14. The REFLECTION of rays is their regress, or returning from the furface of fuch bodies, on which they fall, and cannot enter thus, the rays A B falling on the surface CD, are reflected or turned back again in the direction E. (Fig. 6.).

15. MIRRORS, or Speculums, are thofe bodies whofe farfaces are fo very fmooth, as to be impervious to the rays of light which fall on them; and which, therefore, they reflect, fo as to represent the images of the objects expofed to them they are generally made of metal, highly polished, or of glafs polified on one fide, and filvered on the other; and are either plain, convex, or concave.

16. PLAIN MIRRORS are thofe whofe furfaces are perfect planes, and whose section is a right line, as CD (fig. 6) : thefe are viilgarly called looking-glaffes.

17. CONVEX MIRRORS are those whofe furfaces do every way equally rife above the plane of their bafes; the section of these fort of mirrors is a curve, and is either circular, elliptical, parabolical, or hyperbolical. A CD (fig. 4) is a circular fection; and the mirror is the fegment of a globe, or spherical furface, which is that moftly used.

- 18. CONCAVE MIRRORS are thofe whofe furfaces fink down with an uniforin hollownefs below the upper parts; whose section alfo is a curve, as various as the convex ; but the circular form is the most common.

19. REFRACTION of rays is their being bent, or turned out of their courfe, in paffing out of one medium into ano. ther as the ray B C (fig. 2), in paffing into the dense medium A I HD, is refracted, or turned out of its natural course C K, in the direction C E, which is called the refracted ray.

20. The INCIDENT RAY is that which comes from any object, and falls on the refracting or reflecting furface, as BC (fig. 2), or A B (fig. 6).

21. The ANGLE OF INCIDENCE is that which is contained between B C (fig. 4), the incident ray, and F C, perpendicular to the reflecting surface, that is, the angle B C F.

22. The ANGLE OF REFLECTION is that contained between the faid perpendicular F C and the reflected ray C E, that is, the angle F C E.

23. The ANGLE OF INCIDENCE IN REFRACTION is the angle BCF (fig. 2), as in reflection F C E (fig. 4) is the angle of reflection.

24. A LENS is a medium, generally of glass, made to disperse, or collect, the rays of light which pass through it: of these there are various forms, of which the most common are the eight following:

Firft, a Plano-convex, which has one fide plain, the other convex; as A (fig. 3).-Second, Plano-concave, which is plain on one fide, and concave on the other; as B.-Third, a Double Convex, which is convex on both fides; as C.-Fourth, a Double Concave, which is concave on both fides; as D.Fifth, a Menifcus, which is convex on one fide, and concave on the other; as E.-Sixth, a Plain Lens, which is flat on both fides, and of equal thickness in all its parts; as F.Seventh, a Flat Plano-convex, whose convex fide is ground into several flat furfaces; as G.-Eighth, a Prism, which has three flat fides; and, when viewed end-ways, appears like an equilateral triangle; as H.

Glaffes ground into any of the foregoing fhapes are called lenfes and a line going through the middle, as I K, is called the axis of the lens.

25. The VISUAL or OPTIC ANGLE, is that angle contained under two lines, drawn from the extreme points of an object to the eye; thus A I B (fig. 8) is the optic angle, or the angle under which the object A B appears to the eye at I.

26. A PENCIL OF RAYS is a double cone of rays, as LONF joined together at the bafe in the lens LN; of

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.)

1

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 muscles 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 second, 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 vifible 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, F GO, is the cryftalline, in the form of a double convex lens, and more convex in the back part: behind this is the vitreous humour KL.

The cryftalline is more dense than the vitreous, and the vitreous inore dense 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 menifcus; as is alfo 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 several muscular fibres, in order to let in more or less 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 F G 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 lefs, 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 stifle 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 short, 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 must ufe convex glaffes; and short-fighted people concave ones.

The ray of light Pp, flowing from the point of the objec P, and the ray R r, flowing from the point R, cross each other at O, and proceeding in the same 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 obferved that every visible point of the object sends forth a pencil of rays, which cross 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 vifion; the most common of which is the following:-take a bullock's eye, while it is fresh, from a newly killed beast, 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 bẹ turned upwards towards A, in order that the ray Pp may fall on the axis of the eye, oppofite C, where alone diftinct vifion is performed. And to view that point of the object R, the pupil of the eye must be turned downwards, to take