124 



Prof. Haidinger on the Optical Properties of 



The inclination of the faces d and d! to the adjacent ter- 

 minal planes appears perfectly equal. 



The whole of the measurements succeeded tolerably well, 

 as the faces though small are smooth and brilliant, with the 

 exception of that marked m(-\-lcx) A/2), which only occurred 

 irregular. 



The triangular plates have a brown colour, which passes 

 into a rich deep orange when they are reduced to powder. 

 When thin they are perfectly transparent ; they have a fine 

 adamantine lustre. ^In order to examine their pleochroism by 

 means of the dichroscopic lens, they are most advantageously 

 attached to a piece of wax by the acute angle at ^, and held 

 so that the edge d d' may be horizontal. The ordinary image 

 is then seen above, and the extraordinary E below, as re- 

 presented in fig. 1. By perpendicular incidence of the light 

 the ordinary ray appears much lighter than the extraordi- 

 nary, and the tint varies wnth the thickness of the plate, the 

 former passing from a pale yellowish- brown, through dark 

 honey-yellow into blood-red, while the latter commences with 

 blood-red and soon becomes opaque, that is, gives a black 

 image. When the crystal is made to rotate towards right 

 or left without deranging the horizontal position of the edge 

 dd\ we observe a gradual increase or decrease of transpa- 

 rency; and it appears most transpa- 

 rent in the ordinary image, when 

 examined in the direction of AA, 

 fig. 2, that is, nearly perpendicular 

 to the edge between m and m', or 

 perpendicular to the axis of the prism. 

 It is least transparent in the direction 

 of the axis BB. From this it follows 

 that only one of the axes of elasticity 

 of double refraction lies in the plane 



of the triangular plate, and that perpendicular or nearly so 

 to the edge d d\ while the other two, which are perpendicular 

 to one another, form with the section of the base, angles 

 such that CMA is about 30°, and CMB about 60°. The 

 colour polarized in the direction AA perpendicular to BB is 

 the brightest, that polarized in the direction BB of interme- 



