cle to show the exact curve of the

different lenses?

Tutor. Yes: and you see that parallel rays falling upon a planoconvex lens (Fig. 6.) meet at a point behind it, the distance of which, from the middle of the glass, is exactly equal to the diameter of the sphere of which the lens is a portion.

James. And in the case of a double conver, is the distance of the focus of parallel rays, equal only to the radius of the sphere (Plate 1. Fig. 8.)?

Tutor. It is and you see the reason of it immediately; for two concave surfaces have double the effect in refracting rays to what a single one has the latter bringing them to a focus at the distance of the

diameter, the former at half that distance, or of the radius.

Charles. Sometimes, perhaps, the two sides of the same lens may have different curves: what is to be done then?

Tutor. If you know the radius of both the curves, the following rule will give you the answer:

"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 : 24=3%, 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 timest 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 burning-glass?

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 redhot 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.