Popular Science Monthly/Volume 1/August 1872/Sight and the Visual Organ
By Dr. A. VON GRAEFE,
LATE PROFESSOR OF THE UNIVERSITY OF BERLIN.
BE the idea what it may, that we form to ourselves of the mysterious tie that links our perception to the life of the soul, so much is undoubted, that the material supplied by the impressions of the senses constitutes the basis on which the soul unfolds; further, that they furnish the nutriment on which our thoughts and conceptions live and grow, and that through them alone is preserved the connection between the invisible "I" and the external world—the soil in which all conscious intellectual activity strikes root.
The child does not come into the world fitted out with elementary notions, as the idealists have taught, but endowed with the capacity for acquiring these ideas. These impressions, coming to it through the senses, furnish the "intellectual fuel" for the first psychical processes. And, obviously for this embryo stage of mental life, the association of the senses of seeing and feeling is of peculiar importance. The richer the world of sensuous impressions is, and the more manifold the relations of sense to sense are, so all the more numerous and varied are our inductions from them. By means of a process of collecting and comparing, compound ideas are evolved out of simple ones, and the normal, logically organized mental life attains an ever-higher development, while, by the inexhaustible activity of the senses, it receives a never-failing supply of fresh material for the perfecting of its psychical structure.
The senses are indeed the gates to the mind, through which aliment passes for its sustenance; but equally, they are the portals through which science must endeavor to penetrate into the mental world. This has often been attempted, though in another manner, as by laying down postulates on the nature of the soul. But, since the beginning of the world, this manner of committing our thoughts to the guidance of metaphysical hypotheses has never increased our knowledge even by a hair's breadth.
Fortunately, the majority of thinkers have now struck into the more promising paths of observation and analysis. Essential and most important progress has been made in the knowledge of the human body, since men have ceased to indulge in subtle speculation into the nature of the principle of life, and turned with an undivided spirit of inquiry into the laws of organic appearances. And since men have applied themselves to trace with care the psychical manifestations of perception into the world of ideas, and to do the utmost in their power to discover the laws which there govern, there has arisen another science, forcing itself more and more on our notice as it daily proves itself to be possessed of an inherent vitality. I refer to the science of psychology.
Such being our starting-point, the operations of the perceptions acquire a wider significance. Resting on this increased significance, I now venture to bring before you the structure and functions of that organ which, from the enormous amount of material it is the means of bringing to the mind, takes a prominent place in the part assigned to the operation of the senses. If I succeed in heightening a little your interest in this organ, or even only in reanimating in some of you the sentiment of happiness which must fill every grateful child of the Creator, when, on awaking in the morning, he joyfully greets the light of day, I shall have earned a rich reward for a trifling exertion.
Suppose, as shown in Fig. 1, that the brain which reposes in the cavity of the cranium, and is the bodily organ of consciousness, runs off, at one spot of its complicated structure, into a cord-like process, which lengthens till it reaches the surface of our body, when it then spreads out again in an umbellar form. Imagine, further, this whole process, including its roots, endowed with a specific sensitiveness, by virtue of which it responds with a luminous sensation to every irritation applied, and you have a fundamental idea of the nervous part of the visual organ.
Before proceeding further, let us first become better acquainted with these parts. A, indicates the brain; B, the aforesaid process, or in other words the optic nerve, which, passing through an aperture in the cranium, stretches on till it reaches into the orbit, where it spreads out into that expanse C, which, under the well-known term of retina, turns its surface to the outer world. Lastly, X is the point where the process is inserted in the brain, the letter X meaning to indicate the still unknown extent of its connections.
When I said, above, that every point of the whole mechanism, on being irritated, produced a luminous sensation, I meant that the irritation was conducted to the brain, and called forth this sensation in that sole source of conscious impressions. It is the same irritation coming in contact with the organ of consciousness, which we observe in the nerves of touch, with this difference that, in the above case, the quality of feeling differs; it is luminous or colored. The mere producing of this sensation of the luminous does not in any way depend on the nature of the irritation. Squeezing, pinching, pulling, chemical or electric irritations which give rise to the sensations of warmth or pain in a nerve of touch, call forth in the optic apparatus, by virtue of its specific sensitiveness, only a feeling of light, accompanied by neither pain nor heat.
A, Brain; B, Visual Nerve; C, Retina; X, Boot of the Optic or Visual Nerve.
You ask how men have arrived at the knowledge of these things, seeing the mechanism in question is almost entirely removed from any direct investigation. First, then, the umbellar expansion of the optic nerve, the retina, enables us to make experiments; this retina being in such close contact with the eye, the optical part of the visual organ, that it is accessible to every sort of mechanical irritation. You have yourselves, consciously or unconsciously, often made such experiments, when you watched the circles and sparks of fire and light, which become visible on rubbing or pressing your eyes through their lids, or striking them with a hard substance. Here the eye, as an optical apparatus, remains passive. Just as a man, who sees, is aware of the phenomena even in the deepest darkness, so is also a blind man, as long as the retina is endowed with its specific sensitiveness or sensory power, by which it responds to every irritation with a luminous sensation. Even after blindness, this dancing of sparks of fire and light may be kept up by continual irritation in the eye to such a tormenting extent, that, in order to prevent it, we usually cut the optic nerve just behind the eye, when the sparkling and scintillating cease.
Although less directly, the optic nerve, as well as the retina, is accessible to our observation. Hence we ascribe certain scintillations, visible to us in a rapid motion of the eye, to a twist of the nerve; surgical operations, dating from a period when narcotics were not employed, have likewise shown that contact with this nervous cord produces only sensations of light, not those of pain.
Lastly, we can point out the seat of the root, or the central termination of the optic nerve, by anatomically tracing the fibrils of the visual nerve into this tract; and partly, too, by an analysis of the phenomena observable in healthy and diseased states. When the brain has been excited by a narcotic, and the irritation is transmitted to the aforesaid tract, there arise sensations of light, which, combining with ideas of luminous objects, simultaneously excited, are transformed into what we call phantasms. The same thing takes place when the blood, as in fever, heats the brain; or when that part of the organ is excited from other causes. And thus it is with our visual impressions during dreams, or even in a half-waking condition.
But all this does not constitute any relation between sensation and the objects of the external world; that is, proper sensory action, whether it be the gay visions that surround us in the intoxication caused by opium; the comic phantasmagoria that hashish conjures up; the compact shapes that belladonna brings so near us; the airy forms seen in our dreams, or the scintillations produced by pressure, they all proceed from irritation of the special sensory power, and it is indifferent to the brain whether it receives its impressions from direct vision, or only from internal influences. All those operations, therefore, which proceed from direct irritation of the nervous part of the visual organ, without the medium of the eye and of light, under the term subjective vision, are opposed to those phenomena produced by the media of eye and light, and known as objective vision.
Great as are the influences of this subjective sight for the refreshment of our brain during sleep, and powerfully as they affect the temperament of the blind, they cannot connect us with the outer world. The yellow light which floods our field of vision, on rubbing the retina, is of no use to light up external objects. Hence, when, some years ago, a man pretended to recognize a delinquent who had attacked him in the night, by the sparks of fire produced by a blow on his eye, and founded an accusation thereon, it was, of course, unjustifiable, although the authorities consulted did not declare against the impossibility of the fact.—Baron Münchausen went still further in the use to which he put those visual sparks; for, when attacked by bears in the night, he not only struck out light enough by which to prosecute the chase, but fire, too, for his guns with the same blow.
We cannot entirely overlook the question whether sensations of light can be produced with the assistance of any other mechanism in the body but that of the visual nerve. As only the part where sensation originates is endowed with specific sensory action, so the irritation of that part can alone produce impressions of vision; but this irritation may be imparted to it by other parts of the brain, or by other nerves. It has been already stated that irritations of the brain produced by narcotics are transmitted only by proximity to the terminal extremity of the optic nerve. At the same time, it may be that the irritation proceeds from another nerve, from a nerve of touch or of hearing, and, penetrating to the brain, affects it so strongly as to send on the concussion to the optic centre. It is this that takes place when, after having listened to disagreeable sounds, you are seized with certain sensations in the nerves of touch, for instance, in those of the teeth; or, having gazed into the bright light, you become aware of a tickling sensation in the nose, causing you to sneeze. In a word, it is here a question of so-called sympathy, to be explained by transmission of the irritation from one nerve to the other.
The disposition to such sympathetic sensation is increased by a general irritability of the nervous system; while, in a calmer state of the nerves, the excitations run more regularly in the paths directly affected by the originating causes. In this manner, those indirectly provoked visual impressions which preponderate in circumstances of sickness and disease are augmented.
In these indirect visual sensations, as in the direct excitation of the mechanism of the visual nerve, only subjective sight has been treated of, cut off from from every relation with the outer world. We are quite ready to attach credit to the fact that, at exhibitions of somnambulism, when the natural irritability of nervously-disposed individuals is heightened, subjective visual impressions are produced in an unusual degree. Should, however, any connection with surrounding objects be founded on these results, or any transmission of the specific sensory action into other parts, as, for instance, transmitting to the skin of the abdomen the power of producing objective visual perceptions, such as are necessary for reading, these assurances are to be ranged in the same category of physiological blunders as the Münchausen hunting-story.
By what means, then, does the mechanism of the visual nerve, which we have hitherto regarded merely as the instrument of subjective sight, become a practical bridge between our ideas and the outer world, and a medium of the true and accredited operation of the senses? I answer, by a normal relation to a definite irritation proceeding from an object. This, which we might call the adequate sensory irritation, is light.
Let us consider the general relation between light and the organ of sight. Unable to discover with certainty the nature of light, it is explained in physics as being the undulating motion of an elastic body called ether, diffused throughout the universe. According to this, the irritation by light represents the shock of the undulations of ether on the irritable nervous matter, and at once takes its place among the mechanical excitations mentioned above when speaking of subjective sight.
The cord of the optic nerve is insensible to the undulations of the ether; the peripheral expansion of the retina is alone susceptible of irritation by light. This peculiarity has to do with the arrangement called the terminal apparatus, with which it is now proved every nerve is furnished. The nervous cords themselves are preeminently conductors; their irritation, when it does take place, necessarily produces impressions which come under the head of qualitative, for the eye, therefore, of the quality of luminous sensations; those impressions do not, however, stand in any closer relation to the adequate sensory irritation, and may be, as far as we are concerned, quite devoid of sensitiveness.
According to physics, light is the same species of motion in the ether as warmth; only, in order to affect the retina, the undulations must take place within certain limits of rapidity. Relatively they possess the greatest velocity in the violet-colored part of the spectrum, and the least in the red. In the same proportion as the velocity of the undulations diminishes, does light become invisible, and only dark rays of warmth are emitted, while on its being more highly heated, from the increased rapidity of the undulations, it reaches the glowing-point, that is, it emits rays of red light.
From this we see that the idea of light depends essentially on the organization of the retina. Were it different from what it is, did it possess any susceptibility for ethereal vibrations of a less degree of velocity than those at the red end of the spectrum, then we should call that light which we now term a dark warmth.
In certain cases of natural color-blindness, the susceptibility of irritation in the retina is quite undeveloped for the extremest red of the spectrum.
Now, while the light coming from external objects irritates the retina variously according to its color and power, the impressions made by luminous objects are also very various, and herein lies the first link of connection with the outer world.
It is only the visual organs of the lower animals which lose themselves in such a general and vague relation to the surrounding ocean of light and color. The organ which now occupies our attention has a far higher design to serve, viz., to awaken a perception of separate objects, and of their peculiar forms and colors. Were the retina as you see it in Fig. 1, a surface curving outward, then such a design could not be fulfilled; for every part would receive light from all the points of the outside world. In order to fulfil this condition, every individual point of the retina must enter into a separate and individual relation with the light proceeding from a point beyond it; nor till this takes place can the irritation of each separate spot of the retina produce a peculiar impression corresponding to the presence of the object-point; in short, according to the optical expression for such a relation, a picture of the outside world must be painted on the retina.
And this is indeed what takes place. As, on the one hand, the retina stands as the terminal apparatus of the optic nerve; on the other hand, it acts as a shade subservient to optical purposes; a screen, on which a perspective picture of the outside world is projected. If you compare it with the dull glass on which the picture in the camera-obscura falls, or the prepared plate in the photographer's camera, you have a correct notion of what I mean. As, in the photographer's camera, the picture falls on the sensitive plate, and is impressed on it by means of chemical changes produced by light, so in the eye it falls on the sensitive plate of the retina, whose irritations are telegraphed to the brain in due form.
We henceforth have to consider this image painted on the retina as the real object of the operations of the senses.
But how does the picture imprint itself on the retina? This is done by an optical apparatus close behind the retina and in connection with it; and, in short, by means of that mechanism known to us as the eye.
If we compare the retina with the sensitive plate in a camera-obscura, we shall perceive that the eye has indeed an undeniable resemblance with this well-known optical instrument, the camera-obscura.
S, Sclerotic; C, Cornea; L, Crystalline Lens; K, Aqueous Humor; K', Vitreous Humor; A, Choroid; N, Optic Nerve and Retina.
This is essentially a box painted black in the inside, with an opening fitted with a collective lens, turned on the objects of the outside world, and which receives the images produced by this lens on the wall behind. In order to show us the image, the one side of the box is replaced by a dim plate of glass. Now cast a glance at Fig. 2; and first suppose the box to be round; next, instead of the wooden wall, an organic tissue; and the window glazed with a transparent organic coat or tunic, instead of with a crystal lens, and which fulfills the same purpose as a collective lens, and strengthened by one or more lenses placed one behind the other. Farther, suppose, instead of the blackened inside of the walls of the box, the organic sclerotic coat overlaid from the inside by a second dark-colored tissue; and, lastly, the retina at the farther end as sensitive plate, and you have certainly an imperfect, but still, as far as an outline goes, a good general idea of the chief parts of the eye.
To the clearer understanding of these parts, the figure is provided with letters.
The tissue marked at the different sections with an S, is the enveloping tissue called the sclerotic or sclerotic coat.
To the front, overlapped by the above, lies the transparent tissue, the cornea, C, which represents the window, and at the same time contributes essentially to collecting the rays of light. At the back enters the optic nerve, which, spreading out to right and left within the sclerotic, receives the name retina, likewise marked with an N.
The chief business of the lens, L, lying well back, and rendered perfect by the humors (K, aqueous, K', vitreous), which fill the spaces or chambers, is the refraction of the rays, whose admission has already been prepared by the cornea.
And, lastly, overlying the interior surface of the sclerotic, is the choroid with its pigment, being the substitute for the black paint in the camera. You find it marked with an A.
Now, if this eye with its cornea, like a camera-obscura with its window, is turned on the objects of the outer world, we shall behold what Fig. 3 shows us: The light proceeding from a point A, beyond the eye, throws a pencil of rays on the cornea; this is already refracted here and there on the surface of the lens, but in a manner so as to collect all its rays again in the one point a of the retina. This a, then, is the image-point of the object-point A. In the same manner, b becomes the image-point of the object-point B, and all the object-points between A and B will find their image-points on the retina between a and b. In a word, an inverted perspective image of all the objects comprehended in the space A B will be found reflected on the retina.
Let us now examine a little more closely the structure of the eye, together with the object it is designed to serve, taking the separate parts in the direction from without inward.
The sclerotic, a stout and not very elastic coat, wants no further description. On the other hand, however, the cornea, as the transparent window, deserves your whole attention. In the construction of the cornea Nature has had to overcome exceptional difficulties. If you remember how apt every organic body exposed to the air is to fall a prey to desiccation, under the influence of which the optical homogeneousness, on which transparency depends, is lost, you will readily acknowledge the amount of resistance the cornea is enabled to offer. But further, consider that the cornea does not possess a homogeneous structure, but consists of five different, partly compound layers; that it hides in its interior numerous cellular bodies, canals for the passage of the humors, and a network of nerves—and then assuredly you will not refuse your admiration to the optical excellence of this most indispensable of all windows.
A B, Object of Sight; a b, Image on the Retina.
However, the difficult task implied in the structure of the cornea could not be fulfilled without the aid of some extraneous appliances. Thus, two movable covers lie over the eye, namely, the eyelids, whose inner surface is a compound humorous matter a brackish, mucilaginous, fatty solution. After having used our eye for a while, there arises a certain sensation of dryness on the cornea, from exposure to the air; the ever-recurring necessity of renewing the moistness causes us to close the lids, or, as we say, to wink. This is at least the chief design of the dropping of the lids, which besides lend their aid to the periodical exclusion of the irritation of the sight, as in sleep, for defence against the dazzling light, and for protection against the impurities in the atmosphere. The cornea is likewise being continually moistened by the posterior humors. But, with all the expedients and appliances used by Nature, perfect transparency cannot be always preserved: dull spots are formed on the window of the cornea, often causing derangement of vision. Unimportant irritations, which on the surface of the body are not noticed, seriously affect the cornea.
Let us now notice the second coat, the choroid. This we compared to the black coating of paint in the camera-obscura. Thinking of the dazzling and delusive visions which are a consequence of the gradual consuming away of the pigment in the choroid, or which accompany the entire want of it, as with the albinos, we cannot doubt that one essential design of this tissue is to intercept the scattered rays. But, apart from its office of conducting nourishment to the eye, and secreting the humors by means of its numerous blood-vessels, the choroid has a second optical design to fulfil, which now brings us to the characteristic signs of the eye.
As you will perceive from Fig. 4, which gives a section of an eye from life, the choroid, after having accompanied the sclerotic to the edge of the cornea, goes on expanding anteriorly, and from henceforth bears another name, that of the iris, or, as we might say, the rainbow tissue. As this process, which likewise contains a quantity of pigment, lies behind the transparent cornea, it can be observed in all its minutiæ; and, on account of the rayed arrangement adopted by its fibres, it is frequently called the eyeball, or star. The iris is broken in the centre by an opening to which we apply the term pupil, being the visual aperture. It usually seems to be black.
The presence of the iris greatly diminishes the extent of surface designed for the reception of light; the whole pencil of rays that falls on the cornea, as supposed, in Fig. 3, not reaching the retina, but only as shown in Fig. 4, on that section of it which enters the pupil. Though much of the volume of light is thereby lost, the restriction is highly beneficial, by sharpening the image on the retina; for the refraction of the rays is much more equal in the centre than toward the margin.
The iris, however, has a still more important function to perform. It regulates the entrance of the light, being furnished with a muscular apparatus (ciliary muscle), which provides that in strong light the pupil contracts, and in duller light expands. Thus the iris plays the part of a so-called movable diaphragm, a common appliance in optical instruments, used to dull the light for the purpose of seeing better. You cannot but have observed this play of the pupil, and how it accommodates itself to the volume of light; nor can you be ignorant that the iris with its varied coloring from light blue to deepest brown is what we know as the color of the eye. It is perhaps less well known to you, however, that the peculiar pigment required for the darker colors of the iris comes only as we advance in life, and that, therefore, we all commence our earthly course with blue eyes; a fact already known to Aristotle.
The crystalline lens, which is held fast in its place by a very fine tissue, as shown in Fig. 4, from the curve of its surface, and its strong power of refraction, plays an important part by conducting the collected light to the picture on the retina. It has, however, another and extremely important design, which must here be carefully considered.
The requirements made on an optical instrument depending on lenticular effect, are different according as it is expected to project images of nearer or more distant objects. The light with very divergent rays, and proceeding from near objects, is collected to a picture behind the lens, while that of the distant objects falls with almost parallel lines. To return to the camera-obscura, you must draw out the tube with the lens, in other words, remove the latter farther from the intercepting plate if nearer objects are to be impressed, and on the other, push it in, if more distant ones are wanted. The same effects might be produced at equally the same distance by simply substituting lenses of different power. Now, the human eye has to fulfil the requirement of projecting clearly-defined images on the retina, whether they are but a few inches off, or at an immeasurable distance. The eye being strictly subject to lenticular laws, either the space between the lens and the retina must have the power of varying, or the lens itself, by a change of form, must exercise now a stronger now a weaker power of refraction.
The same parts as in Fig. 2, besides: I, Iris; K, Cillary Muscle; L, Suspensory Ligament of the lens (zonula).
The conclusion has now been arrived at, that this power of accommodation depends on a varying curve of the lens.
In order to effect this, a great degree of elasticity, chiefly of the outer capsule, was obviously necessary, and we find this requisite complied with by an admirably delicate structure of concentric layers, according to which its density reaches the minimum at the periphery of the lens, while its aggregate power of refraction is increased, as if it were composed entirely of the strong refracting substance of which the centre consists.
As the power of accommodating its focus rests on this quality of the lens, it is necessarily accompanied by a loss of elasticity with increasing age. The eye of a man of sixty, that sees distinctly at a distance, is unable to distinguish an object five inches off; and, if, against all rule, it see sharply at five inches off, this benefit is counteracted by the inconvenience of seeing distant objects with great indistinctness, The gradual hardening of the lens is accompanied by a decrease of power of adjusting its focus, and is even subject to such small individual fluctuations that, by an exact calculation of its play, we may sometimes arrive at the most indiscreet conclusions respecting the age.
Should the lenticular elasticity no longer admit of a sufficient scope for refraction, we must then either adjust the distance of the objects, as we see a far-sighted individual do, by holding the book proportionably farther off, or we must afford the eye assistance by accommodating it with movable auxiliary lenses, spectacles, which replace the lost power of adjusting forms to the natural eye. This power of accommodating the focus disappears beyond recall if the lens has sustained an injury, or if we remove it entirely, from its having grown turbid. This takes place in the operation for cataract, which is a dimming of the sight from a thickening of the lens.
The lens, however, is not altogether free from optical irregularities, as when the focus of the eye has not been perfectly adjusted for seeing in the distance. Those of you who are short-sighted, on looking at a distant street-lamp, perceive, instead of the clearly-defined image, an irregular circle of light, and you will at the same time observe within that circle a number of peculiar rays and dots, which are nothing but irregularities in the lens, i. e., the reaction of those irregularities on the retina. Even an eye whose sight is quite normal, makes an analogous observation if it directs its gaze to a very fine point of light, as, for example, to a star. Both the star and the atmosphere are equally innocent of the small beams that radiate from it; they are the rays of our own lens which we transplant to heaven. So little aware are we of what takes place in the depth of our sensory organs, or in the immeasurable distances of the universe.
The spaces between the lens and the cornea, as also between the lens and the retina, are filled with a liquid medium, called humors; the latter, which constitutes by far the largest chamber of the eye, is filled with a gelatinous substance called the vitreous humor. This medium likewise contributes essentially to the concentration of the rays of light, as, lying between two curved partitions, they exercise a similar influence as the lens.
The vitreous humor, optically speaking, is, however, not pure; small granular or wavy forms, which all of you at times have seen hovering within your field of view, and which pursues so many hypochondriacal persons on their summer trip to a watering-place, are occasioned by shadows thrown on the retina by a partial, delicate opaqueness in the vitreous humor. Those bodies are so light as only to be perceptible, either in certain effects of light, or on a peculiar exertion in the straining of the attention. By a simple experiment it is possible to make every person acquainted with these guests of his field of view, the so-called muscæ volitantes; only, one must be prepared for those formerly overlooked, but once honored with attention, never again stirring from their post.
In order, likewise, to preserve space for the play of these variations in the form of the lens, in the act of adjustment, it was necessary to surround the lens with a liquid or elastic medium. That the aqueous humor bears a part in moistening the cornea, has already been stated, but in the voluminous vitreous humor we behold the regulator of the shape and tension of the eye. Such an auxiliary medium is of urgent necessity to keep up the regularity of refraction, the quality of tension in the retina as a sensitive surface, and the action of the optic nerve. Some years ago, I was happy in being able to demonstrate that a comprehensive range of diseased conditions and also blindness, the causes of which had been successively sought for in different parts of the eye, simply arose from too great a tension being exercised by the vitreous humor—a discovery which imparted so much the more pleasure, as a suitable remedy was likewise at hand.
Let us imprint on our memories, from the model of a magnified eye, the positions and dimensions of the different parts. For the present, I begin with the cord of the optic nerve, which, as you perceive, does not enter the sclerotica exactly opposite to the cornea. This part, as well as the larger posterior chamber of the eye, is embedded in the orbit, and is therefore not visible externally; while, on the other hand, between the lids, you remark the white of the eye, which is the anterior chamber of the choroid; next the transparent cornea; and away through it the colored iris, with the pupil in the centre.
The black appearance of the latter used to be ascribed solely to the dark layer formed by the choroid for the interior of the eye, absorbing all the light. More patient and minute investigations have, however, proved that the pupil derives its blackness only partially from the above circumstance, and mostly from the refraction of the light. Helmholtz has succeeded in banishing that darkness from the pupil of the human eye. By a simple arrangement, called the speculum oculi, he uses the light which is reflected from the deeper parts of the eye to illuminate the whole of the interior, as also the image itself projected on the retina. This invention exercises an influence not only on the peculiar branch of the oculist, but likewise on the broad field of medical investigation, seeing it affords an insight into the optic nerve, a direct process of the brain, and other structures, which, along with their analogies, were hidden from observation in the body.
The dimensions of the eyeball among strong-sighted individuals is more equal than you perhaps imagine. The apparent differences of size lie almost exclusively in the shape of the slit of the lid. If it has a wide slit, it affords a view over a greater part of the eyeball, and we think it bigger, simply because we see more of it. In the same manner, our judgment is deluded by the different degrees of prominence of the eye. A staring or protruding eye impresses us as being larger, although it is only pressed forward; while in advanced age, or in consuming sickness, the sunken eye is thought smaller.
If the eye really is larger, then the distance between the cornea and the lens will be greater; and if the effects of the refracted light remain the same in the latter, the image will no longer be projected on the cornea, but in front of it. And this is what really takes place in that wide-spread malady called short-sightedness. Here we have especially to note that the mean axis of the eye is too long. There are others called far-sighted eyes, whose visual axis is too short, the image for such eyes falling behind the retina. In order to reestablish the conditions of keen vision in both cases, the effects of refraction must be diminished for the short-sighted, by diverging or concave glasses; for the far-sighted, by collective or convex glasses. Those conditions have nothing to do with the want of the power of adjustment of focus. If you correct the defective construction of the short-sighted eye with a concave glass, and that of a far-sighted with a convex one, the lens—its mobility being preserved—can with their aid accommodate itself to near and distant objects, which neither an old man, deprived altogether of the power of adjustment, nor an individual who has been operated on for the cataract, is capable of doing.
Let us now pursue the analysis of the model in the same order as at the outset. First, then, fold back the cornea with the anterior section of the sclerotica; the margin of the eye to the front is now interrupted by the pupil, and is for the rest formed by the tissue of the iris, and the anterior section of the choroid. The space (at present wanting) in front of the iris-curtain was filled with aqueous humor, which you are to suppose has escaped. Let us now remove the posterior half of the tissue of the visual nerve; the whole eye will then be closed up by the passage of the choroid and iris, which now meets with no break, except anteriorly from the pupil, and posteriorly from the entrance of the optic nerve. If, now, as with the sclerotica, we fold back the anterior division of the choroid along with the iris, in doing which we have an opportunity of convincing ourselves of the true nature of the pupil, and that it is indeed an opening, we then come upon the hard lens lying behind. Taking this away, as also the gelatinous vitreous humor, we at length remove the posterior section of the choroid, and have nothing left but the optic nerve and the retina. Thus we have again arrived at the starting-point of our reflections; and it only now remains for me to bring before you the retina.
Before, however, entering on this division of my subject, let me call your attention to a few fundamental processes in the act of seeing. First, the picture or image on the retina is perfectly sharp only at one particular spot, situated somewhat beyond the optic nerve and exactly opposite the centre of the cornea. The light which falls along the main axis of the eye converges at this point. This spot in the retina is marked by a small hollow. For the rest, it is filled up with a structure of its own; and we have reason for assuming that it furnishes the most exact perceptions, not only on account of the greater optical sharpness of the image, but also on account of the higher energy or activity with which it is endowed. It is this spot we make use of when we desire to go into details; for, if we wish to examine closely into the nature of an object, either we approach it to the eye, or bring the eye to bear on the object; but, in both cases, in such a manner as to cause the image to fall exactly on the hollow of the retina, or on the spot of direct vision. This arranging of a position for an object is what we mean when we speak of adjusting the eye.
The images which are not projected on the spot of direct vision are not sharp; for the necessary light falls on the refracting media more or less obliquely. This, and the decrease in the activity of the sight from the hollow to the sides, explain how the objects, the farther they are removed from the fixed point, appear with so much less clearness and sharpness of outline. Indirect or eccentric vision, as it is termed, makes us aware simply of the presence of objects, by giving us some notion of their shapes; but we are unable to distinguish even the biggest letters, if the image of them should fall only one hair's breadth off that one spot in the retina. In reading, the eye must constantly move onward to the end of the line, the single letters thus gradually imprinting themselves on the direct point of vision. On the other hand, indirect vision offers hints for fixing the object in our eye; it warns us of, and prepares us for, the object previous to our devoting our whole attention to it, and it is further of use in procuring a wide survey, by enabling us to see and examine what lies before us. There are some who only possess direct vision. Any one can put himself in the place of an individual so afflicted, by holding a long tube of small calibre to his eye. You naturally distinguish the most minute objects enclosed within the restricted range of vision; but, deprived of the lateral impressions, you could not guide your steps in the street. In short, you must fancy the image of the external world that is imprinted on the retina, like a picture highly finished in the middle, and only roughly sketched out at the sides.
The distance from the spot of direct vision, at which the objects may be perceived by eccentric vision, has its limits. When looking straight before you, you can just perceive a hand which stretches down the whole length of the face, in the direction of the temples. This is the extremest point from which it is possible for light to fall on the retina; but, if you attempt to go beyond it, the hand disappears, its image not being projected on the retina. The combination of all the extreme points from which, with a set eye, impressions may be received, forms the frame of the field of view, and what is within this frame is itself the field of view.
This space, immovable as regards the head, becomes the arena to which both those excitations produced by the senses, and those belonging to subjective sight, are transposed; and this transposition always takes place in the same direction as that in which a regular act of sight would in any case lie. Although long experience has taught us it is a delusion, still we always place the image projected by a mirror behind it, because the reflected light falls into our eye just as if the object were behind it. But the sparks produced by pressing the eye from the side of the temples, we seem to see in the opposite side of the field of view; but we do so because, in the normal act of sight, the retina is irritated (from the side of the temples) by light falling from the opposite side. By the action of projection, the reversed image naturally regains its upright position in the field of view.
A general irritation of the retina unaccompanied by a perception of objects will give us a light field of view, and, on the other hand, a perfect repose of those parts will give us a dark field of view. The former represents the sense of the repose of a mechanism endowed with the power of action; the latter corresponds to the absence of all mechanism whatever. The feeling of darkness, therefore, results merely from the expansion of your field of view as opposed to your retina, if I may so say; while behind your back you have the feeling neither of light nor of darkness; you simply miss all sensation of light.
Touching the size of the images on the retina, as compared with the objects themselves, I need merely remind you of the rules of perspective. The images on the retina stand in reversed proportion to the distance of the objects. The image of a pencil, held a foot from your eye, covers the trunk of the tree before your window; that of a pea, at a like distance, covers the moon in the sky. If, notwithstanding, we think the moon bigger than the pea, and the tree than the pencil, the reason is that, apart from our being well acquainted with the tree, our judgment is a combination of the size of the image on the retina and the distance of the object. Now, as consciousness is for the most part founded on experience, so just and correct perspective sight is in the main something we acquire. A child will assuredly not appreciate the difference between the pencil and the trunk of the tree in the same degree as an individual who by experience has learned to know the value of his impressions. What the child first knows of the moon is, that he cannot reach it with his hand; "But," as I once heard a child say, "mother can reach it down." Other inferences have helped him to this conclusion. We are so accustomed to merely play with children, that we are easily blinded to the full seriousness of such requests.
We cannot break off these reflections on the image of the retina without making mention of a remarkable spot in the background of the eye—the yellow spot marking the entrance of the optic nerve. All perception whatever is arrested within the bounds of this spot—it is a blind point in our field of view.
The blind spot is by no means so excessively small. At the distance of four paces, it would cover a man's head in the centre of your field of view; and almost a hundred moons in the sky would find room within its bounds. When Mariotte made this important discovery, it caused so much commotion that the experiment had to be repeated before the King of England in 1688. In the endless variations of the experiment, the remarkable fact only received a new confirmation. For the rest, this discovery had almost proved fatal to the doctrine of perception; for, as at that time the optic nerve and the retina being considered as essentially the same, one might deduce, a priori, the inference that, just at that point of entrance at which all the conducting fibrils converged, a heightened sensibility might be argued. But, now proving to be insensitive, the retina itself could no longer be regarded as the regular conductor of the sensation of light. And this was the conclusion to which Mariotte did arrive, transferring the sensibility to the choroid behind the retina, till at length Bernoulli and Haller again restored it to its rights.
This apparent enigma is explained by what I told you of the general relation of light to the visual organ. The part the optic nerve plays is only that of a conductor, while the sensations of the vibrations of the ether, as also of the specific sensory irritation, is committed to the retina, or, more correctly speaking, to its external layer.
Another question is, Why does the existence of the blind spot usually escape our attention? The chief reason is that, as the gap is regularly situated in the same spot in the field of view, the idea has learned to fill it up in the most natural manner, and as is suitable for the connection of objects. For instance, I draw the figure of a cross on the board, and fix my eye on it, so as to cause the centre of this figure enclosing the point of intersection to fall on the blind spot; in this manner I believe indeed that I see a cross, while in reality I only see what lies beyond; fancy supplementing the rest. The cross is a commonly-known figure, and when any two lines take a perpendicular direction toward each other, they as a rule really do intersect each other. The best proof of this being the case is that, when you obliterate all that lies within the district, you still continue to see the cross; and, to make the experiment more elegant, if you place some photograph in the empty space, you do not perceive it, you still continue to see only the cross. You have here, then, a conjunction of objective sensory action and subjective influence, apparently with the help of the central extremity of the optic nerve, which is highly significant for the whole doctrine, and which to a certain degree combines what I have been endeavoring to explain to you on both of those branches of the subject.