1Q4 PHILOSOPHICAL TRANSACTIONS. [ANNO 1782. 



2Q.05 inches; in 1780, 22.Q inches; in 1781, 25.6 inches; and the mean of 

 the 4 years is 26.4 nearly per year. 



letter, said to be dated in 176~6'. This account is quite conformable to what Boscovich himself o-ives 

 in his Opuscula, first publishe 1 at Bassano in 1785. In these volumes there is another tract on ter- 

 restrial aberration, which topic appears to have come under his attention only a little time before 

 publishing his Opuscula, and many years after the same subject had been considered by Mr. Wilson. 

 It is very remarkable however, by the account given by Lalande, and by Boscovich's own account 

 in the two tracts of his Opuscula, that this excellent person proceeds entirely on a radical and con- 

 firmed mistake on both points, which overthrows all his conclusions ; though many of them, parti- 

 cularly those relating to terrestrial aberration, are very extraordinary, and justly and beautifully de- 

 duced from his erroneous principles ; and had they been true, would have led to most wonderful and 

 important discoveries. 



In regard to the first point, Boscovich asserts, and in this Lalande joins him, that the aberration 

 of the axes of the water and air telescope, when the star is seen in the axes of each, must neces- 

 sarily be different, and that the former would give only 15" aberration of a star in the pole of the 

 ecliptic, instead of 20" the well known aberration of Dr. Bradley's sector, in that case. He then 

 concludes that, were this difference actually found by observation, it would constitute the proof of 

 the acceleration of light in the dense medium. In showing how the aberration of the water telescope 

 should so differ, Boscovich considers the ray at entering, at the axis, as still proceeding down 

 in the water in its former absolute direction, though much accelerated. But by Mr. Wilson's papei 

 it is evident that this cannot possibly happen, on the hypotheses admitted. He has shown that the 

 real or absolute path of the ray, before entering, is inclined to the moving axis, and consequently 

 to the surface of the water. On that account its absolute direction, at the moment of entrance, 

 must be changed by refraction, so as to make a less absolute angle than it did before with the moving 

 axis and the water telescope. 



This unavoidable change of the absolute direction of the ray, at entering, is the very important 

 circumstance which Mr. Wilson at last detected, and which delivered him from all his previous mis- 

 conceptions, and on which the whole reasoning on this subject entirely hinges. It appears however 

 from the year 177<>, till 17^5, when Mr. Boscovich published his Opuscula, that this important 

 circumstance had never once occurred to him; in consequence of which it is now well known that 

 all his conclusions, in the two tracts abovementioned, concerning the water telescope, anil terrestrial 

 aberration, are quite erroneous. Boscovich there considers only one effect produced on the ray a^ 

 entering the water telescope, namely the acceleration of its velocity. But Mr. Wilson considers a 

 second and simultaneous effect produced on the ray by refraction in consequence of its known 

 oblique incidences, namely the unavoidable change of its former absolute direction. Accordingly 

 the scope of his paper is to show how these two different but concomitant effects must, according to 

 the Newtonian doctrine of refraction, precisely counteract one another, so as to make the aberration 

 of the water and air telescopes to agree, when the star is seen in the axis of both. This conclusion 

 is the very reverse of that of Boscovich. From never having attended to this second effect produced 

 in the ray, as changing its former absolute direction, Boscovich necessarily concluded the possibility 

 of terrestrial aberration ; and so deduced many curious things from it in his opuscula, all of which 

 are illusions. But whoever reasons about this point, and takes into account this second effect on die 

 ray, must immediately perceive that a terrestrial aberration, in the case of the water telescope, is an 

 impossible thing. 



In the 2d vol. of the Philos. Trans., of the Edinburgh Royal Society, there is a most ingenious 

 paper, on the Motion of Light, by a philosopher and geometer of great eminence, which points out 

 Boscovich's delusion concerning this terrestrial aberration. This he does solely by taking in this 



