The black skin, which hangs within the pupil, performs the office of a shutter that excludes the light -it extinguishes the rays whose reflection would render the image less distinct. The pupil, by contracting or dilating itself in proportion to the intensity of the light, moderates the action of the rays on the retina; and the nerve, placed behind this, communicates to the brain, as before observed, the various concussions it receives, and to which various perceptions correspond.

In the theory of vision there are many curious phenomena, which have exercised the ingenuity of the most learned men. Whence comes it, for instance, that all external objects are painted inverted on the retina, and that we see them, nevertheless, in their real situation? Whence is it, that objects of the greatest magnitude are delineated on the eye with extreme minuteness, and yet we perceive every thing in its proper size? Whence is it, that if we look down from the top of St. Paul's cathedral in London, we see beneath us many thousand houses of that great metropolis, each painted so exactly in our eye in a space hardly three times larger than the head of a pin? So many millions of rays enter by a very small aperture; they are united on the retina at the bottom of the eye, without confusion, and constantly preserving the same order which the points of the objects had that emitted them. Let us ascend a high rock, and observe a fleet in full sail at sea; let us contemplate the sea itself, and what millions of waves may be discovered! Each of these, nevertheless, reflects masses of rays upon our eye, the size of which is so minute. In an extensive country prospect, every tree, and even every blade of grass, emits its rays; without which it would be impossible for us to perceive the uninterrupted verdure of all the fields beneath us. Is it not wonderful too, that we do not see objects double; and

that although we have two eyes, each object still pears but one?

ар

But there is another circumstance to excite our admiration. The objects we behold are not visible to us alone: we have just observed, with astonishment, the number of rays they send on a space so small as that occupied by the pupil of the eye. They all send as many more to every part of the mass of air that surrounds them. Wherever, therefore, we may remove, new rays replace the first, and render the same objects visible to us, that we had seen in our former situation. All the rays necessary to effect this had existed before, and waited only for eyes. But the rays admitted into the eye are not all equally efficacious. Beside these, there are innumerable others, which, being much weaker, are effaced by the splendour of the first, but which are constantly ready to perform the same functions, if

necessary.

But how few are there who reflect on the wonderful phenomena of vision? From the habit of seeing, the instant we open our eyes, we are apt to regard this operation as a thing extremely simple, and easy to be understood. Nevertheless we are still far remote from an ability to explain all the phenomena of vision. We know, indeed, how the image is formed at the bottom of the eye; we know also, what all the parts of the eye contribute to this. But this is not sufficient; for the eye itself can have no idea of what passes within it. It is necessary, therefore, that the impression which the rays make upon it should be propagated to the brain, and that, to accomplish this, the rays paint the image on a network of nerves which correspond to those of the brain. In this manner, the motion impressed by the rays on the nerves of the retina, is transmitted to the brain by the optic nerve; but beyond this we can describe no more; for we do not

perfectly know the nature of the brain, nor the use of its various parts'.

We know enough, however, to induce us to admire and to adore the unspeakable power, and wisdom, and goodness of God. Whatever remains inaccessible to our comprehension, is the work of a Supreme Intelligence, conspicuous in every object, whether in ourselves, or in every thing that surrounds us; an Intelligence, which never ceases to employ the wisest means to accomplish the most benevolent ends.-The great philosopher of antiquity, in his treatise De Naturâ Deorum, was sensible of the wisdom and contrivance displayed in the construction of the eye: "What artificer but Nature, whose direction is incomparable, could so artfully have formed the senses? She has covered and invested the eyes with the finest membranes, which she has made transparent, that we may see through them, and firm in their texture, to preserve the eyes. She has made them slippery and moveable, that they might avoid what would offend them, and easily direct the sight wherever they will. The point of sight, which is called the pupil, is so small, that it can easily shun whatever may be hurtful to it. The eyelids, which are their coverings, are soft and smooth, that they may not injure the eyes, and are made to shut at the apprehension of any accident, or to open at pleasure; and these movements Nature has ordained to be made in an instant. They are fortified with a sort of palisade of hairs, to keep off what may be noxious to them when open, and, when sleep closes them, to be a fence to their repose. Besides, they are commodiously defended by eminences on every side; for,

They who wish to contemplate this subject with philosophical accuracy, should peruse the Rev. Andrew Horn's ingenious essay entitled The Seat of Vision determined.'

on the upper part, the eyebrows turn aside the sweat that falls from the forehead; the cheeks beneath, having a little rising, protect the lower; and the nose is placed between as a wall of separation.'

No. XLVII.

ON SOUND AND THE SENSE OF HEARING.

Auditur sonus, et vox omnis, in aures
Insinuata suo pepulere ubi corpore sensurn.

LUCRETIUS.

Mark how the spirits, watchful in the ear,
Seize undulating sounds, and catch the vocal air.

BLACKMORE.

SOUND may be defined to be a perception of the soul, communicated by means of the ear; or the effect of a collision of bodies, and a tremulous motion in consequence of that collision, communicated thence to the circumambient fluid, and propagated through it to the organs of hearing.

To explain this definition I must observe, that when obtuse bodies move in elastic fluids, they condense that part toward which they move, at the same time that the part from which they recede is rarefied. This condensation, or rarefaction, must produce an undulatory or vibrating motion in the fluid. Thus, if a body, by percussion or otherwise, be put into a tremulous motion, every vibration of the body will excite a wave in the air, which will proceed in all directions so as to form a hollow

sphere; and the quicker the vibrations of the body succeed each other, the less will be the distance between each successive wave. The sensation excited in the mind by means of these waves, which enter the ear, and produce a like motion in a thin membrane, stretched obliquely across the auditory passage, is called sound. But the term is frequently used to imply, not only the sensation excited in the mind, but likewise the affection of the air, or of the sonorous body by which that sensation is produced. Thus, we say, that a sound is in the air, or that a body sounds when struck, although the affection of the air or body is very different from the sensation.

That bodies move or tremble when they produce sound, requires no particular proof: it is evident in drums, bells, and other instruments, whose vibrations being large and strong are, therefore, more perceptible: and it is equally clear, that a similar vibration is excited in the air, because this vibration is communicated through the air to other bodies, that are adapted to vibrate in the same manner : thus bells, glasses, basins, and musical strings, will sound merely by the action propagated from other sounding bodies.

It is established, as well by mathematical reasoning from the nature of an elastic fluid, as from experiment, that all sounds whatever arrive at the ear in equal times from sounding bodies equally distant, when they are transmitted through atmospheric air. This common velocity is 1142 English feet in a second of time. The knowledge of the velocity of sound is of use, for determining the distances of

The same is visible when a stone is thrown into stagnating water: the waves excited thereby continue some time to rise in the place where the stone fell into the water, and are propagated thence in concentric circles, upon the surface of the water, to great distances.