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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 (Platë 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 lines on.

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. With out 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 as night.

CONVERSATION V.

Definitions of the different kind of Lenses of

Mr. Parker's Burning Jens, 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

• Parallel lines are those which being infinitely ex. tended never meet.

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 Ċ d, then they are said to diverge. But if they continually approach towards each other as in moving from c d to C, they are said to converge.

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 ; B is a plano con-. cave, 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 c d a convex surface, of glass for instance, all of which, except the mid., dle one, fall upon it obliquely, and, according to what we saw 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 no

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