126 Accoimt c^^xr Isaac Newton’s Invest^gdtime ' 
found by experience. And hence, the plane VTX, and such 
like planes which are parallel to the plane CFK, may be called 
the planes of perpendicular refraction. And the coast towards 
which the lines KL and VX are drawn, may be called the coast 
of an unusual refraction. 
“ In like manner, crystal of the rock has a double refraction : 
but the differences of the two refractions is not so great and ma- 
nifest as in Iceland crystal. 
‘‘ When the beam ST incident on Iceland crystal, is divided 
into two beams TV and TX, and these two beams arrive at the 
farther surface of the glass ; the beam TV, which was refracted 
at the first surface after the usual manner, shall be asain refracted 
entirely after the usual manner at the second surface ; and the 
beam TX, which was refracted after the unusual manner in the 
first surface, shall be again refracted entirely after the unusual 
manner in the second surface ; so that both these beams shall 
emerge out of the second surface in lines parallel to the first in- 
cident beam ST. 
And if two pieces of Iceland crystal be placed one after ano- 
ther, in such manner that all the surfaces of the latter be pa- 
rallel to all the corresponding surfaces of the former : The rays 
which are refracted after the usual manner in the first surface 
of the first crystal shall be refracted after the usual manner in 
all the following surfaces ; and the rays which are refracted 
after the unusual manner in the first surface, shall be refracted 
after the unusual manner in all the following surfaces. And 
the same thing happens, though the surfaces of the crystals be 
any ways inclined to one another, provided that their planes of 
perpendicular refraction be parallel to one anothey. 
And, therefore, there is an original difference in the rays of 
light, by means of which some rays are, in this experiment, 
constantly refracted after the usual manner, and others constant- 
ly after the unusual manner : For if the difference be not ori- 
ginal, but arises from new modifications impressed on the rays 
at their first refraction, it would be altered by new modifica- 
tions in the three following refractions ; whereas it suffers no 
alteration, but is constant, and has the same effect upon the 
rays in all the refractions. The unusual refraction is therefore 
performed by an original property of the rays. And it re- 
