152 Mr B. Martin’s Experiments on Island Crystal . 
Lastly, it is evident the arch GH (Fig. 2.) is the measure of 
the angle GCH in the crystal (Fig. 1.) and in the right angled 
triangle AGH, there are two parts known, viz. the side A G of 
50° 56', and the angle G A H of 52° 30', therefore the side G H 
will be found to be 45° 20', and such is the inclination of the 
axis CH to the bisecting line CG. These are all the dimen- 
sions of the crystal necessary for our present use ; and now we 
proceed to specify and recount the unusual properties of refrac- 
tion in this sort of crystal. 
The singular effect of island crystal is, that of shewing ob- 
jectp double by a double refraction of light. Diaphanous bodies 
in general, as glass, water, &c. refract a ray of light in its pas- 
sage through them ; but they do not divide that ray into two, 
and so can shew only one , and not two different and equal images 
of the same object, as we always see in the crystal we now speak 
of. 
The manner in which this double refraction is made, is best 
conceived from the figure. Let PO be a beam of light falling 
upon the upper surface AB of the crystal, in a perpendicular di- 
rection, and in the plane FCGH, which bisects the angle ACB. 
Then will this beam at its entrance become divided into two, 
viz. OQ and ON, both which will be in the said plane FG. The 
beam O Q is in the same direction with O P, and goes through 
the crystal without refraction. The other part or beam O N is 
refracted, so as to make the angle QON always equal to 6° 40' 
very nearly, as is found by experiment. 
If the said ray falls any where else in the surface AB, the 
refraction will ever be made in a plane parallel to the plane FG, 
so that the refracted parts will always be in a line in neither sur- 
face, parallel to the line FH ; and always make the same angle 
equal to QON. 
Any ray cd that falls obliquely upon the surface, will be re- 
fracted in the same manner, viz. into the two beams de , and 
df in a plane aDE6 parallel to FCGH, and in an angle edf 
equal to the angle QON. 
From what has been said, it evidently follows, that if Q be a 
point in the lower surface of the crystal, and directly under the 
eye at P, then a ray of light proceeding from any object placed 
