K L. Penfield — Crystal Drawing. 



49 



— f a ; b ; and — c, and — §<? are laid off both on the orthographic 

 and clinographic projections of the axes, and the twinning 

 plane — %a\b: — f c drawn. The value of $, 54° 37', is laid off 

 at p on the equator, measuring in the direction of the arrow 

 from b, and the radius from the center to p makes an angle 

 of 90° at I* with the line — %a\b. The twinning axis, a line 



16 



from the centre at right angles to the twinning plane, is now 

 plotted on the clinographic axes by finding a point T, on the 

 meridian through p, 60° 31/ (the value of p) from the south 

 pole of an imaginary sphere. This is done by locating x, y 

 and z on the graduated ellipses at 60° 31/ from the south pole, 

 and drawing the chords xx 1 ', yy' and zz' parallel, respectively, 

 to chords on the plane of the equator through p and the inter- 

 sections of the a and b axes with the equator. The intersec- 

 tion of the three chords determine the desired point T at the 



Am. Jour. Sci.— Fourth Series, Vol. XIX, No. 109. — January, 1905. 

 4 



