fication that are invaluable. The degree of refraction 

 of a ray of light upon entering a precious stone is char- 

 acteristic of that particular stone. As a rule this re- 

 fraction cannot be determined in the rough stone, 

 on account of the unevenness of the surfaces. To cut 

 the gem into a prism for the purpose of examination is 

 of course out of the question. But the same thing is 

 accomplished by selecting two facets of the cut stone 

 having the proper angle for the examination, and then 

 painting over the other surfaces of the gem. By means 

 of the goniometer the refraction and double refraction 

 even of stones of greatest refractive power may then 

 be determined accurately. 



A little instrument called the dichroscope, so small 

 that it may be carried in the vest pocket, is useful for 

 determining the pleochroism of gem stones. Pleo- 

 chroism is the property of doubly-refractive colored 

 gems showing two different colors, or shades, when 

 viewed at different angles. The instrument con- 

 sists of a metal cylinder " containing a cleavage rhom- 

 bohedron of Iceland spar, and possesses an eyepiece 

 containing a lens at one end, and a small square aper- 

 ture at the other. The eyepiece is held to the eye, 

 and the gem to be examined is placed between the 

 other end of the cylinder and the light. Two images 

 of the square opening at the other end of the dichro- 

 scope may then be seen, and they will appear either of 

 different colors or of absolutely the same color, accord- 

 ing to the nature of the gem stone under examination." 



If the two images are of exactly the same color, no 

 matter in what direction the gem is viewed, the stone 



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