talls inferiorly; that on the left proceeds to the right, and so on with respect to the others: thus an inverted image is formed on the retina.

cat.

4. The iris, by altering the diameter of the pupil, will influence inmediately the quantity of light admitted into the eye. If one eye is closed, and we continue to look at the same object, the pupil of the open eye dilates evidently, and contracts again, as the other is opened, to its former diameter. The iris also intercepts such rays as would fall on parts incapable of refracting them regularly, or such as are directed so obliquely on the cornea as to be too much refracted, admitting only the smaller pencil which enters the eye more in the direction of its axis. This, however, but partially, and only in cases where the opening of the pupil is circular, and where the confusion which would arise from the aberration of the extreme lateral rays may possibly be prevented: it is not the case where the opening is very much extended, oblong, vertical, and, in some circumstances, almost linear, as in the The eccentricity of the pupil can only so far influence the pencils of rays as to make them fall on the anterior vertex of the crystalline, with which it corresponds: the axes of the pupil, and the lens, do not correspond with that of the cornea. It appears from careful observations that this contraction and dilatation are irregular and limited; that by bringing the object nearer to the eye, within a certain distance, the pupil not only ceases to contract, but becomes again dilated; and that, beyond a few yards distance, it also ceases to dilate. In viewing the sun, instead of dilating according to the distance, it contracts most closely, obeying the quantity and intensity of the light, rather than the distance of the object. In viewing a less luminous object, the pupil dilates, when it is more distant, a greater quantity of light being necessary to produce a clear impression; as the object is brought nearer, we require a less degree of light, and the iris contracts to exclude what is superfluous. Thus far the iris may be useful in accommodating the eye to different distances; it may regulate the quantity of light, but it cannot alter the direction. In quiescent vision, the pupil preserves its diameter most steadily.

5. The crystalline lens being found to diminish in density gradually in every direction, approaching the vitreous humor on one side, and the aqueous on the other, Mr. Ramsden supposes that its refractive power is the same with that of the two contiguous substances. Its principal use appears to him to be that of correcting the aberration arising from the spherical figure of the cornea, where the principal part of the refraction takes place. From the constitution of the crystalline he inferred, that it will refract the rays of light without reflecting any of them; so that, although we have two surfaces of the aqueous, two of the crystalline, and two of the vitreous humor, we have only one reflected image, and, that being from the front of the cornea, there can be no surface to reflect it back, and dilate the image on the retina. If the surfaces of the crystalline had been abrupt, there must have been a reflection at each, and an apparent haziness would have interfered with the

distinct view of every luminous object. The smaller density of the lateral parts will not only correct the aberration of the spherical surface of the cornea, but will cause also the focus of each oblique pencil of rays to fall either accurately, or very nearly so, on the concave surface of the retina, throughout its extent. Had the refractive power been uniform, throughout the whole substance of the lens, it might have collected the lateral rays of a direct pencil nearly as well, but it would have been less adapted to the oblique pencils of rays. Also, the gradual increase of aensity in approaching the centre makes the crystalline equivalent to a much more refractive substance of equal magnitude. The principal use of the vitreous humor appears to be that of giving a ready passage to the rays of light, as they are converging into foci on the retina, and of keeping at the same time the surface of the latter uniformly spherical. It would allow a change of figure in the eye, or in the lens, or even a change of place in the latter, supposing there were powers in the living organ adequate to the purpose.

6. Some writers have contended that the retina is not equally sensible in all parts, and that a portion only, near the axis of the eye, is capable of conveying distinct impressions. Com paretti says that distinct vision is effected only in the optic axis, which is moved most rapidly over every point of the object; and that what is seen apparently out of the axis is caused by the direction of the first impression in the axis. We believe, however, that the limits of distinct vision are far more extensive. Dr. Young says, of his own eye, that the visual axis being fixed in any direction, he can see at the same time a luminous object placed at considerable distances from it; the angle, however, differs. The extent of the retina is every way greater than the limits of the field of yiew. The whole extent of perfect vision is little more than ten degrees; or, more strictly speaking, the imperfection begins within a degree or two of the visual axis, and at the distance of 5° or 6° becomes nearly stationary, until, at a still greater distance, vision is wholly extinguished. The imperfection may be owing partly to the unavoidable aberration of oblique rays, but principally to the insensibility of the retina; for, if the image of the sun itself be received on a part of the retina remote from the axis, the impression will not be sufficiently strong to form a permanent spectrum, although an object of very moderate brightness will produce this effect when distinctly viewed. The motion of the eye has a range of about 55° in every direction, so that the field of perfect vision, in succession, is by this motion extended to 110°.

7. It results from some experiments of Haldat's, made by producing an artificial strabismus, that the opinion, which limits the position in which an image can be be seen distinctly to a point at the bottom of the eye, is by no means reconcileable with actual observation. For, in an artificial strabismus, one of the impressions falling on a part without the visual axis, ought not to produce any perception of the object; this we know not to be the case, From this

fact alone we may conclude that the place of the image is not necessarily confined to the axis, but that many points of the surface of the retina are capable of conveying an impression of it. As the angle is increased, the perfection of the image may be lessened; but we do not lose the perception of it until its position is such, that none of the rays proceeding from it directly can be brought to converge on the posterior hemisphere of the globe. This would appear to conform also with our ideas of the use of the extent of the retina, for which, if the field of vision was so extremely limited, we could assign no reason. The points of it, at a distance from the axis, may be less favorably disposed, but are not perhaps less susceptible of being affected. The whole of the retina is of such a form as to receive the most perfect image on every part of its surface, that the state of each refracted pencil will admit; and the varying density of the crystalline ⚫ renders that state more capable of delineating such a picture than any other imaginable contrivance could have done.' Dr. Young has given a diagram, representing the successive images of a distant object filling the whole extent of view, as they would be formed by the successive refractions of the different surfaces. But in opposition to the observations given above, respecting the decreasing sensibility of the retina remarked by Dr. Young, it has been observed that, on comparing the impressions produced by rays parallel to the optic axis with those by rays much inclined to this axis, they have appeared to differ in intensity only in a degree corresponding to the diminution in the extent of the opening of the pupil, produced by the obliquity of its plane to the luminous rays, and by the obliquity of the rays themselves to the refracting substances through which they pass. At the most, the difference of the clearness of the impression is not such as it would be if it depended on a diminution of the sensibility of the retina, proportionate to its distance from the optic axis. Notwithstanding the influence of the causes just mentioned, the light of a candle passing into both eyes, when their axes are artificially inclined, so that the images make angles of 15° to 25° with the optic axis, suffers no apparent diminution of brightness. This fact certainly gives to the field of distinct vision a more considerable extent than that usually assigned it. The point of the retina, which corresponds to the optic axis, may possibly be the place of most perfect vision; not because it is endowed with a greater sensibility than other parts of the retina, but from its being in the exact focus of the refractive powers of the eye, and the only point where the image can be impressed with every perfection.

8. In considering the sensibility of the retina, the office of the pigmentum must not be overlooked. In the human subject this is always more or less dark. In animals, where the pigmentum is more than of one color in the same eye, the lighter portion is always placed at the bottom of the eye, including the entrance of the optic nerve in its sweep; the color varies in different animals, but has always a brilliant surface. Probably the light has a greater effect on the retina

in eyes which have a white pigmentum, than in such as possess a dark one. Hence all animals see more or less distinctly in the dark, according as their lucid tapetum approaches nearer to a white or black color. Man, in whom it is dark, sees very imperfectly in a light where a cat, or dog, would perceive objects with tolerable clearness. We may observe that, when either of the latter look at us in the dark, the whole pupil is enlarged and illuminated; but in a full light there is no such appearance. Here there must be a reflection of light from the bottom of the eye to produce the effect; and the reflected light is always of the same color with the tapetum. Those individuals of our species who have a light pigmentum, see much better with less light than those who have it dark. In the Albino, where the coloring matter is exceedingly thin, or wholly deficient, the common day-light is far too powerful to admit of distinct vision. When he attempts to examine the qualities of an object with precision, the eye-brows are knit, and thc eye-lids kept almost shut. In the twilight he can see more plainly, as the luminous rays are then not too intense for the very sensible retina. The ferret is destined, from its mode of life, to see in dark places; and its pigmentum is naturally white. The rays which pass through the transparent retina are disposed of according to the reflecting powers of the pigmentum. In inan, who requires distinct vision in a moderate light, rather than the power of seeing where light is almost wholly wanting, the pigmentum is dark, and the rays are absorbed, and entirely lost; therefore, in such eyes, it can add nothing to acuteness of vision, and a considerable quantity of light is required to produce an adequate impression on the retina. The rays are then lost in the pigmentum, and the accuracy of the image is no way impeded. In animals, who require a great acuteness of sight, the rays, reflected from a light and brilliant surface, again impress the retina, and increase the power of vision. The interval of time is too short, and the distance between the points they may strike in their double passage too minute, to occasion any indistinctness of the image. Distinct vision requires that the object should be fixed, and not allowed to move over the surface of the retina. To accomplish this object, the muscles of the globe are employed in the manner above described. We believe the impression made on the retina by the luminous rays to be in some degree permanent, and the more so as the light is stronger. The duration may vary probably from one hundredth of a second to nearly a second. Hence, the well-known phenomenon of the circle of light in revolving a lighted stick. If the object is painfully bright, the sensation is more permanent, and vanishes at last gradually,

9. Images of what we see are pictured inverted on the retina, and much controversy has arisen as to the cause of our perceiving the objects erect. If it be allowed that we judge of the situation of each luminous point by the direction of the rays it transmits, it follows that we must see bodies as we really do see them, in their proper position. The opinion that we really see objects reversed, and correct the sensation by

experience and judgment, derived from the other senses, is liable to very numerous objections. The chick just hatched knows where to direct its bill; and persons born blind, who have suddenly gained their sight, see objects in their proper position. We do not see the picture on the retina, but the object itself in the direction of each of the rays which conveys to us the sensation, or, to speak more correctly, in the direction of the axis of that pyramid which a pencil of divergent rays forms in proceeding from any point of an object to the eye. Bishop Berkeley explains the supposed difficulty in another way; he does not allow that we can estimate the situation of parts or objects by the decussation and direction of the rays of light, as the mind neither perceives the intersections of the radious pencils, nor pursues the impulses they give in right lines: without perceiving them it cannot form a judgment, and it cannot perceive them without a consciousness of such perception. The situation of visible objects must be entirely relative, and depend on the place which they occupy with regard to each other. And, as all visible objects are inverted at the same instant, eacn will be in the same relative situation on the retina as it is in actually. Thus the terms of above and below are arbitrary expressions, by which it is agreed to call upper what corresponds to the heavens, and lower what corresponds to the earth. Now it is evident that, at the bottom of the eye, the situation of these is inverted; the earth is above, and the heavens below. We call that the lower end of an object which is nearest the ground; and, the image of a man's feet being in contact with the image of the earth on the retina, we naturally infer that they are in contact with the actual earth; the head being more remote from the earth, we suppose that it is higher. The confusion has arisen from mixing the ideas derived from the different sensat ons of sight and touch. 'You say,' to use the words of Dr. Berkeley, 'the picture of the man is inverted, and yet the appearance is erect. I ask you what mean you by the picture of the man, or, which is the same thing, the visible man's being inverted? You tell me it is inverted because the heels are uppermost, and the head undermost. Explain me this: you say that, by the head being undermost, you mean that it is nearest to the earth; and, by the heels being uppermost, that they are farthest from the earth. I ask, again, what earth you mean? You cannot mean the earth that is painted on the eye, or the visible earth; for the picture of the head is farthest from the picture of the earth, and the picture of the feet nearest the picture of the earth; and, accordingly, the visible head is farthest from the visible earth, and the visible feet nearest to it. It remains therefore that you mean the tangible earth, and so determine the situation of visible things with respect to tangible things, which is absurd, and perfectly unintelligible.'

10. Two distinct images are certainly painted, one upon each eye, and yet we only perceive a single object. Many explanations have been given of this phenomenon; the most satisfactory is, that in the two eyes there are corresponding parts of the retina, which are probably suscep

tible of the same impression in equal degree, and convey it to the sensorium in that equal degree: hence, as long as similar points of the images fall upon the corresponding points of the retina, the perception of the same object is single. It is double, for the same reason, whenever the disposition of the visual axes is deranged; and, by an artificial pressure on one of the eyes, we may so displace its visual axis, or point of most perfect vision, that the two images shall not fall on those parts of the retina of the two eyes usually impressed simultaneously; a double image is the consequence. In order to preserve the simplicity of perception, when we look at an object brought nearer to us, we make them converge towards it by means of the external muscles of the eye, which is further ad usted to the decreasing distance by some other of its powers, so as to convey a single and distinct image of the object.

11. In the year 1793 Dr. Young made some, observations on the structure of the eye, and its provisions for adjustment, among which are accounts of the theories of adjustment, proposed by various earlier writers. Of these we shall say nothing, as a reference to the anatomical description of the eye, and other remarks already detailed, would at once refute the- greater part of them. It was the opinion of Dr. Young that rays of light, passing from objects at a small distance, could only be brought to foci on the retina by a nearer approach of the crystalline to a spherical form; this change, he believed, was effected by the muscularity of the lens. In the following year, some observations of John Hunter on this subject were published by Mr. Home, from which it appears, that he had for many years entertained a notion, that the crystalline humor was enabled, by its own internal actions, to adjust itself, so as to adapt the eye to different distances. Mr. Hunter had instituted some experiments, but died before he had made sufficient progress to draw any conclusion. In the same year Dr. Hosack, in a paper on vision, controverts Dr. Young's deductions with regard to the muscularity of the lens, and attributes the effects produced in adjustment to the actions of the muscles. He assumes, as the necessary consequence of contraction in these muscles, that the axis of the eye will be elongated, and the elastic cornea rendered more convex; both which circumstances would tend to preserve distinctness of vision with regard to near objects. In order to prove that the eye is capable of having its focal adjustment considerably varied by external pressure, he applied the common speculum to his own eye, and, by increasing the pressure of it considerably, was enabled to see objects distinctly, though placed much nearer than the natural focal distance. The means bere made use of to ascertain the fact do not appear to us very accurate. In the Croonian lecture for 1725 Mr. Home relates a series of experiments and observations made by himself and Mr. Ramsden, from which he concludes, that the eye has a power of adjusting itself to different distances, when deprived of the crystalline lens; and that, therefore, the supposed fibrous, and laminated structure of that lens, is not intended to alter its form, but to prevent reflections in the

passage of the rays through the surfaces of media of different densities, and to correct spherical aberrations; that the cornea is elastic, capable of being elongated one-eleventh of its diameter; that the tendons of the four straight muscles terminate in forming a lamina of the cornea; and that, in changing the focus of the eye from seeing with parallel rays to a near distance, there is a visible alteration produced in the figure of the cornea, rendering it more convex; and, when the eye is again adapted to parallel rays, the alteration by which the cornea is brought back to its former state is equally visible: The exertion required to adjust the eye to near distances, and the ease with which it is adapted to remote objects, proves that the first was a positive action, and the second a relief. The defect of elasticity in the cornea, inferred to arise from age, is applied to explain the changes of vision which take place in advanced life. By some further experiments Mr. Ramsden and Mr. Home were induced to abandon the opinion that the adjustment is produced solely by the alteration of the convexity of the cornea, which might probably be sufficient when the lens was removed, but not when the eye is entire. Mr. Home assumes that the action of the straight muscles will elongate also the axis of the eye, and produce an effect upon the crystalline lens, and ciliary processes, pushing them forward in proportion as the cornea is stretched. Granting these two last changes, Mr. Ramsden computed that the increase of the curvature of the cornea may be capable of producing one-third of the effect, and that the change of place of the lens, and elongation of the axis of vision, sufficiently account for the other twothirds of the quantity of adjustment necessary to make up the whole.

In the year 1800 was published an excellent paper by Dr. Young, on the mechanism of the eye, in which he examines, with great acuteness and accuracy, the different opinions on this subject. It is impossible for us here to give an abstract of his observations; we must refer the reader to the paper itself, for the detail of all the proofs by which he endeavours to establish his opinion of an alteration in the figure of the crystalline, and give here only the general conclusions drawn from his investigations. The arguments in favor of an increase of the convexity of the crystalline lens are of two kinds; some of them are negative, derived from the impossibility of imagining any other mode of performing the accommodation without exceeding the limits of the actual dimensions of the eye, and from the examination of the eye, in its different states by several tests, capable of detecting any other changes if they had existed for example, by the application of water to the cornea, which completely removes the effects of its convexity, without impairing the power of altering the focus, and by holding the eye, when turned inwards, in such a manner as to render any material alteration of its length utterly impossible. Other arguments are deduced from positive evidence of the change of form of the crystalline, furnished by the particular effects of refraction and aberration, which. are observable in the different states of the eye; effects which furnish a

direct proof that the figure of the lens must varyi its surfaces, which are nearly spherical, in the quiescent form of the lens, assuming a different determinable curvature, when it is called into exertion. The objections which have been made to this conclusion are founded only on the appearance of a slight alteration of focal length in an eye from which the crystalline had been extracted; but the fact is neither sufficiently ascertained, nor was the apparent change at all considerable and even if it were proved that an eye without the lens is capable of a certain small alteration, it would by no means follow that it could undergo a change, five times, or ten times as great.'

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12. Motions of the eye are either external or internal. The external motion is that performed by its four straight and two oblique muscles, whereby the whole globe of the eye changes its situation or direction. The spherical figure of our eyes, and the loose connexion to the edge of the orbit by the tunica conjunctiva, which is soft, flexible, and yielding, does excellently dispose them to be moved this or the other way, according to the situation of the object we would view. By the membranes the eye is connected to the edge of the orbit, which, being soft and flexible, they do in such a manner as not in the least to impede its necessary motions; and that great quantity of fat placed all round the globe, betwixt it and the orbit, lubricates and softens the eye, and renders its motions more easy: hence arise the three following remarkable observations:

i. When nature has denied the head any motion, it is observable that she has, with great care and industry, provided for this defect. To this purpose belongs the surprisingly beautiful and curious mechanism observable in the immoveable eyes of flies, wasps, &c. They nearly resemble two protuberant hemispheres, each consisting of a prodigious number of other little segments of a sphere, all which segments are perforated by a hole, which may be called their pupil, in which this is remarkable; that every foramen, or pupil, is of a lenticular nature, so that we see objects through them topsy-turvy, as through so many convex glasses: they even become a small telescope, when there is a due focal distance between them and the lens of the microscope by which they are viewed. Leuwenhoek's observations make it probable that every lens of the cornea supplies the place of the crystalline humor, which seems to be wanting in those creatures; and that each has a distinct branch of the optic nerve answering to it, upon which the images are painted: so that as most animals are binocular, and spiders for the most part octonocular, so flies, &c., are multocular, having in effect as many eyes as there are perforations in the cornea, by which means (as other creatures with but two eyes are obliged, by the contraction of the muscles above-enumerated, to turn their eyes to objects) these have some or other of their pupils always ready placed towards objects nearly all around them: whence they are so far from being denied any benefit of this noble and most necessary sense of sight, that they have probably more of it than other crea