( 9«« ) 
Nor is this only ufcful to difcover the focus from the 
other propofed data, but from the focus given, we may 
thereby determine the diftance of the Objeft, or from 
the focus and diftance given, we may find of what Sphere 
it is requifue to take another Segment, to make any 
given Segment of another Sphere caft the beams from 
the diftance d to the focus f As likewife from the Lens, 
focus, and diftance given, ro find the ratio of Refra&i- 
on, or of m to n, requifite to anfwer thofe data. AH 
which it is obvious, are fully determined from the Equa- 
tion we have hitherto ufed, viz. pdpr=drf^<dpf 
~ pr pf for to find d the Theorem is 
_ — ttM —i the diftance of the Objeft. 
rf m if — n r 
d r f 
For i the Rule is —j — i _ > = p 
s f> d r dfyfr pr f \ 
But for p will be — p which latter 
d erfyj f r f 
determines the ratio of Refra&ion, m being to n as i r£ p 
to p. 
I {hall not expatiate on thefe Particulars, but leave 
them for the exercife of thofe that are defirous to be in- 
formed in Optical Matters, which I am bold to fay are 
comprehended in thefe three Rule, as fully as the moft 
inquifitive can defire them, and in all poffible cafes ; re- 
gard being had to the Signs and as in the former 
cafes of finding the focus. I fliall only {hew two confide- 
rable ufes of them ; the one to find the diftance where- 
at an Obje£t, being placed {hall by a given Lens be repre- 
fcntedin a Species as large as the Objedt itfelf, which 
may be of Angular ufe, in drawing Faces, and other 
things in their true Magnitude, by tranfmittihg the 
Species by a Clafs into a dark Room, which will not 
only give the true Figure and Shades, but even the Co- 
lours themfelyesj almoft as vivid as the Life. In this 
cafe 
