1Q'2 PHILOSOPHICAL TRANSACTIONS. [ANNO 1782. 



give the apparent distance of the star from the pole at that time as affected by 

 aberration. The error of the line of collimation would not affect the result in 

 this way, being the same in the observation both above and below the pole* 



* Though Mr. Wilson did not bring forward this interesting tract till the year 1782, yet it is well 

 known that, so long ago as 1770, his attention was drawn to this subject by a particular view 

 which had occurred to him when reading over Dr. Bradley's admirable paper on aberration, in the 

 Phil. Trans. It then struck Mr. Wilson that, in reasoning concerning the relative motion of the 

 ray from the star, no account had been taken of its having finally to pass through the aqueous and 

 vitreous humours of the eye on its way to the retina, in order to produce vision This unavoidable 

 and ultimate motion of the ray seemed to him to have some relation to the subject. 



His idea was, that when looking into Dr. Bradley's sector, we perceived the image of the star in 

 the centre of the field, at that instant a straight line, joining that centre at the centre of the retina, 

 must coincide with the axis of the eye produced: because, by the laws of vision, an object must 

 always appear in the direction of the optical axis and the eye which beholds it. But, as the image 

 could not be seen in this direction unless the ray, when moving in the aqueous and vitreous humours, 

 passed relatively along the axis of the eye, he was led to think that the velocity of the ray, in those 

 dense fluids, was that which was justly deducible from the angle of aberration, shown by Dr. 

 Bradley's sector ; and not its velocity in air; as had been hitherto imagined. 



Considering now that as the celebrated Romer, by a method wholly different from aberration, had 

 nearly ascertained the velocity of light in the ethereal spaces, it now occurred to Mr. Wilson, that, 

 by comparing the results afforded by these two different methods, it might be determined whether 

 light was accelerated or not in the denser medium according to the ratio resulting from the New- 

 tonian doctrine of refraction. This view, of resorting to the principles of aberration, for deciding 

 experimentally in a question of such high moment to optics and to general physics, arose in Mr. 

 Wilson's mind in the way now stated, and so long ago ; as appears by the outlines of it published in 

 the London Chronicle of the 4th December 1770, under the signature X. 



Not long after this however, on further consideration, he found that some conclusions had been 

 too hastily adopted ; a circumstance the less surprising, as it will appear in the sequel that the very 

 same were fallen into, without ever having been corrected, by philosophers and geometricians of 

 the first eminence ; though now known to be entirely false. On this account it may be both instruc- 

 tive and entertaining to trace a little the history of this intricate subject, and to shew the steps by 

 which Mr. Wilson was led to a just comprehension of it. This we have been enabled to do by se- 

 veral late communications with him, during his present residence at Hampstead. 



While he took it for granted, that the aberration of the axis of the eye must differ from that 

 of Doctor Bradley's sector, yet he became soon sensible that the consequent displacement of the 

 image of the star, as perceived by the eye looking in, would be much more minute than at first he 

 imagined, by reason of the near proximity of the eye to the field of the sector. Still however this 

 displacement seemed sufficient to give rise to phenomena of a very peculiar kind, some symptoms 

 of which might be detected by a very close attention to the image. He supposed the case when tne 

 aberration of a star, near the pole of the ecliptic, lay at right angles to a horizontal wire passing 

 through the centre of the field, when the telescope turned in a vertical circle. Then by making the 

 wire gradually to approach the image, this he concluded ought to disappear when at some small 

 distance from the wire; and, when brought to coincide with the wire, it ought to appear visible 

 upon it, instead of being hid behind it. Such indeed would be the necessary consequences of the 

 premises he now went on, by reason of the displacement of the image that would be occasioned 

 by the eye. 



But as Dr. Bradley has never mentioned any symptoms of phenomena so very peculiar, Mr. 



