seem to be above it. For example, on April 20, 1837, the moon was eclipsed before the sun had set. The mean diameter of both the sun and moon being rather less than 33', it follows that when we see the lower edge of either of these luminaries apparently just touching the horizon, in reality the whole disk is completely below it, and would be altogether hidden were it not for the effect of refraction. The day is consequently materially lengthened.

The sun and moon often appear flattened when near the horizon. This is easily accounted for on the principle just stated. The rays from the lower edge pass through a denser layer of the atmosphere, and are therefore refracted about 4' more than those from the upper edge: the effect of this is to make the vertical diameter appear about 4' less than the horizontal, and so distort the figure of the disk into an oval shape.

The sun and moon often appear larger when near the horizon than when high in the sky. This is not caused by refraction, but is a mere error of judgment. At the horizon we compare them with various terrestrial objects which lie between them and us, while aloft we have no association to guide us, and we are led to underrate their size. On looking at them through a tube, the illusion disappears. The moon should naturally appear largest when at a great altitude, as it is then at a less distance from us.

The dim and hazy appearance of the heavenly bodies when near the horizon is caused not only by the rays of light having to pass through a larger space in the atmosphere, but also by their traversing the lower and denser part. The intensity of the solar light is so greatly diminished by passing through the lower strata, that we are enabled to look upon the sun at that time without being dazzled by his brilliant beams.

Twilight. The glow of light after sunset and before sunrise, which we term twilight, is caused by the refraction and reflection of the sun's rays by the atmosphere. For a time after the sun has truly set, the refracted rays continue to reach the earth; but when these have ceased, he still continues to illuminate the clouds and upper strata of the air, just as he may be seen shining on the summits of lofty mountains long after he has disappeared from the view of the inhabitants of the plains below. The air and clouds thus illuminated reflect back part of the light to the earth. As the sun sinks lower, less light reaches us until reflection ceases and night ensues. The same thing occurs before sunrise, only in reverse order. The duration of twilight is usually reckoned to last until the sun's depression below the horizon amounts to 18°; this, however, varies with the latitude, seasons, and condition of the atmosphere. Strictly speaking, in the latitude of Greenwich there is no true night for a month before and after the summer solstice, but

constant twilight from sunset to sunrise. The sun is then near the Tropic of Cancer, and does not descend so much as 18° below the horizon during the entire night. The twilight is shortest at the equator and longest toward the poles, where the night of six months is shortened by an evening twilight of about fifty days and a morning one of equal length.

Diffused light.-The diffused light of day is produced in the same manner as that of twilight. The atmosphere reflects and scatters the sunlight in every direction. Were it not for this, no object would be visible to us out of direct sunshine; every shadow of a passing cloud would be pitchy darkness; the stars would be visible all day; no window would admit light except as the sun shone directly through it, and a man would require a lantern to go around his house at noon. This is illustrated very clearly in the rarified atmosphere of elevated regions, as on Mont Blanc, where it is said the glare of the direct sunlight is almost insupportable; the darkness of the shadows is deeper and denser; all nice shading and coloring disappear; the sky has a blackish hue, and the stars are seen at midday. The blue light reflected to our eyes from the atmosphere above us, or more probably from the vapor in the air, produces the optical delusion we call the sky. Were it not for this, every time we cast our eyes upward we should feel like one gazing over a dizzy precipice; while now the crystal dome of blue

smiles down upon us so lovingly and beautifully that we call it heaven.

ABERRATION OF LIGHT.-We have seen that the places of the heavenly bodies are apparently changed by refraction. Besides this, there is another change due to the motion of light, combined with the motion of the earth in its orbit. For example: the mean distance of the earth from the sun is ninetyone and a half millions of miles, and since light travels 183,000 miles per second, it follows that the time occupied by a ray of light in reaching us from the sun is about 8 min.; so that, in point of fact, when we look at the sun (1), we do not see it as it is, but as it was 8 min. since. If our globe were at rest, this would be well enough, but since the earth is in motion, when the ray enters our eye we are at some distance in advance of the position we occupied when it started. During the 8 min. the earth has moved in its orbit nearly 20", so that (2) we never see that luminary in the place it occupies at the time of observation.

Illustration.-Suppose a ball let fall from a point P, above the horizontal line A B, and a tube, of which A is the lower extremity, placed to receive it. If the tube were fixed, the ball would strike it on the lower side; but if the tube were carried forward in the direction A B, with a velocity properly adjusted at every instant to that of the ball, while preserving its inclination to the horizon, so that when the ball, in its natural descent, reached B, the tube

would have been carried into the position BQ, it is evident that the ball throughout its whole descent would be found in the tube; and a spectator referring to the tube the motion of the ball, and carried

Fig. 38.

P

ABERRATION OF LIGHT.

along with the former, unconscious of its motion, would fancy that the ball had been moving in an inclined direction, and had come from Q. A very common illustration may be seen almost any rainy day. Choose a time when the air is still and the drops large. Then, if you stand still, you will see that the drops fall vertically; but if you walk forward, you will see the drops fall as if they were meeting you. If, however, you walk backward, you will observe that the drops fall as if they were coming from behind you. We thus see that the drops have an apparent as well as a real motion.