A photometer for measuring photometric magnitudes in a liquid medium includes a housing adapted to be introduced into the medium for on-site measurements; and light-emitting measuring beam transmitter and receiver that are arranged behind opposing measuring windows in a measuring slot in the housing, in which the slot is open to the medium. A measuring beam is generated in a measuring conduit in the measuring slot. A reference conduit is included for a reference measurement which is separate from the measuring conduit. At least one carrier part for the measuring beam transmitter and the measuring beam receiver is provided to move on a given path in the housing. The carrier part is constructed to move on the given path from the measuring conduit to the reference conduit and back, for measuring intensity values of the measuring beam in the measuring conduit and in the reference conduit.