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James. Why in these latitudes particularly? Tutor. Because with us the sun is never in the zenith, s, or directly over our heads; and in that situation alone, his true place in the heavens is the same as his apparent place.

Charles. Is that because there is no refraction when the rays fall perpendicularly on the atmosphere ?

Tutor. It is: but when the sun (Plate 1. Fig. 5.) is at m, his rays will not proceed in a direct line mor, but will be bent out of their course at o, and go in the direction o s, and the spectator will imagine he sees the sun in the line

son.

Charles. What makes the moon look so much larger when it is just above the horizon, than when it is higher up?

Tutor. The thickness of the atmosphere, when the moon is near the horizon, renders it less bright than when it is higher up, which leads us to suppose it is farther off in the former case than in the latter; and because we imagine it to be farther from us, we take it to be a larger object than when it is higher up.

It is owing to the atmosphere that the heavens appear bright in the day time. Without an atmosphere, only that part of the heavens would appear luminous in which the sun is placed; in that case, if we could live without air, and should stand with our backs to the sun, the whole heavens would appear as dark að night.

CONVERSATION V.

Definitions-Of the different kind of Lenses-Of Mr. Parker's Burning Lens, and the effects produced by it.

Tutor. I must claim your attention to a few other definitions; the knowledge of which will be wanted as we proceed.

A pencil of rays is any number that proceed from a point.

Parallel rays are such as move always at the same distance from each other.

Charles. That is something like the definition of parallel lines.* But when you admitted the rays of light through the small hole in the shutter, they did not seem to flow from that point in parallel lines, but to recede from each other in proportion to their distance from that point.

Tutor. They did; and when they do thus recede from each other, as in this figure (Plate 1. Fig. 6.) from c to cd, then they are said to diverge. But if they continually approach towards each other, as in moving from cd to c, they are said to converge.

* Parallel lines are those which being infinitely extended

never meet.

James. What does the dark part of this figure represent ?

Tutor. It represents a glass lens, of which there are several kinds.

Charles. How do you describe a lens?

Tutor. A lens is a glass ground into such a form as to collect or disperse the rays of light which pass through it. They are of different shapes, from which they take their names. They are represented here in one view, (Plate 1. Fig. 7.) A is such a one as that in the last figure, and it is called a plano-convex, because one side is flat, and the other convex; в is a plano-concave, one side being flat, and the other is concave; c is a double-convex lens, because both sides are convex; D is a double-concave, because both sides are concave; and E is called a meniscus, being convex on one side, and concave on the other; of this kind are all watch glasses.

James. I can easily conceive of diverging rays, or rays proceeding from a point; but what is to make them converge, or come to a point?

Tutor. Look again to the figure (Fig. 6.) now a, b, m, &c. represent parallel rays, falling upon с d a convex surface, of glass for instance, all of which, except the middle one, fall upon it obliquely, and, according to what we said yesterday, will be refracted towards the perpendicular.

Charles. And I see they will all meet in a certain point in that middle line.

Tutor. That point c is called the focus: the dark part of this figure only represents the glass, as c d n.

Charles. Have you drawn the circle to show the exact curve of the different lenses?

Tutor. Yes: and you see that parallel rays falling upon a plano-convex 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-convex, 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 an

swer:

"As the sum of the radii of both curves or convexities is to the radius of either, so is dou

ble 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 éven 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 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

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