( 490 ) 
By elimination of 2): 7 we find 
(azo foo — 402.402)” + 4 (420 f11 + 411 S02) (doe Sta + 411 fon) =O. 
By a simple computation this expression is reduced to the sym- 
bolic form 
a2 D2 — 2 (ab) (fy) ar by =0, 
where a? =0? and f2?=g¢?. 
& T Zz zt 
If in the preceding equations we put «2, =0, then we have 
for=0 or a%, For + 4 a2 411 Au + 44? foo = 9. In the former case 
a line of a} is at right angles to a line of f?. Inthe latter case the 
substitution agg = 224a,, furnishes the condition 
— fog + 2t fii + fog = 9, 
from which ensues that one of the two isotropical lines belongs to 
each of the pairs; then again a line of a? is at right angles to a 
line of fe: 
The consideration of the orthogonal pair of rays of the involution 
2 De 
a* + Af? kj 
leads to a simultaneous covariant. 
This pair is indicated by 
(@ HFD) ALB HÀSE) =O, 
or by 
data f= 0, 
so by 
hi da fi=0. 
8. It is a matter of course, that to the cubic form 
em re ry) J . a ’ ay) 7 
a® == 430 a + 3 a9) oo 4) = a dg vy * Es 403 ws 
ct 
belong only invariants with an even number of symbols, 1. e. of 
even degree in the coefficients. 
