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Qf the Sides of tf the Planes of pep- -‘<br />
ndicul.ar WefraC’iion. For one and the h-m 1<br />
r is here rcfixi;.kd fomCtilrLes.ifter the ufual,<br />
kmctimes der the unuS~~1 mmser, ac- :<br />
coding to the ‘Pofi;ion which its Sides hve to ’<br />
the Oryilals. Pf’ the iides of the $lia~;s are pofited<br />
the Same way to both Cryftals, it is refixfted<br />
after the f&e nix~ner in tkrn both: Cut<br />
if that fide of the Ray Which 1001.3 ton7ards<br />
the Coait of the unufuiil Refi*attioil of the firil-<br />
Crgtfial, be 90 Degrees from that fide of the<br />
fame Ray n:hich 1001~s towards the Goaft of the<br />
unnfual Pdi-nC”Lim of the ~cconcl C~yihIs (which<br />
Elily be ef3iettCd hy varying the .Pofition Of the<br />
feccnd c@.tal to the flirit, and by cDllfequence<br />
to the Rq7s of Light) tlie Ray Ml be refrat-ted<br />
after feveral manners in the feveral Cryftals.<br />
There is nothing more required to cktermiiic<br />
whether the Rq7s of Light which fall upon the<br />
fecond CryM, Ihall be refraCted after the ubl<br />
or after the unuC~1a1 manner, but to turn about<br />
this CryM, Ib that the Coafi of this Cryflal’s<br />
un~fw~~J Refra&ion may be on this or on that<br />
fide of the Ray. And therefore every Ray may<br />
be confider’d’ as having four Sides or Quarters,<br />
two of which oppofite to one another incline<br />
the Ray to he rcfrakd after the unufbal manner<br />
, as often as, either of them are turn’d to-<br />
UTards the Co& of unufual RefraAidn ; and the<br />
other m701 whenever either of them are turn’d<br />
towards the Coai’r of unufual RefraAion, do not<br />
incline it to be otherwife refraAed than after<br />
the ufual manner. The two firfi may therefore<br />
be call’d the Sides of unufual<br />
And
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