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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.

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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 I. 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 EC 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.

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