“…Recently, two contributions have dealt with this possibility again: Friedrichs and Grover59 evaluated the intensity of the rings by light reflectance measurement for the determination of iron as Prussian blue in air pollution studies; Du et aZ. 60 measured the light absorption of the rings, cellulose acetate filters being used instead of ordinary filterpaper. In order to achieve even better transparency of the supporting matrix, the filters were moistened with paraffin oil before the final photometric measurement, and in this way…”