light cannot be concentrated without accumulating a proportional quantity of heat; hence concave mirrors

have obtained the name of burning mirrors. If a burning taper be placed in the focus, the ray which falls in the direction of the axis of the mirror will be reflected back in the same line; but two other rays, drawn from the focus, and falling on the mirror at B and F, will be reflected to A and E.Therefore the rays which proceed from a light placed in the focus of a concave mirror fall divergent upon it, and are reflected parallel; it is exactly the reverse of the former figure, in which the rays fell parallel on the mirror, and were reflected to a focus. In other words, when the incident rays are parallel, the reflected rays converge to a focus; when the incident rays proceed from the focus, they are reflected parallel; this is a very important law of optics.

ON KEFRACTION AND COLOURS.

Refraction is the effect which transparent mediums produce on light in its passage through them. Opaque bodies reflect the rays, and transparent bodies transmit them; but it is found that if a ray, in passing from one medium into another of different density, fall obliquely, it is turned out of its course. The power which causes the deviation of the ray is not fully understood; but the appearances are the same as if the ray (supposing it to be a succession of moving particles, which is for this purpose the most convenient way of considering it) were attracted by the denser medium more strongly than by the rarer. Let us suppose the two mediums to be air and water; when a ray of light passes from air into water, it appears to be more strongly attracted by the latter.

If then a ray, AB, fall perpendicularly on water,

the attraction of the water acts in the same direction as the course of the ray; it will not therefore cause a deviation, and the ray will proceed straight on to E; but if it fall obliquely as the ray C B, the water will attract it out of its course. Let us suppose the ray to have reached the surface of a denser medium, and that it is there affected by its attraction. If not counteracted by some other power, this attraction would draw it perpendicularly to the water at B, towards E; but it is also impelled by its projectile force, which the attraction of the denser medium cannot overcome; the ray, therefore, acted on by both these powers, moves in a direction between them, and instead of pursuing its original course to D, or being implicitly guided by the water to E, proceeds towards F, so that the rays appear bent or broken.

If a shilling be placed at the bottom of an empty teacup, and the teacup at such a distance from the eye that the rim shall hide the shilling, it will become visible by filling the cup with water. In the first instance, the rays reflected by the shilling are directed higher than the eye, but

D FE

B

when the cup is filled with water, they are refracted by its attraction, and bent downwards at quitting it, so as to enter the eye. When the shilling becomes visible by the refraction of the ray, you do not see it in the situation which it really occupies, but an image of it higher in the cup; for as objects always appear to be situated in the direction of the rays which enter the eye, the shilling will be seen in the direction of the refracted ray at B. The manner in which an oar appears bent in water is a similar effect of refraction. When we see the bottom of a clear stream, the rays which it reflects, being refracted in their passage from the water into the air, will make the bottom appear more elevated

than it really is, and the water will consequently appear more shallow. Accidents have frequently been occasioned by this circumstance; and boys who are in the habit of bathing should be cautioned not to trust to the apparent shallowness of water, as it will always prove deeper than it appears.

The refraction of light prevents our seeing the heavenly bodies in their real situation. The light they send to us being refracted in passing into the atmosphere, we see the sun and stars in the direction of the refracted ray. If the sun were immediately over our heads, its rays falling perpendicularly on the atmosphere would not be refracted, and we should then see it in its true situation. To the inhabitants of the torrid zone, where the sun is sometimes vertical, its rays are then not refracted. There is, however, another obstacle to see the heavenly bodies in their true situation, which affects them in the torrid zone as well as elsewhere. Light is about eight minutes and a half in its passage from the sun to the earth; therefore, when the rays reach us, the sun has quitted the spot he occupied on their de parture; yet we see him in the direction of those rays, and consequently in a situation which he had abandoned eight minutes and a half before. In speaking of the sun's motion, we mean his apparent motion, produced by the diurnal rotation of the earth, for the effect being the same, whether it be our earth or the heavenly bodies which move, it is more easy to represent things as they appear to be, than as they really are. The refraction of the sun's rays by the atmosphere renders the days longer, as it occasions our seeing an image of the sun, both before he rises and after he sets; for below the horizon he still shines upon the atmosphere, and his rays are thence refracted to the earth. So likewise we see an image of the sun before he rises, the rays that previously fall upon the atmosphere being reflected to the earth.

In passing through a pane of glass; the rays suffer two refractions, which being in contrary directions, pro

duce nearly the same effect as if no refraction had taken place.

E

A

B

A A represents a thick pane of glass seen edgeways. When the ray B approaches the glass at c it is refracted by it; and, instead of continuing its course in the same direction, it passes through the pane to D; at that point, returning into the air, it is again refracted by the glass, but in a contrary direction, and in consequence proceeds to E. Now the ray B C and the ray D E being parallel, the light does not appear to have suffered any refraction; for if a ray of light passes from one medium into another, and through that into the first again, the two refractions being equal and in opposite directions, no sensible effect is produced; for the direction is the same, and the little space by which the ray is thrown to one side, is necessarily less than the thickness of the medium, and the thickness of a pane of glass is too little to be worth considering. But this is the case only when the two surfaces of the refracting medium are parallel to each other; if they are not, the two refractions may be made in the same direction, and come to a focus at a point beyond the lens.

Lenses are of various forms as here represented.

B

A

D

E

is called a plane-convex, from having one side flat, and the other spherically rounded; B is a plane concave, having one side spherically hollow; c is a double-convex, and has both sides spherically rounded; D is a doubleconcave, with both sides hollow; E is a meniscus (so called from its moon shape,) and has one side convex, and the other concave. The property of those which have a convex surface is to collect rays of light to a focus ; and those which have a concave surface to disperse them.

We shall next explain the refractions of a triangular piece of glass called a prism. The sides are flat; it

cannot therefore bring the rays to a focus, nor can its refraction be similar to that of a flat pane of glass, because it has not two sides parallel. The refractions of the light, on entering and on quitting the prism, are both in the same direction.* On entering the prism P, the ray is refracted from в to c, and on quitting it, from c to D. If the window-shutters be closed, and a ray of light, admitted through a small aperture, fall upon a prism, it will be refracted, and a spectrum, A B, representing all the colours of the rain

brilliant colours; but the fact is, that the colours are not formed by the prism, but existed in the ray previous to its refraction; for the white rays of the sun are composed of coloured rays, which when blended together, appear colourless or white.

Sir Isaac Newton, to whom we are indebted for the most important discoveries respecting light and colours, was the first who divided a white ray of light, and found it to consist of an assemblage of coloured rays, which formed an image upon the wall, such as is exhibited, in which are displayed the following series of colours-red, orange, yellow, green, blue, indigo, and violet. Now a prism separates these coloured rays by refraction. It appears that the coloured rays have different degrees of refrangibility; in passing through the prism, therefore they take different directions, according to their susceptibility of refraction. The violet rays deviate most from their original course;

* This will at once appear, as in the case of the lens, by drawing perpendiculars to the surface of the prism where the ray enters and quits it.