by Refra£lion, as defperate” (Optics^ 2 d Edit. 
p. 91.)- 
However, as it has been proved by inconteftable 
Experiments, that this Diffipation of the Rays of 
Light, from whatever Caufe it proceeds, in palling 
out of one Medium into another, is not accidental 
and irregular; but that every fort of homogeneal 
Rays, whether more or lefs refrangible, confidered 
apart, are refradted according to fome conftant uni- 
form and certain Law; and as the Removal of fo 
great an Impediment as this of unequal Refradion 
in the Rays of Light, is of great Importance to the 
Science of Dioptrics, and abfolutely necelTary to its 
further Advancement j we have thought it worthy 
of a careful Examination, whether, in fome Cafes at 
leaft, it might not be poffible for contrary Re- 
fradions fo to corred each other’s Inequalities, as to 
make their Difference regular j and if this could be 
conveniently elfeded. Sir Ifaac Newton has acknow- 
ledged, “ there would be no farther Difficulty” 
{7hih Tranf. N®. 8 8). 
Now, upon a due Confideration of this Subjed, 
we have found it poffible, by proper Methods and 
Expedients, to redify thofe Errors which proceed 
from the different Degrees of Refrangibility in dif- 
ferent Rays, palling from one Medium into another ; 
admitting only this well-known and eftablilhed Prin- 
ciple, upon which we ground our Reafoning, 
“ That the Sines of Refradion of Rays differently 
refrangible, are one to another in a given Pro- 
portion, when their Sines of Incidence are equal” 
iPpticSi 2 d Edit. p. 66.). And our prefent Defign is, 
to Ihew what Advantage thi-s will yield towards 
im- 
