( lyi ) 
Moreover F H being to H I as F G to twice G I, the 
Square of F H will be to the Square of H F, as the 
Square of F G to four times the Square of G I. There- 
fore, by Compofition, as the Square of F H to the 
Square of F I or of AC, lb is the Square of F G to the 
Square of F I together with three times the Square of 
G I, and fo likewife is the Excefs of the Square of F G 
above the Square of F H, which equals the Excefs of 
the Square of I H above the Square of I G, to three 
times the Square of G I ; for as one Antecedent to one 
Conlequent, fo is the dilFerence of the Antecedents to 
the difference of the Confequents. Hence in the laff: 
place, the Square of half F H will be to the Square of 
A M, as the Excels of the Square of I H above the 
Square of I G to three times the Square of I G, or as 
the Excels of the Square of the Sine of Incidence above 
the Square of the Sine of Refra(5tion, to three times 
the Square of the Sine of Refradrion, 
Another 
