8. For any function $f(x)$, in the same Cartesian coordinate system, the graphs of $y_{1}=f(x-19)$ and $y_{2}=f(99-x)$ are always symmetric with respect to the line $\qquad$.
8. $x=59$.

Let $x^{\prime}=x-59, y^{\prime}=y$, then $y_{1}^{\prime}=f\left(x^{\prime}-40\right), y_{2}^{\prime}=f\left(40+x^{\prime}\right), y_{1}^{\prime}$ is symmetric to $y_{2}^{\prime}$ about the line $x^{\prime}=0$, thus $y_{1}$ is symmetric to $y_{2}$ about the line $x=59$.