pencil of rays. By a medium is meant any pellucid or transparent body, which suffers light to pass through it. Thus air, water, and glass are media.

Of Refraction. Where rays of light, after passing through one medium, on entering another medium of different density, are bent out of their former course, and change their direction, they are said to be refracted. Thus Sa, (Fig. 1.* Plate III.) is a ray which, when it enters the medium A B, instead of proceeding in the same direction, a n, it is made to move in the direction a x.

If the rays of light, after passing through a medium, enter another of a different density, perpendicular to its surface, they proceed through this medium in the same direction as before, thus the ray represented by Pa, proceeds to b, in the same direction. But if the ray enter obliquely to the surface of a medium, either denser or rarer than that in which it moved before, it is made to change its direction, in passing through that medium. Of this sort of refraction there are two

cases.

1. If the medium, which the ray enters, be denser, it moves through it in a direction nearer to the perpendicular drawn to its surface. Thus, Sa, supposed to be in air, upon entering the denser medium, A B, glass, or water, instead of proceeding in the same direction, is bent into the direction, ax, which makes a less angle with the perpendicular, Pb.

2. When a ray of light passes out of a denser into a rarer medium, it moves in a direction farther from the perpendicular. Thus, if xa were a ray of light passing through glass or water, A B, it will, on arriving at the rarer medium, move in the direction a S, which makes a greater angle with the perpendicular. Refraction is greater or less, that is, the rays are more or less bent or turned aside from their course, according as the second medium, through which they pass, is more or less dense than the first. Thus light is more refracted in passing from air to glass, than from air to water, because glass is two or three times denser than water.

The refraction of light is thus shewn; take an empty bason into a dark room, make a small hole in the window-shutter, so that a ray of light may proceed to the bottom at a given point: mark this spot: then without disturbing any thing, pour water into the bason, and the ray, instead of proceeding to the point marked, will be bent out of its first direction, and be found at another point nearer the side.

In repeating the experiment, if a piece of looking-glass be laid at the bottom of the bason, the light will be reflected from it; and will be observed to suffer the same kind and degree of refraction, in going out, as in coming in, only in a contrary direction.

If a few drops of milk be put into the water so as to take away its transparency, and if dust be raised in the room by sweeping a carpet, &c. the rays will be rendered much more visible. Another experiment is shewn on this subject, and may be repeated very easily: put a shilling into an upright bason or pan when empty, and let a person who is to observe the experiment walk backward, till he just lose sight of the money by the side of the vessel. Now pour water into the bason, and the observer will see the shilling most distinctly, though neither he nor it has been removed from their places. Parallel rays of light are such as move always at the same distance from each other: such are those represented at a b, &c. fig. 2. Now if these fall upon a plano-convex lens, that is, a lens, one of whose sides is flat, and the other convex, they will be so refracted as to unite in a point, f, behind it, called the focus, the distance of which from the centre of the glass, is called the focal distance, which is equal to the diameter, or to twice the radius of the sphere, from which the lens is supposed to be cut.

When parallel rays, A B, fig. 3, fall upon a double convex lens, that is, a lens, both of whose sides are convex, they will be refracted so as to meet in a focus, f, whose distance is equal to radius, or the semi-diameter of the sphere from which it is taken.

It is evident from the figure, that all the rays of the sun

which fall upon the surface of a convex lens are collected at the focus f; of course the force of all their heat is collected into that part, and is in proportion to the common heat of the sun's rays, as the area of the glass is to the area of the focus. As the one may be 10, or 100, or 1000 times larger than the other, the heat at the focus may be 10 times, or 100 times, or 1000 times greater than it is at the surface, which points out the cause why glasses of this shape are used, and act, as burning-glasses.

All the rays on each side the centre one cross that centre ray, and diverge from it to the contrary sides, in the same manner as they converged in coming to it. And if another glass, FG, of the same convexity as AB, be placed in the rays at the same distance from the focus, it will refract them so, as that, after going out of it, they will be all parallel, and proceed in the same manner as they came to the first glass AB; but on different sides of the middle ray, as may be seen by tracing their progress in the figure.

Since rays diverge from a radiant point as from a principal focus, therefore if a candle be placed at ƒ, in the focus of the convex glass FG, the diverging rays in the space FfG will be so refracted by the glass, that, after going out of it, they will become parallel, as is shewn in the figure. If, however, the candle be placed nearer the glass than its focal distance, the rays will diverge, after passing through the glass, more or less, as the candle is more or less distant from the focus: but if it be placed farther from the glass than its focal distance, the rays will converge after passing through the glass, and meet in a point, which will be more or less distant from the glass, as the candle is nearer to or farther from its focus; and where the rays meet, they will form an inverted image of the flame of the candle. This may be made evident by placing a paper at the point where the rays meet, see fig. 4 and 5.

If an object, ABC, fig. 6, be placed beyond the focus F, of the convex glass def, some of the rays, which flow from every point of the object, on the side next the glass, will fall upon

it; and after passing through it, they will be converged into as many points on the opposite side of the glass, where the image of every point will be formed, and consequently the image of the whole object, which will be inverted. Thus the rays Ad, Ae, Af, flowing from the point A, will converge in the space daf; and by meeting at a, an image will be formed there of the point A. The rays Bd, Be, Bf, flowing from the point B, will be united at b, and those from C at c, and so of all the intermediate points between A and C. If the object ABC be brought nearer to the glass, the picture abc will be removed to a greater distance; for then, more rays flowing from every single point, will fall more diverging upon the glass; and therefore cannot be so soon collected into the corresponding points behind it. If the distance of the object ABC be equal to the focal distance of the glass, the rays of each pencil will, as we have seen, be so refracted by passing through the glass, that they will go out of it parallel to each other, and then there will be no picture formed. But where a picture is formed, it will be as much larger or less than the object, as its distance from the glass is greater or less than the distance of the object; so that if ABC be the object, c ba will be the picture; or if cba be the object, ABC will be the picture.

When parallel rays as a bed, &c. fig. 7, pass through a concave lens, as AB, they will diverge, after passing through a glass, as if they had come from a radiant point x, in the centre of the convexity of the glass; the point is called by writers on Optics, the imaginary or virtual focus. Thus the ray after going through the glass will on coming out at g, go on in the line gh; and the ray b in the direction mn, and so of the rest. The centre ray c, falling perpendicularly upon the middle of the lens, suffers no refraction in passing through it; but goes on in the same rectilinear direction, as if no glass had been in the way. If the lens had been concave only on one side, and the other side quite flat, the rays would have diverged, after passing through it, as if they had come from a radiant point at double the distance of x from the lens; that is, as if the point

had been at the distance of a whole diameter of the glass's convexity.

Of Reflexion. Def. 1. When rays of light strike against a surface, and are sent back again from the surface, they are said to be reflected. 2. The incident ray is that which comes from any luminous body, and falls upon the reflecting surface, as ED, fig. 8, and BA is the reflected ray. 3. The angle of incidence, is that which is contained between the incident ray BC, and a perpendicular BH to the reflecting surface in the point of reflexion, viz. CBH. 4. The angle of reflexion is that contained between the said perpendicular HB, and the reflected ray BA, viz. HBA.

When a ray of light falls upon any body, it is reflected, so that the angle of incidence is equal to the angle of reflexion; this is the fundamental fact upon which all the properties of all kinds of mirrors depend. If for instance a ray of light from the sun S, fall upon the mirror ED, at the point B, it will be reflected into the line BA; because the angle CBH is equal to the angle ABH, or what is the same thing, the curve line ei is equal to the curved line ix, which are the measures of the angles just named.

When the parallel rays a b, Cd, ef, fig. 9, fall upon a concave mirror AB, they will be reflected back from the mirror, and meet in a point m, at half the distance from the surface b df, of the mirror, from C, the centre of concavity: for they will be reflected, at as great an angle from the perpendicular to the surface of the mirror as they fall upon it, with regard to that perpendicular, but on the other side of it. Let C be the centre of the concavity of the mirror AB, and let the parallel rays a b, Cd, ef, fall upon the points b, d, f. Draw the lines Cb, C d, and Cf, from the centre, and these will be perpendicular to the surface of the mirror, because they proceed to it like so many radii from the centre to the circumference of a circle. Now if the angle C bm be made equal to the angle Cba, bm will be the direction of the ray a b, after it has been reflected from the point b of the mirror. The same