  This paper treats the strict semi-stability of the symmetric powers $S^k E$
of a stable vector bundle $E$ of rank $2$ with even degree on a smooth
projective curve $C$ of genus $g \geq 2$. The strict semi-stability of $S^2 E$
is equivalent to the orthogonality of $E$ or the existence of a bisection on
the ruled surface $\mathbb{P}_C(E)$ whose self-intersection number is zero. A
relation between the two interpretations is investigated in this paper through
elementary transformations. This paper also gives a classification of $E$ with
strictly semi-stable $S^3 E$. Moreover, it is shown that when $S^2 E$ is
stable, every symmetric power $S^k E$ is stable for all but a finite number of
$E$ in the moduli of stable vector bundles of rank $2$ with fixed determinant
of even degree on $C$.
