90. R.A. 186° 27'; dec. N. 14° 23'; in Virgo: very faint. 91. R.A. 186° 37′; dec. N. 14° 57'; above the preceding fainter than the preceding.

92. R.A. 257° 38'; dec. N. 43° 22′; diam. 5'; between the knee and left leg of Hercules: a beautiful nebula, bright in the centre, and surrounded with great nebulosity: resolved into stars by Sir W. Herschel.

93. R.A. 113° 48′ 35′′; dec. S. 23° 19′ 45′′; diam. 8': between the Great Dog and the Ship: a mass of small stars. 94. R.A. 190° 10′ 46′′; dec. N. 42° 18′ 43′′; diam. 24'; above Cor Caroli: bright in the centre, with a diffused nebulosity.

95. R.A. 158° 3′ 5′′; dec. N. 12° 50′ 21′′; in the Lion, above : very faint.

96. R.A. 158° 46'; dec. N. 12° 58'; near the preceding: fainter than the preceding.

97. R.A. 165° 18' 40"; dec. N. 56° 13'; diam. 2′; near ß Great Bear: very faint: another near it, and another near y.

98. R.A. 180° 50′ 49′′; dec. N. 16° 8′ 15′′; above the north wing of Virgo: very faint.

99. R.A. 181° 55′ 19′′; dec. N. 15° 37′ 12′′; on the north wing of Virgo: brighter than the preceding: between two stars of the seventh and eighth magnitude.

100. R.A. 182° 59′ 19"; dec. N. 16° 59′ 21′′; in the ear of corn of Virgo: brighter than No. 98.

101. R.A. 208° 52'; dec. N. 55° 24′ 25′′; diam. 7′; between the left hand of Bootes and the tail of the Great Bear: very faint: discovered by Mechain: mottled nebulosity, according to Sir W. Herschel.

102. Between o Bootes and Draconis: very faint: discovered by Mechain.

103. Between ε and d Cassiopeia: a mass of stars.

CHAPTER XIII.

ON THE ABERRATION OF THE STARS, AND ON THEIR PROPER MOTIONS.

THE aberration of the fixed stars is a small change of place in the heavens which they seem to undergo, and by which they appear to describe, in the course of a year, an ellipsis or circle, the greatest diameter of which is about forty seconds. This remarkable fact was discovered, near the middle of the last century, by the celebrated Dr. Bradley, formerly Regius Professor of Astronomy at Greenwich.

Before

In Chapter IV., when describing the mode of finding the parallaxes of the fixed stars, I have given a brief detail of the circumstances which led to this discovery, and the observations from which the aberration of the stars was deduced. perusing the following illustrations of this subject, it may not be improper for the reader to reperuse what was there stated in reference to this point, particularly the illustration of this phenomenon given in the description of Fig. 7 (p. 62). It is there stated that Dr. Bradley and his friend Mr. Molyneux were very much perplexed at the result of their observations; since, instead of observing a motion indicating an annual parallax, they found a result directly opposite to what they expected. Many theories and conjectures were proposed to solve the appearances, but nothing satisfactory was elicited, till one day, when Dr. Bradley was enjoying the amusement of sailing about on the Thames, he observed that every time the boat tacked, the direction of the wind, estimated by the direction of the vane, seemed to change. This immediately suggested to him the cause of the phenomenon which had so much perplexed him, and he ultimately found it to be an optical illusion, occasioned by a combination of the motion of light with the motion of his telescope while observing the polar stars; a discovery of no inconsiderable importance, and which will immortalize the name of this sagacious and indefatigable astronomer. He perceived that, if light is propagated in time, the apparent place of a fixed object will not be the same when

T

the eye is at rest, as when it is moving in any other direction than that of the line passing through the eye and the object; and that, when the eye is moving in different directions, the apparent place of the object will be different.

We see an object in consequence of the rays of light proceeding from it striking our eyes, and we see the place of the object in the direction in which they proceed. If light be in motion and the eye at rest, the object will appear in its real place, provided no refracting medium intervene; but if the eye be in motion, and this motion in a different direction from that of the rays of light, the object will not be seen in its true position. Let us suppose the earth in its circuit round the sun just arrived opposite to a fixed star, which sends off rays perpendicularly to the direction of the earth's motion. The eye of the spectator meets the ray, and, as he perceives not his own motion, he supposes the light to be moving in a different direction; as when we sail along a winding river, certain objects on the banks appear to pass by us in different directions. The eye misses the perpendicular ray, but meets an oblique one, and thence receives the impression of the light in the direction which results from this compound motion; namely, in the diagonal of a parallelogram, the sides of which represent the real motion of light. The spectator sees the star in its true place only when he is approaching ceding from it in a straight line. When moving in any other direction, the star appears a little in advance of its true position; and these apparent changes in the situation of the heavenly bodies, occasioned by the annual motion of the earth, are distinguished by the aberration of light. They are common, to a certain extent, to all the celestial orbs, and are only more perceptible and striking in the case of the fixed stars. In consequence of this aberration during the revolution of the earth round the sun, the stars appear, according as they are situated in the plane of the ecliptic, or in its poles, or somewhere between them, in the first case, to deviate in a straight line to the right or left of their true place; in the second, to describe a circle, or something nearly approximating to it; and in the third, an ellipse about that point which observation determines to be their real situation.

or re

This subject requires a little degree of attention in order to a clear understanding of it. Perhaps the following illustra

tions may in some measure render it plain to the general reader.

Suppose A B, in the following figure, to represent a part of the orbit of the earth, and C B a ray of light descending from a star upon the earth's orbit, A B; if the eye be at rest at B, the object will appear in the direction BC; but if the eye be moving from A towards B, and light be propagated with a velocity that is to the velocity of the eye (or of the earth's motion) as C B to B A, that particle of it by which the object Fig. 74. 1 C

4

2 *

B

L

K

I H

A will be discerned when the eye comes to B will be at C when the eye is at A; the star, therefore, will appear in the direction A C; and as the earth moves through the equal parts of its orbit, A H, HI, I K, &c., the light coming from the star will move through the equal divisions C d, de, e f, fg, g B, and the star will appear successively in the directions H 1, I 2, K 3, L 4, B 5, which are parallel to A C; so that when

the eye comes to B, the object will be seen in the direction B 5.

The following is an explanation of this phenomenon as given by Sir John Herschel. Suppose a shower of rain to fall perpendicularly in a dead calm; a person exposed to the shower, who should stand quite still and upright, would receive the drops on his hat, which would thus shelter him; but if he ran forward in any direction they would strike him in the face. The effect would be the same as if he remained still, and a wind should arise of the same velocity and drift them Fig. 75. A

B

R

P

S

Q E

against him. Suppose a ball to fall from a point A (fig. 75) above a horizontal line E F, and that at B were placed to receive it the open mouth of an inclined hollow tube P Q; if the tube were held immovable, the ball would strike on its lower side; but if the tube were carried forward in the direction E F 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, C, the tube should have been carried into the position R S it is ev