Weak banks dragged the Athens bourse 0.64 percent lower yesterday after a ratings downgrade on heavyweights National Bank of Greece and Alpha Bank by international credit rating agency Fitch. Brokers said the downgrade acted as an opportunity for a blue chip correction while small- and mid-caps continued to outperform. The Fitch announcement discouraged buying, especially at a time when the two banks have been under selling pressure recently, said Andreas Giakoumakis, Value Capital Securities analyst. National Bank slipped 1.73 percent to 27.30 euros, bringing its losses for the last three sessions to more than 8.5 percent. Alpha Bank fell 2.99 percent to 20.78 euros. Banks fell 1.81 percent. The downgrade was more of an excuse for a correction which swept through the rest of the market, said another broker. The Athens benchmark general index fell 0.64 percent to 2,747.91 points. Blue chips lost 1.24 percent to 1,494.65 points. Mid-caps on the FTSE/ASE-40 index rose 0.57 percent and small-caps advanced 0.88 percent. Turnover was 301.02 million euros. Winners beat losers 190 to 150.(Reuters) Santamouris, whose research team has constructed such window panes, explains the process: Electrochromatic panes regulate their translucency by means of an electrical current, so that the right amount of solar radiation enters, heating the room to a comfortable temperature.