A photometer for determining the transmission of a liquid sample in a microcuvette (7, 20, 22) has a cavity for a capillary layer of the liquid sample with a free liquid surface extended transversely of the principal plane of the cavity. The cavity is initially prepared with a reagent to achieve a reaction, which affects the transmission of the liquid sample for indicating its contents of a predetermined substance. The photometer has a holder (6) for the microcuvette and measuring means (9, 10) for measuring the transmission of a bundle of rays directed towards the microcuvette. The cuvette holder (6) is mounted on bearings for vibration in a direction having a component, which extends in the plane of the free liquid surface and is parallel with the principal plane of the cavity, thereby achieving a mixing to accelerate the dissolution and the reaction.