′. Then the straight lines
and
′ will be said to be 'normal' to each other. This definition of the normality of straight lines can be given in general terms thus: Two straight lines in the same space are said to be normal to each other when they are respectively occupied by normal rects lying in the same moment of the corresponding time-system.
47.3 Continuing the notation of [47.2] let
′ be the level containing
