<P> In regard to evidence that still survives of the earlier history, manuscripts written by Newton in the 1660s show that Newton himself had, by 1669, arrived at proofs that in a circular case of planetary motion, "endeavour to recede" (what was later called centrifugal force) had an inverse - square relation with distance from the center . After his 1679 - 1680 correspondence with Hooke, Newton adopted the language of inward or centripetal force . According to Newton scholar J. Bruce Brackenridge, although much has been made of the change in language and difference of point of view, as between centrifugal or centripetal forces, the actual computations and proofs remained the same either way . They also involved the combination of tangential and radial displacements, which Newton was making in the 1660s . The lesson offered by Hooke to Newton here, although significant, was one of perspective and did not change the analysis . This background shows there was basis for Newton to deny deriving the inverse square law from Hooke . </P> <P> On the other hand, Newton did accept and acknowledge, in all editions of the Principia, that Hooke (but not exclusively Hooke) had separately appreciated the inverse square law in the solar system . Newton acknowledged Wren, Hooke and Halley in this connection in the Scholium to Proposition 4 in Book 1 . Newton also acknowledged to Halley that his correspondence with Hooke in 1679 - 80 had reawakened his dormant interest in astronomical matters, but that did not mean, according to Newton, that Hooke had told Newton anything new or original: "yet am I not beholden to him for any light into that business but only for the diversion he gave me from my other studies to think on these things & for his dogmaticalness in writing as if he had found the motion in the Ellipsis, which inclined me to try it ..." </P> <P> Since the time of Newton and Hooke, scholarly discussion has also touched on the question of whether Hooke's 1679 mention of' compounding the motions' provided Newton with something new and valuable, even though that was not a claim actually voiced by Hooke at the time . As described above, Newton's manuscripts of the 1660s do show him actually combining tangential motion with the effects of radially directed force or endeavour, for example in his derivation of the inverse square relation for the circular case . They also show Newton clearly expressing the concept of linear inertia--for which he was indebted to Descartes' work, published in 1644 (as Hooke probably was). These matters do not appear to have been learned by Newton from Hooke . </P> <P> Nevertheless, a number of authors have had more to say about what Newton gained from Hooke and some aspects remain controversial . The fact that most of Hooke's private papers had been destroyed or have disappeared does not help to establish the truth . </P>

Who formulated and tested the law of universal gravitation