Mr B. IV^irtin's 'Experiments on Island Crystal . 155 
in that point Q, will in its passage through the crystal be divid- 
ed into the two parts or beams QO, QS, of which the part QO 
proceeds directly to the eye without refraction ; but the other is 
refracted to S, making the angle OQS equal to QON, in the 
same plane FG. 
A ray of light NO in the substance of the crystal will be 
equally refracted into the air at each surface, that is, the refract- 
ed parts OP, NZ will be parallel to each other ; therefore since 
the ray QS is parallel to NO, it will be refracted into ST paral- 
lel to OP, and so the point Q (or any object placed in it) will be 
seen by the eye by two different rays of light OP, ST. Conti- 
nue the line TS till it cuts the line FH in R ; then will the ob- 
ject at Q be shewn at Q by the ray QP unrefracted, and at R 
by the refracted ray QST, and therefore it must appear double . 
Let Eg bisect the acute angle at L, which will therefore be 
perpendicular to the line FH ; through Q and R draw two lines 
VW, and XY, parallel to Lg* ; then it will follow, that all the 
points Q in the line YW will be refracted to the line YX, and 
consequently the image of the said line will be double, or it will 
appear in the two parallel lines VW and YX. 
If the crystal were made to move round upon the point Q, 
the primary image of an object at Q would appear at rest, while 
the secondary image at R would be seen to move in a circle 
round it. 
If the object be a right line, then the distance between the 
two lineal images, viz. QR, will be the greatest of all when the 
plane of refraction FG is in a direct position before the eye ; the 
images being always necessarily parallel, it is plain, that while 
the crystal moves round upon the point Q, the moveable image 
XY will approach nearer to the primary image in the line VW, 
till at last it coincides with it, and both the images appear as one 
line 9 and that will be when the said plane FG is in a right posi- 
tion to the eye. 
By continuing the motion of the crystal, the secondary image 
YX will separate from the other quiescent one VW, and appear 
on the other side, increasing the distance till it becomes equal to 
QR again, when the plane FG is direct before the eye as at 
first ; and upon turning the crystal another quarter of a circle, 
