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because they will then appear to descend, as form are owing to a dilatation of the being situated before the pupil, or at least branches of the arteria centralis retine." * before the place of intersection of the The general phenomena of muscæ volitanpencils.”* From these observations, it is tes present themselves in the eyes of every obvious that De la Hire regarded the person which we have examined, whether muscæ volitantes as a ropy substance in the young or old. They may be seen in dayaqueous humour, which being heavier than light by looking at the sky through a very that fluid, moved about with the motion of small pinhole in a piece of brass or card; the head, but finally descended. He was and at night by looking at a candle through obviously unacquainted with the mode in a lens of short focal length. The luminous which they throw what he calls their distinct field will then be seen covered with what images on the retina, or with the cause of have been called twisted semi-transparent the parallel black threads with which they tubes or fibres of different sizes, and various are bounded.

little globules, sometimes separate, someAlthough a century and a half has elapsed times attached to the tubes, and sometimes since De la Hire's time, no new light has apparently within them. These various been thrown on this subject. Dr. Porter- objects have their centres or axes luminous, field † has copied, without any acknowledg- and on each side of the centre or axis are ment, the statements and views of Dela two black lines, beyond which coloured Hire. He describes the muscce as resem- lines or fringes, parallel to the black ones, bling the knots of a polished fir board, and as are seen in the larger fibres or tubes. Ali accompanied with certain irregular veins these bodies have a motion even when the which proceed from each spot, and which, as eye is fixed, some of them moving faster, well as the spots themselves, change their some farther than others. Some of the order and disposition. He considers them fibres are twisted as if several knots had as produced by small dense diaphanous been tied upon them, and at the various particles and filaments that swim in the flexures of the knot black spots are seen aqueous humour before the crystalline, and corresponding with the parallel black lines he regards the distinct pictures of them on already mentioned.

The action of light the retina of long-sighted persons as pro- upon the eye seems to stir them up as if duced by the rays which pass through the they had been previously at rest in some dense particles having suffered a greater re- fixed position; for the field of view is always fraction than those which pass by them, so most free from them when the eye is first as to be converged to foci on the retina. applied to the hole or lens. This explanation of the distinctness of the All these phenomena are generally invisipictures is wholly inconsistent with optical ble in ordinary light, excepting, perhaps, the principles, and shews that the author had no knots in the fibres, which, when they are knowledge of the manner in which bodies pretty large, are still seen when all the rest placed within the eye-ball are seen as if have disappeared. These knots, therefore, they had an external existence. Mr. Mac- are the only objects which really obstruct kenzie of Glasgow, in his able work on the ordinary vision, and are alone entitled to diseases of the eye, has treated this subject the name of muscæ. It is only in diverging at considerable length. He describes the light, such as that which diverges from a muscæ as resembling minute twisted semi- small aperture from the focus of a small lens, transparent tubes, partially filled with glo- or from the convex cylinder of fluid which is bules, which sometimes appear in motion. formed when the eyelids are nearly closed, These globules he considers to be blood that the globules and transparent fibres are passing through the vessels of the retina, or seen ; and hence it is certain, that the black of the vitreous humour; but he maintains lines and fringes are the phenomena of the that all the motions of the muscæ " are mere-inflexion or diffraction of light which are ly apparent,” and that they "possess no real never seen but in divergent rays. motion independent of the general motion of But the existence of these fringes estabthe eye-ball,” and hence he concludes that lishes a still more important fact. All " they must be referred either to the retina muscce accompanied with fringes must be itself, including, of course, the three laminæ situated at a greater or less distance from of which it is composed, or to the choroid the retina, and are therefore entirely harmcoat. The probability is,” he adds, “ that less. The black spots which have also been the semi-transparent muscæ of a tubular called muscæ, which have no fringes, and

which never change their place, are insensible

spots in the retina, and are justiy sources of * Smith's Optics, vol. ii. Rem. p. 5. + Treatise on the Eye. 1830. Vol. ii. pp. 74-80. * Diseases of the Eje. 1830. Pp. 748-750.

great alarm to the patient, as the frequent called musca, which are, correctly speaking, symptoms of amaurosis.

nothing more than the accidental accumulaThat the muscæ are situated in the vitre- tion of filaments and globules into knots or ous humour, and at different distances from bunches or groups. the retina, is evident from the different We have already had occasion to mendiameters, and different degrees of distinct- tion that the vitreous humour is contained ness of their shadows and fringes in diver- in separate bags or cells. The filaments or gent light. If any existed in the aqueous muscæ must have their motions limited to humour, they would be invisible from the the cell in which they happen to be placed. faintness of the fringes which they form, and The one which we have examined never hence they would be less injurious to vision quits the field of view, and is confined to a even if they were to collect themselves into triangular space which just comes up to the knots. In order to demonstrate these views axis of vision. By placing the head in vaby specific experiments, we have only to rious positions and observing the place of use two beams of divergent light, obtained the musca when it has risen to the upper from two lights placed before the eye, and part of its cell we might ascertain pretty observe the double shadows which are thus accurately the form of the cell itself, and formed of all the muscæ in the field of view. the distance of every part of it from the Those in the front part of the vitreous retina. humour have their double images very dis It is not easy to form any rational contant, those in the middle of it have their double jecture respecting the cause and purpose of images much nearer; while those near the the numerous filaments by which the muscæ retina have their two images almost over- are produced. Were they fixed, or regulapping each other. But if we measure the larly distributed, we might regard them as distance of the two lights from each other, transparent vessels which supply the vitreand also from the eye when the two images ous humour ; but, existing as they do in of any of the transparent filaments or par-detached and floating portions, they resemticles are just in contact, we may determine ble more the remains of some organized the size of the filament and its exact position structure whose functions are no longer reas well as distance from the retina.

quired. In making this experiment, we first deter Mr. Mackenzie informs us that “ few mined that the angle of apparent magnitude symptoms prove so alarming to persons of of the shadow of a filament was 8', and conse- a nervous habit or constitution as muscæ quently that it subtended this angle at the volitantes, and they immediately suppose centre of the retina. We then found that that they are about to lose their sight by the two images were in contact when the cataract or amaurosis.” The preceding delights were five inches separate and forty- tails prove that the muscæ volitantes have nine inches from the retina. Now, if we no connexion with either of these diseases, take the radius of the retina as 0.524 inches, and are altogether harmless. This valuathe diameter of the shadow of the filament ble result has been deduced from a reconwill be 0.00122, or 1-820th of an inch, and dite property of divergent light which has the distance of the fibre from the retina will only been developed in our own day ;-and be 0.0118, or 1-85th part of an inch. which seems to have no bearing whatever

According to De la Hire and Porterfield, of a utilitarian character. And this is but the muscæ change their place, but according one of numerous proofs which the progress to Mr. Mackenzie these apparent motions of knowledge is daily accumulating that the are an illusion. We have found, however, most abstract and apparently transcendental from numerous trials, that the muscæ truths in physical science will sooner or later change their place, and that, when the head add their tribute to supply human wants, is kept steady, in different positions they and alleviate human sufferings. Nor has always appear to descend, -- that is, they in science performed one of the least imporreality ascend, and consequently float in the tant of her functions, when she enables us, vitreous humour. They shift their place either in our own case or in that of others, with every motion of the head, and it is to dispel those anxieties and fears which are owing to this cause that the long slender the necessary offspring of ignorance and filaments in moving up and down are thrown error. into folds or knots like a coiled snake. A very remarkable optical affection of

As the transparent filaments and small the eye has received the name of hemiopsy globules which can be rendered visible in or half vision. Dr. Wollaston and M. the youngest and healthiest eyes by using Arago were occasionally subject to it, and divergent light are absolutely invisible in we know many persons who have had it in ordinary light, they cannot properly be a slight degree. The defect consists in

being able to read only one half of a word Cases of double, treble, and multiplied when the eye is directed to it. Dr. Wol. vision have been described. Double vision laston, who experienced it twice after tak- often arises from want of power in the ing violent exercise, saw only one half of a muscles to direct the optic axes to objects man whom he met, and could read only at certain distances,-from long or short' i

-son the latter half of JOHNSON upon a sight when the object is small, in which case sign. At another time the loss of sight it is corrected by proper glasses,—or from was on the left side of the word, and had imperfections in the structure of the cornea he looked at the same name he would have or the crystalline, in which case one of the seen only JOHN — At certain distances images will disappear when we place a small from the eye one of two persons should opening before the imperfect part, or meredisappear, and by a slight change of place ly hold up the point of the finger in front either in the observer or the person observ- of the eye till we find that it eclipses the ed, the figure that had vanished would reap- imperfect portion which is diseased. Trepear, while the other would disappear in ble, quadruple, and multiplied vision neceshis turn. This possible case, which we pub- sarily arises from irregularities in the corlished many years ago, has recently occur-nea or in the crystalline, and by the means red to a patient of Mr. Smee's, who, when already referred to we may obliterate one he “ meets two people in the street, is only or all of the imperfect images, and make conscious of one being present, till he use of the best. hears, to his astonishment, the other speak.” Having thus given a popular description A friend of Mr. Smee's " has often told of the human eye, and of the optical changes him that during derangement of his digest- or diseases to which it is subject, we shall ive organs, he is subject occasionally for an now proceed to explain how these changes hour at a time to a derangement of vision may be counteracted by artificial means. somewhat similar to that just mentioned. We have already referred to various methods In reading, at such times, the half inch of of producing distinct vision when the eye or print directly in the line of vision is invisi- some of its parts are in an abnormal conble, so that he is compelled, as it were, to dition, and we shall therefore confine ourread a little behind his direct sight all along, selves at present to those conditions of the and he tells me that the sensation is partic- healthy eye induced by age, or by causes ularly distressing."

which do not injure the organ or any of its In this singular case it is obvious that the parts, but merely change their form, their foramen centrale was insensible to light, density, or their refractive power. while the surrounding retina possessed its In a perfect eye we can obtain distinct usual sensibility. This phenomenon is ex- vision of objects at all distances from the actly the counterpart of one which we de-eye, between four inches and the distance of scribed several years ago. In a case where the stars. This distinctness is obtained by the whole retina had been rendered insensi- adjusting the eye as it is called to the disble by a blow on the head, we found that tance of the object which we examine, just vision was perfect over the space occupied as in using a telescope, or an opera-glass, we by the foramen centrale. When a person must make it longer, or pull out the tube was near the patient he could see only his next the eye, when we look at near objects, nose or his eye, or a small portion of his or make it shorter by pushing in the same face or figure, but he could recognise a tube when we look at distant objects. But friend at a distance when the whole of his as the eye has no tubes, this adjustment face was included within the base of a cone must be obtained by other means which have whose angle was 410* In the case of Dr. not yet been discovered. Some writers Wollaston we are not distinctly told to suppose that the muscles of the eyeball what letter of JOHNSON he directed his at- elongate the eye to see near objects, others tention, but we have no doubt that, in every than the crystalline lens is moved forwards case which has been described, it is either for the same purpose, others that the lens the retina, properly so called, or the foramen is muscular, and becomes more or less conconsidered as an opening, or as a part of vex, and others that the contraction and the retina free of several of its laminee, dilatation of the pupil produces the adjustthat is insensible to light. In the case of ment. From many experiments on the Mr. Smee's patient it was obviously the subject we have been led to the following retina that was insensible.

results :

1st, That the contraction of the pupil, when See Reports of British Association, 1848, pp. ths eye is adjusted to near objects, does not 48, 49.

produce distinct vision solely by the diminu

tion of the aperture, but by another action! But whatever be the process by which we which accompanies it.

see objects at different distances, science has 2dly, That the eye adjusts itself to near taught us how to see them at all distances, objects by two actions, one voluntary de- whether the power of adjustment has been pending on the will, and the other involun- denied to us at our birth, by giving us short tary depending on the stimulus of light fall- or long sight, or has been lost by age or ing upon the retina.

any other cause. Spectacles, and reading. 3dly, That when the voluntary power fails, glasses, and eye-glasses, are the valuable inthe adjustment may be effected by the stimu- struments by which we are able to read and lus of light.

work when we can see nothing distinctly Hence we have been led to infer that the within a yard of us. They enable us to see mechanism by which we contract the pupil the faces of our friends in the same apartdraws forward the lens, and removes it from ment or across a table, to enjoy the beautithe retina, producing, perhaps, at the same ful in external nature or in art, and to count time, as M. L. Vallée thinks, a small advance the stars in the firmament when we can in the cornea, and an increase in its curvature. hardly see with distinctness a few inches be

The whole subject of the construction of fore us, and are obliged to bring close to the the eye, of the various modes by which the eye every object which we examine. Those adjustment of it may be effected, and of the only can understand how miserable must method by which it is made achromatic, or have been the condition of the aged and the the colours produced by refraction reduced, shortsighted before the invention of spechas been treated by M. L. Vallée with great tacles, who have themselves long experienced ability, though we cannot concur in many of the great blessings which they confer. his results. Several reports upon his book Those who have had the good fortune to have been made by committees of the Insti- possess two eyes of equal power, and to tute, who, though they have controverted his have enjoyed distinct and undisturbed vision opinions, have done justice to the zeal and dnring the early half of their life, are the talent with which he has carried on his in- most likely to appreciate the benefit which vestigations. Our limits will not permit us is derived from glasses. Between the ages to give any analysis of M. Vallée's work, of thirty and fifty, such persons begin to exand we shall content ourselves with referring perience a change in their sight, which generto one of the leading points of his theory, ally shows itself in a difficulty of reading in namely, the condition of the vitreous humour. candle light the smallest print in a newspaHe takes it for granted on very slight ex- per. This change, which is the commenceperimental grounds, the truth of which we ment of what is called longsightedness, from do not admit, that the vitreous humour is objects being best seen at a distance, arises not homogeneous, as all other writers have from a change in the crystalline lens, by believed, but diminishes in density from the which its density and refractive power, as crystalline lens to the retina, thus furnishing well as its form, are changed. It generally the means of correcting the colours of re- begins at the margin of the lens, and takes fraction. In so far as experiments have several months to go round it, during which been made, the density of the vitreous the vision is imperfect, and receives no aid humour in the ox and other animals, has from glasses. While this change is going been found to be the same throughout, and on, the eye requires to be managed with if it is not so in man, M. Vallée is bound to much care,—to be protected from strong show that it is not, by examining it when and sudden lights, and to be used with motaken from different parts of the mass. It deration. The general health, also, should is now admitted that the vitreous humour be attended to, in so far as healthful exeris contained in cells like honey in the honey- cise and the state of the stomach can procomb, and in the phenomena of muscæ voli- mote it. When the change has gone round tantes we have seen proofs of such a struc- the crystalline and reduced it equally from ture, which we consider quite incompatible its previous plumpness to a Aatter or less with the idea of a density diminishing to convex lens, the patient will derive immediwards the retina, or of any regular change ate benefit from the use of convex glasses ; of density whatever. If the fluid is in closed and it now becomes an important act in his cells, it is impossible that laminæ of equal life to obtain those which have the most valdensity could exist in each cell in curves uable properties, and are best suited to the concentric with the crystalline. But inde- state of his eyes, that is, which shall counpendent of this difficulty, the fibres and teract the degree of flattening which has tak

floating within the cells, would ne- en place in his crystalline lenses. cessarily bring the fluid in each of them to Spectacles should be made of glass of the the same homogeneous condition. lowest dispersive power, or, what is better still, of rock crystal, (Brazilian quartz called er, or number, as it is called by opticians, pebbles,) which has a lower dispersive power of the glasses which we are to use. Till than any kind of glass, and therefore gives very lately, no accurate method of determinrefraction with less colour. The lenses ing the proper number has been adopted. should be as thin as possible, and to have The optician takes up a book, with print of no more thickness at their edges than is ne different sizes, and makes the purchaser try cessary to keep them firmly in their frames. several pairs of spectacles, and decide upon The form of the lenses should be double those which appear to suit him best. He convex, and the radii of the outer and inner makes the trial, and generally decides for surfaces as 6 to 1 in glass, and as 14 to 1 in himself, though we have known cases where rock crystal, in order to produce the least the optician decided for him, and insisted spherical aberration, and consequently the upon the purchaser taking a pair of spectacles most perfect image on the retina. As the which gave him pain in using, assuring him, eyes are placed at different distances in dif- of what never happened, that his eyes would ferent persons, it is a matter of essential get accustomed to them. The first person, importance to have the lenses at such a dis- in so far as we can learn, who constructed tance from each other that their centres shall and used an apparatus, which he calls a visbe in lines drawn from a point, at the dis- ometer, for determining the focal length of tance at which we wish to read, or draw, or each eye, was Mr. Salom of Edinburgh. He work with them, to the centres of the pupils

. called many years ago upon the writer of In order to determine this, ascertain at what this article with his instrument, and asked distance from the eye the lens will be placed us to examine it. We did examine it, and when it rests in its proper position in its did not scruple to say that it was an instruframe on the nose, and also the distance be- ment which seemed to answer the purpose tween the eyes, that is, the centres of the for which it was intended. The publication pupil when they are directed to a point at of this simple notice not only gave great of the distance at which we wish to use them. fence to the opticians, but what was more These three distances will obviously give the strange, everybody who read it came to the distance of the centres of the lenses from conclusion that we had recommended Mr. each other, which must always be less than Salom's spectacles ! Since that time the atthe distances between the centres of the pu- tention of men of science, and of scientific pils. To find the distance of the centres of opticians, has been called to the subject, and the lenses, draw an isosceles triangle, the the method of Mr. Salom is now beginning two sides of which are equal to the distance to come into use both in England and on the of each pupil from the point to be seen dis- Continent. The following extract from the tinctly, while the third side or base is equal admirable lecture “ On the Philosophical to the distance between the pupils when the Instruments and Processes as represented in eyes view that point. Then set off on each the Great Exhibition," by Mr. Glaisher, side of the triangle, from each end of the who was not acquainted with what had been base, the distance of the centre of the lenses, done in Scotland, will shew the value of the or of their frames, from the pupil, and the instrument which we recommended :-" Of distance of these points will be the distance spectacles a large number were exhibited, of the centres of the lenses required. Very distinguished only in the British portion for little attention has been paid by opticians to their various mountings, without any atthis most essential point in the construction tempt for the improvement of the lenses of spectacles, and we do not know one case themselves as applicable to the peculiarities in which it has been accurately attended to of vision. I beg here to be clearly underby means of the process we have mentioned. stood, that I do not consider either shortOn the contrary, we have optical books sightedness, or the flattening of the eye by before us,* in which the distance between age, as peculiar. To meet such ordinary the centres of the pupils and the centres of states of the eye, the glasses exhibited were the lenses is made exactly the same. In this ample ; but I consider a malformation of case each eye looks through the part of the the eyes such, that one eye would require one lens on the inner side of the centres of the form of lens, and the other eye another form lens, so that prismatic or coloured vision of lens, as peculiar. At the time of the will be the necessary consequence,

Éxhibition, I did not know one optician in When these matters have been determin- London to whom I could refer any one so ed, or rather before they have been deter- afflicted with any chance of relief. The Exmined, we must find the focal length or pow-hibition did not make such person known in

England, but it has given the Jury the op* Dr. Kitchener, in his book entitled The Econo portunity of making the want known, and my of the Eyes, figs. 1, 2, 3.

gladly I avail myself of this opportunity to

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