It is well known that Paul Mahlo (1883-1971) developed a systematic hierarchy of inaccessible cardinals of the type $\pi\_{a,b}$ where $\pi\_{1,b}$ enumerates the strongly inaccessible cardinals, $\pi\_{2,b}$ enumerate the fixed points of $\pi\_{1,b}$ and so on. My question is, where can I find the original paper of Mahlo in English? If this doesn't exist, are there any good expositional articles on this accessible online?