Chapter D: D, an apparatus for holding a small tube, that the spectrum given by (2)

An organ which is the instrument of so many nice discriminations as is the eye must, of course, present the most delicate adjustment in its parts. So much has in recent times been learnt of the nature of its mechanism; of the relation between the impressions made upon it and the judgments formed by the mind therefrom; of the illusions which its very structure produces; of the defects to which it is liable; and of its wonderfully refined physiological elements—that a branch of science sufficiently extensive to require a large part of a studious lifetime for its complete mastery has grown up under the hands of modern physiologists, physicists, and psychologists. To some of the results of their labour we would invite the reader’s attention; and in order to render the account of them intelligible, we must, to a certain extent, describe “things new and old.”

_THE EYE._

FIG. 234.—_Vertical Section of the Eye._
]

The form of the human eye and the general arrangement of its parts may be understood by referring to Fig. 234, which is a section of the eyeball. It has a form nearly globular, and is covered on the outside by a tough firm case, A, named the _sclerotic coat_, which is, for the most part, white and opaque. This covering it is which forms what is commonly termed the “white of the eye;” but in the front part of the eyeball it loses its opacity, and merges into a transparent substance, termed the _cornea_, B. The cornea has a greater convexity than the rest of the exterior of the eyeball, so that it causes the front part of the eye to have a somewhat greater projection than would result from its general globular form. This sclerotic coat—with its continuation, the cornea—serves to support and protect the more delicate parts within, and is itself kept in shape by the _humours_, which fill the whole of the interior. The greater space is occupied by the _vitreous humour_, C; but the space immediately behind the transparent cornea is filled with the _aqueous humour_, D. The latter is little else than pure water, and the former is like thin transparent jelly. The cavities containing these two humours are separated by the transparent double convex lens, E, called the _crystalline lens_, which, in consistence, resembles very thick jelly or soft gristle. The outward surface of this lens has a flatter curvature than the inner surface. Immediately in front of the crystalline lens is found the _iris_, F, which may be described as a curtain having in the middle a round hole. The iris is the part which varies in colour from one individual to another—being blue, brown, grey, &c.; and the aperture in its centre is the dark circular spot termed the _pupil_.

The general disposition of the parts of the eye with regard to light will be most easily understood by comparing it with an optical instrument, to which it bears no little resemblance, namely, the _camera obscura_, so well known in connection with photography. We may picture to ourselves a still more complete resemblance, by imagining that the lens of the camera is single, that we have fixed in front of it a watch-glass, with the convex side outwards, and that we have filled with water the whole of the interior of the camera, including the space between the watch-glass and the lens. The _focussing-screen_ of the camera corresponds with the inner surface of the back of the eyeball, about which we shall presently have more to say. Now, even if the camera had no lens, but were simply a box filled with water, and having in front the watch-glass, fixed in the manner just mentioned, we could obtain the images of objects on the screen, as a consequence of the curvature of the watch-glass. It would, however, in this case, be necessary to have the camera much longer, or, in other words, the rays would be brought to a focus at a greater distance than if we put in the glass lens, which would, thus placed in the water, cause the rays to converge to a focus at a much shorter distance, although its effect when surrounded by water would be less powerful than in the air. There we see the effect of the crystalline lens of the eye in bringing the rays to a focus within a much shorter distance than that which would be required had there been present only the curved cornea, and the aqueous and vitreous humours of the eye, which are but little different from pure water in their optical properties.

If we _focus_ the camera by adjusting the distance between the lens and the screen so as to get a distinct image of a near object, we should find, on directing the instrument to a distant one, that the image would be blurred and indistinct, and the lens would have to be moved nearer to the screen; or we could get the image of the distant object distinct by replacing the lens by another lens in the same position, but having some flatter curvature. It is plain that the same object would be gained if our lens could be made of some elastic material, which, on being pulled out radially at its edges, could be made to assume the required degree of flatness without losing its lenticular form. Now, it is precisely with an automatic adjustment of this kind that the _crystalline lens_ of the eye is provided, for the lens is suspended by an elastic ligament, G, by the tension of which its surfaces are more flattened than they would otherwise be; but when the tension of this ligament is relaxed, by the action of certain delicate muscles which draw it down, the elasticity of the lens causes it to assume a more convex form.

FIG. 235. _Section of Retina._
]

These optical adjustments give, on the inner surface of the coats of the eye, a more or less perfect real image of the objects to which the eye is directed, and it is on the back part of this inner surface that the network of nerves, called the _retina_, H, is spread out. The sclerotic coat, already spoken of, is lined internally with another, named the _choroid_, which is composed of delicate blood-vessels, intermingled with a tissue of cells filled with a substance of an intensely black colour. It is upon this last layer that the delicate membrane of the retina is spread out between the choroid and the vitreous humour.

The retina is, in part, an expansion of the fibres of the optic nerve over the back part of the eyeball. If we suppose the globe of this cut vertically into two portions, and so divide the front from the back part of the eye, the retina would be seen spread out on the concave surface of the back part, and in the middle of this part, opposite the crystalline lens, would be seen a spot in which the retina assumes a yellowish colour, and in the centre of this, a little round pit or depression. The spot is called the _macula lutea_, or _yellow spot_, and the little central pit, which is of the highest importance in vision, is termed the _fovea centralis_. A little way from the yellow spot, and nearer the nose, is a point from which a number of fibres are seen to radiate, and this is, in fact, the part at which the optic nerve enters the eyeball, and from which it sends out its ramifications over the retina. This part, for a reason which will shortly appear, is called the _blind spot_.

When the minute structure of the retina is examined by the microscope, its physiological elements are found to undergo very remarkable modifications at the yellow spot. In the retina, although the total thickness does not exceed the 1/80th part of an inch, no fewer than eight or ten different essential or nervous layers have been distinguished. Fig. 235 rudely represents a section. The lowest stratum, A, which is next the choroid, and forms about a quarter of the total thickness, is formed of a multitude of little rod-shaped bodies, _a_, ranged side by side, and among these are the conical or bottle-shaped bodies, _b_. This lowest stratum of the retina is called the _layer of rods and cones_. At their front extremities the rods and cones pass into very delicate fibres, which, going through an extremely fine layer of fibres, B, are connected with a series of small rounded bodies, which form the layer of _nuclei_, C, separated by a layer of nervous fibres, D, from a granular layer, E, in front of which is a stratum of still finer granules, F, underlying a layer of ganglionic nerve-cells, G, of a larger size than any of the other elements, and these ganglionic cells send out numerous branching nerve-fibres, forming the layer H. Finally, on the front surface of the retina there is a thin stratum formed of fibres, which issue from the optic nerve, K, Fig. 234, and in fact constitute the expansion of this nerve on the inner surface of the eyeball. The terminations of some, at least, of these nerve-fibres have been traced, and have been found to form junctions with those branching from the ganglionic cells.

FIG. 236.
]

Of the part played by each of these delicate structures in exciting visual impressions little is yet known. How light, or the pulsations of ether, if such there be, is ultimately converted into sensation will probably for ever remain a mystery, although it is quite likely that the kind of visual impression which is conveyed by each part of the elaborate structure of the retina may ultimately be distinguished. One curious result of modern investigation is that _light falling directly upon fibres of the optic nerve is quite incapable of exciting any sensation whatever_. Light has no more effect on this nerve and its fibres than it would have on any other nerve of the body if exposed to its action. The apparatus of rods, cones, and other structures are absolutely essential to enable light to give that stimulus to the optic nerve which, conveyed to the brain, is converted into visual sensations. So if this apparatus were absent in our organs of vision, in vain would the optic nerve proper be spread out over the interior of the eyeball: we should be no more able to _see_ with such eyes than we are able to see with our hands.

We now invite the reader’s careful consideration to the diagram, Fig. 236, which is a section of the retina through the yellow spot. The upper part of the figure is the front, and the deep depression is the little pit already spoken of—the _fovea centralis_. The lowest dark line represents the basement membrane of the retina, and immediately above is seen the layer of rods and cones, and the various strata already spoken of are represented in their due order in the marginal parts of the diagram. Now observe the remarkable modifications of the nervous structures in the neighbourhood of the _fovea centralis_, some of which are visible in the diagram. In the first place, the cones are there much longer, more slender, and more closely set, so that there is a far greater number of them on a given surface; but the rods are comparatively few, and are, in fact, not found at all under the floor of the little pit. The layer of _nuclei_, into which the cones extend, is thinner, and is found almost immediately below the anterior surface, for all the other layers thin out in the fovea in a very curious manner. It is, however on the margin of the fovea that the stratum of ganglionic cells, G, Fig. 235, attains its greatest thickness, for there it is formed by the superposition of eight or ten cells, being here thicker than any other layer, while it is so thinned off towards the margin of the retina that it no longer forms even a continuous stratum. This layer, however, becomes much thinner _in the fovea_, which contains, in fact, but few superposed cells. The tint of the yellow spot is said to be derived from a colouring matter, which affects all the layers except that of the cones. The centre of the yellow spot, where the _fovea centralis_ is situated, is extremely transparent, and is so delicate that it is very easily ruptured, and has frequently been taken for an aperture.

We should not have risked wearying the reader with these details concerning the little pit in the centre of the retina had it not possessed an extreme importance in the mechanism of the eye, a fact which he will at once appreciate when we say that _of the whole surface of the retina, the only spot where the image of an object can produce distinct vision is the fovea centralis_. Since this is undoubtedly true, it follows that the physiological elements which we there find are precisely those which are most essential for producing this effect. The case may be exemplified by recurring to the comparison of the eye with a photographer’s camera, by supposing his screen to be of such a nature that only on one _very small spot_ near its centre could a distinct image be possibly obtained of just one point of an object. Such a defect in his camera would render the photographer’s art impossible, and this defect (if it may be so called) in the eye would render it almost equally useless, had not an adjustment, which more than compensates for it, been afforded in the extreme _mobility_ of our organs of vision. This adjustment is so perfect that people in general do not even suspect that the image of _each point_ of an object which they distinctly see must be formed on one particular spot on the retina—a spot about one-tenth of the diameter of an ordinary pin-head! We may venture, without any disrespect to the reader, to assume that the chances are that it is new to him to learn how each letter in the lines beneath his eye must successively, but momentarily, form its image in the very little pit in the centre of his retina; and the chances are at least a hundred to one that, even if aware of this, he has passively received the statement, and that he has not made the least attempt to _realize the truth for himself_. Yet nothing is easier. Let him request a friend to slowly peruse some printed page, while he meanwhile intently watches his friend’s eyes. He will then perceive that before a single word can be read there is a _movement_ of the eyeballs, which are, quite unconsciously to the person reading, so directed that the image of each letter (for the area of distinct vision is incapable of receiving more than this at once) shall fall upon the only parts of the retinæ from which a distinct impression can be conveyed along the optic nerve. Thus it is that the eye, without any conscious effort of the observer, is directed in succession to the various points of an object, and it is only by an effort of will in fixing the eyes upon one spot that one becomes aware of the blurred and confused forms of all the rest of the visual picture. Yet so readily do the eyeballs turn to any part of the indistinct picture on which the attention is fixed, that it is not improbable a person unversed in such experiments, wishing to verify our conclusions by looking, say, at one spot on the opposite wall, will be very apt, in thinking of the features of the rest of the picture, to direct his eyes there, and then declare that he, at least, sees no such vague forms. If such be his experience, the correction is easy. He has only to ask some one to watch closely his eyes while he repeats the experiment, and after a few trials he will succeed in maintaining the requisite immobility of the eyeballs—a condition upon which the success of many such experiments depends.

FIG. 237.—_Muscles of Eyes._

The muscles of the eyeballs viewed from above:—B, the internal rectus;
E, the external rectus; S, the superior rectus; T, the superior
oblique, passing through a loop of ligament at U, and turning
outwards and downwards to its insertion at C. The inferior rectus
and the inferior oblique are not visible in the figure: the superior
rectus is removed from the right eyeball in order to show the optic
nerve N.
]

This extreme mobility of the eyeballs more than compensates for the loss of the clear and well-defined picture, for it calls into action one of the most sensitive of all the impressions of which we are capable, and one which possesses in so high a degree the power of uniting with our other sensations, that this sixth sense has been, as already stated, utterly overlooked, except by the more modern students of the nature of our sensations. It is usually termed the _muscular sense_, and to it are due some of the nicest distinctions of impressions of which we are capable. The muscles of every part of our frame take their part in producing impressions in our minds, and those of the eyeballs have a very large share in furnishing us with ideas of forms and motions. Fig. 237 is a diagram showing the general arrangement of these muscles; and their anatomical designations, which need not much concern us at present, are given beneath the figure. The wonder is, that the sensations arising from the relative conditions of parts so few, should afford us the immense variety of notions referrible for their origin to these muscles only. We take one example in illustration. Suppose we watch the flight of a bird, at such an elevation that no part of the landscape comes into the field of view at all; and that, again, we follow with the eye, under similar circumstances, the path of a rocket. We can unhesitatingly pronounce the motions unlike, and yet in each case there was no visual impression present but that of the object focussed upon the yellow spot. But the movement of _the muscles in one case_ was different from that in the other. Nay more, we can form such a judgment of the motion as to pronounce that the object followed such and such a curve—we may recognize the parabola in one path, and the circle, perhaps, in the other. And this kind of discrimination arises from the fact, that when we have, maybe times without number, previously looked at parabolas and circles, in diagrams perhaps, the muscles of the eyeballs have performed just the same series of movements, as point after point of the line was made to form its image on the yellow spot. This is not the only class of impressions that these muscles are capable of affording; there is, for example, little doubt that they aid us in estimating distance. But space will not permit further discussion of this subject.

Although the blurred and indefinite retinal picture may be compensated, and perhaps more than compensated, by the readiness with which the eyes move, it is, of course, possible that greater precision and delicacy of visual impression over the whole surface of the retina might be consistent with a still greater increase of our powers of perception. There are instances in which the absence of finish, as it may be termed, in all but one little spot in the picture, proves a real inconvenience and a sensible deprivation. Perhaps a friend calls our attention to the fact that a balloon is sailing through the air, or some fine morning, hearing in the fields the blithe song of the sky-lark, we look up and vainly try to bring the small image upon the place of distinct vision. Now, if an image which falls upon any other part of the retina is perceived, even indistinctly, an instant suffices to direct the eyes into the exact position requisite for clear vision—an example of the marvellous precision with which impressions are put in relation to each other by the unconscious action of the brain. But while an image on the fovea, only 1/6000th of an inch diameter, produces a distinct sensation, it is found that if the image falls on the retina at a point some distance from the yellow spot, the image must be 150 times larger in order to produce any impression; and it is in consequence of the image of balloon or bird not having the requisite size to give any impression to the less sensitive portion of the retina, that we grope blindly, as it were, until by chance the image falls near the yellow spot, when the tentative motion of the eyeballs is instantly arrested, and the image fixed. On the other hand, the field of indistinct vision which the eye takes in is extremely wide, for bright objects are thus perceived, even when their direction forms an angle laterally of nearly 90° with the axis of the eye; and, if the object be not only bright, but in motion, its presence is noticed under such circumstances with still greater ease. Thus, an observer scanning the heavens would have a perception of a shooting star anywhere within nearly half the hemisphere. The range is, however, less than 90° in a vertical direction.

We have said that the fibres of the optic nerve, entering the back part of the eyeball, at K, Fig. 234, ramify over the anterior surface of the retina in fibres which form a layer of considerable relative thickness. The light, therefore, first encounters these nerves, and only after traversing their transparent substance does it reach the deeper seated layer of rods and cones, where it excites some action that is capable of stimulating the optic nerve. These rods and cones might naturally be supposed to be merely accessory to the fibres of the optic nerve, had we not the following conclusive evidence that the cones play a necessary part in the action, and that it is only through them that light acts upon the optic nerve:

FIG. 238.
]

1. The cones are more developed and more numerous in the spot where vision is most distinct.

2. The “blind spot” is full of fibres of the optic nerve, but is absolutely insensible to light, and is without rods or cones.

3. We can distinguish an image on the fovea, having only 1/6000th of an inch diameter; but on the other parts of the retina the images must have larger dimensions. It is found that the size of the smallest distinguishable images agrees nearly with the diameters of the cones at the respective parts.

To some readers the fact will doubtless be new, that a considerable portion of the eye is quite insensible to light, namely, that portion already designated as the “blind spot.” A simple experiment, made by help of Fig. 238, will prove this. Place the book so that the length of the figure may be parallel to the line joining the eyes, and let the right eye be exactly opposite the white cross, and at a distance from it of about 11 in. If the left eye be now closed, while with the right the cross is steadily viewed so that it is _always_ clear and distinct, the white circle will completely disappear, and the ground will appear of a uniform black colour. In order to insure success, the observer must be careful not to _look at_ the white circle, but at the cross, and some persons find this more difficult than others. The position of the blind spot in the eye has been already mentioned, and its significance in showing the insensibility to light of the fibres of the optic nerve has been pointed out. In the table of the dimensions of some parts of the eye, which, for convenience of reference, is given together below, it will be seen that the diameter of the blind spot is considerable compared with the size of the retina, its greatest diameter being about 8/100 in. The length on the retina of the image of a man at a distance of 6 ft. or 7 ft. is not greater than this, so that in a certain position with regard to the eye a person would, like the white circle, be quite invisible. In like manner, by looking steadily in a certain direction with one eye, the image of the full moon may be made to fall upon the blind spot, and the luminary then becomes invisible, and would be so even if its apparent diameter were eleven times greater; so that if we suppose eleven full moons ranged in a line, the whole would be quite invisible to a person looking towards a certain point of the sky at no great angular distance from them.

The following are the dimensions in English inches of some parts of the eye:

In.
Diameter of the entrance of the optic nerve 0·08
Distance of centre of optic nerve from centre of yellow spot 0·138
Diameter of _fovea centralis_ 0·008
Diameter of the nerve-cells of the retina 0·0005
Diameter of the _nuclei_ 0·00003
Diameter of the rods 0·00004
Diameter of the cones in yellow spot 0·00018
Length of rods 0·0016
Length of cones in yellow spot 0·0008
Thickness of retina at the back of the eye 0·0058

By means of an instrument to be presently described, the ophthalmoscope, it is possible to view directly the whole surface of the retina, and to observe the inverted images of the objects there depicted. It is thus observed that it is only on the parts near the yellow spot that the images are formed with clear and sharp definition. Away from this the definition is less perfect; and besides the diminished sensitiveness of the retina, this circumstance contributes to the vagueness of the visual picture, although the falling off in clearness of vision at a very little distance from the yellow spot is far more marked than the loss of definition in the image there formed.

Until within the last few years it has been most confidently asserted by many authors that the eye, considered as an _optical instrument_, is absolutely perfect, and entirely free from certain defects to which artificial instruments are liable. Thus Dr. W. B. Carpenter states, in his “Animal Physiology” (1859): “The eye is much more remarkable for its perfection as an optical instrument than we might be led to suppose from the cursory view we have hitherto taken of its functions; for, by the peculiarities of its construction, certain faults and defects are avoided, to which all ordinary optical instruments are liable.” Among the imperfections which are completely corrected in the eye, he names “spherical aberration” and “chromatic aberration”—both of which give rise to certain defects in optical instruments. But by recent careful investigations it has been conclusively shown that the eye is not free from chromatic aberration; that it has defects analogous to spherical aberration; and that there are, besides, certain optical imperfections in its structure, which are avoided in the artificial instruments. Professor Helmholtz, one of the most distinguished of German mathematicians, physicists, and physiologists, whose great work on “Physiological Optics” is the most complete treatise on the subject which has ever appeared, is so far from considering the eye as possessed of all optical perfections that he remarks that, should an optician send him an instrument having like _optical_ defects, he would feel justified in sending it back. The defects which may be traced in the eye, _considered as an optical instrument_, do not, however, he admits, detract from the excellence of the eye _considered as the organ of vision_.

When we find that Sir Isaac Newton pointed out the chromatic aberration of the eye two centuries ago—when we find that D’Alembert, in 1767, proved that the lenses of the eye might have as great a dispersive power as glass without the want of achromatism necessarily becoming noticeable—when we find that the celebrated optician Dolland, the inventor of the achromatic lens, showed that the refractions which take place in the eye all tend to bring the violet rays towards the axis more than the red—when we find that Maskelyne the astronomer, Wollaston the physicist, Fraunhofer the optician, and other scarcely less distinguished men of science, have made actual measurements of the distances of the _foci_ in the human eye for the different rays of the spectrum—when we find how these defects have so long ago been observed, examined, and measured as to their amount—the persistence with which writer after writer has asserted the achromatism of the human eye appears so extraordinary, that it can only be accounted for by the prevalence of the preconceived notion that the eye is absolutely perfect—a notion not without its reason and grounds, in the fact of the exquisite adaptation of the organ of sight to the needs of humanity.

Although the want of achromatism in the eye thus escapes ordinary notice, it is, on the other hand, easy to render it evident by simple experiments. If, for example, we view from a certain distance the solar spectrum projected on a white screen, it will be found that, when we see the red end quite distinctly, the violet end will, at the same time, appear vague and confused, and _vice versâ_. The author believes that the following very simple experiment will at once convince any person that the fact is as stated. Procure a small piece of blue or _violet_ stained glass, and another piece of _red_ glass, and, having cut out of an opaque screen a rectangular opening, say ½ in. long and ¼ in. wide, place the glasses close to it, so that one-half the opening is covered by the red glass and the other half by the violet glass, the two being placed so that, on looking through the screen, a violet square and a red square are visible. The opaque screen may be made of black paper, cardboard, or tinfoil, and the edges of the opening must be cut perfectly even. On looking through this arrangement, held at a distance of about two feet from the eye, both squares may be seen distinctly by a person of ordinary vision; but, at a distance of five inches from the eye, he will find it impossible to see the squares otherwise than with vague and ill-defined edges. This is because the crystalline lens cannot adapt its curvature so as to bring the rays from the object to a focus on the retina. Now, by trial, the nearest distance at which each of the coloured squares becomes visible may be found, and it will be observed, that the violet square is first sharply defined at a less distance than the red, whereas, if the eye brought the red and violet rays to a focus at the same point, the smallest distance of distinct vision would coincide in both cases.

The reader may observe the same fact for himself, in even a still simpler manner, by turning to Fig. 238, page 461. When the white circle is viewed by one eye, at a distance of about a foot, and an opaque screen, such as a coin, is held close to the eye, so that the pupil is half covered by it, the one side of the white circle will appear bordered by a narrow fringe of blue, and the other side by a narrow fringe of orange. If the opaque screen be shifted from one side of the pupil to the other, the colours will change places, the orange appearing always on the same side of the white circle as the screen is held before the eye. The same appearances are presented in a still more marked degree when the full moon is made the subject of the experiment.

The diagram, Fig. 239, shows the course of the red and violet rays from a luminous point, A, the refraction being supposed to take place at B_{1} B_{2}. The violet rays after refraction form the cone, B_{1}, E, B_{2}, and E is their focus; the red rays form the cone, B_{1}, F, B_{2}, and have a focus at F. The position of the retina would be intermediate between E and F, and is indicated by C_{1}, C_{2}. It will be noticed that the violet rays cross, and are received on the retina in the same circle, G G, so that the colours, then blended, would be separately imperceptible; but the point would produce a diffused circular image of the blended colours.

In viewing an object—the moon, for example—the accommodation of the eye is like that indicated in the diagram. The distinct image due to the red rays would be formed behind the retina, and that due to the violet rays would be in front of it. In the image on the retina the most intense rays—such as the orange, yellow, and green—are those which are blended by the adjustment of the eye, and the red and violet form images more out of focus (to use a common expression), and a very little larger than the more intense image. We might expect that a white disc would therefore appear with a fringe of colour, resulting from a mixture of red and violet; but the fringe is too narrow, and the colour itself too feeble, to become perceptible. When, however, the pupil of the eye is half covered, the red and violet images are displaced in different directions, the position of the retina being too far forward for the one, and too far back for the other. The coincidence therefore ceasing, the colours show themselves at the margins of the image.

FIG. 239.
]

The non-perception under ordinary circumstances of the chromatic aberration of the eye is largely due to the greater intensity of the colours which differ least in their refrangibilities. The clearness of our vision does not, therefore, practically suffer from this defect of the eye. Professor Helmholtz constructed lenses which rendered his eyes really achromatic, and looking through these when the pupil was half covered, no coloured fringes were seen at the edges of dark or light objects, or when the objects were looked at with an imperfect accommodation of the eye. He was, however, unable to detect any increase of clearness or distinctness of vision by the correction.

The eye is also subject to other aberrations and irregular refractions, which are special to itself; for example, with moderately illuminated objects the crystalline lens produces images apparently well defined, and nothing is visible to suggest the absence of uniformity in its structure. But when the light is intense, and concentrated in a small object surrounded by a dark field, the irregular structure of the crystalline lens shows itself in the most marked manner. Every one must have noticed the appearance presented by the distant street-lamps on a dark night, and by the stars. The latter we know to be for us mere points of light, and their images produced by perfect lenses would also be mere points; instead of which we see what seem to be rays issuing from the star, an appearance which has given rise to the ordinary representation of a star as a figure having several rays. That no such rays actually do emanate from the real star may be easily proved: first, by concealing the luminous point from view, by means of a small object held up as a screen. If the rays had any existence outside of the eye, they would still be seen; instead of which, the whole of them disappear when the luminous point, or, in the case of the street-lamp, when the flame, is covered by the screen. A second proof that the origin of the phenomenon is in the eye, and not in the object, is afforded by the fact that if, while attentively observing the rays, we incline the head, the rays turn with the eyes, so that when the head is resting on the shoulder the ray which appeared vertical becomes horizontal. The cause of these divergences from the regular image lies in the fact of the crystalline lens being built up of fibres which have refractive powers somewhat different from that of the intermediate substance. These fibres are arranged in layers parallel to the surfaces of the crystalline lens, and the direction of the fibres in each layer is generally from the centre to the circumference; but towards the axis they form, by bending, a kind of six-rayed figure, as shown in Fig. 240, which represents the arrangement of the fibres of the external layers of the lens. In the outermost layers the branches of the star-shaped figure are subdivided into secondary branches, which give rise to more complicated figures. When we view by night a very brilliant but small light, even these subdivisions may be traced in the radiating figure.

FIG. 240.
]

The light which enters the eye is partly absorbed by the black pigment of the choroid, and partly sent back by diffused reflection from the retina through the crystalline lens and pupil. The image of a luminous body as depicted on the retina of another person cannot be seen by us under ordinary circumstances, because, by the principle of reversibility already mentioned as of universal application in optics, the rays which issue from the retinal images are refracted on leaving the eye, and follow the same paths by which they entered it, so that they are sent back to the object. An observer cannot see the retinal image of a candle in another person’s eye, unless he allows the rays to enter his own, and this cannot be done directly, because the head of the observer would be interposed between the candle and the eye observed, and the light would then be intercepted. By holding a piece of unsilvered plate glass vertically, we may reflect the light of a candle into the eye of another person, and then the light thrown out from the retinal image of the candle will, on again meeting the surface of the glass, be in part reflected to its source, and in part pass through the glass, on the other side of which it may be received into the eye of an observer. The positions of the observed and observing eye may be described as exactly opposite to and near each other, while the candle is placed to one side in the plane separating the two eyes, and the glass is held so that it forms an angle of 45° with the line joining the pupils. Under these circumstances the observer may see the light at the back of the eye, but he will not be able to distinguish anything clearly, because his own eye cannot accommodate itself so as to bring to a focus the rays coming from the retina of the other, since these rays are refracted by the media through which they emerge. But, by means of suitable lenses interposed between the two eyes, the retina and all its details may be distinctly seen and examined. Such an arrangement of lenses and a reflecting surface constitute the instrument called the _ophthalmoscope_ (οφθαλμος, _the eye_) of which there are many forms, but all constructed on the principle just indicated. This principle was first pointed out by Helmholtz, who described the first ophthalmoscope in 1851.

FIG. 241.—_Ruete’s Ophthalmoscope._
]

Ruete’s ophthalmoscope is represented in Fig. 241. The parts of the instrument are supported on a stand, C, and about the vertical axis of this the column, D, and the arms, H and K, can turn freely and independently; E is a concave metallic mirror, about 3 in. in diameter, and having an aperture in its centre through which the observer, B, looks. The arm, H, merely carries a black opaque screen, which serves to shield the eye of B from the light of the lamp, and to reduce, if required, the amount of light passing through the aperture in the mirror. The arm, K, which is about a foot in length, carries two uprights which slide along it, and in each of these slides a rod bearing a lens, which can thus be adjusted into any required position. The instrument is used in an apartment where all light but that of the lamp can be excluded. In the instrument just described an inverted image is obtained, which is sufficient for ordinary medical purposes, but this construction does not allow of the examination of retinal images, which is best performed with an instrument having a plane mirror.

The appearance presented by the back of the eye when viewed in the ophthalmoscope is represented in Fig. 242. The retina appears red, except at the place where the optic nerve enters, which is white. On the reddish ground the retinal blood-vessels can be distinguished; A, A, A, branches of the retinal artery, have a brighter red colour, and more strongly reflect the light than the branches, B, B, B, of the retinal vein. Among these, and especially towards the margin, are seen, more or less distinctly, the broader vessels of the choroid. Above the optic nerve and a little to the right may be observed the _fovea centralis_.

FIG. 242.
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During the last twenty years the ophthalmoscope has been the chief means of extending the knowledge of oculists regarding the diseased and healthy conditions of the eye. In this way the substance of the lens and the state of the humours can be directly seen, the causes of impaired vision can be discovered, and the nature of many maladies made out with certainty. This modern invention, by which the interesting spectacle of the interior of the living eye can be observed, has therefore been far from proving a barren triumph of science. Many insidious maladies can thus be detected, and may be successfully treated before the organ has become hopelessly diseased. In some cases the ophthalmoscope gives the most certain evidence of the existence of obscure and unsuspected diseases of other parts of the body.

_VISUAL IMPRESSIONS_.

Everybody knows that, however well the flat picture of an object may imitate the colours and forms of nature, we are never deceived into supposing that we have the real object before us. There must, therefore, be something different in the conditions under which we see real objects from those under which we view their pictures. The most favourable circumstances for receiving an illusive impression of solidity from a flat picture, is when we view it from a fixed position and with one eye. This is because one means by which we unconsciously estimate distances depends upon the changes in the perspective appearances of objects caused by changes in our point of view. In many cases these changes in the perspective are the only means we have of judging of the relative distances of objects. But there is another circumstance which is still more intimately connected with our perception of solidity. Each eye receives a slightly different image of the objects before us (unless these be extremely remote), inasmuch as they are viewed from a different point. When the objects are very near, the two retinal images may differ considerably, as the reader may convince himself by viewing with each eye, alternately, objects immediately before him, while the other eye is closed, and the head all the while motionless. The nearer objects will plainly appear to shift their positions as seen against the back-ground of the more distant objects; and a somewhat more careful observation will reveal changes of perspective, or apparent form, in every one of these objects. An extreme case is presented in that of a playing card, or thin book, held in the plane which divides the eyes. The back or the face, the one side or the other, will be seen, according as the right or the left eye is opened. If we close the left eye, the displacement and change of apparent form produced by a slight movement of the head are sufficiently obvious; a movement of the head 2½ in. to the left causes a decided change in the relative positions of adjacent objects. It is plain, however, that it is precisely from a point 2½ in. to the left that the left eye views these objects, and hence the perspective appearance seen by the left eye must have the difference due to this shifting of the point of view.

On the other hand, if one looks at a picture, or flat surface, placed immediately in front, no change in the relative positions of its parts is discernible by viewing it with either eye alternately. Not but that there is a difference in the retinal images in the two cases, but there is an absence of any point of comparison by which the change may be judged. If we take a photograph of a statue, it will, when viewed by one or the other eye, present the difference of the retinal images which is due to a flat surface; the parts of the photographic image will be of slightly different proportions as seen by each eye. If, instead of the photograph we have before our eyes a statuette, each eye will see a quite different view: the right eye will see a portion which is invisible to the left eye, and _vice versâ_, and, in fact, we shall see more than half round the object. Here, then, we have certain differences of the retinal pictures when solid objects are viewed, and these differences by innumerable repetitions have, unconsciously to ourselves, become associated with notions of solidity, of something having length, breadth, and depth, or thickness. The marvellous delicacy of these perceptions will be alluded to hereafter.

Let us suppose that the lenses of two cameras are fixed in the positions occupied by the two eyes, and that a photograph is taken in each camera, the subject being, for example, a statuette. It is obvious that the differences of the two photographs would correspond with the differences of the two retinal images, and that, if a person could view with the right eye only the photograph taken in the right-hand camera, and with the left eye the left-hand photograph only, there would be formed on the retinæ of his eyes images very nearly corresponding with those which the actual object would produce, and the result would be, if these retinal pictures occupied the proper position on the eyes, that the impression of solidity would be produced, which is called the _stereoscopic effect_.

This may be done without the aid of any instrument, as almost any person may discover after some trials with nothing but a _stereoscopic slide_, if he can succeed in maintaining the optic axis of his eyes quite parallel. In such a case he will observe the stereoscopic effect by the fusing together, as it were, into one sensation, of the impression received by the right eye from the right photograph, with that received by the left eye from the left photograph. But as each eye will, at the same time, have the photograph intended for the other in the field of view, the observer will be conscious of a non-stereoscopic image on each side of the central stereoscopic one. These outside images are, however, very distracting, for the moment the attention is in the least directed to them, the optic axes converge to the one side or the other, losing their parallelism, and the stereoscopic effect vanishes, because the images no longer fall in the usual positions on the retinæ. It is, in consequence, only after some practice that one succeeds in readily viewing stereoscopic slides in this manner, but the acquirement is a convenient one when a person has rapidly to inspect a number of such slides, for he can see them stereoscopically without putting them in the instrument. Many persons, however, find great difficulty in acquiring this power. In such cases it is well to begin by separating the two photographs by means of a piece of cardboard, covered with black paper on both sides. When this is held in the plane between the eyes, each eye sees only its own photograph, and the observer is not troubled with the two exterior images. After a little practice in this way, the cardboard may usually be dispensed with, and the observer will insensibly have acquired the habit of viewing the slides stereoscopically, without any aid whatever.

FIG. 243.—_Wheatstone’s Reflecting Stereoscope._
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Instruments have, however, been contrived which enable one to obtain the desired result without effort; and one form of these is now tolerably well known to everybody. The first stereoscope was the invention of Wheatstone. The reflecting stereoscope is represented in Fig. 243, and consists essentially of two plane metallic mirrors inclined to the front of the instrument at angles of 45°, so that in each of them the observer sees only the design which belongs to it. The rays reach the eyes as if they came from images placed in front of the observer; and the two images having the proper differences, produce together the impression of solid objects.

FIG. 244.
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Brewster’s stereoscope—which is far more widely known than Wheatstone’s—has two acute prisms, or, more usually, two portions of a convex lens are cut out, and placed with their margins or thin parts inwards, and they thus produce the same effect as would be obtained by combinations of a prism with a convex lens. Another very common form of the stereoscope has merely two convex lenses. The effect of the convex lenses is to increase the apparent size of the images by diminishing the divergence of the rays emitted by each point, producing the appearance of larger designs seen at a greater distance. The effect of the prism is to give the rays the direction which they would have if they proceeded from an object placed in a position immediately between the two designs, and an additional element by which we estimate distance, namely, the convergence of the optic axes, is made to aid in the illusion, when the rays proceeding from the two different pictures have approximately the inclination that they would have if they emanated from real objects at the place where the image is apparently formed. The box or case in which the lenses or lenticular prisms are placed takes various forms. One of the most common is represented in Fig. 244, but the stand on which it is mounted is not a necessary part of the instrument, although it is sometimes convenient. A handsome form is met with as a square case, enclosing a number of photographic stereoscopic views mounted on an endless chain in such a manner that they are brought successively into view by turning a knob on the outside. When an instrument of this kind is fitted up with a series of the beautiful landscape transparencies, which are produced by certain continental photographers, a more perfect reproduction of the impressions derived from nature, exclusive of colour, cannot be conceived. We seem to be present on the very spots which are so truthfully depicted by the subtile pencil of the sunbeam; we feel that we have but to advance a foot in order to mix with the passengers in the streets of Paris or of Rome, and that a single step will bring us on the mountain-side, or place us on the slippery glacier; at our own fireside we can feel the forty centuries looking down upon us from the heights of those grand Egyptian pyramids, and find ourselves bodily confronted with the mysterious Sphinx, still asking the solution of her enigma. The truth and force with which these stereoscopic photographs reproduce the relief of buildings are such, that when one sees for the first time the real edifice of which he has once examined the stereoscopic images, it no longer strikes him as new or unknown; for he derives from the actual scene no impression of form that he has not already received from the image.

FIG. 245.
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But of all subjects of stereoscopic photography the glaciers are, perhaps, those which best show the power of the instrument as far surpassing all other resources of graphic presentation. The most careful painting fails to convey a notion of the strange glimmer of light which fills the clefts of the ice, seen through the transparent substance itself. The simple photograph commonly presents nothing but a confused mass of grey patches; but combine in the stereoscope two such photographs, each formed of nothing but slightly different grey patches, and a surprising effect is at once produced: the masses of ice assume a palpable form, and the beautiful effects of light transmitted or reflected by the translucent solid reveal themselves. Another very beautiful class of subjects for stereoscopic slides is found in those marvellous instantaneous photographs, which seize and fix the images of the waves as they dash upon the shore. Here a scene which has tasked the power of the greatest painter is brought home to us with such force and vividness that we all but hear the wild uproar of the breakers.