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580

Atmospheric Illusions.

through the lower strata, K. L, until it reaches M, where the angle of incidence is so small that it is reflected, and traversing N and o by refraction, it meets the eye of the spectator, A, at P. According to the law of visual direction, the object from which the light

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proceeds is seen inverted in the direction of the line, P O z. It therefore appears to the spectator as a perfectly reflected image, and conveys the impression that it is a reflection from the smooth surface of water.

The Fata Morgana, observed occasionally on the coast of Sicily, may be explained on similar principles.

These optical phenomena are not confined to warm climates. They have been observed in the Arctic regions. Scoresby noticed on the coast of Greenland that in certain states of the atmosphere the rocks appeared to be inverted and refracted in a symmetrical form, appearing like ruined castles, obelisks, and monuments, some of them surmounted by turrets, battlements, and towers, while in other cases large masses of rock were apparently suspended in the air at a great height above the summits of the mountains to which they referred. These effects were no doubt due to refraction.

On one occasion, in 1822, Mr. Scoresby saw, off this coast, the in verted figure of a ship in the air. By the aid of a telescope, he was able to discover that it was his father's ship, which was at the time below the horizon. This opinion proved correct. At the time of

Prismatic Refraction.

581 the observation the ships were thirty miles apart. As in atmospheric refraction, objects appear raised above their true position, owing to the bending of the rays of light in passing from a rarer to a dense medium.

811. As it is the obliquity with which a ray meets the surface which, in any case of refraction, determines the degree of bending, a body, seen through a medium of irregular surface, appears so distorted as not to be recognizable. It is because the two surfaces of ordinary window-glass are not, as in the case of plate-glass, perfect planes, and parallel to each other, that objects seen through a common window, appear generally more or less out of shape. Hence the beauty and utility of plate-glass windows, now in general use. The refraction or bending of light is most interestingly exemplified by the effect of a prismshaped piece of glass, a cross section of which is represented in fig. 190. A ray from a, entering the prism at b, is there bent towards the internal perpendicular, and takes the direction, b C, then

d

Fig. 190.

on emerging again at c, it is bent away from the external perpendicular, and thus with its first deviation doubled, goes on to d.

The law of refraction is well illustrated by what is called a multiplying glass, that is to say, a flat piece of glass, a b c (fig. 191), having many distinct faces cut upon it at angles with each other. If any small object, a coloured bead for instance, be placed at d,

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Fig. 191.

an eye at e will see as many beads as there are distinct surfaces or faces on the glass; for, first, the light, d a, passing perpendicularly, and therefore straight through, will form an image as if no glass intervened; then the rays from d to the surface, b, will be bent by

582

Decomposition of White light.

the oblique surface, and will show the object as if it were in the direction, be; and the light falling on the still more oblique surface, c, will be still more bent, and will reach the eye in the direction, ce, exhibiting a similar object also in that direction-and so of all the other surfaces. If the eye were at d, and the object at e, the result would still be the same. A plate of glass roughened, or cut into cross furrows, becomes an effectual screen or window-blind, by disturbing the passage of light through it so that the objects beyond it are not distinguishable.

"A beam of white light thus refracted is resolved into beams

of the seven so-called primary colours seen in the rainbow;
which, on being again collected and blended, become white
light as before."

812. It is a very singular fact connected with the bending or refraction of light, that a beam of pure white light from the sun admitted into a darkened room by a small opening in the windowshutter, and made to pass through a prism, instead of bending all together and appearing still as the same white light, is divided into many beams, which, falling on the white wall, are seen to be of different and most vivid colours. This solar spectrum, as it is called,

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Fig. 192.-V, Violet. I, Indigo. B, Blue. G, Green. Y. Yellow. 0, Orange. R, Red. formed upon a screen, consists of an endless variety of tints of colour, of which the seven in the figure are the most prominent, shading into each other imperceptibly. That these coloured rays are really the components of the original white beam, is proved in several ways. First, if the seven colours which appear in the spectrum be painted separately round the rim of a wheel, and the wheel be then turned rapidly, the individual colours cease to be distinguished, and only a white band appears. Second, if the rays

The Colours of the Spectrum.

583

of the spectrum produced by a prism be again gathered together by another prism reversed, or by a lens, they reproduce white light as before.

According to Sir D. Brewster, the seven are resolvable into three primary colours,--blue, yellow, and red, the green, orange, and violet being compounds of these primary colours. There is a difficulty in admitting this theory in the fact that when the com pound colours are separately passed through another prism they are not resolved into the primary. Thus green remains green and cannot be decomposed into blue and yellow.

If we mix together two coloured substances very finely powdered, one blue and the other red, the mixture will have a purple or violet tint, the differently coloured particles being no longer distinguishable. On examining the coloured powder with a prism, it will be separated into two distinct powders, one red and the other blue.

Green tea owes its colour generally not to any green colouring matter, but to a mixture of Prussian blue and a yellow colouring substance in very fine powder. The naked eye perceives only the green tint, but the separate colouring matters may be distinguished by the aid of a microscope.

The following experiment shows that the sensation produced on the eye by the two component colours may be the same as if the compound colour issued from the object. If a tube containing a yellow liquid (a weak solution of the acid chromate of potassium) is immersed in a jar containing a blue liquid (sulphate of copper), and the jar held opposite the light, the eye will perceive at one time the two primary colours, blue and yellow, and the compound colour, green, in the immersed tube.

Mr. Clark Maxwell has furnished an explanation of this curious experiment. He has proved that this yellow solution cuts off the blue end of the spectrum, leaving only the red, orange, yellow, and green, while the blue solution cuts off the red end, leaving the green, blue, and violet. Hence, the green rays only pass through both, all the other rays being absorbed or quenched by one or the other solution.

813. When Newton first made known the phenomenon of the many-coloured spectrum, and some of the extraordinary conclusions to which it led, great surprise was universally excited, for the common conception of unmixed purity was that of white light. The extension and importance which subsequent researches have given to the prismatic decomposition of light, are so great that we

584

Dispersion of Light.

must return to this experiment and its consequences, at a later part of the section.

All transparent substances, when bending light strongly, produce more or less a separation of colour; but it is an important fact, that the quality of merely bending a beam, or of refraction, and that of dividing it into coloured beams, or of dispersion, are distinct qualities, not having the same proportion to each other in different substances. As a general rule, the length of the spectrum under prisms of equal angles, serves to determine the relative amount of dispersive power in transparent bodies. Sulphide of carbon has a highly dispersive as well as refractive power. When used as a prism, it produces a perfect spectrum on a large scale, with a splendid array of colours. The prismatic colours are frequently produced by the light traversing the glass bottle in which this liquid is con. tained. Newton, from not being aware of this difference in the refractive and dispersive powers of bodies, concluded that a perfect large refracting telescope could never be made: he assumed that the bent light would always be coloured, and so would render the images indistinct. We now know, however, that by uniting lenses of two or more kinds of glass, we may obtain the requisite bending of light without dispersion. This very important discovery was made by the eminent optician Dollond, the first maker of the achromatic telescope.

The diversified colours of the substances in nature depend upon their fitness, from texture or other peculiarity, to reflect or transmit certain modifications of common light; the different colours not being parts of the body itself. We have to explain in a future page that the vivid colours of the rainbow are merely the white light of the sun, reflected to us after being refracted and modified by the transparent drops of falling rain, and that the sparkling appearance of rubies and emeralds, which we see in a cut-glass lustre, is a phenomenon of the same kind; and we shall learn that by scratching the surface of a piece of metal so as to have a given number of fine lines in a given space, as mother-of-pearl shell has, we can cause the same substance to appear in splendid iridescent colours.

"To transparent substances, as glass, such form may be given as to cause all the rays of light which pass through them from any one point, to converge or bend so as to meet again in another corresponding boint beyond them and pass onwards,-the body itself, from the required shape

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