OF LOGICAL CLASS-FREQUENCIES, ETC. 
101 ) 
The plan and elevation of the complete tetrahedron are thus both squares of side 
=: 0’5. Blit, comparing equations (C) with (A), the edge 
§ 16. Case (1). 
(a/3) is 
truncated unless 
Ih + Pi = 
(ay) 
?? 
Pi + Pi = 
(aS) 
Pi + P2 = 
(^y) 
9? 99 
11 
(/3S) 
9 9 9 9 
Pi = Pi- 
(Sy) 
9 9 9 9 
P-2 = Pi^ 
Pi = P-2 = Pi ■ 
= 0-5. 
In this case the congruence-surface reduces to the fundamental tetrahedron, 
fig. Icq* the planes (A) only passing through its edges and not truncating them. The 
form is more clearly shown by fig. Ih, which is drawn from a photograph of an actual 
* This and the similar following figures are drawn in orthographic projection. The picture-plane is 
parallel to the axis of z, and its trace on the plane of xy makes an angle of 30° with the x axis. The 
generators lie in planes jierpendicular to the picture-plane and the jilane of xy, l)ut make an angle of 45 
with the latter jdane. The observer must therefore imagine himself to be looking down, on the model. 
