LIGHT. 63 



to differentiate between elements present not only on our globe, but 

 in other celestial bodies likewise. 



Double refraction. A plate of glass will not interfere with read- 

 ing printed matter over which the plate is laid. But in trying to 

 read through several varieties of transparent crystalline substances a 



FIG. 23. 



Double refraction. 



peculiar phenomenon is noticed ; as then each letter will be dupli- 

 cated, as shown in Fig. 23, where a piece of Iceland spar (crystallized 

 calcium carbonate) is placed over reading matter. 



If a pinhole be made through a card, and the card be placed over a 

 crystal of Iceland spar and held before the eye toward the light, there 

 will appear to be two holes, with light shining through each. If 

 the crystal be made to rotate in a plane parallel with the card, then 

 one of the holes will appear to remain nearly at rest, while the 

 other rotates about it. These facts show that a ray of light on enter- 

 ing the crystal is divided into two parts, one of which obeys the 

 law of regular refraction, and is called the ordinary ray, while the 

 other, which does not, is termed the extraordinary ray. This power 

 of certain substances to refract light rays in two directions is known 

 as double refraction. . 



In all crystals which produce double refraction there is one direc- 

 tion (in some even two directions) in which an object, looked at 

 through the crystal, does not appear double. The line through 

 which double refraction is suspended is called the optic qxis, and is a 

 line around which the molecules appear to be arranged symmetrically. 



Polarization. The semitransparent mineral tourmaline is another 

 substance refracting double. Two rays, the ordinary and the ex- 

 traordinary, are formed when a ray of light is passed through a plate 



