15 



ABKKHATK'N 



ABE1WA! 



1 



the earth w v s A the apparent course of the sun or the ecliptic, I r 

 the ui. of the earth, A * v the equator ; whence, if the sun move 

 in the direction indicated by the arrows, v U the vernal equinox, s the 

 summer solstice, A the autumnal equinox, and w the winter solstice. 



Flf. . Hi- 



The circle which bounds the whole figure is the Kbhtial rotor*, and as 

 the star > ZVoconu is very nearly on that colure, we will suppose it to 

 be at B Let i be the spectator's zenith ; that is, let him be on such a 

 part of the earth that the plumb-line falls in the direction z B, the 

 point I will, by the motion of the earth, be carried round the dotted 

 circle. The meridian is the moving circle passing through P and z, 

 and as the figure stands, the real star and the pole of the ecliptic 

 on the meridian. First, let the time be the vernal equinox, or let the 

 sun appear at v then the point w being 90 behind v, the star will 

 appear to be thrown towards w, and ite apparent place is r. Simi- 

 larly, I, a, and ic are the apparent places corresponding to 8, A, and w, 

 the dotted part of the ellipse being supposed to be bent over on the 

 other side of the sphere. Pig. 4 is taken from fig. 5, and is the course 

 of the star, as it will appear to the spectator at E, s Q P being bent, so 

 that z is over his head, and P behind him. Let us now suppose him in 

 the situation of Bradley, with an instrument capable only of measuring 

 changes in the polar distance, the time being the winter solstice. As 

 the star appears to move from w to r, which takes place between this 

 and the vernal equinox, the polar distance will increase from day to 

 day after the vernal equinox it will decrease, and continue to do so 

 until the autumnal equinox ; after which, it will increase again til 

 winter solstice This is precisely the phenomenon observed by Bradley, 

 who clearly perceived that it could not be attributable to the annual 

 parallax of the star. To illustrate this, let us suppose that when the 

 earth is at Q (jig. 3), we look at the star in the direction Q o, and when 

 it U at p we look at it in the direction P r. Draw p H parallel to Q o : 

 the spectator, who imagines himself at rest, will, if he observe* the 

 star at these two epochs, see the difference of position corresponding to 

 the ingle H P r, at least if the distance of the star be not so great as to 

 render that angle imperceptible to his instruments. This however will 

 frfr place in the plane passing through the star and p Q, whereas, the 

 effect observed by Bradley took place in a direction perpendicular to 



' i. .' 1 -..::' 



The displacement of y Dratonu, observed by Bradley at Kew, was 

 confirmed by subsequent observations which he made at Wanstead, in 

 Essex, with an instrument of similar construction. The question now 

 occurred, to what was the phenomenon attributable? At first he 

 suspected that it arose from some irregularity in the instrument, or 

 deviation of the plumb line, and afterwards from some nvt" 

 conical motion of the earth's axis. These, however, he found would 

 not account for the observed displacement of the star. " At last, wlicn 

 he despaired of being able to account for the phenomenon which he 

 had observed, a satisfactory explanation occurred to him all at onc 

 when he was not in search of it. (Thomson, ' History of th.- BojsJ 

 Society,' p. 846.) He accompanied a pleasure-party in a sail u|>-ii tin 

 river Thames. The boat in which they were was provided with a mast 

 which had a vane at the top of it. It blew a moderate wiiul, and th. 

 party sailed up and down the river for a considerable time. Dr. Bradley 

 remarked, that every time the boat put about, the vane at'the top ol 

 the boat's matt shifted a little, as if there had been a slight change in 

 the direction of the wind. He observed this three or four tunes 

 without speaking; at last ho mentioned it to the sailors, and ex- 

 prtssod his surprise that the wind should shift so regularly everv time 

 they put about. The sailors told him that the wind had not shift.-.! 

 but that the apparent change was owing to the change in the directioi 

 of th* boat, andaasured him that the same thing invariably happenec 

 in all cases." While tendering on the circumstance, it occurred to 

 him that the velocity of light, combined with the motion of the earth in 

 ita orbit, must produce a similar minute change in the apparent places 

 of the stars, and thus he was led to his great discovery of aberrat i. .,,. 



The greatest aberration, as we have observed, is parallel to the 

 ecliptic, and is the greatest semi-diameter of the apparent aniin.i 

 ellipse of a star. Thin ought to be the same for all stars, if the ray* 

 which come from them move with the same velocity. Hitherto i 



Th original tnlborltT for thli account In mid to be Dr. Koniwm (' Nitura 

 PUlowpbr,' rol. IT. p. 029 , who probably rrcflred It from Bradlrjr binuelf. 



has nof been discovered that the greatest aberrations of different stars 

 differ by more than may reasonably be imputed to instrumental errors ; 

 we must therefore conclude that, as far as we know, the light ... 

 star moves with the same veloc ity. 



Bradley, in the early period of his researches, fixed the maximm*. 

 alue of aberration at 20'-25, but ultimately he adopted 20"-0 as the 

 most probable value. Delambre, from observations of the eclipses of 

 Jupiter's satellites, determined it to be 20'-2SS. Bessel, by a dw- 

 cussion of Bradley's observations ('Fundaments Astronomue, 1 

 ound the maximum value to be 20" 7080, but in the ' Tabula Regio- 

 montantc' (1830), he has adopted Delambre'a result. In recent time* 

 various astronomers have investigated the value of the constant c 

 aberration, and have obtained results which generally fluctuate between 

 these two extremes. In the Introduction to the British AM., 

 Catalogue of Stars' (1844), Mr. Bailey has compared togeth. r .-,11 th. 

 most trustworthy Menninationt, and hence deduces 20 

 most probable value. The result generally adopted by astronomers in 

 the present day U due to M. Strove, Director of the Imperial Observa- 

 tory of Pulkowa, who has found the maximum aberration to amount to 

 20"-445 by a discussion of observations recently made at that observa- 

 tory with a transit instrument placed in the prime vertical. 

 INSTRUMENT.] This result agrees very nearly with that obtained aboiii 

 the same time by Dr. Peters, who, from observations made 

 fulkowa observatory with a vertical circle by Ertel [CiRCLl], determined 

 he maximum value of aberration to be 20 '481. 



The aberration of light has furnished astronomers with what has been 

 always regarded a desideratum since the establishment of the true system 

 of the universe by Copernicus an incontrovertible proof of the 

 motion of the earth. In recent times, M. Koucault, an eminent I 

 physicist, has given two distinct demonstrations of the diurnal i, 

 winded on the doctrine of rotation. [GYROSCOPE ; Pww 



It is well known that solar light is composed of several eo 

 which admit of separation from one another. The light of the star* 

 is also compounded of several colours ; for though different tinges 

 predominate in different stars, no one gives a perfectly pure colour. 

 The phenomenon of aberration proves that these different light* mov 

 with the same velocity ; for two light*, moving with different velociti 

 from the same star, would give different quantities of aberration ; that 

 is would make differently coloured images of the star in diil.-r.-Mt 

 places that image being nearest to the real place of the star, the eoloor 

 of which moves with the greatest velocity. But as no indication of 

 such an appearance is observable in the very best telescopes, we are, 

 therefore, bound to conclude, that all the different coloured light of 

 which white light U composed, moves with the same velocity. For the 

 determination of the velocity of light from the aberration, see LIGHT. 



We have hitherto considered only the case of a star which has no 

 motion of its own; let us now take that of a planet, com.t. or th,. 

 moon, which moves while the earth moves. Let the planet im.v.- t,.,m 

 A to a and the earth from E to e, in the time which it takes the light 

 to move from the planet to the earth. Then, by what 

 has been said, the earth at e receives the ray AC , which 

 is imagined to be in the direction M ; and if the plane! 

 had remained fixed at A, AeB would have been the 

 aberration, or the angle contained between ita true and 

 its apparent direction. But in the meanwhile the planet 

 has moved to a, and if light were transmitted instan- 

 taneously would appear in the direction e a. Hence 

 a e B is the aberration ; that is, to the former angle, the 

 planet's motion round e, during the passage of the light, must be m 

 or subtracted, according as the earth and planet move m the 

 or same directions. The greatest aberration of Mercury is nearly one 

 minute ; that of the moon, only two-thirds of a Mmd. 1 > "". 

 which lias no motion of ito own, the rule for a star in the ecliptic may 

 be applied ; recollecting, however, that as a line drawn from th 

 the earth is always at right angles, or very nearly so, to the direct, 

 of the earth's motion, the aberration is always at ite greatest value, or 

 nearly so, and is nearly 20J". 



In the preceding account we liave omitted two circumstance*. 

 wonl.t only have perplexed the reader. Kir,tly .-v.-ry star change, 

 place on account of the preceion of the equinoxes. [P 

 This was known to Bradley, who was therefore obliged to allow for Us 

 change, l*fore he could pretend to assign that arising from any offer 

 phenomenon; secondly, the motion of the earth not being perfectly 

 circular, but -lightly elliptical, the quantity of aberration must I,- a 

 little modified on that account. The effect of this wul be seen in the 



" ' \ BERRATION, in Optics. The most perfect mirror, or l.-u... whi.-li 

 OOnM !.- ,',,:U.. would 1* one in which all the rays which come in,,,. 

 one point should be reflected or refracted to another point. Owing to 

 the practical difficulties in the way of forming such a mirror or lens 

 the spherical form is adopted, of which it can only be said, that instead 

 of returning to a ixrint all the rays coming from a point, it condenses 

 so many oftiiem near a particular point that an apparent mingeis 

 formed at that point. The point near which most rays are colle 

 is called the focus, and the distance at which a ray cutaany line passin 

 through the focus U called ita aberration with respect to that 1m-. 

 For a discussion of this subject, see LENS. Again, when light 

 refracted through any transparent medium, ita different colours 



