3 io SCIENTIFIC APPARATUS. 



flection, are now established in the same horizontal plane with the 

 position of the crystal, and the crystal is adjusted in the usual way 

 with the edge to be measured perpendicular to that plane ; and 

 the signals are also sighted in the usual manner with one of the 

 faces. A flat sheet of paper having previously been laid under 

 the board carrying the instrument and fixed by weights or pins, a 



A 



line A is ruled on the paper along the straight edge of the board. 

 Keeping the crystal as nearly as may be over the same spot, the 

 signals are brought into coincidence for the second face, and a 

 line B ruled as before, intersecting with A in a point c. The 

 angle A c B, in which the two lines, corresponding to the two 

 positions of the instrument, intersect, is that between normals to 

 the faces, and may be determined by the aid of a pair of com- 

 passes. 



