WH-? NATIONAL LIBRARY OF MEDICINE Washington Founded 1836 U. S. Department of Health, Education, and Welfare Public Health Semce H OBSERVATIONS ON VISION. bf DA VID HOSACK, M. D. F. L. S. READ BEFORE THE ZIOYAL SOCIETY OF LONDON, MAY 1, 1794, AND FUBLIBBED IN THEIR TRANSACTIONS OF THE SAME TEAR. COMMUNICATED TO TBS 80CIBTT, BY GEORGE PEARSON, M. D. F. R. S. Sec. NEW-YORK: PRINTED BY C. S. VAN WINKLE, No. 122 Water-street. • ' 1813. fcr tlu Maf. . Jfy.3. For Hu Mai. andP/u/.Ryistrr OBSERVATIONS ON VISION. By what power is the eye enabled to view objects dis- tinctly at different distances ? As the pupil is enlarged or diminished according to the greater or less quantity of light, and, in a certain degree, to the distance of the object, it would readily occur that these different changes of the pupil would * When the author was prosecuting his medical studies in London, in 1794, the following paper was written, and submitted to the private examination of Sir Joseph Banks, Dr. Andrew Marshall, Dr. George Pearson, the late Dr. Robert Willan, Dr. William Woodville, and Mr. George Adams, the late cele- brated optician. By their advice it was communicated to the Royal Society, and was published in the Philosophical Transactions for the same year. Since that time various publications have appeared upon the same subject. In some of these, the writers, passing over in profound silence the observations of the author, have taken to themselves the credit of having given the first illustration of the manner in which the external muscles of the eye operate in adapting it to the different distances of objects. He excepts from this imputation the late Dr. George Gregory and William Nicholson, Esq. The former, in his Economy of Na- ture, and the latter, in his Journal, have candidly noticed, and liberally acknow- ledged the claims of those who have preceded them. Upon a future occasion the author proposes to submit to the public some additional observations, and a summa- ry view of the facts and illustrations which have lately been adduced in support of the doctrine contained in the present paper. D. H. New-York, February 22,1813. 1 i account for the phenomena in question. Accordingly, ana tomists and philosophers, who have written upon this subject, have generally had recourse to this explanation. Amusing myself with these changes of the pupil, as a mat- ter of curiosity, by presenting to the eye different objects at different distances, I soon perceived that its contraction and dilatation were irregular and more limited than had been supposed: i. e. that approaching the object nearer the eye, within a certain distance, the pupil not only ceased to con- tract, but became again dilated; and that beyond a few yards distance, it also ceased to dilate: these circumstances im- mediately occurred as objections to the above explanation ; for were it from the contraction and dilatation of the iris alone that we see objects at different distances, I naturally concluded it should operate regularly to produce its effects; but if to view an object at a few yards distance it be enlarg- ed to the utmost extent, surely it must of itself be insufficient to view one at the distance of several miles ; for example, the heavenly bodies. Another difficulty here presents itself: in viewing the sun, instead of dilating, according to the distance, it contracts, obej ing rather the quantity, or intensity of the light, than the distance of the object. Knowing no other obvious pow- er in the eye itself of adapting it to the different distances of objects, it occurred to me to inquire, whether the combined action of the external muscles could not have this effect. I first proposed this query to an optician of eminence in Lon- don,* and who has written expressly on this subject. I re- peated the same question to a celebrated teacher of anato- my.f Encouraged by their replies, I have since attended more particularly to the subject, and hope my inquiries have * Mr. George Adams. t Dr. Andrew Marshall, of Thavies Inn. 5 not been altogether unsuccessful. As introductory to a more distinct view of what I have to advance, it appears necessa- ry to premise the following observations, relative to those general laws of vision which are more particularly connected with this part of the subject, and to which we shall have oc- casion of frequent reference. 1st. Let ABC, (see plate 1st, fig. 1.) be an object plac- ed before the double convex lens D E, at any distance great- er than the radius of the sphere whereof the lens is a seg- ment ; the rays which issue from the different points of the object, and fall upon the lens, will be so bent by the refrac- tive power of the glass as to be made to convene at as many other points behind the lens, and at the place of their con- course they will form an image or picture of the object. The distance of the image behind the glass varies in propor- tion to the distance of the object before the glass ; the image approaching as the object recedes, and receding as that ap- proaches. For if we suppose, (fig. 2.) A and B, two radiat- ing points, from which the rays A C, A D, and B C, B D, fall upon the lens C D, it is manifest, that the rays from the nearest point A diverge more than those from the more dis- tant point B, the angle at A being greater than that of B;t consequently the rays from A, whose direction is A E and AF when they pass through the glass, must convene at some point (as G) more distant from the lens than the point H, where the less diverging rays B K and B L from the point B are made to convene ; which may also be proved by experi- ment with the common convex glass.f It will be necessary to have this proposition in view, as we shall afterwards have occasion to use it in showing that by varying the distance between the retina and the ante- * Euclid, Book I. Prop. 21. t See Kepler, Diopt. Postul. Smith's Optics, Gravesandp, ic. 6 rior part of the eye we are enabled to see objects at different distances. 2d. If an object, as A B, (fig 3.) be placed at a proper distance before the eye (E,) the rays which fall from the se- veral points of the object falling upon the cornea pass through the pupil, and will be brought together by the refractive pow- er of the different parts of the eye on as many correspond- ing points of the retina, and there paint the image of the ob- ect, in the same manner as the images of objects placed be- fore a convex lens are painted upon the spectrum, placed at a proper distance behind it; thus the rays which flow from the point A are united on the retina at C, and those which proceed from B are collected at D, and the rays from all the intermediate points are convened at as many intermediate points of the retina; on this union of the rays at the retina depends distinct vision. But supposing the eye of a given form, should the point of union lie beyond the retina, as must be the case with those from the less distant object, agreea- ble to the preceding proposition; or should they be united before they arrive at the retina, as from the more distant ob- ject, it is evident that the picture at the retina must be ex- tremely confused. Now, as the rays which fall upon the eye from radiating points at different distances have differ- ent degrees of divergence, and the divergence of the rays increasing as the distance of the radiating point lessens, and, vice versa, lessening as that increases ; again, as those rays which have greater degrees of divergence, viz. from the near- er objects, require a stronger refractive power to bring them together at a given distance than what is necessary to make those meet which diverge less, it is manifest, that to see ob- jects distinctly at different distances, either the refractive power of the eye must be increased or diminished, or the distance between the iris and retina be varied, correspond- 7 ing with the different distances of the objects ; both of which probably take place, as will hereafter appear.* Having then established these as our premises, we shall next examine the different principles which have been em- ployed for explaining vision at different distances. Most writers upon this subject refer this power of the eye to the contraction and dilatation of the iris. Within certain limits this would, upon first examination, as already observed, appear to be the case, since the pupil enlarges as the object is further removed from the eye, and again contracts as it is brought near. The extent of this principle I have already pointed out; but I suspect we also err in attributing to the difference of distance what are only effects of different quan- tities of light, a circumstance in which it is the more easy to commit error, as they are generally proportionate one to the other ; i. e. as the object is near we require a less degree of light, and to exclude what is superfluous the iris contracts ; but as it is more distant, a greater quantity of light becomes necessary, and the iris dilates : thus far we see the use of the enlargement or diminution of the pupil, as the object is more or less distant. But distinct vision does not consist in the quantity of light alone, though too much or too little would obscure the image. It is also necessary that the rays which flow from the ob- ject should fall upon the retina in a certain direction, to form a distinct picture ; but surely the greater or less quantity of light, the greater or less number of rays, which it is only * Facile enim intelligitur, quo longius radii adveniunt, eo magis esse paralle- ls; eo minus ergo differre ab axi, et eo minoribus viribus cornea et lentis crys- talline in focum cogi. Ut enim corpus magis distat, ita sub minori angulo radii adveniunt. Contra si corpus conspicuum valde vicinura fuerit, radiorum ab eo advenientium angulus'est major, et adeo magis divergentes in oculum incidiint, et viribus egent refringentibus majoribus omnibus densioribus. Halltr. Ekm. Phys. lib. xvi. u the property of the iris to diminish or increase, cannot alter the direction. But there is still another argument to prove, that the con- traction or enlargement of the pupil is not of itself sufficient to produce distinct vision at different distances, viz. that the myopes, whose pupil contracts and dilates as in other eyes, are still unable to adapt the eye to different distances ; and the means by which this is remedied certainly does not con- sist in a larger or smaller aperture for the rays to pass through, but a power of altering their direction, which the change in the shape of the eye had rendered too convergent. The same fact is also observable in those who squint; the pupil in both eyes equally contracts and dilates, but still the vision of one eye is less perfect than the other. Another principle upon which it has been attempted to explain this power of the eye, is a supposed change in the convexity of the crys- talline lens: the ancients had some obscure notion of it, but it has been lately pursued by Mr. Thomas Young, in a paper published in the Philosophical Transactions of London, for 1793. He has endeavoured to demonstrate the existence of muscles in the crystalline lens, and by their action to ac- count for distinct vision at different distances. This opinion deserves here the more particular examination, having met the attention of the Royal Society, and thereby likely to in- fluence the general opinion upon this subject. That we may not mistake the meaning of the author, I beg leave to premise his description of the structure of the lens: " The crystalline lens of the ox," he observes, " is an orbicular convex transparent body, composed of a consi- derable number of similar coats, of which the exterior close- ly adhere to the interior; each of these coats consists of six muscles, intermixed with a gelatinous substance, and attach- ed to six membranous tendons. Three of these tendons are anterior, three posterior; their length is about two-thirds of 9 the semidiameter of their coat; their arrangement is that of three equal and equidistant rays meeting in the axis of the ( rystalline ; one of the anterior is directed towards the out- er angle of the eye, and one of the posterior towards the in- ner angle; so that the posterior are placed opposite to the middle of the interstices of the anterior, and planes passing through each of the six, and through the axis, would mark on either surface six regular equidistant rays. The muscu- lar fibres arise from both sides of each tendon, they diverge till they reach the greater circumference of the coat, and having passed it, they again converge till they are attached respectively to the sides of the nearest tendons of the oppo- site surface. The exterior or posterior portion of the six, viewed together, exhibits the appearance of three penni- formi-radiated muscles." In the first place, to say nothing of the transparency of muscles, as an argument against their existence, we must un- avoidably suppose, as they have membranous tendons, which Mr. Young informs us he distinctly observed, that these tendons cannot possess the same degree of transparency and density with the bellies of these muscles; that is, they must possess some degree of opacity, or certainly he could not have pointed out their membranous structure, nor even the tendon itself, as distinct from the body of the muscle; and if they have not the same density, from their situation, and being of a penniform shape, must there not be some irregu- larity from the difference in the refraction of those rays which pass through the bellies of those muscles, and those again which pass through their membranous tendons ? This structure then, of consequence, cannot be well adapted for a body whose regular shape and transparency are of so much consequence. Again, Mr. Young describes six muscles in each layer ; but Leeuwcnhoek, whose authority he admits as accurate, rela- 2 10 live to the muscularity of the lens, is certainly more to be* attended to in his observation of bodies less minute, viz. as to the layers themselves, in which these muscles are found ; which of course are larger, and more easily observed; but, with his accuracy of observation, he has computed, that there are near two thousand laminae; and according to Mr. Young, supposing each layer to contain six muscles, we have neces- sarily, in all, twelve thousand muscles ; the action of which certainly exceeds human comprehension. I hope this will not be deemed trifling minuteness, as it is a necessary and regular consequence, if we admit their existence as described. But secondly, as to the existence of these muscles, I can- not avoid expressing a doubt. With the utmost accuracy I was capable of, and with the assistance of the best glasses, to my disappointment, I cannot bear witness to the same cir- cumstances related by Mr. Young, but found the lens per- fectly transparent: at the same time, lest it might be attri- buted to the want of habit in looking through glasses, I beg leave to observe, that I have been accustomed to the use of them in the examination of the more minute objects of natur- al history. After failing with the glasses in the natural vis- cid state of the lens, I had recourse to another expedient; I exposed different lenses before the fire to a moderate degree of heat, by which they became opaque and dry; in this state it is easy to separate the layers described by Mr. Young; but although not so numerous as noticed by the accurate Leeuwenhoek, still they were too numerous to suppose each to have contained six muscles; for I could have shown dis- tinctly at least fifty layers, without the assistance of a glass, as was readily granted by those to whom I exhibited them. But a circumstance which would seem to prove that these layers possess no distinct muscles, is, that in this opaque state 11 they are not visible, but consist rather of an almost infinite number of concentric fibres (if the term be at all appropriate) not divided into particular bundles, but similar to as many of the finest hairs of equal thickness, arranged in similar order: see fig. 4, 5, and 6, where the arrangement of the layers and fibres has been painted from the real lens of an ox, and that without the assistance of a glass. To observe this fact, any person may try the experiment at pleasure, and witness the same with the naked eye, even separating many layers and their fibres with the point of a penknife. This regular structure of layers, and those consisting of concentric fibres, is unquestionably better adapted for the transmission of the rays of light, than the irregular structure of muscles. It may, perhaps, be urged, that the heat to which I exposed the lens may have changed its structure : in answer to that I observe, it was moderate in degree, and regularly applied; of consequence we may presume, as it appeared uniformly opaque, that every part was alike acted upon: but by boiling the lens, where the heat is, without doubt, regularly applied, we observe the same structure. Thirdly, that it is not from any changes of the lens, and that this is not the most essential organ in viewing objects at different distances, we may also infer from this undeniable fact, that we can, in a great degree, do without it: as after couching or extraction, by which operations all its parts must be destroyed, capsule, ciliary processes, muscles, Sec. Mr. Young asserts, from the authority of Dr. Porterfield, that patients, after the operation of couching, have not the power of accommodating the eye to the different distances of objects ; at present, I believe the contrary fact is almost universally asserted.* * " Et lente ob cataractam extracta vel deposita oculum tamen ad varias dis- tantias videre, ut in nobiliviro video ab;que nullo experiment*) quo earn facultatetn V2 Besides, if the other powers of the eye are insufficient to compensate for the loss of this dense medium, the lens, a glass of the same shape answers the purpose, and which certainly does not act by changing its figure. I grant their vision is not so perfect; but we have other circumstances upon which this can be more easily explained; which will be particular- ly noticed under the next head. It may not be improper also to observe, that the specific gravity of the crystalline, compared with that of the vitreous humour, and of conse- quence, its density and power of refraction, is not so great as has been generally believed. Dr. Bryant Robinson, by the hydrostatic balance, found it to be nearly as eleven to ten. I have also examined them with the instrument of Mr. Schmeisser, lately presented to the Royal Society, and found the same result; of consequence, the crystalline lens is not so essentially necessary for vision as has been represented ; especially, as it is also probable, that upon removing it, the place which it occupied is again filled by the vitreous hu- mour, whose power of refraction is nearly equal. At the same time we cannot suppose the lens an unnecessary organ in the eye, for nature produces nothing in vain; but that it is not of that indispensable importance, which writers upon optics have taught us to believe. Fourthly. Mr. Young tells us, he has not yet had an op- portunity of examining the human crystalline; and grants, that from the spherical form of it in the fish, such a change as he attributes to the lens in quadrupeds cannot take place recuperaverit. Etsi enim tunc ob diminutas vires qua? radios uniunt, aeger lente vitrea opus habet, eadem tamen lens in omni distantia sufficit."—Haller. El. Phys. " La lentille cristalline n'est cependant point de premiere necessite pour la vision. Aujourd'hui, dans l'operation de la cataracte on l'enleve entiSrement et la vision n'en souffre point."- De la Metherie Fues Physiologiques. See alsoDe la Hire, Hambergtr's Physiolog. 13 in that class of animals. The lenses which I have examined in the manner above mentioned were the human, those of the ox, the sheep, the rabbit, and the fish, and in all the same lamellated structure is observable; even in the spherical lens of the fish these lamella? are equally distinct, but without the smallest appearance of a muscle. From these circumstances I cannot avoid the conclusion, that they do not exist; at the same time I am persuaded that Mr. Young met with appearances which he supposed were muscles ; but I am satisfied he will readily acknowledge, that the examination of the crystalline lens in its viscid glutinous slate,"is not only attended with much difficulty, but that the smallest change of circumstances might lead to error ; which I apprehend may, probably, have been the case in that in- stance. Upon examining it after boiling, or exposing it to a gradu- al degree of heat before the fire, when it may be handled with freedom, he will readily observe (without a glass) the nume- rous lamellae, and the arrangement of their fibres, which I have described. Another opinion has been sanctioned by many respectable writers, of the effects of the ciliary processes in changing the shape and situation of the lens; some supposed it to possess the power of changing the figure of the crystalline, rendering it more or less convex ;* others, that it removes it nearer to the cornea ;f and others, that it removed it nearer the retina. I The advocates for these different opinions all agree in at- tributing these effects to a supposed muscularity of the cilia- ry processes. * Des Cartes, Scheinerus, Bidious, Mollinettus, Sarictoritre, Jurin. f Kepler, Zinn, Porterfield. } La C'hariere, Pcrrault, Hartsoeker, Brisseau, and Derham. 11 Of the structure of these processes Haller observes, " la omni certe animalium genere processus ciliares absque ulla musculosa sunt fabrica, mere vasculosi vasculis serpentinis percursi molli facti membrana;" which structure, I believe, at present is universally admitted. But even supposing them muscular, such is their delicacy of structure, their attach- ment, and direction, that we cannot possibly conceive them adequate to the effects ascribed to them. Besides, what we observed of the muscles of the lens itself, also applies to the processes, viz. that they may be destroyed, as in couching or extraction, and yet the eye be capable of adapting itself to the different distances of objects. For a more full refuta- tion of these opinions, see Haller's large work. The Situation, Structure, and Action of the external dluscles.* Upon carefully removing the eyelids, with their muscles, we are presented with the muscles of the eye itself, which are six in number; four called recti, or straight; and two oblique; so named from their direction, (see plate Hd. fig. 1.) A A A A, the tendons of the recti muscles, where they are inserted into the sclerotic coat, at the anterior part of the eye. B, the superior oblique, or trochlearis, as some- times called, from its passing through the loop or pulley con- nected to the lower angle of the orbiter notch in the os fron- tis; it passes under the superior rectus muscle, and back- wards to the posterior part of the eye, where it is inserted by a broad flat tendon into the sclerotic coat. C, the inferior oblique, arising tendinous from the edge of the orbiter pro- * For the accuracy of the representation I have annexed (see plate lid.) I vouch, having been at much pains in the dig«ection ; from which I had the pa ing taken by a most accurate hand, Mr. S. Edwards, a gentleman well km for his abilities in the plates of that admirable work, the Flora Londinensis. 15 cess of the superior maxillary bone, passes strong and fleshy over the inferior rectus, and backwards under the abductor to the posterior part of the eye, where it is also inserted by a broad flat tendon into the sclerotic coat. D D D, the fat in which the eye is lodged. In fig. 2. we have removed the bones forming the external side of the orbit, with a portion of the fat, by which we have a distinct view of the abductor. A B C, three of the recti muscles, arising from the back part of the orbit, passing strong, broad, and fleshy over the ball of the eye, and inserted by flat, broad tendons into the scle- rotic coat, at its anterior part. D, the tendon of the supe- rior oblique muscle. E, the inferior oblique; fig. 3. A, the adductor of the eye. B, the fleshy belly of the superior oblique, arising strong, tendinous, and fleshy from the back part of the orbit. C, the optic nerve. D and E, the recti muscles. The use ascribed to these different muscles, is that of changing the direction of the eye, to turn it upwards, down- wards, laterally, or in any of the intermediate directions, ac- commodated either to the different situation of objects, or to express the different passions of the mind, for which they are peculiarly adapted. But is it inconsistent with the ge- neral laws of nature, or even with the animal economy, that from their combination they should have a different action, and thus an additional use ? To illustrate this we need only witness the action of almost any set of muscles in the body: for example, in lifting a weight, the combined action of the muscles of the arm, shoulder, and chest, is different from the individual action of either set, or of any individual muscle • or an instance nearer our purpose may be adduced, viz. the actions of the muscles of the chest and belly, making a com- pression upon the viscera, as in the discharge of urine, faeces, &c. But to question this fact would be to question the in- 16 fluence of the will in any one of the almost infinite variety of motions in the human body. I presume, therefore, it will be admitted, that we have the same power over these muscles of the eye as of others, and I believe we are no less sensible of their combined action; for example, after viewing an object at the distance of half a mile, if we direct our attention to an object but ten feet dis- tance, every person must be sensible of some exertion ; and if our attention be continued but for a short time, a degree of uneasiness and even pain in the ball of the eye is experienc- ed ; if, again, we view an object within the focal distance, i. e. within six or seven inches, such is the intensity of the pain that the exertion can be continued but a very short time, and we again relieve it by looking at the more distant ob- jects ; this, I believe, must be the experience of every person, whose eyes are in the natural and healthy state, and accordingly it has been observed by almost every writer upon optics. But the power of this combination, even from analogy, appears too obvious to need further illustration. I shall therefore next endeavour to point out their precise action. Supposing the eye in its horizontal natural position ; I see an object distinctly at the distance of six feet, the picture of the object falls exactly upon the retina ; I now direct my at- tention to an object at the distance of six inches, as nearly as possible in the same line ; although the rays from the first object still fall upon my eye, while viewing the second, it does not form a distinct picture on the retina, although at the same distance as before, which shows that the eye has under- gone some change; for while I was viewing the first object I did not see the second distinctly, although in the same line: and now, vice versa, I see the second distinctly, and not the first; the rays from the first, therefore, as they still fall upon the eye, must ejther meet before or behind the re- IT tina; but we have shown that the rays from the more distant object convene sooner than those from the less distant ob- ject, therefore the picture of the object at six feet falls be- fore, while the other forms a distinct image upon the retina ; but as my eye is still in the same place as at first, the retina has by some means or other been removed to a greater dis- tance from the fore part of the eye to receive the picture of the nearer object, agreeable to the principle, page 6. From which it is evident, that to see the less distant object, either the retina should be removed to a greater distance, or the re- fracting power of the media should be increased: but I hope we have shown that the lens, which is the greater refracting medium, has no power of changing itself. Let us next in- quire, if the external muscles, the only remaining power the eye possesses, are capable of producing these changes. With respect to the anterior part of the eye, we have seen the sit- uation of those muscles; the recti strong, broad, and flat, arising from the back part of the orbit, passing over the ball as over a pully, and inserted by broad flat tendons at the an- terior part of the eye; the oblique inserted toward the pos- terior part, also by broad flat tendons; when they act joint- ly, the eye being in its horizontal position, it is obvious, as every muscle in action contracts itself, the four recti by their combination must necessarily make a compression upon the different parts of the eye, and thus elongate its axis, while the oblique muscles serve to keep the eye in its proper direc- tion and situation. For my own part, I have no more diffi- culty in conceiving of this combination of those muscles than I have at present of the different flexors of my fingers in holding my pen. But other corresponding effects are also produced by this action; not only the distance between the anterior and posterior parts of the eye is increased, but of consequence, the convexity of the cornea, from its great elas- ticity, is also increased, and that in proportion to the degree o 18 of pressure, by which the rays of light passing through it are thence necessarily more converged. But another effect, and one not inconsiderable, is, that by this elongation of the eye, the media, viz. the aqueous, crystalline, and vitreous humours through which the rays pass, are also lengthened, of consequence their powers of refraction are proportionably increased; all which correspond with the general principle. It may, however, be said, that as the four recti muscles are larger and stronger than the two oblique, the action of the former would overcome that of the latter, and thus draw back the whole globe of the eye; but does not the fat at the pos- terior part of the orbit also afford a resistance to the too great action of the recti muscles, especially as it is of a firm consistence, and the eye rests immediately upon it ? Admit- ting then that this is the operation of the external muscles when in a state of contraction, it is also to be observed, we have the same power of relaxing them, in proportion to the greater distance of the object, until we arrive at the utmost extent of indolent vision. But, as a further testimony of what has been advanced, I had recourse to the following experiment, which will show that the eye is easily compressible, and that the effects produced correspond with the principles I have endeavoured to illustrate. With the common speculum oculi I made a very moderate degree of pressure upon my eye, while directing my atten- tion to an object at the distance of about twenty yards ; I Haw it distinctly, as also the different intermediate objects ; but endeavouring to look beyond it, every thing appeared con- fused. I then increased the pressure considerably, in conse- quence of which I was enabled to see objects distinctly at a much nearer than the natural focal distance; for example, I held before my eye, at the distance of about two inches, a printed book ; in the natural slate of the eye I could neither 19 distinguish the lines nor letters; but upon making pressure with the speculum I was enabled to distinguish both lines and letters of the book with ease. Such then I conceive to be the action and effects of the external muscles, and which I apprehend will also apply in explaining many other phenomena of vision; some of those it will not be improper at present briefly to notice. First. May not the actions of those muscles have more or less effect in producing the changes of vision which take place in the different periods of life ? At the same time the origi- nal conformation of the eye, the diminution of its humours, and, probably, of the quantity of fat upon which the eye is lodged, are also to be taken into the account. But the ex- ternal muscles becoming irregular and debilitated by old age, in common with every other muscle of the body, are not only incapable of compensating for these losses, but cannot even perform their wonted action, and thus necessarily have con- siderable influence in impairing vision. Again, does not the habit of long sight so remarkable in sailors and sportsmen, who are much accustomed to view objects at a great distance, and that of short sight, as of watchmakers, seal-cutters, &c. admit of an easy solution upon this principle ? as we know of no part of the body so susceptible of an habitual action as the muscular fibre. Secondly. How are we to account for the weaker action of one eye in the case of squinting ? That this is the fact has been well ascertained ; Dr. Reid* upon this subject observes, that he has examined above twenty persons who squinted, and found in all of them a defect in the sight of one eye. Porterfield and Jurin have made the same observation. * See his Inquiry into the Human Mind, page 322, 20 The distorted position of the eye has, I believe, been ge- nerally attributed to the external muscles ; but no satisfacto- ,v reason has ever been given why the eye, directed towards an object, does not see it distinctly at the same distance as with the other. The state of the iris here cannot explain it, as it contracts and dilates in common with the other; nor can we suppose any muscles the lens might possess could have any effect, as they are not at all connected with the nature of this disease. But the action of the external muscles, I apprehend, will afford us a satisfactory explanation. When the eye is turn- ed from its natural direction, for example, towards the inner canthus, it is obvious that the adductor muscle is shorten- ed, and its antagonist, the abductor, lengthened ; consequent- ly, as the abductor has not the same power of contracting itself with the adductor, when the eye is directed towards an object, their power of action being different and irregular, the compression made upon the eye and its humours must also be equally irregular, and therefore insufficient to produce the regular changes in the refraction and shape of the eye we have shown to be necessary in adapting it to the different distances of objects. The effects produced by making a partial pressure upon the eye with the finger, or specu- lum oculi, before noticed, would also appear to favour this explanation. Thirdly. May it not in part be owing to the loss of this combined action of the external muscles, and the difficulty of recovering it, that the operation of couching is sometimes unsuccessful, especially when the cataract has been of long standing? This cannot be attributed to the iris, for it, per- inps, dilates and contracts as before: nor to the muscles of the lens, for they are removed ; nor to the state of the nerve, for it is still sensible to light; and yet the patient cannot wf objects distinctly ; and it is not an uncommon circum- 21 atance, even when the operation succeeds, that the sight is slowly and gradually recovered. Instances have occurred, Mr. Bell* observes, of the sight becoming gradually better for several months after the operation. When we have been long out of the habit of combining our muscles in almost any one action of life, as walking, dan- cing, or playing upon a musical instrument, we in a great measure lose the combination, and find a difficulty in reco- vering it, in proportion to the length of time we had been de- prived of it; but the individual action of each muscle re- mains as before. Thus, probably, with the muscles of the eye. A variety of facts of a similar nature must present themselves to every person conversant in the science of op- tics, which may admit of a similar explanation. I have thus endeavoured, first, to point out the limited ac- tion of the iris, and of consequence the insufficiency of this action for explaining vision. Secondly, to prove that the lens possesses no power of changing its form to the different distances of objects. Thirdly, that to see objects at different distances, corresponding changes of distance should be pro- duced between the retina and the anterior part of the eye, as also in the refracting powers of the media through which the rays of light are to pass. And, fourthly, that the combined action of the external muscles is not only capable of produ- cing these effects, but that from their situation and structure they are also peculiarly adapted to produce them. Is it not then consistent with every principle in the eco- nomy of nature and philosophy, seeing the imperfections of the principles which have hitherto been employed in explain- ing the phenomena in question, to adopt the one before us, until (agreeably to one of the established rules in philosophiz- * See hi? System of Surgery 22 ing) other phenomena occur, by which it may be rendered either more general, or liable to objections ? I have now finished what was proposed. I have declined entering into an extensive view of the structure of the eye, or any of the general principles of optics, as those subjects have been more ably treated in the works already cited, and thus would certainly have destroyed every claim to attention, which these few pages in their present form may possibly possess; and if I should be so fortunate as to succeed in es- tablishing the principle I have proposed, for explaining the phenomena dependent upon this more important organ of our body (if any part possesses a pre-eminence in nature,) I also hope it may, in abler hands, admit of some practical appli- cation, in alleviating the diseases to which its delicate or- ganization so particularly exposes it.* * Since the above pages have been written, I have found, upon consulting some •f the earliest writers, that the effects of the external muscles did not altogether escape their attention ; at the same time they had no distant idea of their action: I must therefore disclaim the originality of the thought, although I had never met with it before the circumstances already noticed, of the insufficiency of the iris, had suggested it. If, however, I have succeeded in pointing out the precise action of those muscles, and its application to the general principles of vision, in which, I believe, I have never been anticipated, it will be the height of my wishes. WZ 1*13 c.|. S3 &J ^H ^m ■>v?V^" *l^.."- *.*.*,-.