SECT. XXI. PROPERTIES OF ICELAND SPAR. 181 



as fluids, gases, glass, &c., and a few regularly crystallized 

 minerals, it is refracted into a single pencil of light by the laws 

 of ordinary refraction, according to which the ray, passing through 

 the refracting surface from the object to the eye, never quits a 

 plane perpendicular to that surface. Almost all other bodies, 

 such as the greater number of crystallized minerals, animal and 

 vegetable substances, gums, resins, jellies, and all solid bodies 

 having unequal tensions, whether from unequal temperature or 

 pressure, possess the property of doubling the image or appear- 

 ance 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 different velocities, and are more or 

 less separated, according to the nature of the body and the direc- 

 tion of the incident ray. Whenever a ray of natural light is thus 

 divided into two pencils in its passage through a substance, both 

 of the transmitted rays are polarized. Iceland spar, a carbonate 

 of lime, which by its natural cleavage may be split into the 

 form of a rhombohedron, possesses the property of double refrac- 

 tion in an eminent degree, as may be seen by pasting a piece of 

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

 from the eye. The hole will appear double when held to the 

 light (N. 205). One of these pencils is refracted according to the 

 same law as in glass or water, never quitting the plane perpen- 

 dicular to the refracting surface, and is therefore called the ordi- 

 nary 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 extra- 

 ordinary 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 tourmaline, it will be found that, as the tourmaline 

 revolves, the images vary in their relative brightness one in- 

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

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

 nately at each quarter revolution, proving both rays to be 

 polarized. For in one position the tourmaline transmits the 

 ordinary ray, and reflects the extraordinary ; and, after revolving 

 90, the extraordinary ray is transmitted, and the ordinary ray is 



