The apparatus comprises a square, metallic vessel holder (11) with twenty-four chambers (13) arranged to smoothly take up in an upright position changeable, transparent reaction vessels used to carry biological reaction liquids of about 100 µl. At the side walls of the chambers (13) are arranged two connectors (21, 22) with associated boreholes. Within these holes are removably fixed first (32) and second optic fibers (31). These fibers are connected in such a manner, that the axial extensions of the fibers (31, 32) meet one another approximately at a same point of the chamber's axle, and in such a manner, that light escaping from the first fibers (32) cannot reach the second fibers (31) directly. The open ends of the fibers are connected to a common light source and individually to associated light receivers, respectively.