some Carbonates of the Calcite Group. 331 



be rotated until the symmetrical pattern is obtained and 

 then the indices may be read off: as before. This is done, 

 as he points out, by first turning the plot so that the line 

 joining the center with the point P, which is to become the 

 center of the projection, will be perpendicular to the 

 straight (longitude) lines of the net and then by moving 

 all the points through the necessary angle along the hyper- 

 bolas (of latitude). It can be done quite simply by work- 

 ing on tracing paper or cloth over the net. The gnomonic 



\ 7~x" 



Fig. 9. — The " symmetrical ' ' pattern obtained by rotating the projection 

 of fig. 3 through an angle sufficient to throw the (110) plane to infinity. 

 The indices of the planes can now be read from their coordinate positions. 



net, as given by Hilton, cannot be used because it does not 

 extend to great enough angles. 



The computations necessary for constructing a 65° net 

 have been given. 14 The values necessary to extend it to 

 80°, as is required for this purpose, may be obtained in 

 the same way. A reduction of such a net is given in fig. 8. 

 If this is enlarged photographically so that it is exactly 70 

 cm. on a side, it will serve as a net for a sphere of 5 cm. 

 radius. 



In order to determine the indices of the reflecting 

 planes in a photograph through a cleavage piece of calcite, 



14 G. F. H. Smith, Zs. Kryst., 39, 142, 1904. 



