8. If the function $f(x)=\lg \left[\left(k^{2}-1\right) x^{2}-(k+1) x+1\right]$ has a range of $\mathbf{R}$, then the range of real number $k$ is $\qquad$ .
8. $\left[1, \frac{5}{3}\right]$ If $k=1$, then $f(x)=\lg (-2 x+1)$ is valid: If $k=-1$, then $f(x)=\lg 1=0$ is not suitable; When $k \neq$ $\pm 1$, let $\Delta=(k+1)^{2}-4\left(k^{2}-1\right) \geqslant 0$, and $k^{2}-1>0$.