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double. We have already stated that, when tinct vision will only be obtained by shutthe image of a point falls upon the retina, ting one of our eyes. A very remarkable it is seen in the direction of a line perpen- case of this kind came under our observadicular to the retina at the point of its inci- tion about half a century ago. The two dence. This is the law of visible direction. eyes could not fix their axes on the same When an image of the same point falls upon point, so that the patient, who was a boy the retina of the other eye, it is seen also in at school, was unable to read, and obliged the direction of a line perpendicular to the to abandon his studies. The oculists of that retina at the point of incidence. Two images day supposed that it was an affection of the of the point are therefore seen at the same retina, or the commencement of amaurosis, instant, at the extremity of the optical axis and employed without any advantage the of each eye, or at the point of most distinct ordinary means of cure. When the boy vision in the centre of the foramen; but the was made to look through a telescope, which eyes instantly direct their axes, or converge he did with one eye, he saw objects distinctly, them, to the same point till the two images so that it was obvious that there was no real of the point are coincident, and the distance disease in the eye, and that his inability to of the point of convergence from the eyes is the visible distance of the point. Hence we have the law of single vision for points, and the law of visible distance.

read or see distinctly, which he described by the term dazzling, arose solely from his being unable to fix the axes of his eyes on one point. He was sent to sea-bathing for a month or two, and returned to school perfectly cured.

The law of vision for visible objects is entirely different from that for points. A visible object cannot, in all its parts, be seen Mr. Alfred Smee refers very generally to single at the same instant of time. If the cases somewhat analogous to the preceding, object is a line half an inch long, we cannot where "the globes of the eye wander during see it single with both eyes. The two illness, and are not directed to the same images cannot be made to coincide. When points of sight," an "effect," he says, "which the right-hand extremities or points are coin- is particularly distressing, as two or more cident or seen single, the left-hand extremities representations appear to jump over each are not, but are seen double; and when the other. For this reason,' ," he continues, axes converge upon the left-hand extremities "bed-rooms are now frequently papered or points, the right-hand extremities of the with a tertiary colour, without any distinct line are seen double. The line appears sin- pattern, so that the overlapping may not gle from the rapidity with which the eyes produce any positive change. After railcarry the point of their convergence over way accidents I have seen cases where this the whole of the line. When the visible want of concert of the axes of the eyes has object is a plane surface, only one point of been very distressing to the patient, but it is seen single and distinct, the point of the malady has not proved of any permaconvergence of the axes running over every nent importance.' part of it, and giving us the idea of space in If we rightly understand the preceding two dimensions. When the visible object description, the author means that the axes is a solid, or a body in relief, such as a cube, are either converged to a point nearer or only one point of it is seen single and dis- more remote than the papered wall, so that tinct, the two eyes converging their axes to the images of the patterns did not coincide, the near and to the remote parts of it in but overlapped. This can hardly be called succession, and thus giving an idea of the a want of concert. It is merely an inability different distances of its parts by the vary- in the muscles to obtain a single vision of obing angle of the optic axes. The law of jects at a certain distance, and it is singular vision for points, lines, and solids, furnishes that none of the persons Mr. Smee refers to, us with the true theory of the stereoscope. happened to separate the patterns on the It not only explains all the phenomena of paper-hangings so far as to unite the separatvision, but enables us to calculate them with ed pattern with the one to which it approachas much accuracy as the positions of the ed, and thus make the wall appear to apheavenly bodies. proach or recede from the patient according As all visual phenomena with two eyes as his optical axes were converged to a point depend on the power of converging the optic near or more remote than the wall. It is axes upon every point of an object in suc- from this inability of the eye to fix its opcession, it is obvious that any defect in the tical axes that drunkards see double, and muscular apparatus of the eyeball which that persons recovering from fever see the prevents us from doing this, must be fatal papered walls of their bed-rooms standing to distinct vision. The two images will flutter, as it were, upon each other, and dis

*The Eye in Health and Disease, p. 79, Art. 101.

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the head.*

nea.

The cornea, as we have already said, resembles a small watch-glass. It has a refractive power greater than water, or than the aqueous humour with which it is internally in contact. The form of the cornea has generally been considered spherical; .but M. Chossat discovered, not many years ago, by projecting a magnified profile of it on the wall, that it was elliptical, a fact which had been observed by Sir Isaac Newton in the eye of the sheep, but which he had never published. It is quite possible that the spherical form may not be the normal since cases of conical cornea have been observed, and also cases in which the form of it is cylindrical.

out, or approaching to them, or receding sions, which sufficiently accounted for the from them, and, when the axes are fixed at broken and multiplied images of luminous any distance, moving with every motion of objects. About this time the late Dr. Lyell published a thesis, in which he ascribed Having thus described the general struc- this disease to an extreme thinness of the ture of the eye, and the more important cornea; in consequence of which, it was phenomena of monocular and binocular pushed outwards into a conical shape by the vision, we shall now proceed to describe the pressure of a superabundant quantity of optical changes which take place in the aqueous humour, and he therefore proposed sound eye, the phenomena which these to cure it by evacuating that humour. changes produce, and the means by which Upon examining, however, as we had been their effects may be corrected or removed. requested to do by the late Dr. Henry, one In this inquiry we shall begin with the cor- of Dr. Lyell's patients, in the Manchester Infirmary, who had derived no benefit from having her eye tapped about forty times, we found that the cornea had not been protruded, but had the same irregularities which were seen in Mr. Wardrop's patient, and which we have since found in every other case that has come under our notice. In one of these cases, which was that of a woman from Morpeth, who came to the Royal Infimary of Edinburgh for advice, the writer of this article was requested to contrive for her special glasses to enable her to make some use of her eyes, both of which were affected with this disease in its worst form. The cornea had a very irregular surface, like the scar of a wound, and also such a degree of convexity as to produce great short-sightIn the first of these optical diseases, edness. Concave lenses were, of course, emnamely, conical cornea, the cornea is ex- ployed to correct this last defect, and after tremely prominent, and has, when seen finding out the smoothest portions of the laterally, the form of a cone. When the conical surface, very small apertures were patient looks at a luminous body, such as a placed upon each lens of the spectacle opcandle placed at a distance, he sees several posite to these smooth parts, so to allow the images of it more or less distinct. In ex- images of external objects to be formed on amining, many years ago, the eye of one of the retina by these parts alone, without mixMr. Wardrop's patients, we found that the ing with, and being injured by the impercornea, when carefully viewed in profile, had, fect images formed by the other parts. in all its sections, a regular curvature, be- The patient was thus able to perform cercoming more convex at the apex like a hy- tain kinds of work which she could not do perbola, a form which could produce no de: before. rangement in the refraction of the rays of light. Conceiving, therefore, that the surface of it was not uniform, we found this to be the case by observing the image of a small taper reflected from different parts of it. The image suffered such changes in its size and shape, as to indicate the presence of a number of round eminences and depres

one,

*Mr. Smee has certainly not seen a paper On the knowledge of distance given by binocular vision, published in the Edin. Transactions for 1844, vol. xv. P. 663, and reprinted in the Phil. Magazine for 1847, vol. xxx. p. 305, in which the effects of what Mr. Smee calls want of concert are described, and many singular phenomena explained. See this Journal, vol. xvii. p. 195, where a brief notice of them is given, and the remarkable case of Dr. Christison

quoted.

As the unnatural convexity of the cornea necessarily produces short sight in this disease, Sir W. Adams conceived the idea of removing this part of the complaint, by extracting the crystalline lens, and in one case carried it into effect. The vision of the patient was no doubt improved, in so far as as short sight was concerned, but the effects of the corrugated surface still remained. The same result, however, would have been gained by the use of a concave lens, and it would be greatly surprising if any other oculist should adopt the principle of this extraordinary practice, and dig out the interior of our eyes to save the expense of a pair of spectacles. When we find one medical practitioner making forty demands upon the aqueous humour, and another extracting the

↑ Memoirs of Sir Isaac Newton, vol. i. pp. 218, 420. crystalline lens to cure an incurable disease,

we learn the value of the smallest sprink-spectacles; but if the patient is long or short ling of optical knowledge in those to whom sighted, the remedy for cylindrical vision is we confide the most precious of our bless- to use cylindrical glasses, or glasses in which ings. the horizontal and vertical focus is such as

In extreme cases of conical cornea, where to correct the error and give distinct vision. the patient is virtually blind from the irre- Mr. Airy, our distinguished Astronomergular refractions of the corrugated surface, Royal, to whom astronomy and optics owe or where vision is necessary to his happiness, so many obligations, was the first who or to his maintenance, we venture to recom-studied this imperfection of vision in consemend a method of alleviating it, which has quence of discovering it in his own eyes, and neither been tried nor suggested. Artificial he succeeded in curing it by a cylindrical eyes have been long used to hide an eye lens, which, as he is short-sighted, required which has either been destroyed or disfigur- to be concave on one side, and cylindrical on ed by disease. The artificial eye, which is the other. We have now before us two merely a concave disc of glass, imitating the lenses of this kind, one for cylindrical and iris and the pupil, is placed within the eye- short-sighted eyes, viz., concavo-cylindrical, lids, and partakes so completely of the na- and another for long-sighted eyes, namely, ture of the eyeball, that, with the exception convexo-cylindrical, which were constructed of the variation of the pupil, the imitation under Mr. Airy's directions, and presented of the natural eye is perfect. If we then by him to the writer of this article. A botapply a transparent artificial eye above the tle or a test tube filled with fluid is a cylinconical cornea, and place between it and the drical lens, and two such bottles placed corrugated surface a drop of thick fluid, such cross-wise are equivalent to a convex lens. as albumen, or gelatine, or balsam, of nearly In order, therefore, to examine preparations the same refractive power as the cornea, all in a bottle, we should look at them through the irregularities of refraction at the outer a cylindrical lens whose axis is at right surface of the cornea will disappear, and angles to the axis of the bottle, or through good vision will be obtained. If there are another bottle of the proper size. irregularities in the inner surface of the cornea, they will produce comparatively little effect, being in contact with the aqueous humour, which approximates to the cornea in refractive power.

The cornea is subject to another disease very injurious to vision, namely, a partial opacity, forming one or more white spots either on its surface or between the laminæ of which it consists. These spots vary in intensity from a slight nebulosity to a degree of white opacity almost impervious to light. When they are the result of local irritation or the effect of inflammation, or disease of the adjacent parts, they often disappear with the causes which produce them. In young persons they frequently grow less and less with the growth of the cornea: but when the membrane is really disorganized it

Another optical disease of the eye, and a very rare one, may be called cylindrical cornea, in which the curvature is different in a horizontal and in a vertical direction; and, therefore, the pencils of light incident in a horizontal plane will have their focus at a greater or less distance than those which are incident in a vertical plane. Vision consequently must, in such cases, be very imperfect. This irregular structure is recognised only is not easy to effect a cure.* When these in the imperfect vision it produces, and it cannot be corrected either by convex or concave spectacles. It is possible that it may exist in the crystalline lens alone, or both in the crystalline and cornea, but the determination of this point is of no practical importance, as the method of rendering the vision perfect is in this case independent of the The action of the cornea in refracting the locality of the imperfection. The most obvi- rays incident upon it is often disturbed by ous method of correcting it is to look through the thickening of the secretion which lubria narrow linear aperture placed horizontally, cates it, and enables the eye-ball to move if our horizontal vision is most perfect, or without friction in its socket. In certain through the same aperture placed vertically states of the body this secretion is in such a if our vision in a vertical plane is the best, viscid state that when the eyelids move and it might be useful to try the effect of placing it inclined 45° to the horizon. This

spots are small and semi-transparent, the injurious effect on vision might be removed by placing opposite to them, on plane or other glasses, opaque spots of the same size, or by placing a small aperture of the same form as the good part of the cornea opposite that part.

Dr. Smith of Cambridge says that he has been

plan, however, would be advantageous only told that these opacities may be cured by blowing in those cases where the vertical or horizon-pounded glass every day into the eye!-Treatise on tal vision did not require the aid of ordinary Optics, Vol. ii. p. 5, Remarks.

.

The Sight and how to See.

November,

lamina and fibres of which it is composed are kept in optical contact by a secretion supplied either by special vessels, or obtained by absorption from the aqueous humour. That the capsule of the crystalline freely absorbs distilled water, and also water from the aqueous humour, is a fact established by experiment, and we are therefore entitled to suppose that it is preserved in its healthy state by the water which it thus absorbs.

over the cornea, by that beautiful provision | the beautiful contrivance not referred to by of nature by which it is kept smooth and natural theologians, that the injurious effect clean, the lubricating fluid which is pushed of the vertical descent of the lubricating fluid into a ridge between the eye-lids, does not is' counteracted by the eyelids opening hori quickly recover a convex surface. state of the cornea is incompatible with deli- dency of the fluid to form vertical currents. This zontally, and consequently effacing the tencate microscopical observations, and espe- Had the eyelids opened vertically, the ver cially with the vision of horizontal lines, and tical ridges would have been increased, and its existence and effect may be ascertained vision proportionally impaired. by viewing the expanded image of a luminous point held close to the eye. shutting the eyelids and again opening them in the condition of the crystalline lens, Another optical disease of the eye, which After has been little studied, arises from a change slowly, we shall ascertain by observing the which, if not carefully watched, often terluminous ridge which crosses the expanded minates in cataract. disc of light, whether the disc recovers its ally takes place at all ages, but especially at original uniform mottled appearance quickly that period when the eyes begin to require This change occasionor slowly. If the luminous line produced the aid of glasses, and when its focal length by the fluid, accumulated between the eye- is affected by a general change in the density lids, continues to be visible, and the general and refractive powers of the lens. The surface mottled and spotted, the lubricating secretion should be excited by exposing the eye to the vapour of hartshorn, raised by pouring a few drops on the surface of hot water. The secretion will now flow copiously, the cornea will be swept clean by the less viscid fluid, and the vision of the observer greatly improved. This moveable fluid on the surface of the cornea generates another imperfection of vision. When undisturbed by the eyelids, it descends in vertical lines by the action of gravity, and the mi- the incident light suffers no reflexion in passnute ridges thus formed obliterate and ren- ing through its countless junctions; but if When this supply is properly regulated, der indistinct all horizontal lines seen by the the aqueous humour should contain too eye, while they have a tendency rather to much albumen, or too little water, then the improve the vision of rude vertical lines. supply will be too scanty, the touching faces If we take a striped pattern, therefore, of of the teeth and lamine will separate, and if any fabric, and bend part of it into zontal direction, while the rest remains ver- light, like the palest opalescence, will be rehori- this change is general, a sort of bluish-white tical, the vertical part will always appear flected from the lens. If this desiccation of the most distinct. Hence in viewing in the the lens is local, as it most frequently is, it microscope lined objects, such as the delicate will shew itself by prismatic colours, and scales of moths, &c., the lines should be irregular luminous images surrounding the placed parallel to the direction of the de- candle, and produced by the reflexion and scending fluid, when the position of the ob- decomposition of the light by the separated server's head is either vertical or oblique. faces of the laminæ, and the action of the If the axis of the lenses is vertical, and the fibres. eye looks downward, the lubricating fluid with a small hole in it, we may so place the will collect irregularly at the apex of the hole as to exclude all the light except what If we now take a plate of brass, cornea, and injure vision. If the axis of the passes through the the diseased part. When lenses is horizontal, and the observer's head this position is found, the eye can see nothing, in its natural position, the fluid will descend because the sound part of the lens is shut up. in vertical lines; but if the observer lies on If, on the contrary, we take a small-headed his back and looks into the microscope up-pin, and place the head of it so as to prevent wards, (a position not very favourable for any light from falling upon the diseased part research,) the fluid will flow equally in all of the lens, while the sound part receives directions, from the apex to the margin of rays from any object, the vision will be perthe cornea, and the part of the cornea oppo- fect. Now, if this local affection extends site the pupil will be smooth and well fitted itself through the lens, it will become dry for distinct vision. We may here notice

* The image of the sun or of a candle reflected from a small glass sphere.

and even indurated, and when seen from
reflected at the separated faces of the teeth.
without, it will appear white from the light
The lens is now totally disorganized, and the

only method of restoring the eye to its functions, is to remove the lens by extraction.

at the end of nine months, the disappearance of the prismatic colours occasioned by the lamina coming again into optical contact. The eye thus threatened with the disorganisation of its lens, has continued sound for nearly thirty-five years, and since that time. done much hard work.

Our limits will not permit us to do more than notice the polarising structure of the lenses of man and other animals. In the human lens the optical figure produced in polarized light consists of four luminous sectors arising from the increasing density of the lens towards its centre. In the lenses of some animals there are two sets of luminous sectors, indicating two variations of density, and in other lenses three sets of sectors, indicating three variations of density. The optical effect of such a singular mechanical condition of the lens has not been investigated.

If, on the contrary, the lens is supplied too freely with water from the aqueous humour, it becomes a soft mass by the gradual absorption of the fluid, and swells and bursts its capsule. In this state it is equally unfit for vision, and requires to be removed as before. In order to study the change which an excess of water produces on the crystal line, we have placed the lenses of various animals in distilled water, and watched the progressive changes which they underwent. The water first passes through the capsule and surrounds the lens on all sides. Its refractive power is gradually increased by the albumen which it dissolves, and it is absorbed unequally by different parts of the surface of the lens. The surface thus becomes irregular with hollows at one place and heights at another; and in the living eye the consequence of this must be to form a brok- Another optical disease of the eye is one en image of the candle when it is placed at a which possesses a very high degree of interconsiderable distance from the eye,— -an effect est. Dr. Smith of Cambridge describes it which is invaribly produced at a certain age, in the following words: "People," says when the eye has begun to experience a he, " in growing old, are often troubled with change in its humours. As the absorption the appearance of dark irregular spots advances, these irregular images increase, continually flying before their eyes, like and if the supply of water is not checked, the lens swells and bursts.

If these be the cause of hard and soft cataract, the first step towards the cure of the disease is to ascertain, by taking out a small portion of the aqueous humour, whether it contains too much or too little albumen, which the measurement of its refractive power will readily determine. In either case, it might be advisable, by a partial evacuation of the aqueous humour, to reduce the quantity of the diseased secretion, in the hope that a healthier one might be supplied; but if the disease should continue, distilled water, or a solution of albumen, should be injected into the aqueous chamber, to restore the humour to its proper condition.

These measures, however, should be resorted to only when a considerable degree of disorganisation has taken place. If the disease is attended to in its earlier stages, a cautious use of the eyes, an attention to diet and general health, and local applications, such as friction, galvanism, and other stimulants, will, we are persuaded, seldom fail in effecting a cure.

We have had occasion to study a case in which the lamina of the lens began to separate, and produce a mass of prismatic colour round the moon, or any luminous object. During its progress, great attention was paid to the state of the stomach, and the eye preserved from strong lights; and the patient had the satisfaction of observing,

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flies, especially in looking at bright objects, such as white paper, or the sky light." From the resemblance of these spots to flies, they have been called musca volitantes, the name which is everywhere given to this disease.

M. De la Hire, in his work entitled Differens Accidens de la Vue, describes these musca as of two kinds, some permanent and fixed, which he ascribes to small drops of extravasated blood on the retina; and others as volatile, or flying about and changing their place, even though the eye be fixed. These moving specks, which are the true muscœ volitantes, he describes as resembling the knots of a deal board, some parts of them being very clear, and surrounded with dark threads, and accompanied with long fillets of irregular shapes, which are bright in the middle, and terminated on each side by parallel black threads. After shaking the head suddenly, and fixing the eye upon an object, the musca appear to descend gradually. In order to account for these irregular fillets and spots, De la Hire supposes that the aqueous humour is "sometimes troubled with some little mothery ropy substance; some parts of which, by the figures of their little surfaces, or by refractive powers different from the humour itself, may cast their distinct images upon the retina. He supposes them in the aqueous humour rather than in the vitreous, because of its greater fluidity for a freedom of descent, and

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