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It is owing to the atmosphere that the heavens appear bright in the daytime. 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 as night.

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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 that proceed from a point.

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

any number

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 c d, 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.

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

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

* Parallel lines are those which being infinitely extended never meet.

Charles. How do


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-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.

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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 iniddle 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

represents the glass, as c dn. Charles. Have you drawn the cir

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