48 



E. I. Gal'perin et al. 



and the angle 9? with the vertical, which varies from to 180°. The first 

 essential is to learn to plot on the net the diiections of the rays along the 

 co-ordinates so that we can remove the co-ordinates from the net. 

 Let us examine the main methods of working with a WulfF net. 



280 



270 



260 



90- a^ 



100 



250 



190 180 '70 



Fig. 2. Constructing the direction from given co-ordinates (a = 162°, (p = 54°) by 



means of a Wulff net. 



1. To construct on the grid directions of which the co-ordinates are 

 given. Let the co-ordinates of direction be a =^ 162° and 9? = 54°. In order 

 to construct this direction on the net (see Fig. 2) we must count off an angle 

 equal to the azimuth of our direction on the tracing paper in a clockwise 

 direction round the outer circle of the grid, and mark point oc with a^. Rotating 

 the tracing paper we bring the point which we have just obtained onto one 

 of the diameters of the ])rojection circle. For example, let this be the horizontal 



