( 15<5 ) 
M P being equal to P C, M L is equal to L V ; and 
K W being let fall perpendicular to LV, M W is equal 
to three times L W. But now if the incident Ray E F 
be produced to X, the Angle under M L K being equal 
to that under C K F, or to that under E F K, F X fliall 
be equal to LV, equal to twice. L W ; and the Angle 
under KM L being-equal to that under K F G ; fince 
K W is perpendicular to MW, F G lliall be to twice 
M W as M K to K F, or as the Sine of Incidence to 
the Sine of Refradlion : whence M W being equal to 
three times L W, F X fliall be to F G as the Sine of 
Incidence to three times the Sine of Refradlion. 
Moreover, M W being equal to three times L W, 
the Square of M W wilF be equal to nine times the 
Square of L W^ and the Redlangle under V M L, or 
the Redlangle under C M A, that is, the Excels of the 
Square of K M above the Square of K A, will be equal 
to eight times the Square of LW ; therefore the Square 
of L W or the Square of half F X will be to the Square 
of K L, or of KA, as the Excefs of the Square of K M 
above the Square of K A to eight times the Square of 
K A, that is, as the Excefs of the Square of the Sine 
of Incidence, above the Sine of Refradlion to eight 
times the Square of the Sine of Refradlion. 
) 
'»■ 
-Another 
1 
