of Light; and a new Theory of Fision. 4Z\ 



and stationary, like a wall, with this difference, that the power 

 of reflection is not so nnuch in the resistance of the fluid as in the 

 dislike of the rays to pass through it. 



By my hypothesis, and by no other that I have ever met with, 

 we can earisfactorilv account for the double refraction in Iceland 

 crystal, and some othersubstances,that has excited so much atten- 

 tion and caused so many speculations among the philosophers; for 

 if we can suppose a fluid on the surface of a medium consisting of 

 two substances, each of which possesses distinct properties, there 

 can be no difficulty in adding to it a third substance possessing 

 a property of its own which is different from those of the other 

 two. Almost every refracting medium possesses the power of 

 refraction in a different degree to that of every other medium ; 

 water refracts less than glass ; and even in glass itself, we find 

 the quantum of refraction is more or less according to its quality, 

 that is, according to the substance of which it is composed. Now 

 it is very possible that Iceland crystal may be composed of two 

 substances which refract the rays of light in different degrees, 

 and which substances, though they are mingled together, are not 

 united, so that the fluid adhering to it is composed of three di- 

 stinct substances, instead of two as in glass, one of which reflects 

 and the other two refract the rays of light, but in different de- 

 grees. 



If we admit this, the phaenomenon is explained ; for the only 

 difference between glass and Iceland crystal is, that a§ the lat- 

 ter has two refracting substances, and glass only one, it exhibits 

 two images of the same object for every one that is shown in 

 glass ; for all the other phsenomena that I have ever heard of in 

 Iceland crystal* are merely dependent upon its figure, and glass 

 of the same shape will exhibit the same phaenomena, except that 

 there will be only one, two or three images in glass, where there 

 are two, four and six images in the crystal. 



The properties peculiar to Iceland crystal, as I understand 

 them, are, that it exhibits sometimes 



1. Only one image of an object. 



2. Two images of the same object. 



3. When the plate is thin, the two images are very close to 

 each other, and their distances increase in proportion to the 

 thickness of the plate. 



4. It exhibits six images of one object. 



5. When any object is viewed through two pieces of Iceland 

 crystal a little distant from each other, they exhibit but two 

 images in one situation, and four in another. 



• I have never seen any Iceland crystal, nnd consc<jiien.ly have had no 

 opportunity of trying the eip«rinicnt» myself". 



6. In 



