![[Figures 1-3, optical diagrams]](hosack_1-3.gif)
1st. Let ABC [fig 1] be an object placed before the double convex lens DE, at any distance greater than the radius of the sphere of which the lens is a segment: the rays which issue from the different points of the object, and fall on the lens, will be so bent by the refractive power of the glass, as to be made to convene as many other points behind the lens, and at the place of their concourse they will form an image or picture of the object. The distance of the image behind the glass varies in proportion to the distance of the object before the glass; the image approaching as the object recedes, and receding as that approaches.
For if we suppose [fig 2] A and B two radiating points, from which the rays AC, AD, and BC, BD, fall on the lens CD, it is manifest that the rays from the nearest point A diverge more than those from the more distant point B, the angle at A being greater than that of B; consequently the rays from A, whose direction is AE and AF when they pass through the glass, most convene at some point, as G., more distant from the lens than the point H, where the less diverging rays BK and BL from the point B are made to convene; which may also be proved by experiment with the common convex glass. 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 anterior part of the eye, we are enabled to see objects at different distances.
2d. If an object, as AB (fig. 3) be placed at a proper distance before the eye, E, the rays which fall from the several point of the object falling on the cornea pass through the pupil, and will be brought togther by the refractive power of the different parts of the eye on as many corresponding points of the retina, and there paint the image of the object, in the same manner as the images of objects placed before a convex lens are painted on 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 objeject, agreeable to the preceeding proposition; or should they be united before theuy arrive at the retina, as from the more distant object, it is evident that the pictre at the retina must be extremely confused. Now as the rays which fall on the eye from radiating points at different distances, have different degrees of divergence, and the divergence of the rays increasing as the distance of the radiating point lessens, and vice versa, lessening as that increass; again, as those rays which have greater degrees of divergence, viz. from the nearer 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 se objects 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, corresponding with the different distances of the objects; both of which probably take place, as will hereafter appear.
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 process, muscles, &c. Mr. Young asserts, from the authority of Dr. Porterfield, that patients, after the operation of courching, 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 extractam vel depositam oculum tamen ad varias distantias videre, ut in nobili viro video absque allo experimento que eam facultatem recuperaverit. Etsi enim tunc ob diminutas vires quae radios uniunt, aeger lente vitrea opus habet, eadem tamen lens in omni distantia sufficit." -- Haller, El. Phys."La lentille cristalline n'est cepandant point de première necessité pour la vision. Aujour d'hui, dans l'opération de la cataracte on l'enlève entièrement, et la vision n'en souffre point." De la Metherie Vues Physiologiques. See also De la Hire, Hamberger Physiolog.
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 on which this can be more easily explained; which will be particularly 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 consequence 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 11 to 10. I have also examined them with the instrument of Mr. Schmeisser, lately presented to the R.S., 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 on removing it, the place which it occupied is again filled by the vitreous humour, 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 writers on optics have taught us to believe. ...
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 (Des Cartes, Scheinerus, Bidlous, Mollinettus, Sanctorius, Jurin); others, that it removed it nearer to the cornea (Kepler, Zinn, Porterfield); and thers, that it removed it nearer the retina (La Chariere, Perrault, Hartsoeker, Brisseau, and Derham). The advocates for these different opinions all agree in attributing these effects to a supposed muscularity of the ciliary processes. Of the structure of these processes Haller observes 'In 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 attachment and direction, that we cannot possibly conceive them adequate to the effects ascribed to them. Besides ... 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 refutation of these opinions, see Haller's large work.
For the accuracy of the representation I have annexed (in figs. 7, 8, 9,) I can vouch, having been at much pains in the dissection; from which I had the painting taken by a most accurate hand, Mr. S. Edwards, a gentleman well known for his abilities in the plates of that admirable work, the Flora Londinensis.On carefully removing the eyelids, with their muscles, we are presented with the muscles of the eye itself, which are 6 in number; 4 called recti, or straight; and 2 oblique; so named from their direction (see fig. 7) AA AA, 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 sometimes called, from its passing through the loop or pulley connected to the lower angle of the orbiter notch in the os frontis; it passes under the superior rectus muscle, and backwards 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 process 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. DDD, the fat in which the eye is lodged.
The use ascribed to these different muscles, is that of changing the direction of the eye, to turn it upwards, downwards, laterally, or in any of the intermediate direcations, accommodated either to the different situation of objects, or to express the different passions of the mind, for which they are pecularly adapted. But is it inconsistent with the general 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 compression on the viscera, as in the discharge of urine, foeces, &c. But to question this fact would be to question the inluence 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 10 feet distance, 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 experienced; if again we view an object within the focal distance, i.e. within 6 or 7 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 objects; this I believe must be the experience of every person whose eyes are in the natural and healthy state, and accordingly has been observed by almost every writer on optics. But the power of this combination, even from analogy, appears too obvious to need further illustration. I shall therefore next endevour to point out their precise action.
Supposing the eye in its horizontal natural position; I see an object distinctly at the distance of 6 feet, the picture of the object falls exactly on the retina; I now direct my attention to an object at the distance of 6 inches, as nearly as possible in the same line; though the rays from the first object still fall on my eye, while viewing the 2d, it does not form a distinct picture on the retina, though at the same distance as before, which shows that the eye has undergone some change; for while I was viewing the first object I did not see the 2d distinctly, thorugh in the same line: and now, vice versa, I see the 2d distinctly, and not the first; the rays from the first therefore, as they still fall on the eye, must either meet before or behind the retina; but we have shown that the rays from the more distant object convene sooner than those from the less distant object, therefore the picture of the object at 6 feet falls before, while the other forms a distinct image on the retina; but as my eye is stil in the same place as at first, the retina has by some means or other been removed to a greater distance from the fore part of the eye to receive the picture of the nearer object, agreeable to the principle before-mentioned. From which it is evident, that to see the less distant object, either the retina should be removed to a greater distance or the refracting power of the media should be increased: but I hope we have shown that the lens, which is the greatest refracting medium, has no power of changing itself.
Let us next inqure, if the external muscles, the only remaining power the eye possesses, are capable of producing those changes. With respect to the anterior part of the eye, we have seen the situation of those muscles; the recti strong, broad and flat, arising from the back part of the orbit, passing over the ball as over a pulley, and inserted by broad flat tendons at the anterior part of the eye; the oblique inserted toward the posterior part, also by broad flat tendons; when they act jointly, the eye being in its horizotal position, it is obvious, as every muscle in action contracts itself, the 4 recti by their combination must necessarily make a compression on the different parts of the eye, and thus elongate its axis, while the oblique muscles serve to keep the eye in its proper direction and situation. For my own part, I have no more difficulty 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 elasticity, is also increased, and that in proportion to the degree 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 4 recti muscles are larger and stronger than the 2 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 posterior 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 on it? Admitting then that this is the operation of the external muscles when in a state of contraction, it is also to be observed that we have the same power of relaxing them, in proportion to the greater distance of the object, till 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 on my eye, while directing my attention to an object at the distance of about 20 yards; I saw it distinctly, as also the different intermediate objects; but endeavouring to look beyond it, every thing appeard confused. I then increased the pressure considerably, in consequence of which I was enabled to see objects distinctly at much nearer than the natural focal distance; for example, I held before my eye, at the distance of about 2 inches, a printed book; in the natural state of the eye I could neither distinguish the lines nor letters; but on 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 phaenomena of vision; some of those it will not be improper at present brriefly to notice. First, may not the action 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 original conformation of the eye, the diminution of its humours, and probably of the quantity of fat on which the eye is lodged, are also to be taken into the account. But the external 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 considerable influence in impairing vision. Again, does not the habit of long sight so remarkable in sailors and sportsmen, who are as 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 on this principle? as we know of no part of the body so susceptible of an habitual action as the muscular fibre.
2dly, How are we to account for the weaker action of one eye in the case of squinting [cross-eye]? That this is the fact has been well ascertained; Dr. Reid (Se his Inquiry into the Human Mind, page 322) on this subject observes, that he has examined over 20 persons that squinted, and found in all of them a defect in the sight of one eye. Porterfield and Jurin have made the same observation. The distorted position of the eye has I believe been generally attributed to the extername muscles; but no satisfactory 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 turned form its natural direction, for example, towards the inner canthus, it is obvious that the adductor muscle is shortened, and its antagonist, the abductor, lengthened; consequently, 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 on the eye and its humours must also be equallyi 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 on the eye with the finger, or speculum oculi, before noticed, would also appear to favour this explanation.
3dly, May it notin 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 perhaps dilates and contracts as before: nore 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 see objects distinctly; and it is not an uncommon circumstance, even when the operation succeeds, that the sight is slowly and gradually recovered. Instances have occurred, Mr. Bell (see his System of Surgery) 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, dancing,k or playing on a musical instrument, we in a great measure lose the combination, and find a difficulty in recovering it, in proportion to the length of time we had been deprived of it; but the individual action of each muscle remains 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 optics, which may admit of a similar explanation.
I have thus endeavoured first, to point out the limited action of the iris, and of consequence the insufficiency of this action for explaining vision. 2dly, to prove that the lens possesses no power of changing its form to the different distances of objects. 3dly, that to see objects at different distances, corresponding changes of distance should be produced 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 4thly, that the combined action of teh external muscles is not only capable of producing 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 economy of nature and philosohpy, seeing the imperfections of the pirnicples which have hitherto been employed in explaining the phenomena in question, to adopt the one before us, till, agreeable to one of the established rules in philosophizing, 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 establishing the principle I have proposed, for explaining the phenomena dependent on 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 application, in alleviating the diseases to which its delicate organization so particularly exposes it.
Since the above pages have been written, I have found, on consulting some of the earliest writers, that the effects of the external muscles did not altogether escape their attention; at the same time they had no distinct idea of their action: I must therefore disclaim the originality of the though, though 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.