respecting Double Hefraction, i29S 
and the moveable fixed in the same crystal, and another to make 
both moveable, 
8. The moveable image doth not move at random, but al- 
ways about the Jixed^ which, while it turneth about, it never 
describeth a perfect circle but in one case. 
9. Dioptricks teach, that a diaphanous body having one only 
surface, sends from one object but one image refracted to the 
eye, and, having more surfaces than one, it represents one 
image in each. But, whereas in our substance there occurs 
but one plain superficies to the eye, and yet a double image of 
one object, it concerned us to consider whence this double im- 
age might be caused. Two ways offered themselves to us, re- 
flection and refraction. How reflection could perform it, was 
difficult to find. For, having dulled the clearness of the two 
plain sides of our crystalline prism, thereby to make them un- 
fit for reflecting the light, the rays being directed through its 
upper . and lowermost superficies, the image still appeared 
double. Again, two species appearing through a great prism, 
upon breaking of the same into pieces, and so reducing it into 
divers smaller ones, it came to pass, that through each of these 
lesser portions the same object was seen always double. Whence 
I collected, that if it should be said that one of the images pro- 
ceeded from the reflection of the plain sides^ the former of these 
experiments would discountenance that assertion. But then 
if another should derive the cause from some internal reflection 
of the surfaces of this body, certainly the same effect would not 
have been found in every one of its parts ; but the double ap- 
pearance that was exhibited in the smallest portion, would have 
be6n multiplied in a greater bulk. 
“ Reflection, therefore, not satisfying, we recurred to refrac-. 
tion. But whereas ’tis known, that no image can pass through 
two diaphanous bodies of a different nature but by refraction, 
and that one image supposeth one refraction, it did follow, that, 
if refraction were made the cause of this phenomenon, there 
would be a double refraction for a double image. And foras- 
much as the appearances of our Iceland crystal are not of the 
same kind, but one of them is fixed, the other moveth, we shall 
distinguish the refractions themselves v/hich refract the double 
rays arriving to the eye, and call the one which sends the fixed 
image refracted to our sight, Usual; the other, which transmits. 
