154 Mr B. Martin's Experiments on Island Crystal. 
the said plane FG is again right to the eye, and two images 
coincide in one as before. In short, the moveable image YX will 
always be parallel to VW, during the whole rotation of the crys- 
tal, about the centre Q. 
Sir Isaac Newton calls the plane FCGH and all others par- 
allel to it, planes of perpendicular refraction. Let FCGH be 
such a plane described out of the crystal (Fig. 3.) and let PO be 
a ray of light falling upon it in O ; through the point O, draw 
the perpendicular BN, and upon it as a centre describe the 
arches AB, IK ; from the point A where the arch cuts the ray, 
draW AD perpendicular to BN, and it will be the sine of the 
angle of incidence AOB ; let this be divided into five equal 
parts, and make EK (the sine of an arch IK) equal to three of 
those equal parts, and through the point K draw OL, which 
will be one of the refracted parts of the beam, and OM will be the 
other part. It is found by experience, that the first or most re- 
fracted part OL is always regular in its refraction, or that its 
sine of refraction EK is in the constant proportion of three to 
five to the sine of incidence AD, however the angle of incidence 
AOB may vary. 
But the other part of the beam OM is refracted in an irregu- 
lar and uncertain manner, as is also found by experience. We 
shall, therefore, for the future, speak of the refraction of Island 
Crystal as distinguished into two kinds, regidar and irregular. 
When two pieces of crystal are placed one over the other, 
and in similar positions, that is, with their planes of perpendicu- 
lar refraction FCGH, fc g h, parallel to each other (Fig. 4.), 
then an incident ray A B is refracted through the first regular- 
ly in the beam B B, and irregularly into the beam B E, as be- 
fore has been said. At the nether surface F H of the first piece, 
the two rays will be refracted into two others DK and El pa- 
rallel to each other, and to the incident ray AB. 
In this state they fall upon the lower piece fcg h , and are 
refracted through it without further division ; but the ray KL is 
refracted in the regular manner, and the ray IM in the irregu- 
lar one, and at the lower surface/^, they are both refracted in- 
to the air parallel to each other, as before. And this would be 
constantly the case with any number of crystals placed in like 
manner. 
