one or more of the muscles of the eye being shortened or elongated; and by the derangement of their action the pupil is consequently carried out of the proper axis of vision. 133. This defect is cured by a very simple operation. The shortened or elongated muscle is cut down by a skilful operator. The false position of the pupil is immediately rectified by the division of the disordered muscle that induced obliquity by its irregular action. The divided muscle in healing shortens or lengthens, and during the healing process suits itself to the exigencies of the case, and in a short time re-unites. 134. Why when we look for some time at a white spot upon a black ground, and afterwards direct the eye to a white ground, do we perceive a black spot? Because the retina of the eye has become insensible in the point which was formerly fatigued by the white light. In the same manner, after the retina has been some time without acting in one of its points, whilst the others have acted, the point which has been in repose becomes of an extreme sensibility, and on this account objects seem as if they were spotted. 135. In this manner it is explained why, after having looked for a long time at a red spot, white bodies appear as if spotted with green; in this case the retina has become insensible to the red rays, and we know that a ray of white light, from which the red is substracted, produces the sensation of green. ! "Each friend by fate snatched from us, is a plume 136. Why does a flash of light from the eye generally follow from a violent blow on that organ or the surrounding parts? Because light, or some degree or modification of light, is the only impression of which the eye is susceptible; and therefore every effect upon the parts of the eye which are essential to vision only must be a sensation of light. 187. A violent blow on the eye appears as an instantaneous flash of light, and this effect is produced, not only when the eye itself is struck, but when there is a violent concussion of the head, which we may suppose to be transmissable to the surrounding parts of the eye, and so to the eyeball itself. If a person walking hastily and heedlessly in the dark strikes his forehead violently against an object of resistance, then instantly, and before any pain is felt in the part which has received the stroke, a flash of light is seen by both eyes, brighter in proportion as the stroke is more violent. Thus, any sudden agitation produced in the exterior coats of the eyeball, from whatever source it may arise, always tells upon the sentient part of the eye as light, and never as pain or any other sensation. 138. Why cannot a person distinguish minute objects in the water with his head likewise immersed in the fluid? Because, although there is sufficient difference between the density of the humours of the eye and that of the air, to bring the rays transmitted by the latter to a focus on the retina, there is not sufficient difference between the density of these humours and that of water, to do the same by rays transmitted through this fluid, so that such rays are not brought to a focus sufficiently soon. Hence divers in some places are in the habit, when they descend into the water, of using extremely convex glasses, in shape almost like the lens of fishes, and turning their eyes by this means, as it were, into those of an aquatic animal. 139. Why are the eyes furnished with lashes? Because they serve to ward off insects, and to protect the eye from particles floating in the air. When the eyelashes are humid, the little drops of moisture decompose the rays of light, and, even independently of humidity, they also partly resolve the light passing into the interior of the eye. 140. When the eyelids are placed near each other, and the eyelashes admit only a small quantity of light to pass at a time, the eyelashes, by separating into pencils "And as he spoke a big round drop A witness that we all believe."-BLOOMFIELD. the light which penetrates the eye, make bodies in ignition appear during the night as if they were surrounded with luminous rays. This appearance does not take place if the eyelashes are inverted, or merely turned in another direction. The vision of those persons who have lost their eyelashes is always more or less mperfect. 141. Why are the eyes protected by eyebrows? The eyebrows have many uses. The projection which they form protects the eye against external violence; the hairs on account of their oblique direction, and the oily matter with which they are covered, prevent the perspiration from flowing towards or irritating the surface of that organ: they direct it towards the temple and the root of the nose. The colour and the number of hairs of the eyebrows have an influence upon their use. They have generally some relation to the climate. The inhabitants of hot countries have them very thick and black; the inhabitants of cold countries may have them thick, but they are rarely black. The eyebrows protect the eyes from excess of light, particularly when it comes from above; this effect is rendered still more conspicuous by the knitting of the brows. 142. Of what use are eyelids? They cover the eye during sleep, and preserve it from the contact of extraneous particles flying about in the air, which might injure it; they defend it from sudden shocks, by their almost instantaneous closure, and by their habitual motions, which are renewed at nearly equal intervals, they preserve it from the effects of long-continued contact of the air. The eyelids also moderate the force of a too brilliant light, and prevent the passage of any more of this fluid than what is necessary for vision without offending the eye. On the contrary, when the light is feeble, we separate the eyelids to a considerable distance, in order to admit the passage of as great a quantity of light as possible to the interior of the eye. 143. Why is the involuntary closing of the eyelid more advantageous than if it depended upon our will? Because, if the closing of the eyelid depended upon voluntary "Give every man thine ears, but few thy voice; Take each man's censure, but reserve thy judgment."-SHAKSPERE. power, we might have gone to sleep forgetting to have closed it, thereby exposing the eye to considerable danger. But under the involuntary arrangement the lid is made to fall over the eye as drowsiness comes on, is kept there during sleep, and in the inorning, owing to the delicacy of its structure, it transmits a sufficient quantity of light to the eye to arouse sensibility, and we awake. 144. How is the process of hearing conducted? The folds of the outward ear conduct to its entrance, and into an outer passage, which, after running inward a short distance, is closed by a membrane called the drum. of the ear is put upon the stretch, more or less, according to the force or loudness of the sound, as shown by the trick of calling loudly under the pretence of whispering in the ear. Behind the drum, in the hard bone, or the rock-like wall of the inner passage, are two small openings termed mastoid, which communicate with the inner cavities or chambers of the ear. There are two other openings closed by a thin transparent membrane, and the chambers excavated, as it were, in the rock-like bone, are, with the mastoid cells, filled with fluid. Within these chambers (called the labyrinth and semi-circular canals) the nerve of hearing spreads out between the folds of a most delicate membrane, receiving the impressions conveyed by the atmosphere or other conducting media, and communicating them to the sense of hearing in the brain. 145. Why do infants hear indistinctly? Because the bones of their ears are soft and cartilaginous; and of |