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plane on the other fide of the perpendicular OL, the 
rays will diverge from fome point f on the other fide 
of the fecond furface : the violet ray O V being more 
refraCted from the perpendicular VP, than the red 
is from the perpendicular R p. 
And it is evident, that if the diftance (OT or Ot) 
of the point of incidence from the edge of a prifm, 
the angle of incidence LOS, and the angle of the 
prifm (OTV or OfV) are given, together with the 
refractive powers of the media, the lines OV, OR, 
will be given in magnitude and pofition. And thence 
the diftance VR being given, with the angles of re- 
fraction at the fecond furface, the points, F or /, to 
which the rays converge, or from which they diverge, 
will be given. And their locus, or the Curve in 
which all thefe points are found, may be affigned ; 
whether the angle of the prifm is conftant, and the 
angle of incidence is variable, or the contrary ; and 
whether the rays are refraCted, or, at a certain obli- 
quity, come to be reflected by the fecond plane. 
io. If it is further required that the extreme, and 
all the intermediate, rays which meet at F (in Fig. 3.) 
fhould thenceforth remain united in a colourlefs pen- 
cil : through the point of convergence F draw (by 
the lemma) the line ZX, making the angles ZFR, 
ZFV, fuch that their difference RFV being the 
given angle of convergence, their fines may be as the 
lines of refraCtion of the red and violet rays, when 
they pafs from a given denfer medium GKH into the 
air, at a common angle of incidence : and H F G 
perpendicular to ZX will be the line in which the 
furface of that medium mud cut the plane of refrac- 
tion, when the rays RF, VF, are refraCted into the 
fame 
