VOL. v.] PHILOSOPHICAL TRANSACTIONS. 547 



ject ; it concerned us to consider, whence this double image might be caused. 

 Two ways offered themselves to us, reflection and refraction. How reflection 

 could perfarm it, was difficult to find. For, having dulled the clearness of tlie 

 two plain sides of our crystalline prism, to make them unfit 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 the same into pieces, and so reducing it into divers 

 smaller ones, 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 proceeded 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, cer- 

 tainly the same effect would not have been found in every one of its parts, but 

 the double appearance, that was exhibited in the smallest portion, would have 

 been multiplied in a greater bulk. 



Reflection therefore not satisfying, we recurred to refraction. But as it is 

 known, that no image can pass through two diaphanous bodies of a different 

 nature, but by refraction, and that one image supposes one refraction ; it fol- 

 lows, that if refraction were made the cause of this phenomenon, there would 

 be a double refraction for a double image. And forasmuch as the ap- 

 pearances of our Iceland crystal are not of the same kind, but one of them is 

 fixed, the other moving, we shall also distinguish the refractions themselves, 

 which 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 the 

 moveable to the eye, unusual. And hence, namely, from this peculiar and 

 notable property of the double refraction in this Iceland stone, we have not 

 scrupled to call it disdiaclastic. 



This being supposed, it will not be irrational to suspect, that these two re- 

 fractions proceed from different principles. For, since it is commonly known 

 from dioptrics, that an object, by visual rays affecting the eye, exhibits some 

 image on the superficies of the diaphanous body, which image is single, as 

 long as the superficies is so, and the upper plain parallel to the lower; as also, 

 that if the eye remaining steady, the diaphanous body be moved, that image 

 remains alway fixed, as long as the object, whence it comes, remains unmoved. 

 Wherefore in this transparent substance, the image which appears fixed may 

 proceed according to the ordinary laws of usual refraction; but that which 

 moves, and is carried about according to the motion of the diaphanous body, 

 while the object remains fixed, shows an unusual kind of refraction, liitherto un- 

 observed by dioptricians. 



3 z 2 



