( <*<5 ) 
in ^ le |y me a b > an ^ (by the known Laws of Mecha- 
mcks) be turned out of the Way into the Diredion * a 
inftead of at: When it is got to b, being aded upon’ 
tn the Oiredion ^ 4 , its new Direction will become 
b b : At e, by the Power aCting in the Line e c, it will 
irnoXpi ? lrea ‘° n r t0fc ’ and la %> » d « will go 
into the Glafb in the Line dd, continuing in that ftreigiy 
Line whil ft it moves in that Medium. 8 ' 
Now if the Lines i 1, ii, 3 3, », c, b, a, be infinitely 
near (as they mult be fuppofed to be) the Ray, in- 
ftead of being broken into the feveral ftreight Lines 
ab,bc, and <r d will be bent into the Curve abed- and 
die emergent Ray d d will make the fame Angle with 
it Incident Ray R r as it the Refraction had been made 
at once at the Point », which Point may be confidered 
as in the Surface G G, becaufe MM has been fuppofed 
extremely near that Surface: Then alfo may Refradi- 
ons be confidered in graft and Rays trac’d, in all Op- 
tical Propofitions, as if there were no fuch Curve a, 
what we have been deferibing. ’ 
Again, let D {Fig. 13 ) reprefent the denfe Me- 
dium or Glafs, and A the rare Medium or Air • R a 
a Ray of Light coming out of the denfe Medium in. 
to the Rare, m the Direction Rr, i n which it may 
be refracted (as for Example, in an Angle of to De 
grees With the Perpendicular P ). Let MM be the 
Line which limits the Attraction of the Surface G G 
which Attraction is exerted in Lines tending perpen- 
dicularly from MMtqG G. As foon as the Ray of 
Light has emerged at a, it is attracted in the Di- 
rection a P, and therefore diverted front the Line 
f r , into the new Direction a a; at b, it is turned 
into the Line b b ; at c, into the Line c c ; and at 
d , in- 
1 
