By the constitution of the eye it is necessary, in order to produce distinct vision, that an image or picture of the object be formed at the bottom of the eye; and it is a point which experience and observation demonstrate, that the formation of an image at the bottom of the eye is necessary to perfect vision. The image itself can be shown at the back of a sheep's or bullock's eye. The lenses of a teletelescope, and the humours of the eye, bear a complete resemblance to one another, in their figure, their position, and in their power over the rays of light, viz. in bringing each radiation to a point at the right distance from the lens; which, in the eye, is at the exact place where the nervous membrane is spread to receive it. You may make the experiment and reason upon it yourselves by means of two or three glass lenses of different convexities, by holding them before a candle or a window, and seeing their images on a wall. In telescopes there is an imperfection, owing to radiations of light of different energies being separated into different colours in passing through glass lenses, thereby tinging the object, especially the edges of it, as though it were viewed through a prism. To correct this inconvenience, had been long a desideratum in optics. At last it came into the mind of a sagacious optician, to inquire how it was managed in the eye; in which there was exactly the same difficulty to contend with as in the telescope. Observation soon taught him, that, in the eye, the evil was cured by combining lenses composed of different kinds of substance, which possessed different refracting powers. As though, in your preceding experiment you had had lenses of the same convexity but of different kinds of glass, so that from this last cause the distances of the image would be different. He borrowed the principle, and produced a correction of the defect, by imitating, in lenses combined of different materials, the effects of the different humours through which the rays of light pass before they reach the bottom of the eye, and hence the Achromatic Tele scope. Two other things are wanted to the eye, which are not wanted, at least in the same degree, in the telescope; and these are the adaptation of the organ first, to different degrees of light; and, secondly, to the vast diversity of distance at which objects are to be viewed by the same eye. In the telescope the first is effected by diminishing the opening of the object glass, and the last by pushing in or drawing out the tubes of the eye glass. In the natural eye, the hole or aperture through which the light enters, is so formed as to contract or dilate itself for the purpose of admitting a greater or less number of rays at the same time. The chamber of the eye is a camera obscura, which, when the light is too small, can enlarge its opening; when too strong, can again contract it; and that without any other assistance than that of its own exquisite machinery. The second difficulty, the adaptation of the same organ to the perception of objects that lie but a few inches from the eye, and objects which are placed at a considerable distance from it, is removed by the action of certain muscles, called the straight muscles. Whenever the eye is directed to a near object, three changes are produced in it at the same time, all severally contributing to the adjustment required. The cornea, or outermost coat of the eye, is rendered more round and prominent; the crystalline lens underneath is pushed forward; and the axis of vision, as the depth of the eye is called, is lengthened. In different animals the eye exactly conforms itself to the wants and other powers of the animal. In birds, for instance, which procure their food by means of their beak, the distance between the eye and the point of the beak being small, they have the power of seeing very near objects distinctly. On the other hand, from being often elevated much above the ground, living in air, and moving through it with great velocity, they require, for their safety, as well as to assist them in descrying their prey, a power of seeing at a great distance. Two peculiarities, therefore, are found in the eyes of birds, both tending to facilitate the change upon which an adjustment of the eye to different distances depends. The one is a bony, yet, in most species, a flexible rim or hoop, surrounding the broadest part of the eye; which, confining the action of the muscles to that part, increases the effect of their lateral pressure upon the orb, by which pressure its axis is elongated for the purpose of looking at very near objects. The other is an additional muscle, to draw the crystalline lens back, and to fit the same eye for viewing very distant objects. By these means, the eyes of birds can pass from one extreme to another of their scale of adjustment, with more ease and readiness than the eyes of other animals, who do not require the same variation. The eyes of fishes, also, compared with those of terrestrial animals, exhibit certain distinctions of structure, adapted to their state and element, and the iris in the eyes of fish does not admit of contraction, because the diminished light in water is never too strong for the retina. In considering vision as achieved by the means of an image formed at the bottom of the eye, we can never reflect without wonder upon the smallness, yet correctness, of the picture, the subtility of the touch, the fineness of the lines. A landscape of five or six square miles is brought into a space of half an inch diameter; yet the multitude of objects which it contains, are all preserved; are all discriminated in their magnitudes, positions, figures, colours. The prospect from a hill is compressed into M the compass of a sixpence, yet circumstantially represented. The examination of the eye is a cure for atheism. Besides that conformity to optical principles which its internal constitution displays, and which alone amounts to a manifestation of intelligence having been exerted in the structure; besides this, which forms, no doubt, the leading character of the organ, there is to be seen, in every thing belonging to it and about it, an extraordinary degree of care, an anxiety for its preservation, due, if we may so speak, to its value and its tenderness. It is lodged in a strong, deep, bony socket, composed by the junction of seven different bones, hollowed out at their edges. In some few species, as that of the coatimondi, the orbit is not bony throughout; but whenever this is the case, the upper, which is the deficient part, is supplied by a cartilaginous ligament; a substitution which shows the same care. Within this socket it is imbedded in fat, and without it is sheltered by the eye-brows; an arch of hair, which, like a thatched penthouse, prevents the moisture of the forehead from running into it. But it is still better protected by its lid. This defends the eye, wipes it, and closes it in sleep. Are there, in any work of art whatever, purposes more evident than those which this organ fulfils? or an apparatus for executing those purposes more intelligible, more appropriate, or more perfectly sagacious? In order to keep the eye moist and clean (which qualities are necessary to its brightness and its use,) a wash is constantly supplied by a secretion for the purpose; and the superfluous brine is conveyed to the nose through a perforation in the bone as large as a goose-quil; and it is observable, that this provision is not found in fish,-the element in which they live supplying a constant lotion to the eye. Thus, in comparing the eyes of different kinds of animals, we see, in their resemblances and distinc tions, one general plan laid down, and that plan varied by divine omniscience, with the varying exigences to which it is to be applied. ***** LETTER LXXV. Construction of the Ear. MY ESTEEMED CHILDREN, The Ear is no less artificially and wonderfully adapted to its office, than the eye. Its general form, both external and internal, is sufficient to show that it is an instrument curiously adapted to the reception of those affections of the air called sound; that is to say, already knowing that sound consists in pulses of the masses of the air, we perceive, in the structure of the ear, a suitableness to receive impressions from this species of action, and to propagate the impressions to the brain. of an external ear, This structure consists (the conca,) calculated, like an ear-trumpet, to catch and collect the pulses of air; in large quadrupeds, turning to the sound, and possessing a configuration, as well as motion, evidently fitted for the office of a tube which leads into the head, lying at the root of this outward ear, the folds and sinuses therefore tending and conducting the air towards it: of a thin membrane, like the pelt of a drum, stretched across this passage upon a bony rim: of a chain of moveable, and infinitely curious, bones, forming a communication, and the only communication that can be observed, between the membrane last mentioned and the interior channels and recesses of the skull of cavities, similar in shape and form to wind instruments of music, being spiral or portions of circles of the eustachian tube, like the hole in a |