I have known that the Fundamental matrix is $\operatorname{rank}(2)$,and all the epipolar lines meet at the epipoles.
Given this identity $(e')^T\cdot (Fx)= \mathbf 0$, in order to make this equation set up for all $Fx$, we should set $e'$ value to be zero.
But, why $e'$ is called right space of the $F$, or left space? In other words, can we divide the Fundamental matrix up into $3$ by $5$ matrix ,with left-most column vector and right-most column vector $\mathbf 0$?