CRYSTALL OGRAPHY MINERAL OGY. 3 1 5 



the crystal, and originally placed in a line with the o-i8o 

 diameter of a graduated circle, by which the motions of the stage 

 carrying the crystal may be read. A more convenient form has 

 been imparted to the instrument by Brezina, who has further 

 increased the delicacy of the method by substituting a compound 

 calcite plate, formed by the symmetrical union of two plates, on 

 each of which the optic axis is slightly inclined to the normals of 

 the plates. The theory of the stauroscope was completely investi- 

 gated by Grailich. Since in this instrument rays of light nearly 

 parallel have to be employe^, it is not immediately applicable to 

 one of the more important uses for which it would be otherwise 

 adapted; namely, for determining the principal sections of the 

 crystals which are presented in the section of a rock as seen in 

 the microscope. Though only approximate results can in the 

 generality of cases be obtained from such microscopic sections, 

 even these are of great importance. Such approximate results 

 may be obtained by applying to the microscope a goniometer eye- 

 piece, in which one wire revolves in the focus of the first lens 

 around the centre of the field, while another wire is fixed in a 

 position parallel to the trace of the plane in which the light is 

 polarised by the polarising Nicol. The position of the fixed wire 

 may be accurately determined by adjusting it in optical coincidence 

 with one of the edges of a very minute crystal of mesotype, or of 

 some orthorhombic mineral presenting an elongated prism, which 

 is mounted on a glass slide and moved by the rotation of the 

 stage of the microscope into such a position that no colour is seen 

 either when the Nicols are crossed or when the analyzer is slightly 

 turned to the right or left. If now a transparent crystal in a rock 

 section in which some crystallographic direction is approximately 

 known, replace in the field the crystal of mesotype, the known 

 line or direction in it can be brought into optical coincidence with 

 the fixed wire in the focus of the eye-piece. Should the light be 

 depolarised by the crystal, this would prove that the determined 

 direction in it must be inclined to its principal sections; since 



