Aqueous somatotropin compositions are presented that contain less than about 10% of a biologically active somatotropin, water, and glycerol in an amount effective and at a pH effective to maintain the somatotropin substantially completely stable. The rotor of the swivel drive and a first part of the position measuring device are connected to the shaft in a rotatably fixed manner. A diffractive optical element is disposed on the optical axis of return light in the photodetection optical system and provided with an annulus. The female end has a locking socket extending laterally within the female end.