THE ANALYSIS OF LIGHT. 341 



law of refraction, is called the " ordinary"' ray ; the second, which follows a 



different law, ij called the "extraordinary" ray. 



^ If we look at a small object, a3 a 



dot, a letter, or a line, through a 

 plate of glass, it appears single ; but 

 if a plate of Iceland spar be sub- 

 stituted, a double imago will be per- 

 ceived, as two dots, two letters, two 

 lines, etc. This result of double re- 

 fraction is represented in Fig. 27 T. 

 Crystals of many other substances, 

 such as mica, the topaz, gypsum, etc., 



possess the property of double refraction, but not in so remarkable a degree 



as Iceland spar. 



__ , , In all these crystals, there are one or more directions along 



what are the ' . , , 



axes of double which objects when viewed through them appear single; 



re.raction? these directions are termed the Imes, or axes of double re- 



fraction. In the case of Iceland spar, there is one axis of double refraction, 

 i. c, one direction along which objects when viewed appear single ; this is in 

 the direction of the line A B, Fig. 275, which joins the two obtuse three- 

 sided angles. If the summits A and B be ground down and polished, no 

 double refraction will occur in looking through the crystal in this direction. 

 To what is the '^'^^^ ^^^ phenomenon of double refraction is due entirely to 

 phenomenon of the molecular structure of the medium through which light 

 tion due? passes, is proved by taking a cube of regularly annealed glass, 



which produces but one refracted ray, and heating it unequally, 

 by subjecting it to pressure : a change is thereby affected ui the arrangement 

 of its parts, and double refraction takes place. 



What is polar- 701. When a ray of light has been reflected 

 ized light? £j.Qj^ ll^g surface of a body under certain 

 special conditions, or transmitted through certain trans- 

 parent crystals, it undergoes a remarkable change in its 

 properties, so that it is no longer reflected and refracted 

 as before. The effect thus produced upon it has been 

 called polarization, and the ray or rays of light thus af- 

 fected are said to be polarized. 



•What are the The uame poles is given in physics in gen- 

 poiesofabody? ^^^^ ^^ ^l^g gjj^g ^j, ^^j^ of any body which 



enjoy, or have acquired any contrary properties. 



Thus, the opposite ends or sides of a magnet have contrary properties, in- 

 asmuch as each attracts what the other repels. The opposite ends of an elec- 

 tric or galvanic arrangement are, for like reasons, denominated poles. So also 

 iu the case of light, the raya which have been reflected or transmitted under 



