The ball of the eye, as now described, is situated in a bony cavity, called its orbit, composed by the junction of seven different bones, hollowed out at their edges. This cavity is in all the vacant spaces filled with a loose fat, which serves as a proper medium for the eye to rest in, and as a socket in which it may move. It is sheltered by the eyebrows, which are provided with hair, to prevent the descending sweat of the forehead from running down into it. As a still farther protection to this delicate organ, it is furnished with the eyelid, which, like a curtain, is drawn over it with inconceivable swiftness, for its security, on the approach of danger. I also serves to wipe it from superfluous misture, and to cover it during sleep. In the upper part of its orbit, it is furnished with a gland, to supply it with water sufficient to wash off dust, and to keep its outer surface moist, without which the cornea would be less transparent, and the rays of light would be disturbed in their passage; and the superfluous water is conveyed to the nose through a perforation in the bone.

For the purpose of enabling the eye to move in its socket, six muscles are provided. These are admirably contrived to move it in every direction, upwards or downwards, to the right or to the left, or in whatever direction the occasion may require; and thus we are spared the trouble of turning our heads continually towards the objects we wish to inspect. If we want to look upward, one of these muscles lifts up the orb of the eye; if we would cast our eyes to the ground, another muscle pulls them down. A third muscle moves the globe outwards towards the temples, and a fourth draws it towards the nose. A fifth, which slides within a cartilaginous ring, like a cord over a pulley, and is fastened to the globe of the eye in two points, makes it roll about at pleasure. A sixth lies under the eye, and is designed to temper and restrain, within proper bounds, the action of the rest, to keep it steadily fixed on the object it beholds, and to prevent those frightful contortions which otherwise might take place. By these, and a multitude of other mechanical contrivances, all acting in harmonious combination, the eye, as a natural telescope and microscope, is made to advance, to recede, to move to the right and to the left, and in every other direction; and to view near and distant objects with equal distinctness; so that a single eye, by the variety of positions it may assume, performs the office of a thousand.*

The utility of these several movements, and the pain and inconvenience which would be suffered, were any of them wanting, can scarcely be pears in its natural state, and exhibits the relative positions of the Cornea, Iris, and Pupil.

• Files and other insects, whose eyes are immoveable, have several thousands of distinct globes in each eye. See note page 38.

conceived, by any one whose eyes have always remained in a sound state. We are so much ac customed to the regular exercise of our visual organs, that we seldom reflect on the numerous delicate springs which must be set in action, before the functions of vision can, with ease, bo performed. But were any one of the muscular organs, now described, to fail in its functions, we should soon experience so many inconveniences, as would throw a gloom on all the other comforts of life; and convince us, how much we are indebted, every moment, to the provident care and goodness of our Beneficent Creator, for thousands of enjoyments which we seldom think of, and for which we are never sufficiently grateful."With much compassion, as well as astonishment at the goodness of our loving Creator," says Dr. Nieuwentyt," have I considered the sad state of a certain gentleman, who, as to the rest, was in pretty good health, but only wanted the use of those two little muscles that serve to lift up the eyelid, and so had almost lost the use of his sight being forced, as long as this defect lasted, to shove up his eyelids every moment, with his own hands."t

How admirable, then, is the formation of the eye, and how grateful ought we to feel at the consideration, that we are permitted to enjoy all the transporting pleasures of vision, without the least perplexity or effort on our part! If the loss of action in a single muscle produces so many distressing sensations and efforts, what would be the consequence if all the muscles of the eye were wanting or deranged? And is it man that governs these nice and intricate movements? or is it the eye itself, as a self-directing machine, that thus turns around, seasonably and significantly, towards every visible object? Man knows neither the organs of vision, nor the functions they ought to perform. The eye is only an unconscious machine, in the hands of a Superior Intelligence, as a watch, or a steam engine, is in the hands of a mechanic. It is God alone who constantly performs its movements, according to certain laws, which he has submitted to our inclinations and desires; "for in him we live and move." We are desirous to see certain objects around us: this is all the share we have in the operations of our eyes; and without per plexing our understanding, without the least care or managenent, in regard to any of the functions, we can, in a few moments, take a survey of the beauties and sublimities of an extensive landscape, and of the glories of the vault of heaven. Thus, the Divine Being operates not only in this, but in a thousand different ways, in the various senses and contrivances which belong to our ani❤ ma system; and yot, thoughtless and ungrateful man often inquires, in the language of doubt

↑ Nieuwentyt's Religious Philospher, vol. 1, p. 238.

and hesitation, "Where is God my Maker?" He is in us, and around us, directing every movement in our animal frame to act in harmony with the surrounding elements, and to minister to our enjoyments; and it is only when his exquisite operations are deranged by external violence, that we feel inconvenience or pain.

Such are only a few general outlines of the structure of the eye: for no notice has been taken of the numerous minute veins, arteries, nerves, lymphatics, glands, and many other par ticulars which are connected with this organ. But all this delicate and complicated apparatus, in the structure of the eye, would have been of no use whatever for the purpose of vision, had not a distinct substance been created to act upon it, exactly adapted to its nature and functions. In order that the eye might serve as the medium of our perceptions of visible objects, light was formed, and made to travel from its source at the rate of 195,000 miles in a second of time. This prodigious velocity of light is, doubtless, essential to the nature of vision; since it actually exists, and since we find that it radiates with the saine swiftness from the most distant visible star, as from the sun which enlightens our system. To abate the force of this amazing velocity, its particles have been formed almost infinitely small-a circumstance which alone prevents this. delightful visitant from becoming the most tremendous and destructive element in nature. Dr. Nieuwentyt has computed, that, in one second of time, there flows 418,660,000,000,000,000,000,000,000,000,000,000,000,000,000* particles of light out of a burning candle, which number contains at least 6,337,242,000,000 times the number of grains of sand in the whole earth, supposing every cubic inch of the earth to contain a million of grains. It has been justly remarked, by Mr. Ferguson and other authors, that "if the particles of light were so large, that a million of them were equal in bulk to an ordinary grain of sand, we durst no more open our eyes to the light, than suffer sand to be shot point blank against them, from the mouth of a cannon." It may also be remarked, that the property which all bodies possess, of reflecting light, is essential to the purpose of vision, without which, the splendid and variegated scene of nature would be changed into a dreadful gloom; and were the rays of light of one uniform colour, and not compounded of various hues, one object could not be distinguished from another, and the beautiful aspect of our globe would instantly disappear. Thus we see, that the eye is adapted to light, and light to the eye; and in this admirable adaptation the wisdom of the Creator is strikingly displayed. For light has no effect upon the ear, or upon any other organ of sensation; so as to

• See Appendix, No. V

produce a perception of visible objects; as, on the other hand, the undulations of the air have no effect upon the eye, so as to produce the sensation of sound. The eye did not produce the light, nor did the light form the eye; they are perfectly distinct from each other, yet so nicely adapted in every particular, that had any one quality or circumstance been wanting in either, the functions of vision could not have been per formed in the manner in which they now operate, which strikingly demonstrates, that one and the same Intelligent Being, possessed of a wisdom beyond our comprehension, formed the curious structure of the eye, and endued the rays of light with those properties of colour, motion, and minuteness, which are calculated, through the medium of this organ, to produce, in sentient beings, the ideas of visible objects. And, surely, he never intended that such exquisite skill and contrivance should be altogether overlooked by rational beings, for whose pleasure and enjoyment all this benevolent care is exercised.

Manner in which vision is performed.

Let us now attend a little to the manner in

which vision is performed, by the medium of light acting on the organs of sight. If we take a common convex glass-a reading glass for example-and hold it at some distance from a candle or a window sash, placing a piece of white paper behind the glass, at the distance of its focus, the image of the candle or sash will be painted on the paper, in an inverted position. This experiment may be performed, with a better effect, by darkening a room, and placing the convex glass in a hole cut out of the window shutter. when the rays of light flowing from the objects without, and passing through the glass, will form a picture of the objects opposite the window, on the white paper, adorned with the most beautiful colours. In a manner similar to this, are the images of external objects depicted on the back part of the inner coat or membrane of the eye. The rays of light, proceeding in all directions, from surrounding objects, and falling on the eye, are transmitted through the pupil; and being refracted by the different humours, (particularly by the crystalline humour, which acts the part of a convex lens,) they converge to a focus on the retina, where the images of visible objects are painted in an inverted position; and, by means of the optic nerve, these images are conveyed to the mind.

The following figure will perhaps more distinctly illustrate this point. Let a, b, c, x, y, represent the globe of the eye, and A, B, C, an object at a certain distance from it. Now, it is well known that every point of a visible object sends out rays of light in all directions; and, therefore, a certain portion of the rays which flow from the object A B C, will fall upon the cornea, between and y, and, passing through the

aqueous humour, m, n, and the crystalline humour , P, and the vitreous humour, D, E, will be converged to a focus on the retina, and paint a distinct picture, a b c, of the object A BC, in an inverted position. The rays from the point A of the object, after being refracted by the different humours, will be brought to a point at a; those from B will be converged at b; and those from Cat c; and, of course, the intermediate rays between A B, and B C, will be formed between a b, and b c, and the object will become visible by means of its image or representation being painted on the retina, in all the colours and proportions which belong to it. If we take a bullock's eye, and cut off the three coats from the back part, and put a piece of thin white paper over that part, and hold the eye towards the window or any bright object, we shall see the image of the object depicted upon the paper, and in an inverted position, as stated above.

In order that we may more distinctly perceive the wonders of vision, and the numerous circumstances on which it depends, let us suppose ourselves placed on an eminence, which commands a view of a variegated and extensive landscape. Let us suppose ourselves stationed on Arthur's seat, or on the top of Salisbury Crags, in the vicinity of Edinburgh. Turning our face to the north-west, the city, with its castles, spires, and stately edifices, presents itself to our view. Beyond it, on the north and west, a beautiful country, adorned with villas, plantations, and fertile fields, stretches as far as the eye can reach, till the view is bounded by the castle of Stirling, at the distance of more than thirty miles. On the right hand, we behold the port of Leith, the shipping in the roads, the coast of Fife, the isles of Inchkeith and of May, and the Frith of Forth, gradually losing itself in the German ocean. If we suppose the length of this landscape to be forty miles, and its breadth twenty-five, it will, of course, comprehend an area of a thousand square miles.

The first circumstance which strikes the mind, is the immense multitude of rays of reflected light which flow, in all directions, from the myriads of objects which compose the surrounding scene. In order to form a rude idea of this infinity of radiations, I fix my attention on a single object I direct my eye to Nelson's monument, o the Calton hill. From the parapet at the top, a thouand different points send forth a thousand differ

ent cones of rays, which, entering my eye, rep. der the different parts of it distinctly visible, besides myriads of rays from the same points, which flow in every other direction through the open spaces of the atmosphere which surround them. How many thousands of millions, then, of different radiations, must be issuing forth every moment from the whole mass of the monument! And if one object pours forth such a flood of rays, how immense must be the number of radiations which are issuing from all the objects which compose this extensive landscape! Myriads of rays, from myriads of objects, must be crossing each other in an infinity of directions, so that the mind is confounded at the apparent confusion which seems to exist in this immensity of radiations; yet every ray passes forward in the crowd, in the most perfect order, and without being blended or confused with any other ray, produces its specific effect on every eye that is open to receive it. But this is not all: these millions of rays which flow from the minutest points of the surrounding scene, before they can produce the sensation of vision, and form a picture of the landscape on the retina, must be compressed into a space little more than one-eighth of an inch in diameter, before they can enter the pupil of the eye; yet they all pass through this small aperture without the least confusion, and paint the images of their respective objects in exactly the same order in which these objects are arranged. Another circumstance demands attention. rays which proceed from the objects before me are not all directed to the spot where I stand, but are diffused throughout every point of the surrounding space, ready to produce the same effect, wherever sentient beings are present to receive them. Were the whole inhabitants of Edinburgh placed on the sloping declivity of Arthur's seat, and along the top of Salisbury Crags, and were millions of other spectators suspended in the surrounding atmosphere, similar sensations would be produced, and a scene similar to that which I now behold, would be depicted in every eye. Amidst the infinity of cones of light, crossing each other in an infinity of directions, no confu sion would ensue, but every spectator, whose eyes were in a sound state, would obtain a correct view of the scene before him; and hence it happens, that, whenever I shift my position to the right hand or to the left, other streams of light enter my eye, and produce the same affect Le

The