64 DOUBLE REFRACTION. 



of phenomena, he was happily led to assign the true law of 

 extraordinary refraction. The important researches of Huy- 

 gens on this subject are contained in the fifth chapter of his 

 " Traite de la Lumiere" 



(82) The property of double refraction is possessed by all 

 crystallized minerals, excepting those belonging to the tessular 

 system, i. e. those whose fundamental form is the cube. It 

 belongs likewise to all animal and vegetable substances, in 

 which there is a regular arrangement of parts ; and, in fine, 

 to all bodies whatever, whose parts are in a state of unequal 

 compression or dilatation. The separation of the two refracted 

 pencils is in some cases considerable, and the course of each 

 easily ascertained by observation ; but it is generally too mi- 

 nute to be directly observed, and its existence is only proved 

 by the appearance of certain phenomena which are known to 

 arise from the mutual action of two pencils. In Iceland spar, 

 the substance in which the property was first discovered, the 

 separation of the pencils is very striking : and, as this mineral 

 is found in considerable masses, and in a state of great purity 

 and transparency, it is well fitted for the exhibition of the 

 phenomena. 



(83) Carbonate of lime, of which Iceland spar is a variety, 

 crystallizes in more than 300 different forms, all of which 

 may be reduced by cleavage to the rhombohcdron, which is 

 accordingly the primitive form. The angles 

 of the bounding parallelograms, CAB and 

 ABD, in the rhombohedron of Iceland spar, 

 are 101 55' and 78 5 7 . Two of the solid 

 angles, at A and , are contained by thre 

 obtuse angles ; while the remaining four are bounded by one 

 obtuse and two acute angles. The line AO, joining the sum- 

 mits of the obtuse solid angles, is called the axis of the rhom- 



