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ray now proceeds in a direction PS, parallel to the line RT, in which it would have proceeded, had it not been refracted at all. It is obvious, however, that this will not be the case, unless the refracting surfaces are parallel.

As a general rule, a ray passing from a rarer to a denser medium is bent towards the perpendicular, and a ray passing from a denser to a rarer medium is bent away from the perpendicular. But this is not always quite true; for certain substances have a stronger refractive power than others of equal, or even greater density.

FIG. 52.

A few curious effects of refraction may now be explained. Let the point A represent a pebble at the bottom of a stream or pond, BC being the surface of the water. A small pencil of rays, c issuing from A, will be refracted at O, away from the perpendicular, so as to reach an eye at E; and the rays will enter the eye in the

same directions as if they came from the point D. The pebble will therefore be seen, not at A where it really is, but at D where it is not; and since every point of the solid bottom on which the pebble rests will be raised in the same way, the water will appear much shallower than it actually is. Have you never found, when wading or bathing in a clear stream, that it was deeper than you supposed? If so, you will now readily understand the cause; and you will also see that it is a good rule, especially if you cannot swim, to make due allowance, in estimating the depth of water, for this singular species of deception.


There is a pretty and easily made experiment illustrative of this subject. Place in the bottom of a cup or basin any small object, such as a shilling or a halfpenny, and stand in such a position that the edge of the vessel shall just conceal the coin from your view. Then let some other person pour water into the basin, while you retain your position. The

coin which before was invisible, will now seem to rise into view, the rays issuing from it being refracted so as to reach the eye. From what has been already explained, it is manifest that the effect will be the same as if the coin were rcmoved from A to D (fig. 52).

Another experiment, still more simple, is equally instructive. If a straight rod be dipped slantingly in water, it appears crooked, or rather it appears to consist of two straight rods, one in the water and one above it, joined together at a very obtuse angle. The reason is, that every point below the surface of the water is seen out of its true position, the displacement increasing as the depth increases.

The atmosphere has not only the power of refracting light, but, since it consists of strata constantly increasing in density as they are nearer the earth's surface, a ray passing slantingly downwards is more and more refracted in each successive stratum. Hence the light from the sun, or from any other heavenly body (unless that body be right overhead) always describes, in the last part of its course, a slightly curvilinear path. In so doing, it tends more and more towards the perpendicular. Now the body from which light comes is seen in the direction in which the rays finally enter the eye, and hence it follows that we see the sun, moon, and stars, a little nearer the zenith than they really are. Their displacement increases as they approach the horizon, and when they are really on the horizon, we see them considerably above it. More remarkable still is the fact that, in the very same way, we actually see them for a short time after they are below the horizon. The rays of light which come from them to us are bent round the earth's convexity, much in the same way as the rays from the coin, in the experiment described above, are bent round the edge of the cup or basin in which it is placed.

Upon refraction depends, in a great measure, the value of those numerous optical instruments, such as spectacles, telescopes, microscopes, magic-lanterns, cameras, and stereoscopes, which have contributed so much to the convenience, instruction, and amusement of mankind. In all these

instruments, one or more refracting lenses are essential. Such lenses are usually made of glass, and are of six kinds, according as their surfaces are convex, plane, or concave. Three of them, the double convex (A), plano-convex (B), and meniscus (C),are thicker towards the

FIG. 53.

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centre than at the edges, and collect parallel rays falling

FIG. 54.

upon them to a common point or focus beyond, as in fig. 54. They are therefore called converging lenses. The other three, the double concave (D), the plano-concave (E), and the concavo-convex (F), which are thickest at the edges, are called diverging lenses, because parallel rays, after being refracted by them, diverge or spread outwards, as if they came from a focus on the same side. This is shown in fig. 55. If the FIG. 55. rays falling upon the lenses are not parallel, the effect is substantially the same; converging lenses tend to gather them, diverging lenses to scatter them. Hence the rays from any object which we wish to examine may be made to enter the eye, by means of a suitable combination of lenses, or of lenses and reflectors together, in any desired direction. By the same means magnified images may be produced of objects too small to be seen by the naked eye; and the dim light from objects far beyond the reach of unassisted vision may be so collected and concentrated as to render these objects distinctly visible. It is thus that the telescope has revealed to us thousands of worlds in the remote regions of space, of whose existence we should otherwise have been ignorant; while, on the other hand, the microscope has made us acquainted with other worlds within and around us, filled with creatures of

inconceivable minuteness, and has extended as far downwards, as the telescope has extended it upwards, our knowledge of the great Creator's handiwork.


WHEN anything can be resolved or decomposed into distinct parts, each part having different properties, we say that it is compound. Now it has been found, astonishing as the fact may appear, that a ray of white light is capable of such decomposition.

Let a small hole be made in a window-shutter, so as to admit into a darkened room a beam of sunlight. This bcam, falling upon the opposite wall, will there illuminate a small spot, as at B (fig. 56), which will be, in fact, a

FIG. 56.

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more or less perfect image of the sun. Suppose now that a glass prism P is interposed. The light will of course be refracted, but it will neither be brought to a focus, as it would be by a converging lens, nor transmitted in a direction parallel to its former course, as it would be by a plate of glass of uniform thickness. According to the laws of refraction already explained, we should expect it to be bent upwards, and to form an image of the sun somewhere towards C. And this really does happen, but another and

unexpected effect is at the same time produced. Instead of a round white spot, we find that the image now assumes an oblong form, extending from C to D, and exhibiting all the colours of the rainbow. Hence it is inferred, that each ray of white light is divisible into rays of different colours, some of which are more refrangible than others, and have therefore been bent farther away from their origi nal course.

The beautiful phenomenon just mentioned has been denominated the solar or prismatic spectrum. Its colours gradually melt into each other, so that it is not possible to say exactly where one ends and another begins. Nor arc the learned quite at one as to their number. It is usual, however, to reckon seven-red, orange, yellow, green, blue, indigo, and violet. The red light, being the least refrangible, occupies the lower part of the spectrum; the violet, being the most refrangible, occupies the upper part. The other colours appear in intermediate positions, as indicated in the figure. Thus a ray of pure white light, passing through a piece of colourless glass, is resolved into distinct and even brilliant colours, and the conclusion is therefore inevitable that these colours must be in the light itself.

Having gone thus far, it is not difficult to conclude, that the colours of all the objects which we see around us, are dependent on the power which these objects have of reflecting particular kinds of light. A rose appears red, because it reflects the red rays, while it absorbs all or nearly all the rest; and the same thing is true of other colours. But if a rose be placed in blue light, it will appear of a dingy bluish colour; dingy, because it reflects but few of the blue rays, its nature being to reflect red. If it reflected no light at all, it would appear totally black. Colour, then, is not anything inherent in the bodies themselves, to which we are wont to attribute it. It exists only in the light which illuminates them, and which they reflect. No body cau appear red or blue, unless there be red or blue rays in the light which falls upon it.

The rainbow is not only one of the most beautiful objects

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