Charles. Look again at the fish in the glass, and you will see through the round part two very large fish, and seeing them from the upper part, they appear of their natural size; the deception is the same as with the shilling in the goblet.

Tutor. The principle of refraction is productive of some very important effects. By this the sun, every clear morning, is seen several minutes before he comes to the horizon, and as long after he sinks beneath it in the evening.

Charles. Then the days are longer than they would be if there was no such a thing as refraction. Will you explain how this happens?

Tutor. I will: you know we are surrounded with an atmosphere which extends all round the earth, and above it about the height of forty-five miles; now the dotted part of Fig. 5. represents that atmosphere: suppose a spectator stand at 8, and the sun be at a; if there were no refraction, the person at s would not see

the rays of the sun till he were situate with regard to the sun in a line s x a; because when it was below the horizon at b, the rays would pass by the earth in the direction b x z; but owing to the atmosphere, and its refracting power, when the rays from breach x, they are bent towards the perpendicular, and carried to the spec

tator at s.

James. Will he really see the image of the sun while it is below the horizon?

Tutor. He will; for it is easy to calculate the moment when the sun should rise and set, and if that be compared with exact observation, it will be found that the image of the sun is seen sooner and later than this by several minutes every clear day.

Charles.

Are we subject to the same kind of deception when the sun is actually above the horizon?

Tutor. We are always subject to it in these latitudes, and the sun is never in that place in the heavens where he appears to be.

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 m o r, 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;

1

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 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

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