136 M. V. von Lang on a Horizontal Goniometer. 



ingredient minerals — the other minerals associated with it 

 being olivine and garnet, as is the case in some specimens of 

 lherzolite, though the two rocks are quite dissimilar in aspect. 

 The following Table exhibits the analysis (1) of the Korn- 

 Kopje rock by Dr. Prevost, in which all the iron is assumed to 

 be in the ferrous condition ; (2) of a rock from the Radauthal 

 (occurring in bastite) in the Hartz, by Streng (Jahrb. Min. 

 1862); (3) of that in the lherzolite of L. Lherz, by Damour 

 {Bui. G4ol. xxix. p. 413) :— 





0) 



(2) 



(3) 



Silica 



53 



54-15 



54-76 



Alumina . . . . 



2-6 



3-04 



4-90 



Ferrous oxide . . 



9-27 



12*17 



9-35 



Manganous oxide . 



. 2 







Magnesia . . . 



. 25-5 



28-37 



30-22 



Lime 



. 6-6 



2-37 





98-97 100-10 99-23 



The excess of lime in the analysis is probably traceable to 

 an augitic mineral, a diopside, probably present as an ingre- 

 dient, although not yet recognized by the microscope in the 

 sections made from the South-African rocks. In fact the mi- 

 croscope, so invaluable as an instrument for pioneering in the 

 realm of petrology, is frequently an untrustworthy guide when 

 too much relied on — that is to say, when the results obtained 

 by it are not checked and confirmed, and in fact supplemented 

 by the more tedious methods of investigation pursued in the 

 laboratory. 



XXII. On a Horizontal Goniometer. By Victor VON Lang*. 

 [Plate VII. figs. 7 & 8.] 



THE instrument represented in fig. 8 owes its existence 

 to the necessity of being able to measure refractive 

 indices at different angles of incidence. This is of great im- 

 portance when one wants, like Professor Stokes, to verify the 

 theoretical formulae of double refraction by experiment, or 

 when one tries, on the contrary, to determine with the aid of 

 these formulae the constants of double refraction from obser- 

 vations on a prism of arbitrary position. 



The measurements I made in order to determine the figure 

 of the wave-surface in quartz near its axis belong to the first 

 kind of researches ; whereas the determination of the refrac- 

 tive indices of gypsum I have just finished gives an example 

 of the second kind of researches, as for that determination one 

 single prism was made use of. 



* Communicated by the Crystallological Society. 



