"As the sum of the radii of both curves or convexities is to the radius of either, so is double the radius of the other to the distance of the focus from the middle point."

James. Then if one radius be four inches, and the other three inches, I say, as 4+3 : 4 :: 6 : 2434, or to nearly three inches and a half. I saw a gentleman lighting his pipe yesterday by means of the sun's rays and a glass, was that a double convex lens?

Tutor. I dare say it was: and you now see the reason of that which then you could not comprehend: all the rays of the sun that fall on the surface of the glass (see Fig. 8.) are collected in the point f, which, in this case, may represent the tobacco in the pipe.

Charles. How do you calculate the heat which is collected in the focus?

Tutor. The force of the heat collected in the focus is in proportion to the common heat of the sun, as the area of the glass is to the area of the focus of course, it may be a

hundred or even a thousand times greater in the one case than in the other.

James. Have I not heard you say that Mr. Parker, of Fleet-street, made once a very large lens, which he used as a burningglass?

Tutor. He formed one three feet in diameter, and when fixed in its frame, it exposes a clear surface of more than two feet eight inches in diameter, and its focus, by means of another lens, was reduced to a diameter of half an inch. The heat produced by this was so great, that 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 risque 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 he 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 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