jmc

The expression inside the square root must be greater than 0 because the denominator cannot be equal to 0. Therefore, $x-1>0$, so $x>1$. The expression inside the logarithm must be greater than 0, so $3-x>0$, which gives $x<3$. Therefore, the interval of $x$ for which the expression $\frac{\log (3-x)}{\sqrt{x-1}}$ is defined is $1<x<3$, which is $\boxed{(1,3)}$.