Imatges de pàgina
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which one cone, LON, has its vertex O in one point of an object; and the other cone, L F N, has its vertex F in that point of the object. The middle line O F is called the axis of that pencil, (fig. 5.)

SECT. I..

OF VISION.

Direct vifion is the faculty of fight, and is occafioned by the-rays of light proceeding from an object, and paffing through the humours of the eye, where they form the image of the object on the back part, or bottom of the eye. In order to understand which, it will be neceffary to explain the figure and construction of the human eye.

ABCE is the eye. It is of a spherical figure, by which means it is easily moved any way in its focket, by mufcles appointed for that purpose; the fore part at A (fig. 1) is more convex than any other part. The eye is enclosed in three membranes: the outermoft is called the Sclerotica; the fecond, the Tunica Choroides; the fore part of which is called the Iris, which confifts of many fibres, like fo many radii: the third membrane, or innermoft coat, is called the Retina, which is nothing but the optic nerve, fpread over the bottom of the eye upon this membrane the images of visible objects are formed.

In these three membranes are contained the three humours of the eye: the firft, HAI, is called the aqueous

humour,

humour, which is a thin watery liquor; the fecond, FG O, is the cryftalline, in the form of a double convex lens, and more convex in the back part: behind this is the vitreous humour K L.

The cryftalline is more denfe than the vitreous, and the vitreous more denfe than the aqueous humour: the three humours together form a compound lens, which refracts the rays of light, iffuing from an object PR, to the bottom of the eye; and there paints its image p r, upon the retina, in an inverted position.

The aqueous humour is in the form of a meniscus; as is also the vitreous humour. The fore part of that membrane called the sclerotica is called the cornea, as at A, and that part adjoining is called the white of the eye. Within the cornea is that coat called the uvea; in the middle of this is a hole O called the pupil, to let in the rays of light: this pupil is contracted or dilated by feveral mufcular fibres, in order to let in more or lefs light, as found convenient.

D is the optic nerve, which, coming from the common fenforium in the brain, is expanded all over the concave back furface of the eye, and thus forms the retina. This nerve is not fituated in the middle of the eye, but lies nearer the fide E, in that part next the nose.

The crystalline FG has a ring of fibres round its edge, by which means it can be drawn more or lefs convex, and the distance AC is thereby made greater or less, in order to form the image pr, upon the retina, for distinct vifion. This ring of fibres is called the ligamentum ciliare, the back part of which is black, in order to ftifle the rays which fall upon it. The eye is moved in the head by several muscles in the sclerotica.

If the image of an object do not fall upon the retina at pr, the vifion will be confused; if it fall fhort, or nearer FG, as is the cafe with fhort-fighted people, then a concave lens that makes the rays more diverging will bring it to the retina.

VOL. I.

3 G

If

If the rays of light do not unite, so as to form the image of the object, till they get beyond the retina, as is the cafe with most old people, then a common convex lens of a proper form will make them converge fooner, and fo form the image upon the retina; therefore long-fighted people muft ufe convex glaffes; and short-fighted people concave ones.

The ray of light Pp, flowing from the point of the object P, and the ray Rr, flowing from the point R, crofs each other at O, and proceeding in the fame straight lines, paint the image of the object PR on the retina, in an inverted pofition, as pr.

Note. Though the rays of light are in the figure reprefented by fingle lines, yet it must be observed that every visible point of the object fends forth a pencil of rays, which crofs each other at O, and paint the image of the object on the retina,

There are many experiments made by philofophers to demonstrate the truth of this theory of vision; the most common of which is the following:-take a bullock's eye, while it is fresh, from a newly killed beaft, and having cut off the three coats from the back part, quite to the vitreous humour, put a piece of paper behind that part, and hold the front of the eye towards any bright object, and there will be an inverted image of the object upon the white paper; which in this cafe ferves as a retina to the eye.

Though the image of the object is inverted in the bottom of the eye, yet we judge it to be erect, being always used to that pofition of the object. By an attentive perufal of the figure, the pofition of the object, with regard to that of the image, may be eafily accounted for. Thus, to view that point of the object P, the pupil of the eye at O must be turned upwards towards A, in order that the ray Pp may fall on the axis of the eye, oppofite C, where alone distinct vifion is performed. And to view that point of the object R, the pupil of the eye must be turned downwards, to take

in the rays of light R r, so that r may fall on the axis of the eye at C.

The diameters of objects are proportioned to the diameters of the images at the bottom of the eye; thus, the angle POR is equal to the angle p Or.

Some of the most common properties of the eye are the following:

1. The eye can only fee a very small part of an object diftinctly at once; therefore the eye must be turned fucceffively to the several parts of the object, that each part may fall in or near the axis of the eye.

2. When an object is feen diftinctly with both eyes, the axes of both eyes are directed to that point; thus the object appears fingle, though it be seen by both eyes at once: but if the axes of both eyes are not directed to the fame point of the object, it will appear double.

3. Few eyes can diftinguish a particle of matter, that fub. tends, at the eye, an angle less than half a minute; and very few can distinguish it when it fubtends a minute. If the distance of two stars in the heavens be not greater than this, they will appear as one.

4. The eyes of young people are more convex than those of old people; and this is the reafon the former can fee an object nigher than the latter.

5. The eyes of short-fighted people are too convex to admit of distinct vifion, when the object is placed at the distance of fix or eight inches, which is the common distance of an object for diftinct vifion. And the eyes of long-fighted people are not convex enough to admit of distinct vision at that distance. In the former cafe, the rays of light converging from an object through the humours of the eye, unite too foon, and before they reach the retina: to remedy which, concave glasses are used, which render the rays more diverging. And in the latter cafe the rays of light do not unite foon enough, in their paffage through the eye, to paint the image on the retina: the remedy of which is the convex glaffes.

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6. Whatever light falls upon the optic nerve at D makes no impreffion; therefore if the image of an object fall thereon it is loft. This place is not at the bottom of the eye, but on one fide, and towards the nofe in each eye; fo that no rays that come from any object can fall upon that part in both the eyes at the fame time.

7. The dimness of fight which generally attends old people arifes from two caufes :-Firft, by the eye growing more flat, and thereby not uniting the rays of light at the retina. Secondly, by the opacity of the humours of the eye, which, in time, lose somewhat of their transparency.

S. As the rays of light flowing from an object are the immediate cause of vision; it must necessarily follow, that where there is no light there can be no vifion.

Of the Properties of Light in general.

It may be neceffary to give a general defcription of the nature and properties of light. Sir Ifaac Newton has demonftrated, beyond a doubt, that the phenomena of vifion are produced by light; which is a matter emitted from the shining body with immense velocity, moving uniformly in straight lines, and acted on by other bodies, fo as to be reflected, refracted, or inflected, in various ways, by means of forces, which act on it in the same manner as on other inert matter.

Every visible body exposed to the light, emits, or reflects a great number of thefe inconceivably fmall particles of matter from each point of its furface; which iffue from it continue ally, in ftraight lines, and in all directions, forming a sphere, of which the illuminated body is the centre. These rays light, as before obferved, entering the eye, and ftriking upon the retina, are the caufe of vifion. And as these rays differ in fubftance, denfity, velocity, or magnitude, they will produce in the mind the idea of different colours.

of

That the particles of light are exceedingly fmall, appears from hence: viz. That if there be a candle lighted, and nothing to obftruct the progrefs of its rays, it will fill all

the

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