[ i8 7 ] 
emerged in his experiment ; and which (being paral- 
lel to the incident light) continued to be colourlefs. 
He adds farther, “ the permanent whitenefs ar- 
gues, that in like incidence of the rays, there is no 
reparation of the emerging rays”: as much as to fay, 
that in his experiment (as in our 6th Figure) the pen- 
cil, in palling or repaffing, is fuppoled to meet with 
furfaces of equal refractive powers, fimilarly lituated. 
The other cafes in which refraded light may re- 
cover its whitenefs, although it emerges not parallel 
to the incident, or may be tinged though parallel to 
it, Sir Ifaac does not treat of: the experiment he had 
made, being fufficient for the purpofes to which he 
applies it. But he allures his readers, that if they 
will argue truely upon his theory, trying all things 
with good inftruments, and fufficient. circumfpedion, 
the expedled event will not be wanting. And the 
fad is, that in all the experiments which have been 
made, if none of the necellary data are wanting, 
the appearance of the emerging light may be certain- 
ly prodided. 
II. 
When a (lender pencil of light is refolded at the 
furface of any medium, the extreme rays, the violet 
and red, and the leveral intermediate rays, each of 
its particular degree of refrangibility, will all diverge 
from, or converge to, the fame phyfical point: or 
when that point, by altering the pofition of the plane, 
is thrown to an infinite distance, will all Oi them be- 
come parallel. And it appears from the roiegoing 
folution, that fuch parallelism may always be efreded, 
C c 2 whatever 
