At OAC we regularly monitor 
              the dust in atmosphere by means of photoelectric measurements of 
              the atmospheric extinction coefficient at various wavelenghts. The 
              effective wavelenghts are those of UBVR photometric system, namely: 
              360, 440, 550, and 640 nm. 
              Below we show some examples of Bouguer's lines obtained during nights 
              having fairly high levels of dust in atmosphere.  |