( <J»8 ) 
M M, there be fuppofed a repellent Force, whofe 
Limits are G G ; then will the Ray by the Repul* 
fion of the Surface M M be bent into the Curve 
abed efg hi klm in the fame Manner as we lhewed 
it would be under the Surface G G, when G p P G was 
confidered as a denfe Medium. Hence it follows 
that a Ray moving in the Air, is reflected from a 
fpecular Surface of Glafs, or any other Mirrour,- 
opake or diaphanous, without touching the faid 
Surface. 
N. B. That the fame Tower may , under different 
Cir cum fiances , attraff to and repel from the 
fame Surface , /ball be made out in the remaining 
Tart of this Taper ; but now taking fitch a Tower 
for granted , we .will proceed in confidering the 
Flexure of Rays of Light. 
Let us luppofe a Prifm A C B (Fig. 15) to have the 
attracting Power of its inferior Surface extend as far as 
the Linetfz m ; if another Prifm G D F (the attracting 
Force of whofe upper Surface extends as far as nn) be 
brought very near to the firft Prifm ; where the at- 
tracting Powers of the Prifms interfere, they will 
deftroy one another, becaufe they act in contrary Di- 
rections; and thereby the Limits of Attraction of 
each of the Surfaces will be contracted ; the Power 
of A B extending no farther than n 0, and that of D F 
no farther than m m , whilll the Space nn mm lofes 
all the Force that it had (and would have upon the 
Removal of either Prifm ) to turn a Ray of Light, 
moving obliquely, out of its Direction. 
Now in this Situation of the Prifms, a Ray of 
Light entering the Surface C B at right Angles, will 
go through the fecond Prifm alfo at Right Angles 
(not 
