was present at Fátima on October 13, states, "We must declare that, although we admit the possibility of some miraculous fact, we were there while having in mind conversations we had earlier with cool-headed persons who were anticipating some changes of colour in the Sun". The villagers in Alburitel were preparing for a Sun miracle too. According to Maria do Carmo, "It was anticipated that the miracle would involve the stars". This is likely due to the fact that in the months of July, August and September people at Fátima claimed the Sun's light dimmed and the sky became dark enough for stars to become visible. This was denied too by many witnesses from the previous months. She also states that on the morning of October 13th, "the people of Alburitel were darkening bits of glass by exposing them to candle-smoke so that they might watch the Sun, with no harm to their eyes." Supernatural explanations, such as those by Father Pio Scatizzi, who argues that observers in Fátima could not be collectively deceived, or that the effect was not seen by observatories in distant places because of divine intervention have been dismissed by critics who say those taking part in the event could certainly be deceived by their senses, or they could have experienced a localized, natural phenomenon. According to Benjamin Radford, "It is of course dangerous to stare directly at the sun, and to avoid permanently damaging their eyesight, those at Fátima that day were looking up in the sky around the sun, which, if you do it long enough, can give the illusion of the sun moving as the eye muscles tire.” Others, such as professor of physics Auguste Meessen, suggest that optical effects created by the human eye can account for the reported phenomenon. Meessen presented his analysis of apparitions and "Miracles of the Sun" at the International Symposium "Science, Religion and Conscience" in 2003. While Meessen felt those who claim to have experienced miracles were "honestly experiencing what they report", he stated Sun miracles cannot be taken at face value and that the reported observations were optical effects caused by prolonged staring at the Sun. Meessen contends that retinal after-images produced after brief periods of Sun gazing are a likely cause of the observed dancing effects. Similarly, Meessen concluded that the color changes witnessed were most likely caused by the bleaching of photosensitive retinal cells.