OPTICAL INSTRUMENTS. 433 



673. Dotible Refraction. We have seen that a 

 plate of tourmaline may stop all rays whose vibrations lie 

 in a certain plane while it allows passage to all rays whose 

 vibrations lie in a plane perpendicular to this. A crystal 

 of Iceland spar shows 

 a different but very 

 important effect upon 

 an incident beam. 

 The retardation of 

 those vibrations whose 

 plane is parallej to the 

 axis (the line joining FIG. 344. 



the two obtuse angles 



of the crystal) is different from the retardation of those 

 vibrations whose plane is perpendicular to the axis. This 

 difference in change of velocity produces a difference in the 

 refraction of the two sets of rays. A beam of light, there- 

 fore, falling upon a crystal of Iceland spar will be gener- 

 ally split into two, producing the effect known as double 

 refraction. 



(a.) A small object, as a dot or line, viewed through a crystal of 

 Iceland spar, will generally show two images formed by light oppo- 

 sitely polarized. If the eye be placed directly above the dot and 

 the crystal slowly turned around, one image known as the ordinary 

 image will remain stationary, while the other known as the extra- 

 ordinary image will revolve about it at a varying distance. The 

 ordinary ray has a constant and the extraordinary ray a variable 

 index of refraction. 



(&.) On looking at the two images formed by double refraction 

 through a tourmaline or any other analyzer, it will be found that 

 there is a marked difference in the brightness of the two images. 

 As the analyzer is turned around, one image grows brighter and the 

 other fainter, the greatest brightness of one being simultaneous 

 with the extinction of the other. 

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