A method and device for optically detecting particles (23) have the following features: (a) a cell wall (9) of rectangular cross-section, made of black glass, is fitted on a longitudinal surface and adjoining transverse surface with an L-shaped heating and cooling element (1); (b) the centre of the transverse surface of the cell wall (9) opposite the transverse surface which forms the support of the cell wall (9) is irradiated by an irradiation device and is observed at right angles to the optical axis of the irradiation device by means of an observation device; (c) the focus of the irradiation device and the focus of the observation device can be moved by a motor to any point in the three-dimensional inner region defined by the cell wall (9) by means of a control device; (d) the surface of the cell wall (9) opposite the optical glass window (11) through which the radiation from the irradiation device enters comprises another optical glass window (11) in the centre thereof; (e) the temperature of the surface of the cell wall (9) is monitored by means of two thermistors (8).