THE STRUCTURE OF THE EYE.
Of all the senses, says an ardent admirer of nature, the sight is certainly that which furnishes the mind with the quickest and most widely-extended perceptions. It is the source of the richest treasures of the imagination, and of our ideas of the beauty, order, and unity of the world around us. How unhappy are those whom a hard fate has deprived of the sense of sight from their birth! Alas! the finest day and the darkest night differ in nothing as far as they are concerned; the light of heaven never brings joy into their hearts. The enamelled beauties of a bed of flowers, the varied plumage of the peacock, the glories of the rainbow are alike unknown to them. They cannot contemplate from the mountain height the beauties of the valley beneath; the fields golden with the harvest, the meadows smiling with verdure, and watered by winding rivers, and the habitations of man dotted about here and there over the surface of this magnificent picture. To them is unknown the sight of the mighty ocean; and the innumerable legions of the cloud army of Heaven are to them as if they did not exist. The impenetrable obscurity which surrounds them allows them neither the contemplation of what is grandest in man's outward aspect, nor even the admiration of those qualities which they themselves would hold most dear.
A strong sentiment of pity should, therefore, animate the breast of every right-thinking man, when he considers the unhappy condition of those who are born blind.
The eye infinitely surpasses in its complexity and beauty of structure all the other organs of sense, and is most unquestionably the most marvellous object that the human mind is capable of examining and understanding. Let us first examine the external parts of this wonderful organ. With what a singular system of entrenchments and defences do we find the eye provided! It is itself placed in the head at a certain depth, and surrounded on all sides by solid bone, so that it is only with the greatest difficulty that it is hurt by accident from without. The eyebrows also play their part as protection to the eye, and prevent the perspiration from entering and irritating the organ. The eyelids too are always ready to rush to the rescue, whether to protect the eye from outward attacks, or to shade it from too strong a light during sleep. The eyelashes not only add to the beauty of the eye, but they shade it from the too brilliant light of the sun, and act as advanced guards to prevent the entrance of dust or any other foreign body with which the eyes might be injured.
But its internal structure is still more admirable. The globe of the eye is almost spherical and measures nearly one inch in diameter. Fig. 1 is a view of the eyeball, showing the details of its structure; the various membranes surrounding it have been cut away in order that it may be better examined. If we commence our examination by the exterior portion of the front, we shall first find immediately beneath the eyelashes a perfectly transparent membrane (C), called the cornea. It is a prolongation of the hard opaque external coating of the eye, called the sclerotic membrane, and marked s in the figure. The cornea is sufficiently hard in its nature to present a strong resistance to any violence from without.
Immediately beneath the cornea and in contact with it is the aqueous humour, a thin transparent liquid occupying a small portion of the front of the eye.
Next comes the iris, a circular disc perforated with a round hole in the middle, and coloured with various shades of blue, brown, and grey.
The opening in the centre, which appears like a black spot when the eye is examined, is not really an object, but simply an aperture, capable of changing its size according to the quantity of light striking the eye. This change of size in the opening, or pupil, as it is popularly called, is effected by the contraction or expansion of the iris, which thus possesses the peculiar property of exactly proportioning the amount of light that enters the eye, so that there is never too much or too little. It is through the pupil that the rays of light proceeding from the various objects around us pass into the interior of the eye, and form an image upon the retina, as will be afterwards explained.
Fig. 1.—Section of the Eye.
Immediately behind the pupil is O, a bi-convex lens to transmit the rays of light to the retina. It is generally called the crystalline lens.
From the crystalline lens to the back of the eyeball, is a space more or less globular in form, containing a gelatinous diaphanous mass somewhat resembling white of egg in appearance, and called the vitreous humour.
Behind the vitreous humour, and immediately opposite the pupil and lens, is the most delicate and important of all the membranes of the eye, the retina, which serves as a screen whereon are received the images of the objects around us. This membrane is an expansion of the optic nerve N leading from the brain, and lines the whole of the interior of the eye. The eye is also enveloped in a second membrane (C), called the choroid, which is impregnated with a black pigment. Round this is wrapped a third membrane, the sclerotic (S), which unites with the cornea in front of the eyeball.
The crystalline lens through which all the rays pass before they reach the retina, possesses the marvellous power of being able to modify its curvature in such a manner as to adapt itself to the distance of the object seen, and thus throw a distinct image on the retina. When we come to talk of the properties of lenses, we shall see that the focus of a lens differs for objects at different distances; if, therefore, the eye were not provided with some such means for altering the focus of the crystalline lens, we should only see objects distinctly at one particular point. The crystalline lens consists of infinite numbers of extremely thin transparent little plates, each of which is in itself composed of fine fibres so united together as to be capable of a small degree of compression or extension. Hence the power of the lens to alter its form according to circumstances. It is calculated that the human eye contains over five millions of the laminæ above referred to. With such wonders is the world of nature replete,—wonders that we daily and hourly pass by without examination.
It is by means of this ingenious and inimitable structure of the eye that external objects pass from the domain of the material world into that of the mind, and become accessible to every faculty of our brain. Of its own accord, and without apparently any effort of our own will, does this marvellous mechanism adapt itself to all the variations of distance and intensity of light, a power possessed by no instrument as yet constructed by the hand of man—being capable, as it is, of distinguishing instantaneously between the distance of the remotest nebulæ and that of the letters forming this page. This wonderful organ, writes Brewster, may be considered as being the sentinel that guards the passage between the world of matter and that of mind, and as the medium through which they interchange all their communications. The optic nerve perceives the objects written on the retina by the hand of nature, and conveys them to the brain in all their integrity of form and colour.
The path of the rays of light and the formation of images upon the retina are shown in the preceding figure. At first sight it will be perceived that the objects thereon depicted are in a reversed position, that is to say, when we look at a view similar to that shown in fig. 2, we should find, if we had any means of observing the positions of objects reflected on our retina, that the flock of sheep coming up the road were at the top of the eye, while the trees, the roof of the house, and the chimney were in the contrary position. Similar reversed images may be seen in dark rooms, by holding a screen before any little crack or pinhole in the door or shutter of the room. In fig. 2 the keyhole of the door is represented as playing the part of a lens. The author, in common with almost every other boy, observed this fact at a very early age, and the idea immediately struck him that it would be only necessary to fix these images to procure exact representations of natural scenery; but in making inquiries into the subject, he found that his juvenile observations had been made a little too late, photography having already gained the end he intended striving for.
Fig. 2.—A Camera Obscura.
Seeing that the images of all objects appear on our retina upside down, the student is naturally disposed to ask how it happens that we do not see them in that position. Physiologists and natural philosophers have advanced numerous theories on the subject. Some, with Buffon, admit at once that it is by habit and education of the eye that we see objects unreversed. Others, like the great physiologist Müller, imagine that as we see everything upside down, and not a single object only, we have no points of comparison, and practically ignore the reversal. The truth, however, appears to be that it is the brain, and not the eye, that possesses the power of determining the real position of what we see. That the eye alone has no power of determining the positions of objects by itself, may be easily proved by showing a person an astronomical object, such as the moon through a telescope. Unless the observer has been already familiarized with the appearance of our
satellite, he will not know whether the image he sees is
reversed or not. It is the brain, therefore, and the
brain only, that has the power of determining the position
of objects around us, without taking into consideration
the reversed picture of them that is depicted on our
retina. The student who takes an interest in the structure
of this important organ, would do well to procure
a sheep's or bullock's eye from the butchers, and dissect
it carefully with a sharp penknife and pair of scissors.
The image formed on the retina may be easily seen by
cutting away the sclerotic and choroid coatings at the
back of the eye.
The ordinary distance of distinct vision for small objects, such as the letters of a book, is from ten to twelve inches. But possibly there do not exist two pairs of eyes in the world whose foci are the same. Even in the same individual it frequently happens that the focal length of the eyes differs considerably. In some persons the focus of the eye is so reduced that they are obliged to bring the object they are examining within six, and even four inches of their eyes, before they can see it. This defect is known ordinarily as short sight, and results from the too great convexity of the cornea and crystalline lens. It is corrected by wearing spectacles with concave glasses. Others again, on the contrary, place the book or object they are looking at, at a greater distance from the eye than that named. Such people are called long-sighted, and the defect results from the too great flatness of the cornea and the crystalline lens. The fault is of course corrected by the use of spectacles containing convex lenses.
Long-sightedness is generally the result of old age, and it may be taken as a fact that the older we grow the flatter becomes the crystalline lens. Hence short-sighted people have been known to recover their sight perfectly as they advance in years through the natural process of the flattening of the crystalline lens. These matters, however, will be more fully treated of when we begin to speak of the properties of lenses of different forms and curvatures.