T&specting Double Refracilm and Rolarhutmi. 127 
mains to be inquired, whether the rays have not more original 
properties than are yet discovered. 
“ Qu. 26. Have not the rays of light several sides, endued 
with original properties ? For if the planes of perpendicular re- 
fraction of the second crystal, be at right angles with the planes 
of perpendicular refraction of the first crystal, the rays which 
are refracted after the usual manner in passing through the 
first crystal, will be all of them refracted after the unusual man- 
ner in passing through the second crystal ; and the rays which 
are refracted after the unusual manner in passing through the 
first crystal, will be all of them refracted after the usual man- 
ner in passing through the second crystal. And, therefore, 
there are not two sorts of rays difiering in their nature from 
one another, one of which is constantly and in all positions re- 
fracted after the usual manner, and the other constantly and iu 
all positions, after the unusual manner. The difierence between 
the two sorts of rays in the experiment mentioned in the 25th 
question, was only in the positions of the sides of the rays to the 
planes of perpendicular refraction. For one and the same ray 
is here refracted sometimes after the usual, and sometimes after 
the unusual manner, according to the position which its sides 
have to the crystals. If the sides of the ray are posited the 
same Avay to both crystals, it is refracted after the same manner 
in them both : But if that side of the ray which looks towards 
the cdast of the unusual refraction of the first crystal be 90® 
from that side of the same ray which looks towards the coast of 
the unusual refraction of the second crystal, (which may be 
effected by varying the position of the second crystal to the 
first, and by consequence to the rays of light,) the ray shall be 
refracted after several manners in the several crystals. There is 
nothing more required to determine whether the rays of light 
which fall upon the second crystal, shall be refracted after 
the usual or after the unusual manner, but to turn about 
this crystal, so that the coast of this crystal’s unusual refrac- 
tion, may be on this or on that side of the ray. And there- 
fore every ray may be considered as having four sides or 
quarters, two of which opposite to one another incline the 
ray to be refraeted after the unusual manner, as often as ei- 
ther of them are turned towards the coast of unusual refraction ; 
