REFRACTION AND POLARIZATION OF LIGHT. 59 



index of refraction. It is about 1£ for water, or more accu- 

 rately, 1*335. With diamond, the ray would be bent in the 

 direct S V, which indicates a much greater amount of re- 

 fraction ; its index is nearly 2£, or correctly, 2.439. The 

 eye at R, looking into a diamond in thctlirection R S, would 

 see an object in the direction of S V, and not in that of § T. 

 The index of refraction has been obtained for many sub- 

 stances, of which the following are a few : 



Air, 



1-000 



Calc spar, 



1-654 



Tabasheer, 



1-211 



Spinel, 



1-764 



Ice, 



1-308 



Sapphire, 



1-794 



Cryolite, 



1-349 



Garnet, 



1-815 



Water, 



1-335 



Zircon, 



1-961 



Fluor spar, 



1-434 



Blende, 



2-260 



Rock salt, 



1-557 



Diamond, 



2-439 



Quartz, 



1-548 



Chromate of lead, 



2-974 



Double Refraction. — Many crystals possess the pro- 

 perty of refracting light in two directions, instead of one, and 

 objects seen through them consequently appear double. 

 This is called double refraction. It is most conveniently 

 exhibited with a crystal of calc spar, and was first noticed 

 in a pellucid variety of this mineral from Iceland, called from 

 the locality Iceland spar. On drawing a line on paper and 

 placing the crystal over it, two lines are seen instead of one — 

 one by ordinary refraction, the other by an extraordinary 

 refraction. If the crystal, as it lies over the line, be turned 

 around, when it is in one position the two lines will come 

 together. Instead of a line, make a dot on the paper, and 

 place the crystal over the dot : the two dots seen will not 

 come together on revolving the crystal, but will seem to re- 

 volve one around the other. The dot will, in fact, appear 

 double through the crystal in every direction except that of 

 the vertical axis, and this direction is called the axis of double 

 refraction. To view it in this direction, the ends must be 

 ground and polished. The divergence increases on passing 

 from a view in the direction of the axis to one at right angles 

 with it, where it is greatest. In some substances, the re- 

 fraction of the extraordinary ray is greater in the latter 

 direction than that of the ordinary ray, and in others it is less. 



What is double refraction? What takes place on revolving a trans- 

 parent rhomb of calc spar over a line or dot 1 In what direction is ther« 

 no double refraction, and in which is it greatest? 



