respecting Double Uefrciction and Polarisatidn. 1^5 
but with some little unevenness. It is ^easily scratched, and by 
reason of its softness it takes a polish very difficultly. It polishei^ 
better upon polished looking-glass than upon metal, and per- 
haps better upon pitch, leather or parchment. Afterwards it 
must be rubbed with a little oil or white of an egg, to fill up its 
scratches ; whereby it will become very transparent and polite. 
But for several experiments, it is not necessary to polish it. If 
a piece of this crystalline stone be laid upon a book, every letter 
of the book seen through it will apipear double, by means of a 
double refraction. And if any beam of light falls either perpen- 
dicularly, or in any oblique angle upon any surface of this crys- 
tal, it becomes divided into two beams, by means of the same 
double refraction. Which beams are of the same colour with 
the incident beam of light, and seem equal to one another in the 
quantity of their light, or very nearly equal. One of these re- 
fractions is performed by the usual rule of optics, the sine of 
incidence out of air into this crystal being to the Sine of refrac- 
tion, as five to three. The other refraction, which may be call- 
ed the unusual refraction, is performed by the following rule, 
‘‘ Let ADBC represent the refracting surface of the crystal, C 
the biggest solid angle at that surface, GEHF the opposite sur- 
face, and CK a perpendicular on that surface. This perpendi- 
cular makes with the edge of the crystal CF, an angle of 19° 3'. 
Join KF, and in it take KL, so that the angle KCL be 6° 40', 
and the angle LCF 12° 23'. And if ST represent any beam 
of light incident at T in any angle upon the refracting surface 
ADBC, let TV be the refracted d)eam determined by the given 
proportion of the sines five to three, according to the usual rule 
of optics. Draw VX parallel and equal to KL. Draw it the 
same way from V in which L lieth from K ; and joining TX, 
this line TX shall be the other refracted beam carried from T 
to X, by the unusual refraction. 
“ If, therefore, the incident beam ST be perpendicular to 
the refracting surface, the two beams TV and TX, into which 
it shall become divided, shall be parallel to the lines CK and 
CL ; one of those beams going through the crystal perpendicu- 
larly, as it ought to do by the usual laws of optics, and the 
other TX by an unusual refraction diverging from the perpen- 
dicular, and making with it an angle VTX of about 6^°, aS is 
