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ffill appear d double. Again,twoj^et'/>/ appearing through a 
great Prifme^upon breaking of the fame into pieces and (o re- 
ducing it into divers (mailer ones, it came to pafs,that through 
each of thefe lefler portions the fame objecft was feen always 
double^ Whence I Collefted, that if it (hould befaid , that 
one of the images proceeded from the Refleftion of the plain 
fides 'y the former of thefe Experiments would di(countenance 
that aflertion. But then if another fhould derive (he caufe 
from fome internal Reflexion of the Surfaces of this Body, 
certainly the fame efFedt would not have been found in every 
oneof its parts ^ but the double appearance, that was exhi- 
bited in the fmalleft portion, would have been multiplied in a 
greater bulk* 
/{^e(?/Vje therefore not fatisfying, we recurred to K^r^iflf/- 
on^ But, whereas 'tis known^tliat no image can pafs through 
two Diaphanous Bodies of a different nature, but by Re- 
fradion , and that one image fuppofeth one Refraftion s 
it did follow, thatjif Refraftion were made the caufe of this 
Tho^namenott.tYitxt would be a double Refra&ion for a double 
image. And, forafmuch as the Appearances of our Ifland* 
Chryftal are not of the fame kind, but one of them is fixr^the 
other moveth, welhallalfo diftinguilh the Refraftions them- 
felves, which refrafl: the double Rays arriving to the Eye, 
and call the one, which fends the Fixt image refraded to our 
h^t, Vfuah^ the other^which tranfmits the Moveable to th^ 
Eye, Vn-ufuaL And hence, namely, from this peculiar and 
notable propriety of the double Refradion in this Kland- 
(tone, we have not (crupled to call it DifdiacUjikl^, 
This being fuppofed, it will not be irrational to fufpeft, 
thatthefe two R^fraftions proceed from different principles 
For, fince it is commonly kuown from Dioptric l{s ^ that an 
Objcft, byvifual rays affefting the Eye, exhibits Tome 1- 
mage on the Snperficies of the Diaphanous Body, which I- 
mage is but one, as long as the Superficies is one, and the up- 
per plain parallel to the lower 3 as alio, that if, the Eye re- 
maining fteady, the Diaphanous Body be moved, that knage 
remains always fixt, as long as the Objcft^ whence it comes, 
remains 
