OF NATITRAL PHILOSOPHY. 31 



tween the object and the eye ; and if a ray or small 

 sunbeam be thrown upon a surface of either of these 

 substances, it will be split into two, making an angle 

 with each other, and each pursuing its own separate 

 course, this is called double refraction. Now, of 

 these images or doubly refracted rays, one always 

 follows the same rule as if the substance were glass 

 or water : its deviation can be correctly calculated 

 by Snell's law above mentioned, and it does not quit 

 the plane perpendicular to the refracting surface. 

 The other ray, on the contrary, (which is therefore 

 said to have undergone extraordinary refraction] does 

 quit that plane, and the amount of its deviation from 

 its former course requires for its determination a 

 much more complicated rule, which cannot be un- 

 derstood or even stated without a pretty intimate 

 knowledge of geometry. Now, rock-crystal and 

 Iceland spar differ from glass in a very remarkable 

 circumstance. They affect naturally certain regular 

 figures, not being found in shapeless lumps, but in 

 determinate geometrical forms; and they are sus- 

 ceptible of being cleft or split much easier in certain 

 directions than in others they have a grain which 

 glass has not. When other substances having this 

 peculiarity (and which are called crystallized sub- 

 stances) were examined, they were all, or by far the 

 greater part, found to possess this singular property 

 of double refraction ; and it was very natural to con- 

 clude, therefore, that the same thing took place in 

 all of them, viz. that of the two rays, into which any 

 beam of light falling on the surface of such a sub- 

 stance was split, or of the two images of an object 

 seen through it, one only was turned aside out of its 



