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CERTAIN things, like glass, water, mica, and ice, allow light to pass directly through their substance. We hold them before the eyes, and see the light very nearly as well as through the air. Such substances we say, are transparent. Other objects, like porce lain or oiled paper, do not permit all the light to pass, and such things, we say, are semitransparent or translucent. Many other things do not permit light to pass through them, and cast shadows behind them when brought into a beam of light. These things cut off all the light, and we call them opaque.

Here is a common glass bottle with straight sides and about three inches (76 millimetres) broad, or as wide as a postal-card (Fig. 14). On one side is pasted a piece of white paper having a perfectly round hole cut in it. On the glass, in the clear space made by the circular opening in the paper, are two lines drawn at right angles, in ink. These two lines divide the circle into four equal parts, and are to serve as guides in some new experiments.

Fill the bottle with clear water up to the hori

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zontal line in the circle, and then, holding the bottle in a small horizontal beam of sunlight from the heliostat, you will see that the light passes directly through the water in the bottle or through the air above the water. To make this more distinct, cut a slit, 17 inch (38 millimetres) long and inch (1 millimetre) wide, in a postal-card, and place this against the side of the bottle, so that the light will pass through the slit. This gives a sharp, clear beam of light, and by studying it carefully, we see that the beam in the air and its continuation in the water

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preserve the same direction. If we place the bottle on the floor or table, and with the mirror send a per

pendicular beam down into the water, we shall see exactly the same thing.

Fig. 15 represents the bottle of water standing upon a table, under a window, where the beam of sunlight enters from the heliostat.

The opening where the light comes in, the mirror, and the reflected beam of light thrown down upon the bottle, are plainly shown in the picture. The postalcard is held in such a position that the beam falls upon

the slit and then enters the bottle. Look into the bottle through the opening in the paper, and see where the beam falls, and then move the mirror and the card till the beam enters the bottle above the water and strikes the water just where the two lines meet in the centre of the circle. Draw the postalcard forward so that some of the light will cross the outside of the bottle, and appears to make a white mark across the paper circle. Study the two beams outside and inside the bottle, and see if you can discern anything peculiar about them. The part of the beam inside the bottle and above the water follows the same direction as the beam outside till it touches the water-line, and then it turns down and takes a new direction. This bending, that takes place when a beam of light passes from air into water, is called refraction. It takes place very generally when light passes from one transparent medium to another, and gives rise to a number of curious matters in regard to light.

Here is a drawing of the beam of light crossing the opening in the paper, and showing how it is bent. It passes through the air above the water,

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in the upper half of the circle, and then takes a new direction through the water in the lower half. You will observe in the drawing dotted horizontal lines extending from B to A, and from C to D. Look at the beam of light carefully, and with a pen mark these places A and C on the edge of the paper circle. Take the bottle to the light and measure off the distances from A to the perpendicular line, or along the line A B in the drawing, and from C to the perpendicular line, or the line C D in the drawing. Make a record of these measurements, and then take the

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