Scr. XXI. DOUBLE REFRACTION. 175 



crystaKzed minerals, animal and vegetable substances, 

 gums, resins, jellies, and all solid bodies having unequal 

 tensions, whether from unequal temperature or pres- 

 sure, possess the property of doubling the image or ap- 

 pearance of an object seen through them in certain 

 directions. Because a ray of natural light falling upon 

 them is refracted into two pencils, which move with dif- 

 ferent velocities, and are more or less separated, accord- 

 ing to the nature of the body and the direction of the 

 incident ray. Whenever a ray of natural light is thus 

 divided into two pencils in its passage through a sub- 

 stance, both of the transmitted rays are polarized. Ice- 

 land spar, a carbonate of lime, which by its natural 

 cleavage may be split into the form of a rhombohedron, 

 possesses the property of double refraction in an emi- 

 nent degree, as may be seen by pasting a piece of paper 

 with a large pin-hole in it, on the side of the spar far- 

 thest from the eye. The hole will appear double when 

 held to the light (N. 200). One of these pencils is re- 

 fracted according to the same law as in glass or water, 

 never quitting the plane perpendicular to the refracting 

 surface, and is therefore called the ordinary ray. But 

 the other does quit the plane, being refracted according 

 to a different and much more complicated law, and on 

 that account is called the extraordinary ray. For the 

 same reason one image is called the ordinary, and the 

 other the extraordinary image. When the spar is turned 

 round in the same plane, the extraordinary image of the 

 hole revolves about the ordinary image which remains 

 fixed, both being equally bright. But if the spar be kept 

 in one position and viewed through a plate of tourma- 

 line, it will be found that as the tourmaline revolves, the 

 images vary in their relative brightness one increases 

 in intensity till it arrives at a maximum, at the same 

 time that the other diminishes till it vanishes, and so on 

 alternately at each quarter revolution, proving both rays 

 to be polarized. For in one position the tourmaline 

 transmits the ordinary ray, and reflects the extraordi- 

 nary; and after revolving 90, the extraordinary ray is 

 transmitted, and the ordinary ray is reflected. Thus 

 another property of polarized light is, that it cannot be 

 divided into two equal pencils by double refraction, in 



