Quadratic tertias and seconds of rays

Abstract

Tertias of rays, which represent the so-called null systems and are completely and uniquely determined by five arbitrary rays, are usually called linear in view of the fact that in each plane and from each point of space there exists a prima of rays that intersect at a single point and are contained in a single plane; such a prima of rays in a plane or emanating from a single center is called linear. But for a system of rays determined, as a parameter, by a special extra-ray, such primas are no longer linear, since the latter must necessarily contain this extra-ray. Therefore, the null systems themselves do not represent linear tertias in this system, but are quadratic tertias.

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