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the finger might, without the smallest injury, be placed in the cone of rays within an inch of the focus.
Fames. 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.
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 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
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 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 F G, 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.