VOL. LXXII.] PHILOSOPHICAL TRANSACTIONS. ]85 



contrary to the most probable issue of the experiment, some unsuspected law 

 should be discovered, we must, according to the rules of induction laid down 

 by that great master in philosophy, so far restrict our general conclusions, and 

 accommodate our ideas to the real condition of things. 



The method of experiment at present alluded to, is that of observing the 

 aberration of the fixed stars with a telescope filled with a dense fluid, such as 

 water, or any other equally limpid and of greater refraction, fitted to bring the 

 rays to a focus by the surface of the medium opposed to the object having a 

 proper degree of convexity. Suffice it at this time to suggest a general notion 

 of the instrument ; and proceed we now to explain in what manner it can assist 

 us in the present inquiry. 



Since aberration, taken in its enlarged sense, depends on the relative velocities 

 of light and of the telescope, if the rays were really to move much faster or 

 much slower in an unusual telescope of this kind, it seems to follow, that the 

 quantity of aberration given in these circumstances, compared with Dr. Bradley's 

 angle, would certainly indicate the new rate of velocity. Such an inference 

 would certainly be just, and it is on these grounds that we propose to inquire into 

 the velocity of the rays, as they move forward in dense media so applied to teles- 

 copes. Granting however, for the sake of argument, that light moves down 

 through such an unusual telescope with an increased velocity, suited to the re- 

 fractive density of the medium, it will by no means happen, that the aberration 

 will be changed on that account. This proposition, which at first view may ap- 

 pear paradoxical, and even contradictory to what has been affirmed above, is 

 however not the less certain, and may serve to show what caution is sometimes 

 requisite in applying general principles to particular cases : for it will be proved, 

 that the aberration in such a telescope will precisely agree with that of Dr. Brad- 

 ley's only in the case of the rays moving swifter in the watery medium than in air, 

 in the ratio assigned by Sir Isaac Newton, and that this sameness of aberration 

 will itself be a proof of light being so accelerated within the telescope. In the 

 illustrations which follow, the reader is supposed not to be wholly unaccustomed 

 to the distinctions between absolute and relative motion, as this will prevent repe- 

 titions and all unnecessary prolixity. 



Let abc (fig. 13, pi. 3,) be the spherical refracting surface of such a telescope 

 as has been described, and let the telescope be supposed to be at rest, or the 

 velocity of light to be infinite with respect to that of the earth, and let gbmf be 

 a line drawn from a star at g, in the pole of the ecliptic, through the centre m 

 of the refracting surface ; the image of the star will be formed somewhere, as at 

 f, in the line bf ; and here the intersection of the cross wires used in observing 

 must be placed. It is evident, that the star will be seen in its true direction fg ; 

 and we must conclude that to be its true direction, because we know that the 



vol. xv. B R 



