inch (1

slit, 1 inch (38 millimetres) long and 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

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,

B

D
FIG. 16.

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 CD in the drawing. Make a record of these measurements, and then take the

it

bottle to the dark room. Place it nearer to the mirror, and let the reflected beam of light fall upon at a different angle, being careful that the beam strikes the water at the centre of the circle. Examine the beam of light in the bottle, and you will observe that it is bent, but at a different angle. Mark the two points where the beam crosses the circle above and below the water, and measure their distances from the perpendicular line, and then compare these distances with those we obtained the first time. Divide the distance between A and B by the distance between C and D, and you will obtain a certain quotient. Divide the two sets of figures obtained the second time (that is, the distance from the edge of the circle to the perpendicular line above by the same below the water), and the quotient or ratio of the one to the other will be exactly the same as before. For instance, if the distance from A to B is four units, the distance from C to D will be three units, and in every experiment this proportion will be the same. In this case, where the light passes from the air to the water, we get a quotient that is one and a third, and this quotient we call the index of refraction. These experiments show us that there is a fixed law of refraction. When the light met the surface of the water at a right angle, it passed

through the water without bending. In such instances the light is said to meet the water in a normal direction. If it meets the water on either side of this normal, it is refracted. Glass, diamonds, mica, and every transparent substance, have their own peculiar refraction. Glass has an index of refraction of 1.5. A diamond has quite another index of refraction,

and it is by comparing these that we are able to prove whether a stone is a real diamond or only an imitation made of glass.

Above is a picture representing the bottle in a new position. The beam of sunlight enters the darkened window, and falls upon a mirror lying flat on the