jkct. in. J DISSERTATION SECOND. && 



or peculiar virtue residing in the opposite sides of the 

 rays of light, which he deduced from the enigmatical phe- 

 nomena of douhly refracting crystals. Here, also, the 

 first step is made toward the doctrine of elective attrac- 

 tions or of chemical affinity, and to the notion, that the 

 phenomena of chemistry, as well as of cohesion, depend 

 oi: the alternate attractions and repulsions existing be- 

 tween the particles of bodies at different distances. The 

 comparison of the gradual transition from repulsion to 

 attraction at those distances, with the positive and nega- 

 tive quantities in algebra, was first suggested here, and is 

 the same idea which the ingenuity of Boscovich after- 

 wards expanded into such a beautiful and complete sys- 

 tem. Others who have attempted such flights, had ended 

 in mere fiction and romance ; it is only for such men as 

 Bacon or Newton to soar beyond the region of poetical 

 fiction, still keeping sight of probability, and alighting 

 again safe on the terra jirma of philosophic truth. 1 



1 The optical works of Newton are not often to be found all 

 brought together into one body. The first part of them con- 

 sists of the papers in the Philosophical Transactions, which 

 gave the earliest account of his discoveries, and which have 

 been already referred to. They are in the form of Letters to 

 Oldenburg, the Secretary of the Society, as are also the an- 

 swers to Hooke, and the others who objpeted to these discove- 

 ries ; the whole forming a most interesting and valuable series, 

 which Dr. Horsely has published in the fourth volume of his 

 edition of Newton's works, under the title of Letters reletting to 

 the Theory of Lights and Colours The next work, in point of 

 time, consists of the Lcctioncs Optica, or the optical lectures 

 which the author delivered at Cambridge. The Optics, in 

 three books, is the last and most complete, containing all the 

 reasoning concerning optical phenomena above referred to. 

 The first edition was in 1704, the second, with additions, in 

 1717. Ncivtoni Opera, Tom. IV. Horsely's edition. 



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