Passage: Until 1909 the ETH was not authorized to grant doctoral degrees, so a special arrangement enabled ETH students to obtain doctorates from the University of Zurich. At that time, most dissertations in physics by ETH students were carried out under the supervision of H.F. Weber, Einstein's former teacher at the Polytechnikum, as it was then called. The University of Zurich had only one physics chair, held by Alfred Kleiner. His main research was focused on measuring instruments, but he also had an interest in the foundations of physics.

Question: Alfred Kleiner was known to focus his research on what?
measuring instruments