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; 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 expériments, it may then begin to be pronounced 
with fuch exceptions as occur.” 
Tuis 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 expreffions 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 OrpENBURGH, dated from Cam- 
bridge in the year 1662, he exprefleth himfelf in the following 
“cc 
words: “ Mr Hook thinks himfelf concerned to reprehend 
me for laying afide the thoughts of improving optics by re- 
fra€tion. What I faid there was in refpect of telefcopes of | 
the ordinary conftrudtion, fignifying that their improvement 
is not to be expected from the well figuring of glafles, as op- 
ticians have imagined. But [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 neceffarily more and more augment the errors of 
the firft refraGtion, yet it feemed not impoflible for contrary 
refractions fo to correét 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 glafles alone, 
but more efpecially by a complication of diverfe fucceflive 
mediums ; as by two or more glaffes 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, 
6é on 
