  Only null one-directions are determined. Therefore no special choice  of parameters along light rays is determined by this structure. The  first-order conformal structure can be measured using only the local  differential-topological structure. Moreover, by a purely  mathematical process involving only differentiation, the first-order  conformal structure determines a second-order conformal structure,  namely, an equivalence class \(K\) of conformally related  symmetric linear connections. 