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without wetting the sides: if the tube is at rest there is no difficulty, it has only to be held upright in the direction AB; but if we must move the tube the matter is not so easy. The diagram shows that the tube must be inclined, or else the drop in the centre of the tube at a will no longer be in the centre of the tube at b; and the faster the tube is moved the more must it be inclined. Now we may liken the drop to rays of light, and the tube to the telescope, and we find that to see a star we must incline our telescopes in this way. By virtue of this, each star really seems to describe a small circle in the heavens, representing on a small scale the Earth's orbit; the extent of this apparent circular motion of the star depending upon the relative velocity of light, and of the Earth in its orbit, as in Fig. 41 the slope of the tube depends

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Fig. 42.-Showing how a ray coming from a star in the direction A B changes its direction, in consequence of the refraction of the atmosphere.

upon the relative rapidity of the motion of the tube and of the drop; and we learn from the actual dimensions of the circle that light travels about 10,000 times faster than the Earth does-that is, about 186,000 miles a second. This velocity has been experimentally proved by M. Foucault, by means of a turning mirror.

451. Now a ray of light is reflected by bodies which lie in its path, and is refracted, or bent out of its course, when

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it passes obliquely from a transparent medium of a certain density, such for instance as air, into another of a different density, such as water.

452. By an effect of refraction the stars appear to be higher above the horizon than they really are. In Fig. 42, AB represents a pencil of light coming from a star. In its passage through our atmosphere, as each layer gets denser as the surface of the Earth is approached, the ray is gradually refracted until it reaches the surface at C, so that from C the star seems to lie in the direction CB.

453. The refraction of light can be best studied by means of a piece of glass with three rectangular faces,

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Fig. 43.-A Prism, showing its action on a beam of light.

called a prism. If we take such a prism into a dark room, and admit a beam of sunlight through a hole in the shutter, and let it fall obliquely on one of the surfaces of the prism, we shall see at once that the direction of the ray is entirely changed. In other words, the angle at which the light falls on the first surface of the prism is different from the angle at which it leaves it. The difference between the angles, however, is known to depend upon a law which is expressed as follows: The sines of the angles of incidence and refraction have a constant proportion or ratio to one another. This

ratio, called the index of refraction, varies in different substances.

For instance, it is—

2'9 for chromate of lead.

2'0 for flint glass.

15 for crown glass.

1.3 for water.

454. If we receive a beam after its passage through the prism on a piece of smooth white paper, we shall see that this is not all. Not only has the ray been bent out of its original course bodily, so to speak, but instead of a spot of white light the size of the hole which admitted the beam, we have a lengthened figure of various colours, called a spectrum.

455. This spectrum will be of the same breadth as the spot which would have been formed by the admitted light, had it not been intercepted by the prism. The lengthened figure shows us, therefore, that the beam of light in its passage through the prism must have been opened out, the various rays of which it is composed having undergone different degrees of deviation, which are exhibited to us by various colours-from a fiery brownish red when the refraction is least, to a faint reddish violet at the point of greatest divergence. This is called dispersion.

456. If we pass the light through prisms of different materials, we shall find that although the colours always maintain the same order, they will vary in breadth or in degree. Thus, if we employ a hollow prism, filled with oil of cassia, we shall obtain a spectrum two or three times longer than if we use one made of common glass. This fact is expressed by saying that different media have different dispersive powers-that is, disperse or open out the light to a greater or less extent.

457. Every species of light preserves its own relative place in the general scale of the spectrum, whatever be

the media between which the light passes, but only in order, not in degree; that is, not only do the different media vary as to their general dispersive effect on the different kinds of light, but they affect them in different proportions. If, for instance, the green, in one case, holds a certain definite position between the red and the violet, in another case, using a different medium, this position will be altered.

This is what is termed by opticians the irrationality of the dispersions of the different media-or shortly, the irrationality of the spectrum.

458. What has been stated will enable us to understand the action of a common magnifying-glass or lens. Thus as a prism acts upon a ray of light, as shown in the above Fig. 43, two prisms arranged as in Fig. 44 would

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Fig. 44-Action of two Prisms placed base to base.

converge two beams coming from points at a and b to one point at c. A lens, we know, is a round piece of glass, generally thickest in the middle, and we may look upon it as composed of an infinite number of prisms. Fig. 45 shows a section of such a lens, which section, of course,

may be taken in any direction through its centre, and a little thought will show that the light which falls on its whole surface will be bent to c, which point is called the focus. If we hold a common burning-glass up to the sun, and let the light fall on a piece of paper, we shall find

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Fig. 45.-Action of a Convex Lens upon a beam of parallel rays. that when held at a certain distance from the lens a hole will be burned through it; this distance marks the focal distance of the lens. If we place an arrow, ab, in front of the lens mn, we shall have an image of an arrow behind at

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Fig. 46.-Showing how a Convex Lens, mn, with an arrow, a b, in front of it, throws an inverted image, a'b', behind it.

ab, every point of the arrow sending a ray to every point in the surface of the lens; each point of the arrow, in fact, is the apex of a cone of rays resting on the lens, and a similar cone of rays, after refraction, paints every point of the image. At a, for instance, we have the apex of a

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