EYE

, the organ of sight, consisting of several parts, and of such forms as best to answer the purpose for which it was formed.

As vision or sight is effected by a refraction of light through the humours of the eye to the bottom or farther internal part of it, where the images of external objects are formed on a fine expansion of the optic nerve, called the Retina, and therefore the sorepart of the eye must be of a convex figure, and of such a precise degree of convexity as the particular refractive power of the several humours require for forming the image of an object at a given focal distance, viz, the diameter of the eye. Hence we sind,

1st; The external part of the eye-ball CD (Plate 2, fig. 8.) is a strong pellucid substance, properly convex, and which, when dried, has some resemblance to a piece of transparent horn, for which reason it is called the Cornea, or horny coat of the eye.

2dly; Immediately behind this coat there is a fine clear humour which, from its likeness to water, is called the Aqueous or watery humour, and is contained in the space between CD and GFE.

3dly; In this space there is a membrane or diaphragm, called the Uvea, with a hole in the middle as at F, called the Pupil, of a muscular contexture for altering the dimensions of that hole, for the adjusting or admitting a due quantity of light.

4thly; Just behind this diaphragm is placed a lenticular-formed substance GE, of a considerable consistence, called from its transparency the Crystalline humour. This is contained in a sine tunic called the Choroides, and is suspended in the middle of the eye by a ring of muscular fibres called the Ligamentum Ciliare, as at G and E; by which means it is moved a little nearer to, or farther from, the bottom of the eye, to alter the focal distance.

5thly; All the remaining interior part of the eye, constituting the great body of it, from GHE to IMK, is made up of a large quantity of a jelly-like substance, called the Vitreous or glassy humour; though it resembles glass in nothing except its transparency; it being most like the white of an egg of any thing.

6thly; On one side of the hinder part of the Eye, as at K, the optic nerve enters it from the brain, and is expanded over all the interior part of the eye to C and E quite around, the expansion being named the Retina. On this delicate membrane, the image IM of every external object OB, is formed according to the optic laws of nature, in the following manner.

Let OB be any very distant object. Then a pencil of rays proceeding from any point L, will fall on the cornea CD, and be refracted by the aqueous humour under it, to a point in the axis of that pencil continued out. Then the radius of convexity of the cornea being nearly 1/3 of an inch; and the sine of incidence in air to that of refraction in the aqueous humour, being nearly as 4 to 3, supposing the rays parallel, or the object very far distant, the focal distance after the first refraction, by the proper theorem mr/(m-n), will be found 1 1/3 inch from the cornea: r being the radius 1/3, and m to n as 4 to 3. |

The rays thus refracted by the cornea, fall converging on the crystalline humour, and tend to a point 1.228 inch behind it; also the radii of convexity in the said humour are 1/3 and 1/4 respectively; and the sine of incidence to that of refraction of the aqueous into the crystalline humour, being as 13 to 12; therefore, by this theorem mdr/(md-nd+nr), the focal distance after refraction in the crystalline, will be 1.02 inch from the fore part of it: where m = 13, n = 12, r = 1/3, and d = 1.228.

The rays now pass from the crystalline to the vitreous humour still in a converging state, and the sines of incidence and refraction being here as 12 to 13, as found by experiment; and since the surface of the vitreous humour is concave which receives the rays, and is the same with the convexity of the hinder surface of the crystalline, the radius will be the same, viz 1/4 of an inch. Therefore the focal distance after this third refraction will be found, by this theorem, mdr/(nd-md+nr), to be 6 tenths of an inch nearly from the hinder part of the cornea: where m=12, n=13, r=1/4, and d=.82; the thickness of the lens of the cornea being nearly 1/5 of an inch.

Now experience shews that the distance of the retina in the back part of the eye, behind the cornea, is nearly equal to that focal distance; and therefore it follows that all objects at a great distance have their images formed on the retina in the bottom or hinder part of the eye, and thus distinct vision is produced by this wonderful organ of optic sensation.

When the distance of objects is not very great, the focal distance, after the last refraction in the vitreous humour, will be a little increased; and to do this we can move the crystalline a little nearer the cornea by means of the ligamentum ciliare, and thus on all occasions it may be adjusted for a due focal distance for every distance of objects, excepting that which is less than 6 or 7 inches, in good eyes. Many are of opinion, however, that this is effected by a power in the eye to alter the convexity of the crystalline humour as occasion requires; though this is rather doubtful.

By what has been said it appears, that rays of light flowing from every part of an object OB, placed at a proper distance from the eye, will have an image IM thereby formed on the retina in the bottom of the eye; and since the rays OM, BI, which come from the extreme parts of the object, cross each other in the middle of the pupil, the position of the image IM will be contrary to that of the object, or inverted, as in the case of a lens.

The apparent place of any part of an object is in the axis, and conjugate focus of that pencil of rays by which that part or point is formed in the image. Thus, OM is the axis, and O the focus proper to the rays by which the point M in the image is made; therefore the sensation of the place of that part will be conceived in the mind to be at O; in like manner the idea of place belonging to the point I, will be referred in the axis IB, to the proper focus B; therefore the apparent place of the whole image IM, will be conceived in the mind to occupy all the space between O and B, and at the distance AL from the eye.

Hence likewise appears the reason, why we see an object upright by means of an inverted image; for since the apparent place of every point M will be in the axis MO at O; and this axis crossing the axis of the eye HL in the pupil, it follows, that the sensible place O of that point will lie, without the eye, on the contrary side of the axis of the eye, to that of the point in the eye; and since this is true of all other parts or points in the image, it is evident that the position of every part of the object will be on the contrary side of the axis to every corresponding part in the image, and therefore the whole object OB will have a contrary position to that of the image IM, or will appear upright.

If the convexity of the cornea CD happens not exactly to correspond to the diameter of the eye, considered as the natural focal distance, then the image will not be formed on the retina, and consequently no distinct vision can be effected in such an eye.

If the cornea be too convex, the focal distance in the eye will be less than its diameter, and the image will be formed short of the retina. Hence the reason why people having such eyes are obliged to hold things very near them, to lengthen the focal distances; and also why they use concave glasses to counteract or remedy the excess of convexity, in order to view distant objects distinctly.

When the eye has less than a just degree of convexity, or is too flat, as is generally the case with old eyes, by a natural deficiency of the aqueous humour, then the rays tend to a point or focus beyond the retina or bottom of the eye; and to supply this want of convexity in the cornea, we use convex lenses in those frames called spectacles, or visual glasses.

Since the rays of light OA, BA, which constitute the visual angle OAB, will, when they are intercepted by a lens, be refracted sooner to the axis; the said angle will thereby be enlarged, and the object of course become magnisied; which is the reason why those lenses are called magnisiers, or reading-glasses.

The dimensions, or magnitude, of an object OB, are judged of by the quantity of the angle OAB which it subtends at the eye. For if the same object be placed at two different distances L and N, the angles OAB, oAb, which in these two places it subtends at the eye, will be of different magnitude; and the lineal dimensions, viz length and breadth, will be at N and at L, as the angle oAb is to the angle OAB. But the surfaces of the objects will be as the squares of those angles, and the solidities as the cubes of them.

It is found by experience, that two points O, L, in any object, will not be distinctly seen by the Eye, till they are near enough to subtend an angle OAL of one minute. And hence when objects, however large they may be, are so remote as not to be seen under an angle of one minute, they cannot properly be said to have any apparent dimensions or magnitude at all; such as is the case of the large bodies of the planets, comets, and fixed stars. But the optic science has supplied means of enlarging this natural small angle under which most distant objects appear, and thereby increasing their apparent magnitudes to a very surprising degree, in the instance of that noble instrument the telescope.

On the other hand, there are in creation an infinity | of objects, of such small dimensions, that they will not subtend the requisite angle, if brought to the nearest limits of distinct vision, viz 6, 7, or 8 inches from the Eye, as found by experience; and therefore to render them visible at a very near distance, we have a variety of glasses, and instruments of different constructions, usually called microscopes, by which those minute objects appear many thousand times larger than to the naked Eye; and thereby enrich the mind with discoveries of the sublimest nature, in regard to creative power, wisdom, and œconomy.

EYE-glass, in Optical Instruments, is that which is next the Eye in using the machine. This is usually a lens convex on both sides; but Eustachia Divini long since invented a microscope of this kind, the power of which he places very greatly above that of the common sort; and this chiefly depending on the Eye-glass, which was double, consisting of two plano-convex glasses, so placed as to touch one another in the middle of their convex surface. This instrument is well spoken of by Fabri in his Optics, and as possessing this peculiar excellence, that it shews all the objects flat, and not crooked, and takes in a large area, though it magnifies very much.

Bull's EYE, a star of the first magnitude, in the Eye of the constellation Taurus, the bull, and by the Arabs called Aldebaran.