INTRODUCTION. 25 



result of some higher and still unknown power, or of " the 

 centrifugal force of the aether, which fills the realms of space, 

 and is rarer within bodies, but increases in density outwards. 

 The latter view is set forth in detail in a letter to Robert 

 Boyle" (dated February 28, 1678), which ends with the 



" To tell us that every species of things is endowed with an 

 occult specific quality, by which it acts and produces manifest 

 effects, is to tell us nothing; but to derive two or three general 

 principles of motion from phenomena, and afterwards to tell 

 us how the properties and actions of all corporeal things follow 

 from those manifest principles, would be a very great step in 

 philosophy, though the causes of those principles were not yet 

 discovered : and therefore I scruple not to propose the prin- 

 ciples of motion, and leave their causes to be found out." 

 Newton's Opticks, p. 377. In a previous portion of the same 

 work, at query 31, p. 351, he writes as follows : " Bodies act 

 one upon another by the attraction of gravity, magnetism, and 

 electricity ; and it is not improbable that there may be more 

 attractive powers than these. How these attractions may be 

 performed I do not here consider. What I call attraction 

 may be performed by impulse, or by some other means unknown 

 to me. I use that word here to signify only in general any 

 force by which bodies tend towards one another, whatsoever 

 be the cause." 



tt " I suppose the rarer rather within bodies, and the denser 

 without them." Operum Newtoni, tomus iv. (ed. 1782, Sam. 

 Horsley,) p. 386. The above observation was made in refer- 

 ence to the explanation of the discovery made by Grimaldi of 

 the diffraction or inflection of light. At the close of Newton's 

 letter to Robert Boyle, February 1678, p. 394, he says: "I 

 shall set down one conjecture more which came into my mind : 

 it is about the cause of gravity." .... His correspondence 

 with Oldenburg (December 1675) shows that the great philo- 

 sopher was not at that time averse to the " a?ther hypotheses." 

 According to these views, the impulse of material light causes 

 the aether to vibrate ; but the vibrations of the aBther alone, 

 which has some affinity to a nervous fluid, does not generate 

 light. In reference to the contest with Hooke, consult 

 Horsley, t. iv. pp. 378-380. 



