Eager to investigate these curious phenomena, Dr. Bradley determined to erect an accurate instrument for himself at \Vanstead, and, with the assistance of Mr Graham, it was ready for use on the 19th August, 1727. After a number of interesting observations, con tinued for the space of a year, from which he satisfied himself completely respecting the general laws of the phenomena, he again directed the whole energy of his mind to discover their cause. A change in the direc tion of the plumb-line which adjusted his instrument, and an elevation of the stars by the refraction of the atmosphere, soon occurred to him as sources of ex planation ; but, being inadequate to account for the phenomena, they were as quickly rejected. In this state of perplexity, the discovery of Roemer concern ing the successive propagation of light, came into his mind, and he instantly saw, that all the phenomena which he had observed, might be occasioned by the motion of the earth in its orbit, combined with the suc cessive propagation of light.
In order to understand this, let us suppose, that a particle of light. coming from a star, is at A when the eye is at 11, and that light moves through the space AC in the same time that the eve, by the annwil mo tion of the earth, moves through BC. By taking any point E, and drawing ED parallel to BA, the particle of light will evidently be at D, when the eye has moved to E, for CD : CA—CE : CB (Euclid, VI Prop. 2.) Let ns now suppose, that AB is a telescope or tithe, moving parallel to itself, then it is manifest that the particle of light NI dl alit ay s appe:ir in thy tube, and con sequently the star from which it conies will be seen in the direction attic tube ; that is, in the direct ions BA, ED, CI', when the eve is at B, E, and C, successively : But the li ht from the star comes in to AC,bccause the real place of the star is in that direc tion ; consequently the aberration, or the difference be tween the real and apparent place of the star, 1)111 be t he angle ACE, which may be computed ti igonometri cally, by having the velocity of light, the velocity of the earth in its orbit, and the place of the star.
The follow Mg Table contains the greatest aberration in right ascension and declination of the principal fixed stars for 1800 Larger tables ma‘ be seen in the Ephe merides de Vienne,177 3, 1781, 1783 ; Eph cm crides 22er lin,177 6 ; Connoissance Temp.s., 1781, 1789, 1790, 1791_ It is a remarkable circumstance, that from the theory of the first satellite of Jupiter, M. La Place has found for the entire aberration, 57" 5, a result exactly the same as that which Bradley deduced from a great number of delicate observations on the fixed stars. From this curious coincidence, La Place concludes, that the velo city of light in all the space comprehended by the earth's orbit, is the same as at the circumference of this orbit, and that the same thing holds with regard to the orbit of Jupiter. See Preface to his Mecanique Cciestr, torn. iv. p. 11. For farther information on this subject, See Phil. Trans. vol. xxxv. p. 637. Id. 1782. p. 58. Alen. Acad. Paris, 1737, p. 205. Mem. Acad. Berlin, torn. ii. 1746, p. 14.. Nov. .1cad Petrop. i. 446. T. Simpson's _Essays on several Subjects, 1740; Traii'e sur l'..Th•rration, par Fontaincs des Clines, 1744 ; Boscovi chii Opera, torn. v. p. 417. 1785. Connoissance des Temps, 1788. Trigonometric, par M. Cagnoli, § 790, 791 ; and Vince's A.Itronomy, vol. i. ch. 22. See As