Prof. Maskelyne and Dr. Lang's Miner alogical Notes. 435 



following remarks to make. A slide parallel to 101, which is 

 easily obtained by cleavage, shows in the polarizing apparatus 

 one of the optic axes; and it is possible by inclining the slide a 

 little to get also the second axis into the field of view. One 

 perceives in this way that the plane of the optic axes is parallel 

 to the plane of symmetry, and that the optical character of the 

 first mean line is negative. As the crystals are also cleavable 

 parallel to the plane (010), although not so perfectly, it is pos- 

 sible to obtain a slide parallel to the plane of symmetry. Such 

 a slide out of a twin crystal gave, with the aid of a polarizing 

 microscope, for the angle which the axes of greatest elasticity of 

 the two individuals make with one another, the approximate value 

 of 47° 40', which by the notation before introduced may be ex- 

 pressed by the equation 



n(100)w = 47°40\ 

 Hence we find 



n (101) = 4° 43'. 



Let N in fig. 9 represent the normal to the plane (101); A, B 

 the direction of the optic axes in the crystal ; A', B' their direc- 

 tion in the air after their refraction at the plane 101; then we 

 find easily 



. AT sin NB' — sin N A' cos 2rcN 

 cos AN = ■ 



js= 



sin NA' sin 2wN 

 sin A'N 



sin AN' 



33 30 



36 30 



56 



60 30 



46 26 



48 8 



1-7943 



1-8028 



ft being the mean coefficient of refraction for malachite. The 

 observed numbers for NA' and NB', and the values of AB and j3 

 which follow from these observations, are, assuming for all colours 

 nN = 4°43', 



Red. Yellow. Blue. 



NA' . . . 29 50 



NB' ... 51 



AB ... 41 54 



P > . . . 1-7886 



These numbers can only be considered as a first approximation 

 to their correct values. 



The optical orientation of malachite may be expressed by the 

 symbol* 



1 b a = 66° 10'. 



8. On some Artificial Crystals of Gold. By Viktor von Lang. 



For the crystals of gold, of which the following is an account, 



I am indebted to Dr. Percy, who received them from Australia. 



* Vide Murmann and Rotter, Sitzunysber.der Wiener Akad. vol. xxxiv. 



