[ l8 5 3 
Obferve likewife, that as, in practice, we mud fit 
the water to the glafs, not the glafs to the water, we 
are to begin by aflliming VR of a convenient mag- 
nitude; and fuppofing the rays V v, Rr, &c. to be 
parallel within the glafs, find the point O to which 
they converge in the water, through which a plane 
may be drawn which fhall fend them, out into the air, 
in a colourlefs pencil OS. 
REMARKS. 
I. 
The 8 th experiment in Sir Ifaac Newton’s optics 
(Book I. Part 2 .) feems to have been made under 
the conditions which are limited by the foregoing 
problem ; though he does not Ipecify thefe conditions. 
For, it is to be prefumed, he did not combine his prifm 
and water at random, but adjufted them fo as to pro- 
duce the expected effeCt. It is obferved likewife, 
that he does not give us a defcription of his experi- 
ment fo particular as, in mod: inftances, he was wont 
to do. He thought perhaps that the confequences he 
deduces from it might fufficiently explain his mean- 
ing; efpecially as he had, in the foregoing propofiti- 
ons, fully eftablifhed the principles of his theory. 
However this be, feveral perfons of fkill and addrefs 
in optical matters, have produced experiments in con- 
tradiction to that of Sir Ifaac, and have affixed mean- 
ings to his conclufions which he never could intend, 
without being grofsly inconfiftent with himfelf: an 
Vol. LIII. C c imputation 
