Abstract: The apparatus comprises a massive body accurately maintained at 37.degree. and having a through horizontal bore in which are disposed in succession a bulb, a first concentrating lens, a test tube containing the sample, an interchangeable optical filter, a second concentrating lens and a photo-electric cell. The test tube is disposed in a blind vertical bore of the body and it contains a small stirrer bar resting on its bottom and made of a magnetic material. A permanent magnet, adapted to be operatively associated with the stirrer bar is rotated below the body, co-axially to the bar, by an electric motor at one of two accurately controlled speeds. The output of the cell is amplified and applied to one inlet of a differential amplifier the other inlet of which receives the output of a memory wherein the response of the cell has been registered at the beginning of a test. The outlet signal from the differential amplifier starts therefore from zero and may be applied to a usual graphically recording apparatus.