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332 I<br />
double Refratiion-: But the difference of, the<br />
two RefraAions is not fo great and manifeit as<br />
in Uland Cvyfial.<br />
When the beam ST incident on Ifland Cryital,<br />
is divided into two beams T’V and TX,<br />
and thefe two beams arrive at the farther Surhx<br />
of’ the Glafs ; the beam TV, which was<br />
refraAed at the firit Surface after the ufual manner,<br />
ihall bc again refra&ed cntircly after the<br />
ufud manner at the fccond Surface ; and the<br />
beam TX, which u7as refratied after the unuiizal<br />
manner in the firit Surface, fhall be again<br />
refraCied entirely after the unufual manner in<br />
the fccoiid Surf&e ; fo that both thefe beams<br />
Ihall emerge out of the fecond Surface in lines<br />
parallel to the firft incident beam ST.<br />
And if two pieces of Iiland Cryfial be plated<br />
one after another, in fuch manner that’ all<br />
the Surfaces of the latter be parallel to all the<br />
correfponding Surfaces of the former : The-<br />
Rays which are refracted after the uf~~al manner<br />
in the fir-it Surf&e of the firft Cryital fhali<br />
be refra&d after the ufual manner in all the<br />
following Surfkes ; and the Rays which are refrn&d<br />
after the unufual manner in the firfi SLKke<br />
, !hall be rcfra&ed after the unufual manner<br />
in all the following Surfaces. And the fame<br />
thing happens, though the Surfaces of the Cry-<br />
MS be any ways inclined to one another, provided<br />
that their Planes of perpendicular RefraEtion<br />
be parallel to one another.<br />
And therefore there is an original difference<br />
in the Rays of Light, by means of which Rome<br />
Rays are inthis Experiment confiantly r&a&-<br />
e&
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