GENERAL EQUATION OF A CUBIC SURFACE. 
55 
Such a combination as 
c 
* # 0 
Z) ^ 0 ^ 
a n * 
(I Ji r 
implies that three lines intersect each other in pairs, i.e., that they form the complete 
section of the surface by their plane, which is a triple tangent plane. 
Here, again, the ro^^^s and columns must represent tiie same lines. 
Such a combination as 
6 I • * 
a I * 
c 5. 
where the rows and the columns necessarily represent different hues, implies that a, c 
and b, d are intersecting pairs, and that h, c and a, d are non-intersecting pairs; but 
the figure does not indicate whether the pairs ct, h and c, d^ intersect or do not 
intersect. 
The whole truth with respect to the intersections of the four lines is not conveyed 
in the above figure. 
When the whole truth is conveyed in the figure 
(Z ' ■ * • o 
c * • 0 • 
h • 0 ■ * 
a 0 • * • 
abed, 
that is, when there are no other intersections among the four lines than those repre 
sented in the figure 
h ■ * 
a * 
c d, 
we shall call the combination a “ double two.” 
