The maximum visibility of the interferences obtainable from two points in a wave field is defined as their degree of coherence γ. By a simple statistical method general formulae are found for deducing γ from illumination data. For any extended lightsource γ is found equal to the amplitude in a certain diffraction image. It does not change by the use of a condensing lens, but depends only on the aperture of the illuminating cone. These properties are applied to the microscopic observation of objects in transmitted light.