u 



4 On the UNEQUAL 



" way of arguing which the nature of things admits of, and 

 " may be looked upon as fo much the ftronger by how much 

 the induction is more general j and if no exception occur 

 from phenomena, the conclufion may be pronounced gene- 

 rally ; but if at any time afterwards any exception fhall 

 " occur from experiments, it may then begin to be pronounced 

 with fuch exceptions as occur." 

 This is the general doctrine which he lays down as applicable 

 in all experimental enquiries ; and fo far was he from confider- 

 ing the particular cafe above mentioned as an exception to this 

 general rule, that from fome exprefhons he makes ufe of, it 

 evidently appears, that he was not without fufpicion, of what 

 has fince been difcovered to be the truth. 



In his fixth letter to Mr Oldenburgh, dated from Cam- 

 bridge in the year 1662, he expreffeth himfelf in the following 

 words : " Mr Hook thinks himfelf concerned to reprehend 

 " me for laying afide the thoughts of improving optics by re- 

 " fraction. What I faid there was in refpect of telefcopes of 

 " the ordinary conftruction, fignifying that their improvement 

 " is not to be expected from the well figuring of glafTes, as op- 

 " ticians have imagined. But I defpaired not of their im- 

 " provement by other conftructions, which made me cautious 

 " to infert nothing that might intimate the contrary. For al- 

 " though fucceflive refractions which are all made the fame 

 " way, do neceiTarily more and more augment the errors of 

 " the firft refraction, yet it feemed not impoflible for contrary 

 " refractions fo to correct each others unequalities, as to make 

 " their difference regular ; and if that could be conveniently 

 " effected, there would be no farther difficulty. Now to this 

 " end I examined what may be done; not only by glafTes alone, 

 " but more efpecially by a complication of diverfe fucceflive 

 " mediums ; as by two or more glafTes or cryftals, with water, 

 " or fome other fluid, between them ', all which together may 

 " perform the office of one glafs, efpecially of the object glafs, 



" on 



