Verfahren zur Feststellung vorzeitiger Flüssigkeitsabgabe beim Probengeben

It is known to detect pre-spots on slide elements using infrared radiation which is reflected therefrom by utilizing an expensive lamp and PbS photocell combination. This invention is based on the surprising discovery that the inexpensive versions of that system, namely, the infrared light-emitting diode and the photosensitive transistor will work effectively only if the light is detected in transmission rather than reflection. Most preferably, two parts of the system are disposed so that the light is directedin a direction which is counter to the flow of liquid through the element.