PHYSICAL AND LITERARY. 87 



fubjca: to thefe repulfive powers, tho* they 

 pafs thro' the fphere of their adion ? Does 

 not the refraaion of light towards the per- 

 pendicular, out of the cel«ftial fpaces into 

 air, even prove that it is attraded by the par- 

 ticles of air? Would it not be extravagant 

 and incongruous beyond meafure, to imagine 

 the cether fo fubtile, in refpea: of light, that, 

 tho' it be driven out of the way by the rays, 

 as air is by common projediles, it is not ca- 

 pable of retarding them fenfibly in their mo- 

 tion from the moft diftant fixed ftars to our 

 eye ? Do not thefe and many other difficul- 

 ties, in the phyfical part of Optics, whofc 

 folution is fought for in vain from any prin- 

 ciples hitherto difcovered, (licw the nccef- 

 (ity of Extending our views and inlarging 

 our ftock of principles by further experi- 

 mental inquiry? Such objeaions are not to 

 be confidered as demonftrations of the falfe- 

 hood of our prefent theory ♦, but as proofs 

 of its narrownefs, partiality and imperfc- 

 aion . 



Des Cartes, contenting himfelf with a 

 fuperficial and inaccurate knowledge of the 

 laws of impulfe, vainly dreamed, that he 

 had got poffeflion of the univerfal caufe 



from 



