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Tutor. It appears one fourth nearer the surface than the object is. Hence a pond or river is a third part deeper than it appears to be, which is of importance to remember, for many a school-boy has lost his life by imagining the water into which he plunged was within his depth.
James. You say the bottom appears one fourth nearer the surface than it is ; and then that the water is a third deeper than it seems to be: I do not understand this.
Tutor. Suppose the river to be six feet deep, which is sufficient to drown you or if we cannot swim : J
the bottom will appear to be only four feet and a half from the surface, in which case you could stand and have the greater part of your head above it ; of course it appears to be a foot and a half shallower than it is ; but a foot and a half is just the third part of four feet and a half.
Charles. Can this be shown by experiment? Tutor. It may I take this large empty
pan, and with a piece of soft wax stick a piece of money at the bottom, but so that you can just see it as you stand ; keep your position, and I will pour in a quantity of water gradually, and tell me the appear
Charles. The shilling rises exactly in the same proportion as you pour in the water.
Tutor. Recollect then, in future, that we cannot judge of distances so well in water as in air.
James. And I am sure we cannot of magnitudes : for in looking through the sides of a globular glass at some gold and silver fish, I thought them very large ; but if I looked down upon them from the top, they appear much smaller indeed.
Tutor. Here the convex or round shape of the glass becomes a magnifier, the reason of which will be explained hereafter. A fish will, however, look larger in water than it really is. I will show you another experiment which depends on refraction :
here is a glass goblet two-thirds full of water; I throw into it a shilling, and place a plate on the top of it, and turn it quickly over, that the water may not escape. What do you see?
Charles. There is certainly a half crown lying on the plate, and a shilling seems swimming above it in the water.
Tutor. So it appears indeed; but it is a deception which arises from your seeing the piece of money in two directions at once, viz. through the conical surface of the water at the side of the glass, and through the flat surface at the top of the water. The conical surface, as was the case with the globular one in which the fish were swimming, magnifies the money ; but by the flat surface the rays are only refracted, on which account the money is seen higher up in the glass, and of its natural size, or nearly so.
ames. If I look sideways at the money I only see the large piece ; and if only at top, I see it in its natural size and state,
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 sur. rounded 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
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 6 x 2; but owing to the atmosphere, and its refracting power, when the rays from b reach x, they are bent towards the perpendicular, and carried to the
spectator 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