( HP y- 
Let A BCD be the given Circle, the given Diameter 
A C, and given Point G ; and let the Ray E F, parallel 
to A C, be refracSted to G. I fay E F is given in Po- 
fition. 
B 
Produce E F to H, and draw the Diameter F I, draw- 
ing likewife I K H, I G. Then is Fi F I the Angle of 
Incidence, and G F I the refracted Angle ; fo that I H 
being perpendicular to F H and I G perpendicular 
to F G, I H is to I G as the Sine of the Angle of 
Incidence to the Sine of the refradFed Angle, and 
the Ratio of I H to I G is given, as likewife the 
Ratio of I K to I G. Therefore I K being perpen- 
dicular to A C the Point I is in a Conic Sec^tion given 
in Pofirion, whole Axis is perpendicular to A C,. 
and one of its Foci is the Point G Confequently 
the Points I and F are given, and laflly the Ray E F 
given in Pofition. 
r 
* See Papp., 1 . 7. prop. 238. Milnes Conic, part. 4. prop. 9. 
‘DETEE- 
