128 Account 0 / Sir Isaac Newton^s Investigations' 
and tlie other two^ Avlienever either of them are turned towards 
the coast of unusual refraction, do not incline it to be otherwise 
refracted than after the usual mannrr. The two first may there- 
fore be called the sides of unusual refraction. 'And since these 
dispositions were in the rays before their incidence on the second, 
third and fourth surfaces of the two crystals, and suffered no al- 
teration (so far as appears) by the refraction of the rays in their 
passage through those surfaces, and the rays were refracted by 
the same laws in all the four surfaces ; it appears that those dis- 
, positions were in the rays originally, and suffered no alteration 
by the first refraction, and that by means of those dispositions 
the rays were refracted at their incidence on the first surface of 
the first crystal, some of them after the usual, and some of them 
after the unusual manner, according as their sides of unusual 
refraction were then turned towards the coast of the unusual re- 
fraction from that crystal, or sideways from it. 
Ever}^ ray of light has therefore two opposite sides, origi- 
nally endued with a property on which the unusual refraction 
depends, and the other two. opposite sides not endued with that 
property. And it remains to be enquired, whether there are 
not more properties of light by which the sides of the rays differ, 
and are distinguished from one another. 
‘‘ In explaining the difference of the sides of the rays above 
mentioned, I have supposed that the rays fall perpendicularly 
on the first crystal. But if they fall obliquely on it, the success 
is the same. Those rays which are refracted after the usual 
manner in the first crystal, will be refracted after the unusual 
manner in the second crystal, supposing the planes of perpen- 
dicular refraction to be at right angles with one another, as 
above, and on the contrary. 
“ If the planes of the perpendicular refraction of the two 
crystals be neither parallel nor perpendicular to one another, 
but contain an acute angle, the two beams of light which emerge 
out of the first crystal, will be each of them divided into two 
more at their incidence on the second crystal. For in this case 
the rays in each of the two beams will some of them have their 
sides of unusual refraction, and some of them their other sides 
turned toward the coast of the unusual refraction of the second 
crystal” 
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