transparent membrane, called the arachnoides, Between the crystalline humor and the cornea is contained a clear transparent fluid, called the aqueous humor; and between the crystalline humor and the posterior part or bottom of the eye is included another clear transparent fluid, which is termed the vitreous humor. The refractive density of the crystalline is greater than those of the humors that furround, it. On the fide next to the nofe a nerve is inferted in the bottom of each eye, about twenty-five degrees from the axis of the cryftalline, which, after entering the eye, is fpread into an exceeding fine coat of network, termed the retina. Lastly; a very black mucus or flime is fpread over all the internal parts of the eye, that are not transparent, except the anterior part of the iris, which, as before obferved, is coloured.

In the figure, the three concentric circles ABC s (fig. 94) represent the coats of the eye, The external coat, or sclerotica, is tranfparent, and more convex between A and B, AKB being the cornea. The fecond tunic, or uvea is fibrous between D and I, and between G and H, and is there called the iris; the hole I H is the pupil. The third coat becomes fibrous between D and E, and between G and F, being there called the ligamentum ciliare, and is attached to the circumference of the lens or cryftalline humor E F. The cavity or chamber A EFB is filled with the aqueous humor, and the chamber DN GFE is filled with the vitreous humor. At N is inferted the optic nerve, the expansion of

T

which, over the internal furface DNG, is the retina.

The manner in which the eye acts upon the rays of light may be thus explained. Let oL represent an object, and suppose a pencil of light to proceed from o, and enter the eye; then, because the cornea is a convex-concave lens, whofe convexity is greatest, (323, c) the rays will be rendered more convergent in paffing through it; and if the crystalline be properly formed, they will be refracted by it into a focus at c on the retina. The fame will happen to the pencil which proceeds from 1, whose focus will be м; and the foci of the intermediate points will be between м and c: confequently an inverted picture or image will be formed on the retina, and fenfation be produced by the action of the light on the expansion of the optic nerve, which from thence is conveyed to the fenforium. And that the parts of the eye are adapted to produce fuch an image, appears likewife from experiment: for if the tunica sclerotica be carefully taken away from the back of the eye of any animal, the inverted picture of external objects may be feen on the thin membranes which remain. Neither is the inverfion of the image any obftacle to the mind's conceiving that the object is erect'; for a focus at м may be confidered as the indication of the existence of a radiant point at L, and a focus at c may indicate the existence of a radiant point at o; and fo of others, the mind contemv plating the object itself, and not the image; befides

which, we have notions refpecting position that are not derived from the fight, whence we judge whether a wall is perpendicular or a plane level, &c. Thefe notions are derived from a perception of the direction in which gravity conftantly acts; to which direction we always refer. Whence it happens, that though the pofition of the eye be ever fo much changed, the idea of the pofition of objects in view remains unaltered. For example; if an w obferver view an upright pole or ftaff, the image of the pole on the retina will be in a line at right angles to the opening of the eyelids, provided he holds his head upright; but if he vary the pofition of his head, the image will be formed in a different pofition, and upon a different part of the retina: notwithstanding which, he conftantly imagines the pole to be erect and unaltered.

Because the foci of rays that differ in divergence x are found at different diftances from the lens, those which diverge lefs coming to a focus fooner than those which diverge more, it is necessary that the eye fhould be adapted fo as to act upon the rays that arrive from points at various distances, and to bring them to a focus upon the retina. The natural structure of the eye is fuch, that parallel rays have their focus on the retina; and when the proximity of any object caufes its rays to fall with a greater divergency, the pupil of the eye contracts and excludes the moft divergent rays, at the fame time that the cryftalline is brought forward, and perhaps rendered more convex by means of the ligamentum

ligamentum ciliare, by which provifions the focus y still falls on the retina. This adjustment of the eye to the distances of objects gives the reason why we cannot view a near and a distant object at the fame time; for, if a hair be held at a few inches distance between the eye and a remote object, fuppose a tree at half a mile distance, the tree will appear confused and indiftinct when the attention is fixed on the hair, and the fame will be the case with the hair when the attention is fixed on the diftant tree.

There are some eyes naturally fo defective, that they cannot effect this adjustment. Those which are replete with humors have the cornea and crystalline too convex, fo that the pencils come to their foci before their arrival at the retina, where they fall in fmall circular spaces inftead of points, and by their interference render the image confused on the other hand, if the humors be deficient in quantity, the cornea and cryftalline are too flat, and the pencils of rays not being fufficiently refracted, arrive at the retina before their union in their foci; whence arifes the fame confufion in the image as in the former cafe. They whose eyes are imperfect in the firft manner are called myopes, from their winking or closing their eyelids, but more commonly near-fighted, because they fee very near objects diftinctly, the divergency of the rays caufing their foci to fall on the retina. They whofe eyes are too flat are called prefbytæ, because the imperfection of the fight of old men

being occafioned by a decay of the humors, is generally of this kind. Both these imperfections may in a great measure be remedied by the use of

,

pro

per fpectacles. Since the rays converge too foon in the eyes of myopes, it is plain that a concave lens interpofed between the object and the eye will cause the rays to fall more divergent, and confequently will prevent their converging to a focus before their arrival at the retina. And the rays may be made to converge fooner in the eyes of prefbytæ, by means of convex fpectacles, fo that they, being already convergent when they enter the eye, will be fufficiently refracted by the cornea and cryftalline to have their focus on the retina, and cause diftinct vifion.

Thefe imperfections are much more frequently A the confequences of habit than is generally imagined. Studious men are generally near-fighted, whereas failors, sportsmen, and others, who are used to fix their attention on remote objects, are more fubject to the contrary defect. The eyes of old men have another defect, namely, rigidity, or a want of the power of adjustment, fo that it often happens that they require concaves for diftant and convex lenfes for near objects, being capable only of feeing objects diftinctly with the naked eye that are at å moderate distance. Every one should avoid the use of spectacles as much as poffible. For, though they render vifion more diftinct, yet, they never fail to increase the defect of the eye, fo as in time

to