506 Prof. Maskelyne and Dr. Lang's Minwalogical Notes. 



the crystals are prismatic pyramids. By means of a second sec- 

 tion perpendicular to the axis c, it was found that the plane of 

 the optic axes is indeed perpendicular to the axis a, and that 

 the character of the mean line (probably the first) coinciding with 

 the axis c is negative. The points of the axes themselves could 

 not be seen, as the angle of the optic axes is too great for this ; 

 but another section, a little inclined to the former one, showed 

 one axis, by which one could see that the negative angle of the 

 optic axis is smaller for red than for blue light. 



The planes of the crystals are not very good for measuring, 

 but the crystals are cleavable parallel to planes (101), and by 

 means of such cleavage-planes I determined the angles 



I 1 . 1 1 = 93° 4i 



I I . 1 1 = 65 18 

 110.110= 90° 50' calc, 



whence we find 



a:b:c= 1 : 0-9856 : 0'9377. 

 As for the axes of optic elasticity, we found 



a lie 6 II a c|| &; 

 the symbol of the optical orientation becomes 



6ca. 



Last summer I had the opportunity of seeing two quite similar 

 specimens in the mineralogical collection of the Berlin Univer- 

 sity. The accompanying label gave, " Gismondine [variete de- 

 crite par Gismondi, qui est la plus rare] : Capo di Bove pres de 

 Rome. Monsignore Lavinio de Medice, Speelo, Roma." 



On the Crystalline Form of Herschelite. By Viktor von Lang. 



It has hitherto been supposed that Herschelite crystallizes in 

 the hexagonal system ; but the optical investigation of the nu- 

 merous specimens in the mineralogical collection of the British 

 Museum showed that Herschelite belongs to the prismatic 

 system, and that its hexagonal forms are only produced by the 

 twin association of six individuals. One has only to grind 

 down the crystals a little, as the top plane is always rounded, to 

 see with the polarizing apparatus the internal structure o*f the 

 crystals as represented in fig. 4, PL VII. Each triangular section 

 is doubly refracting, and gives moreover two optic axes, the planes 

 of which bisect the central angle of each section. The angle of 

 the optic axes is very small, the character of the first mean line 

 negative. 



In making these observations, one has only to take care that 

 the crystal is not too thin, as, the double refraction being not 



