446 TRANSACTIONS OF THE CANADIAN INSTITUTE. (Vou. VIII. 
gnomonic projection was made and each face was plotted by means of | 
values of g and ¢ as measured. The symbols for the different faces 
were then determined graphically. The symbols for the different faces 
may also be calculated from the angular co-ordinates x’ and y’.. Thus:— 
Pps ier y es C= cor 
where the symbols p and q are simple multiples or sub-multiples of 
the co-ordinates p,’ and q,’ of the unit form €1113 and e’ is the tangent 
of the angle the base normal makes with the pole. 
These formule hold for terminal faces but for prisms we have, 
x’=sin ¢ tan g, y’= cos ¢ tan g, 
: x’ sin ¢g 
whence it follows —== — =tan ¢. 
y’ cos¢ 
For positive faces on the projection pp,’=x’—e’. 
For negative faces on the projection pp,’=x’+e’. 
The unit form selected is the pyramid <1113. Since all the good 
bright forms are negative the value of q,’ must be obtained from the 
unit form and this value substituted in the values obtained from the 
formule for the prisms. 
/ 
do 
0.6983 
0.7065 
0.6985 
0.6995 
0.7003 
0.7006 = average q,’ for five readings. ‘ 
/ 
Po 
0.9712 
0.9714 
0.9673 
0.9656 
0.9667 
0.9600 
0.9670=average p,’ for six readings. 
The elements for Erythrite then become, 
Pp.’ =0.9670, q,.’ =0. 7006, ‘e’=0: 2681, 42=74°—59’. 
es 
se? he 
