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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 read or see distinctly, which he described by the visible distance of the point. Hence we the term dazzling, arose solely from his have the law of single vision for points, and being unable to fix the axes of his eyes on the law of visible distance.
one point. He was sent to sea-bathing for The law of vision for visible objects is a month or two, and returned to school perentirely different from that for points. Afectly cured. 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.
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 the head.*
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 nea.
Infirmary, who had derived no benefit from The cornea, as we have already said, re having her eye tapped about forty times, we sembles a small watch-glass. It has a refrac- found that the cornea had not been protrudtive power greater than water, or than the ed, but had the same irregularities which aqueous humour with which it is internally were seen in Mr. Wardrop's patient, and in contact. The form of the cornea has which we have since found in every other generally been considered spherical ; ,but M. case that has come under our notice. In one Chossat discovered, not many years ago, by of these cases, which was that of a woman projecting a magnified profile of it on the from Morpeth, who came to the Royal Inwall, that it was elliptical, a fact which had fimary of Edinburgh for advice, the writer been observed by Sir Isaac Newton in the of this article was requested to contrive for eye of the sheep, but which he had never her special glasses to enable her to make published.f It is quite possible that the some use of her eyes, both of which were spherical form may not be the normal one, affected with this disease in its worst form. since cases of conical cornea have been ob- The cornea had a very irregular surface, like served, and also cases in which the form of the scar of a wound, and also such a degree it is cylindrical.
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 As the unnatural convexity of the cornea light. Conceiving, therefore, that the sur- necessarily produces short sight in this disface of it was not uniform, we found this to ease, Sir W. Adams conceived the idea of be the case by observing the image of a removing this part of the complaint, by exsmall taper reflected from different parts of tracting the crystalline lens, and in one case it. The image suffered such changes in its carried it into effect. The vision of the size and shape, as to indicate the presence of patient was no doubt improved, in so far as a number of round eminences and depres- as short sight was concerned, but the effects
of the corrugated surface still remained.
The same result, however, would have been knowledge of distance given by binocular vision, pub- would be greatly surprising if any other
* Mr. Smee has certainly not seen a paper on the gained by the use of a concave lens, and it lished in the Edin. Transactions for 1844, vol. xv. Oculist should adopt the principle of this ex663, and reprinted in the Phil. Magazine for 1847, vol. XXX. p. 305, in which the effects of what Mr. traordinary practice, and dig out the interior Smee calls want of concert are described, and many of our eyes to save the expense of a pair of singular phenomena explained. See this Journal, spectacles. When we find one medical given, and the remarkable case of Dr. Christison practitioner making forty demands upon the quoted.
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 immer surface of the The cornea is subject to another disease cornea, they will produce comparatively lit- very injurious to vision, namely, a partial tle effect, being in contact with the aqueous opacity, forming one or more white spots humour, which approximates to the cornea either on its surface or between the laminæ in refractive power.
of which it consists. These spots vary in Another optical disease of the eye, and a intensity from a slight nebulosity to a devery rare one, may be called cylindrical gree of white opacity almost impervious to cornea, in which the curvature is different in light. When they are the result of local a horizontal and in a vertical direction; and, irritation or the effect of inflammation, or therefore, the pencils of light incident in a hor- disease of the adjacent parts, they often disizontal plane will have their focus at a great- appear with the causes which produce them. er or less distance than those which are inci- In young persons they frequently grow less dent in a vertical plane. Vision consequent- and less with the growth of the cornea : but ly must, in such cases, be very imperfect. when the membrane is really disorganized it This irregular structure is recognised only is not easy to effect a cure.* When these in the imperfect vision it produces, and it spots are small and semi-transparent, the incannot be corrected either by convex or con- jurious effect on vision might be removed by cave spectacles. It is possible that it may placing opposite to them, on plane or other exist in the crystalline lens alone, or both in glasses, opaque spots of the same size, or by the crystalline and cornea, but the determin- placing a small aperture of the same form as ation of this point is of no practical impor- the good part of the cornea opposite that tance, as the method of rendering the vision part. 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 eye-lids move and it might be useful to try the effect of placing it inclined 45° to the horizon. This plan, however, would be advantageous only told that these opacities may be cured by blowing
* Dr. Smith of Cambridge says that he has been in those cases where the vertical or horizon- pounded glass every day into the eyel— Treatise on tal vision did not require the aid of ordinary Optics, Vol. ii, p. 5, Remarks.
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 horiquickly recover a convex surface. This zontally, and consequently effacing the tenstate of the cornea is incompatible with deli- dency of the fluid to form vertical currents. cate microscopical observations, and espe- Had the eyelids opened vertically, the vercially 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 lumin Another optical disease of the eye, which ous point* held close to the eye. After has been little studied, arises from a change shutting the eyelids and again opening them in the condition of the crystalline lens, slowly, we shall ascertain by observing the which, if not carefully watched, often terluminous ridge which crosses the expanded minates in cataract. This change occasiondisc 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 or slowly. If the luminous line produced the aid of glasses, and when its focal length by the Suid, 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 laminæ and fibres of which it is composed secretion should be excited by exposing the are kept in optical contact by a secretion eye to the vapour of bartshorn, raised by supplied either by special vessels, or obtainpouring a few drops on the surface of hot ed by absorption from the aqueous humour, water. The secretion will now flow copious. That the capsule of the crystalline freely ly,—the cornea will be swept clean by the absorbs distilled water, and also water from less viscid fluid, and the vision of the obser- the aqueous humour, is a fact established by ver greatly improved. This moveable fluid experiment, and we are therefore entitled on the surface of the cornea generates an- to suppose that it is preserved in its healthy other imperfection of vision. When undis- state by the water which it thus absorbs. turbed by the eyelids, it descends in vertical When this supply is properly regulated, 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 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 laminæ will separate, and if any fabric, and bend part of it into a hori- this change is general, a sort of bluish-white zontal direction, while the rest remains ver- light, like the palest opalescence, will be retical, 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 lamine, and the action of the If the axis of the lenses is vertical, and the fibres. If we now take a plate of brass, 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 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 and even indurated, and when seen from
without, it will appear white from the light * The image of the sun or of a candle reflected reflected at the separated faces of the teeth. from a small glass sphere.
The lens is now totally disorganized, and the
only method of restoring the eye to its func- at the end of nine months, the disappearance tions, is to remove the lens by extraction. of the prismatic colours occasioned by the
If, on the contrary, the lens is supplied too laminæ coming again into optical contact. freely with water from the aqueous humour, The eye thus threatened with the disorganiit becomes a soft mass by the gradual ab- sation of its lens, has continued sound for sorption of the fluid, and swells and bursts nearly thirty-five years, and since that time its capsule. In this state it is equally unfit done much hard work. for vision, and requires to be removed as Our limits will not permit us to do more before. In order to study the change which than notice the polarising structure of the an excess of water produces on the crystal- lenses of man and other animals. In the line, we have placed the lenses of various human lens the optical figure produced in animals in distilled water, and watched the polarized light consists of four luminous progressive changes which they underwent sectors arising from the increasing density The water first passes through the capsule of the lens towards its centre. In the and surrounds the lens on all sides. Its re- lenses of some animals there are two sets of fractive power is gradually increased by the luminous sectors, indicating two variations albumen which it dissolves, and it is absorb- of density, and in other lenses three sets of ed unequally by different parts of the sur- sectors, indicating three variations of density. face of the lens. The surface thus becomes The optical effect of such a singular irregular with hollows at one place and mechanical condition of the lens has not heights 'at another; and in the living eye the been investigated. 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, flies, especially in looking at bright objects, the lens swells and bursts.
such as white paper, or the sky light.” If these be the cause of hard and soft cata- From the resemblance of these spots to ract, the first step towards the cure of the flies, they have been called muscæ volitantes, disease is to ascertain, by taking out a small the name which is everywhere given to portion of the aqueous humour, whether it this disease. contains too much or too little albumen, which M. De la Hire, in his work entitled the measurement of its refractive power will Differens Accidens de la Vue, describes these readily determine. In either case, it might muscæ as of two kinds, some permanent and be advisable, by a partial evacuation of the fixed, which he ascribes to small drops of aqueous humour, to reduce the quantity of extravasated blood on the retina ; and others the diseased secretion, in the hope that a as volatile, or flying about and changing their healthier one might be supplied ; but if the place, even though the eye be fixed. These disease should continue, distilled water, or a moving specks, which are the true muscæ solution of albumen, should be injected into volitantes, he describes as resembling the the aqueous chamber, to restore the humour knots of a deal board, some parts of them to its proper condition.
being very clear, and surrounded with dark These measures, however, should be re-threads, and accompanied with long fillets sorted to only when a considerable degree of irregular shapes, which are bright in the of disorganisation has taken place. If the middle, and terminated on each side by disease is attended to in its earlier stages, a parallel black threads. After shaking the cautious use of the eyes, an attention to head suddenly, and fixing the eye upon an diet and general health, and local applica-object, the muscæ appear to descend grations, such as friction, galvanism, and other dually. In order to account for these irrestimulants, will
, we are persuaded, seldom gular fillets and spots, De la Hire supposes fail in effecting a cure.
that the aqueous humour is “ sometimes We have had occasion to study a case in troubled with some little mothery ropy subwhich the laminæ of the lens began to sepa- stance; some parts of which, by the figures
rate, and produce a mass of prismatic colour of their little surfaces, or by refractive round the moon, or any luminous object. powers different from the humour itself, may During its progress, great attention was cast their distinct images upon the retina. paid to the state of the stomach, and the eye He supposes them in the aqueous humour rapreserved from strong lights; and the ther than in the vitreous, because of its patient had the satisfaction of observing, I greater fluidity for a freedom of descent, and