( 6\7 ) 
die fame Angle with the Perpendicular m p that R a 
made with a P. Now as the Lines perpendicular 
to G G drawn from M M are infinitely near, the 
Line abcdefghik l m mud be a Curve • 
and as M M and G G are extremely near, the 
Vertex of fhe Curye ( whofe Tangent is ff parallel 
to G G ) will be lb near the Point I, as to be con- 
idered as co-inciding with it, when we compare the 
Angle of Inddence with that of Reflexion ; then 
alio will the Space between the Parallels pm and Pa 
be fo fir diminiihed, that thofe two Lines may be 
looked upon as co-inciding, the Angles m m p and 
R a P being equal, whether the three Points m. I,a 
co-incide or not. 9 
For thefe Reafons,* for common Ufe, one may 
confider the Reflexion from the under Surface of 
the Glafs as made at once in that Surface at the 
Point I. But when we examine Things ftridtly Ex 
periments as well as the above Reafoning, will lhew ' 
that there is fuch a Curve as we have mentioned! 
See Experiment VI. Fig. roandii, and we lhall 
mention others. 
N. B. If any Point of the Curve a be, &c. be- 
rweena and /I fall below ( or beyond the Line MM. 
the Ray will then go on in a ftraight Line Tangent 
to the Curve in that Point where it leaves the Line 
Now let us fuppofe M e d c b a r M ( in the fame 
tig. ) to be Glais, or any other denfe Medium, and 
m p P R Air, or any other rare Medium, and R a a 
Ray of Light moving in the ra.e Medium towards 
the denfe Medium in the Dired.on R a towards r : if 
inltcad of an Attra&ion at the Surface of tiie Glafs 
Nona MM, 
