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VIII- An Experiment propofed for determining , by the Aberration 
of the fixed Stars, 'whether the Rays of Lighi, in pervading 
diferent Media, change their Velocity according to the Law 
which re fults from Sir Ifaac Newton’s Ideas concerning the 
Caufe of. Refraction ; and for ascertaining their V elocity in 
every Medium whnfe refractive Denfty is known. By Patrick 
, Wilfon, A. M. AJfiflant to Alexander Wilfon, M. D. Pro - 
fejfor of Practical Afironomy in the Univerfity of Glafgow ; 
communicated by the Rev. Nevil Mafkelyne, D . JD. F. R. S* 
AJironomer Royal. 
PON the fuppolition that the refraction of light is caufed 
by a certain action of grofs and fenfible bodies upon it. 
Sir isaac newton has demon ftrated, that the fines ot inci- 
dence and refraCtion, when the rays pafs out of one medium 
into another of different denfity, muff always be in a conftant 
ratio. This conftancy of the ratio of the lines is agreeable to 
an univerfal experience, and has been called the law of refrac- 
tion. Upon the fame grounds he has alfo demonftrated, that 
the velocity of the rays muff be greater in the more refraCting 
medium in the inverfe ratio of the fines. Of this property of 
refraCtion, however, we have hitherto had no evidence in the 
way of experiment. The ideas entertained by Sir Isaac 
newton, from which this property has been deduced, though 
they confefs their great author, by a mod: beautiful fimplicity, 
Read January 24, 1782. 
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