DOUBLE REFRACTION. 



+ Titanite. 



Idocrase. 



Wernerite. 



Paranthine. 



Meionite. 



Subphosphate of Potash. 



Edingtonite. 



+ Apophyllite of Uton. 



+ Superacetate of Copper and Lime. 



Phosphate of Ammonia and Mag- 



nesia. 

 - Hydrate of Strontites. 



Arseniate of Potash. 



Sulphate of Nickel of Copper. 



Somervillite. 

 + Oxahverite. 



In all these crystals the axis of 

 double refraction coincides with the 

 line AB, which is the axis of the geo- 

 metrical solid. 



The following crystals, whose primi- 

 tive form has not been perfectly deter- 

 mined, have also one axis of double 

 refraction. 



Position of the Axes. 



Mica from Kariat. . . Perpendicu- 



lar to the Laminae. 



Mica with Amianthus . Perpendicu- 



lar to the Laminae. 



Muriate of Lime . . Axis of Hex- 



agonal Prism. 



Muriate of Strontian . Axis of Hex- 



agonal Prism. 



Hyposulphate of Lime Axis of Hex- 



agonal Table. 



+ Boracite . . Axis of Rhomb of 90. 

 + Apophyllite surcomposee . Perpen- 

 dicular to the Plate. 



+ Sulph. of Pot. and Iron Axis of Hex- 

 agonal Prism. 



+ Ice Axis of Hex- 

 agonal Prism or Rhomb. 



Cyanuret of Mercury . . . Axis of 



Square Prism. 



Having thus given a list of those re- 

 gular crystals which have one axis of 

 double refraction, we shall now proceed 

 to describe the phenomena which they 

 exhibit, and to explain the law by which 

 the phenomena are regulated. In doing 

 this, we shall begin with crystals which 

 have one negative axis, such as Iceland 

 spar, a mineral which is peculiarly 

 adapted for investigating the pheno- 

 mena of double refraction. 



2. On the Law of Double Refraction in 

 Crystals with one Negative Axis. 



If we grind down and polish the two 

 opposite summits A, B, fig. 1, of a 



rhomb of Iceland spar, so that the faces 

 are perpendicular to the axis A B, we 

 shall find that a ray of light transmitted 

 parallel to A. B is not divided into two 

 pencils. This will be the case whether 

 the ray is incident perpendicularly upon 

 the two faces, or obliquely upon any 

 face not perpendicular to A B, provided 

 that in the latter case the refracted ray 

 is parallel to AB. If, in this latter 

 case, we measure the index 'of refrac- 

 tion of the Iceland spar, we shall find it 

 as follows: 



If we measure the indices of refrac- 

 tion in a direction perpendicular to 

 each of the six faces of the. rhomb 

 (which are all inclined 44 36' 34" to 

 the axis) so that the plane of incidence 

 passes through AB, we shall find them 

 as follows : 



Indices of refrac-^j 



tionperpendicu- 1 1 . 6543 ordinary ray. 

 lar to the faces 1 1.5720 extraor. ray. 

 of the rhomb. J 



If we now grind a face parallel to 

 AB, and measure the indices of refrac- 

 tion in a plane of incidence perpendicu- 

 lar to A B, we shall find them to be the 

 same all round the axis, and to be as 

 follows : 



From these results it follows that the 

 double refraction, or the force which pro- 

 duces it, disappears, or is nothing when 

 the ray acted upon passes along the 

 axis of the crystal ; that it increases with 

 the angle which the ray forms with the 

 axis ; and is a maximum when the inci- 

 dent ray is perpendicular to the axis. 



In order to discover the precise law 

 by which the doubly refracting force 

 increases as the inclination of the inci- 

 dent ray with the axis increases, Huy- 

 gens measured the double refraction at 

 different angles, and found that the re- 

 ciprocal of the index of refraction of the 

 extraordinary ray was measured by the 

 radius of an ellipse whose lesser axis is 



to its greater as 



the reciprocals of the greatest, and the 

 least index of extraordinary refraction. 



In order to make this plain, let us 

 suppose that the rhomb of calcareous 



