Imatges de pàgina
PDF
EPUB

CHAPTER VI.

THE ATMOSPHERE-ITS REFRACTIVE POWER-SPECTRAL

APPEARANCES.

WE see, as well as hear, through the medium of the atmosphere. A knowledge of its action on light is therefore necessary; and, in the absence of this, many and great errors will arise. Without it neither the exact place of very distant objects on the surface of the earth can be determined, nor the position of the heavenly bodies.

All these luminaries appear more elevated than they actually are; because the rays of light, instead of moving in straight lines, are bent by the atmosphere towards the earth. Let, for example, a b, a b, a b, a b, in the following figure, be strata, or very thin layers of the atmosphere, which increase in density towards m n, the surface of the earth. A ray coming from a star meeting the surface of the atmosphere at S, would be refracted, or bent at the surface of each layer, and would consequently move in the curved line S rrrA;

F

and as an object is seen in the direction of the ray that meets the eye, the star which actually is in the direction

[blocks in formation]

This

A S, would seem to a person at A to be in s. refraction, which always acts in a vertical direction, raises objects above their true place; and for this reason, a body at T, below the horizon H A O, would be raised, and seen in V.

To the refractive power of the atmosphere, it is owing that the sun is visible before he rises above the horizon in the morning, and after he sinks beneath it in the evening; and that we sometimes see the moon on her rising totally eclipsed, while the sun is still apparent in the opposite part of the horizon.

SIMPLE EXPERIMENT.

The same effect appears in other cases.

67

If a ray of

light falls on a drop of water, a piece of glass, or a bottle containing a transparent fluid, it does not reach the eye as it would if no one of them had been placed in its way. In these, and all similar circumstances, light is refracted, or diverted from its direct course by the body which is thus interposed. Of this a simple illustration may be given.

If, for instance, you put a cup, having a flower painted at the bottom of the inside, on a table, to stand at such a distance as that the flower may be concealed by the rim, and if any person will pour water into the cup, you will soon see the flower, though you do not move. The reason is, because when you removed so far that the flower was hidden by the rim, the rays reflected by it no longer met your eyes; but when the cup was filled with water, the water refracted or bent them downwards, so as to render the object visible.

It is here, however, to be particularly observed, that when the flower becomes visible by the refraction of the ray, you do not see it in its real situation, but you behold an image of the flower higher up in the cup; for, as objects always appear to be situated in the direction of the rays that enter the eye, the flower will be seen in the direction of the refracted ray. Before the

water was poured in, the flower could not be seen where it was, now it is seen where it is not; for as it is in a denser medium, it appears raised, or nearer to the surface.

To this circumstance it is owing, that an oar, partly in and partly out of the water, appears crooked; that a

fish seems much nearer the surface of the stream than it actually is; and that pools and rivers appear shallower than they really are, so that if eight feet deep, they will seem to be but six. From ignorance or forgetfulness of this fact, many a school-boy and many a traveller have lost their lives. The only way to judge of the depth of water accurately, is to view it from a boat, so that it may be looked down upon perpendicularly; when the rays from the bottom rising perpendicularly, no refraction will take place.

To return now to the flower in the cup. If salt water be poured in instead of fresh, the ray will be more bent; alcohol would refract it more than salt water, and oil more than alcohol; thus different bodies refract light in different degrees. A table of their varied power may be easily found in large works on this subject.

It follows from the circumstances thus illustrated, that certain states of the atmosphere, depending on warmth, humidity, or other causes, greatly affect its ordinary

SPECTRAL ILLUSION.

« AnteriorContinua »