Pagina-afbeeldingen
PDF
ePub

The pura

matter how) to the sense of sight, and to the exercise of that sense, the apparatus by which it is formed is constructed and put together, not only with infinitely more art, but upon the selfsame principles of art, as in the telescope or the camera obscura. The perception arising from the image may be laid out of the question; for the production of the image, these are instruments of the same kind. The end is the same; the means are the same. pose in both is alike, the contrivance for accomplishing that purpose

is in both alike.* The lenses of the telescope, [Plate II. fig. 3, 4.) and the humours of the eye, bear a complete resemblance to one another, in their figure, their position, and in their power over the rays of light, viz. in bringing each pencil to a point at the right distance from the lens; namely, in the eye, at the exact place where ne membrane is spread to receive it. How is it possible, under circumstances of such close affinity, and under the operation of equal evidence, to exclude contrivance from the one, yet to acknowledge the proof of contrivance have ing been employed, as the plainest and clearest of all propositions, in the other?

The resemblance between the two cases is still more accurate, and obtains in more points than we have yet represented, or than we are, on the first view of the subject, aware of. In dioptric telescopes there is an imperfection of this nature. Pencils of light, in passing through glass lenses, are separated into different colors, thereby tinging the object, especially the edges of it, as if it were viewed through a prism. To correct this inconvenience had been long a desideratum in the art. At last it came into the mind of à sagacious optician, to inquire how this matter was managed in the eye; in which there was exactly the same difficulty to contend with as in the telescope. His observation taught him, that, in the eye, the evil was cured by combining lenses composed of different substances, i. e. of substances which possessed different refracting powers.

Our artist borrowed thence his hint; and produced a correction of the defect by imitating, in glasses

* The comparison with the lens of the telescope is not perfectly exact for the crystalline lens is a substance composed of concentric layers, of unequal density, the hardness of which increases from the surface to the centre; and hence possesses a more refractive power than any artificial lens. Mr. Ramsden supposes that this texture tends to correct the aberration occasioned by the spherical form of the cornea, and the focus of each oblique pencil of rays falls accurately on the concave surface of che retina. - Paxtun.

made froin different materials, the effects of the different humours through which the rays of light pass before they .each the bottom of the eye. Could this be in the eye without purpose, which suggested to the optician the only eflectual means of attaining that purpose?

But farther; there are other points, not so much perhaps of strict resemblance between the two, as of superiority of the eye over the telescope, which being found in the laws that regulate both, may furnish topics of fair and just comparison. Two things were wanted, to the eye, which were not wanted (at least in the same degree) to the telescope: and these were the adaptation of the organ, first, to different degrees of light; and, secondly, to the vast diversity of distance at which objects are viewed by the naked eye, viz. from a few inches to as many miles. These difficulties present not themselves to the maker of the telescope. He wants all the light he can get; and he never directs his instrument to objects near at hand.

In the eye, both these cases were to be provided for; and for the purpose of providing for them a subtile and appropriate mechanism is introduced:

*“It does not appear that the hint of this discovery was taken by Mr. Dollond from the structure of the eye, as supposed by our author, but was obtained in a different manner. This circumstance does not however lessen the force of the reasoning. The principle thus applied in the construction of achromatic telescopes, has been since carried still farther, and in its new application, illustrates more strongly, if possible, the point so well insisted on by Dr. Paley, namely, the resemblance between the eye and our optical instruments. In the best achromatic telescopes, composed of the different kinds of glass, according to the discovery of Mr. Dollond, white or luminous objects are not shown perfectly free from color, their edges being tinged on one side with a claret colored, and on the other with a greenish fringe. This remaining imperfection has been got rid of by the combination of solid and fluid lenses in the object and eye-glasses of telescopes. For this beautiful discovery science is indebted to Dr. Blair of Edinburgh, who found that by placing a concave lens of muriatic acid with a metallic solution, between two convex lenses of glass, a combined Jens was formed which refracted rays with perfect regularity and equality. A lens like this has been used with great advantage. The most important point is, however, to consider this improvement in its application to the argument, and it will be seen how much nearer this construction brings the telescope to the eye. In Dollond's telescope there is a combination of solid lenses of different substances.-In Blair's, a combination of fluid and solid ; which is exactly the case in the human eye. The only difference is, that in the eye there is a solid lens between two fluid ones ; and in the telescope a fluid between two solid. The combination is closely siniilar, and the final cause in both probably the same, namely, to correct the unequal refraction of ligh --See Erlinburgh Journal of Science, No. viii p 212 : and Library of Useful k'nouluilge. No i & 12.

(Ed.

J. In order to exclude excess of light, when it is ex cessive, and to render objects visible under obscurer degrees of it, when no more can be had, the hole or aperture in the eye, through which the light enters, is so formed, as to contract or dilate itself for the purpose of admitting a greater or less number of rays at the same time. The chamber of the eye is a camera obscura,* which, when the light is too small, can enlarge its opening; when too strong, can again contract it; and that without any other assistance than that of its own exquisite machinery. It is farther also, in the human subject, to be observed, that this hole in the eye, which we call the pupil, under all its different dimensions, retains its exact circular shape. This is a structure extremely artificial. Let an artist only try to execute the same; he will find that his threads and strings must be disposed with great consideration and contrivance to make a circle, which shall continually change its diameter, yet preserve its form. This is done in the eye by an application of fibres, i. e. of strings, similar, in their position and action, to what an artist would and must employ, if he had the same piece of workmanship to perform. [Plate II. Fig. 5 & 6.]t

* As the rays of light flowing from all the points of an object through the pupil of the eye, by the refraction of the lens and humours of the aye, form an exact representation at the bottom of the eye on the retina ; so the camera obscura, by means of a lens refracting the rays, exhibits a picture of the scene before it on the opposite wall.-Paxton.

† Some eminent anatomists have doubted the muscularity of the iris, and have given very different explanations of its motions, attributing the contraction and dilatation either to the varied impulse of the blood in its vessels, or to its own vita propria. The enlightened physiologist Magendie affirms, that the latest researches upon the anatomy of the iris proves its muscular structure, and that it is composed of two layers of fibres, the external, Plate II. (Fig. 5.) radiated, which dilate the pupil, the other (Fig. 6.) circular, which contract the pupil. The external circular fibres appear to be supported by a species of ring, which each of the radiated fibres contribute to form, and in which they slide during the allernate contractions and relaxations of the pupil.– Paxton.

There is a curious circumstance in the way in which light produces the contraction of the opening of the iris, which strengthens very much the argument derived from design manifested in its structure and adaptation to its purpose. The object of the iris, it is to be observed, has reference to the quantity of light to be admitted upon the retina or expansion of "he optic

It is the state of the retina then which regulates the motions of the iris, and it is the action of the light on the retina which causes those motions and not its action upon the iris itself. This has been shown by a very delicate experiment. Il a ray of light be accurately thrown in such a direction, that it shall fall upon the circle of the iris itself, and not pays through its aperture, no contraction of the aperture takes place; but if it

nerve.

[ocr errors]

II. The second difficulty which has been stated, was the suiting of the same organ to the perception of objects that lie near at hand, within a few inches, we will suppose, of the eye, and of objects which are placed at a considerable distance from it, that, for example, of as many furlongs; (I speak in both cases of the distance at which distinct Vision can be exercised.) Now this, according to the principles of optics, that is, according to the laws by which the transmission of light is regulated, (and these laws are fixed,) could not be done without the organ itself undergoing an alteration and receiving an adjustment, that might correspond with the exigency of the case, that is to say, with the different inclination to one another under which the rays of light reached it. Rays issuing from points placed at a small distance from the eye, and which consequently must enter the eye in a spreading or diverging order, cannot, by the same optical instrument in the same state, be brought to a point, i. e. be made to form an image, in the same place with rays proceeding from objects situated at a much greater distance, and which rays arrive at the eye in directions nearly (and physically speaking) parallel. It requires a rounder lens to do it. The point of concourse behind the lens must fall critically upon the retina, or the vision is confused;* yet other things remaining the same, this point, by the immutable properties of light, is carried farther back when the rays proceed from a near object than when they are sent from one that is remote. A person who was using an optical instrument, would manage this matter by changing, as the occasion required, his lens or his telescopes; or by adjusting the distance of his glasses with his hand or his screw: but how is it to be managed in the eye? What the alteration was, or in what part of the eye it took place, or by what means it was effected, (for if the known be so thrown as to pass through the aperture, and fall upon the retina without touching the iris at all, still a contraction of the iris immediately takes place. So that light upon the iris alone occasions no contraction, although it is the part which really contracts when the same light falls upon a distant part. The design here is too obvious to need being enlarged upon. How could the iris acquire the power of contracting when light falls on another membrane, for the protection of that membrane ? although it does not contract wben the light falls apun itself alone ?--[Ed.

* 'The focus of the refracted rays must fall exactly on the retina, so that the point of vision may be neither produced beyond it, nor shortened so is not to reach it. The latter defect exists in short-sighted persons, from too great convexity of the cornea or lens. The former is the defect of long-sighted persons, in whom there is an opposite conformation of those perts.--Paxton.

's which

govern the refraction of light be maintained, sume alteration in the state of the organ there must be, had long formed a subject of inquiry and conjecture. The change, though sufficient for the purpose, is so minute as to elude ordinary, observation. Some very late discoveries, deduced from a iaborious and most accurate inspection of the structure and operation of the organ, seem at length to have ascertained the mechanical alteration which the parts of the eye undergo. It is found, that by the action of certain muscles (Pl. II. fig. 7.) called the straight muscles, and which action is the most advantageous that could be imagined for the purpose, -it is found, I say, that whenever the eye is directed to a near object, three changes are produced in it at the same time, all severally contributing to the adjustment required. The cornea, or outermost coat of the eye, is rendered more round and prominent; the crystalline lens underneath is pushed forwards; and the axis of vision, as the depth of the eye is called, is elongated. These changes in the eye vary its power over the rays of light in such a manner and degree as to produce exactly the effect which is wanted, viz. the formation of an image upon the relina, whether the rays come to the eye in a state of divergency, which is the case when the object is near to the eye, or come parallel to one another, which is the case when the object is placed at a distance. Can anything be more decisive of contrivance than this is? The most secret laws of optics must have been known to the author of a structure endowed with such capacity of change. It is as though an optician, when he had a nearer object to view, should rectify his instrument by putting in another glass, at the same time drawing out also his tube to a different length.

Observe a new-born child first lifting up its eyelids What does the opening of the curtain discover? The an. terior part of two pellucid globes, which, when they come to be examined, are found to be constructed upon

strict

optical principles; the selfsame principles upon which we ourselves construct optical instruments. We find them perfect for the purpose of forming an image by refraction; composed of parts executing different offices; one part having fulfilled its office upon the pencil of light, delivering it over to the action of another part; that to a third, and so onward; the progressive action depending for its success upon the nicest and minutest adjustment of the parts concerned; yet these parts so in fact adjusted, as to

« VorigeDoorgaan »