Dr, Brewster on some Properties of Light. 105 



laminated structure of the agate, and to indicate that the 

 phenomena of double refraction are produced by an alter- 

 nation of laminae of two separate refractive and dispersive 

 powers. In Iceland spar, one set of the lamina? may be formed 

 by a combination of oxygen and calcium, while the other set 

 is formed by a combination of oxygen and carbon. In chro- 

 mateof leadjthe chromium and oxygen may give one image, 

 while the oxygen and lead give another. In like manner the 

 carbonate of lead, the carbonate of strontites, jargon, and 

 other crystals may give double images, in virtue of similar 

 binary combinations. Of the simple inflammable bodies, sul- 

 phur is the only one which has the property of double refrac- 

 tion, but it will probably be found that it holds a metal or some 

 other ingredient in its composition, which chemists have not 

 been able to discover. 



If the explanation which has now been given of the polar- 

 ising power of the agate should be confirmed by future expe- 

 riments, this property will be considered as a case, though a 

 very curious one, of double refraction ; but if these conjectures 

 should be overturned, the phenomena which we have described 

 must be ranked among the most singular appearances in the 

 wide range of optical science. 



2. On the double Refraction of Chromate of Lead. 



In the course of my experiments on refractive powers, I 

 discovered a double refraction in this metallic salt of such 

 enormous magnitude, that the deviation of the extiaordinary 

 ray is more than thrice as great as that produced by Iceland 

 5par. The ratio of the sines, for both refractions, and the other 



MDCCCXIII. P 



