iron plates were melted in a few seconds: tiles and slates became red-hot in a moment, and were vitrified, or changed into glass: sulphur, pitch, and other resinous bodies, were melted under water: wood-ashes, and those of other vegetable substances, were turned in a moment into transparent glass.

Charles. Would the heat produced by it melt all the metals?

Tutor. It would even gold was rendered fluid in a few seconds; notwithstanding, however, this intense heat at the focus, the finger might, without the smallest injury, be placed in the cone of rays within an inch of the focus.

James. There was, however, I should suppose, some risk in this experiment, for fear of bringing the finger too near the focus.

Tutor. Mr. Parker's curiosity led him to try what the sensation would be at the focus; and be describes it like that produced by a sharp lancet, and not at all similar to the pain produced by the heat of fire or a candle. Substances of a white colour were difficult to be acted upon.

Charles. I suppose he could make water boil in a very short time with the lens.

Tutor. If the water be very pure, and contained in a clear glass decanter, it will not be warmed by the most powerful lens. But a piece of wood may be burned to a coal, when it is contained in a decanter of water.

James. Will not the heat break the glass? Tutor. It will scarcely warm it: if, however, a piece of metal be put in the water, and the point of rays be thrown on that, it will communicate heat to the water, and sometimes make it boil. The same effect will be produced if there be some ink thrown into the water.

If a cavity be made in a piece of charcoal, and the substance to be acted on be put in it, the effect produced by the lens will be much increased. Any metal thus enclosed melts in a moment, the fire sparkling like that of a forge to which the blast of a bellows is applied.

CONVERSATION VI.

Of Parallel Rays-Of diverging and converging Rays-Of the Focus and focal distances.

Charles. I have been looking at the figures 6 and 8, and see that the rays falling upon the lenses are parallel to one another: are the sun's rays parallel?

Tutor. They are considered so: but you must not suppose that all the rays that come from the Vor. III.-D

surface of an object, as the sun, or any other body, to the eye, are parallel to each other, but it must be understood of those rays only which proceed from a single point. Suppose s (Plate 1. Fig. 9.) to be the sun, the rays which proceed from a single point a, do in reality form a cone, the base of which is the pupil of the eye, and its height is the distance from us to the sun. James. But the breadth of the eye is nothing when compared to a line ninety-five millions of miles long.

Tutor. And for that reason, the various rays that proceed from a single point in the sun are considered as parallel, because their inclination to each other is insensible. The same may be said of any other point as c. Now all the rays that we can admit by means of a small aperture or hole, must proceed from an indefinitely small point of the sun, and therefore they are justly considered as parallel.

If now we take a ray from the point a, and another from c, on opposite points of the sun's disk, they will form a sensible angle at the eye; and it is from this angle A E C that we judge of the apparent size of the sun, which is about half a degree in diameter.

Charles. Will the size of the pupil of the eye make any difference with regard to the appearance of the object?

Tutor. The larger the pupil, the brighter will the object appear, because the larger the pupil

is, the greater number of rays it will receive from any single point of the object.And I wish you to remember what I have told you before, that whenever the appearance of a given object is rendered larger and brighter, we always imagine that the object is nearer to us than it really is, or than it appears at other times.

James. If there be nothing to receive the rays (Fig. 8.) at f, would they cross one another and diverge?

Tutor. Certainly, in the same manner as they converged in coming to it; and if another glass FG, of the same convexity as D E, be placed in the rays at the same distance from the focus, it will so refract them, that, after going out of it, they will be parallel, and so proceed on in the same manner as they came to the first glass.

Charles. There is, however, this difference; all the rays, except the middle one, have changed sides.

Tutor. You are right; the ray B, which entered at bottom, goes out at the top b; and A, which entered at the top, goes out at the bottom c, and so of the rest.

If a candle be placed at f, the focus of the convex glass, the diverging rays in the space FfG, will be so refracted by the glass, that after going out of it, they will become parallel again.

James. What will be the effect if the candle be nearer to the glass than the point ƒ?

.

Tutor. In that case, as if the candle be at g (Plate 11. Fig. 10.) the rays will diverge after they have passed through the glass, and the divergency will be greater or less in proportion as the candle is more or less distant from the focus.

Charles. If the candle be placed farther from the lens than the focus f, will the rays meet in a point after they have passed through it?

Tutor. They will: thus if the candle be placed at g, (Plate 11. Fig. 11.) the rays, after passing the lens, will meet at x; and this point x will be more or less distant from the glass, as the candle is nearer to, or farther from its focus. Where the rays meet, they form an inverted image of the flame of the candle.

James. Why so?

Tutor. Because that is the point where the rays, if they are not stopped, cross each other; to satisfy you on this head, I will hold in that point a sheet of paper, and you now see that the flame of the candle is inverted.

James. How is this explained?

of a

Tutor. Let A B C (Plate II. Fig. 12.) represent an arrow placed beyond the focus F, double convex lens de f, some rays will flow from every part of the arrow, and fall on the lens; but we shall consider only those which flow from the points A, B, and c. The rays which come from a, as a d, a e, and a ƒ, will be refracted by the lens, and meet in A. Those