SECT, xxi.] DOUBLE REFKACTION. 203 



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 (1ST. 200). One of these pencils is refracted accord- 

 ing to the same law as in glass or water, never quitting the 

 plane perpendicular to the refracting surface, and is there- 

 fore 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 ole 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 tour- 

 maline, 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 extraordinary ; 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 positions of the doubly refracting bodies in 

 which a ray of common light would be so divided. 



Were tourmaline like other doubly refracting bodies, each 

 of the transmitted rays would be double ; but that mineral, 

 when of a certain thickness, after separating the light into 

 two polarized pencils, absorbs that which undergoes ordinary 

 refraction, and consequently shows only one image of an 

 object. On this account, tourmaline is peculiarly fitted for 

 analyzing polarized light, which shows nothing remarkable 

 till viewed through it or something equivalent. 



The pencils of light, on leaving a double refracting sub- 



