 Other possibilities include an alteration in the density of the passing clouds, causing the sun’s image to alternately brighten and dim and so seem to advance and recede, and dust or moisture droplets in the atmosphere refracting the sunlight and thus imparting a variety of colors".Nickell also suggests that unusual visual effects could have resulted from temporary retinal distortion caused by staring at the intense light of the Sun, or have been caused by a sundog, a relatively common atmospheric optical phenomenon.Nickell also highlights the psychological suggestibility of the witnesses, noting that devout spectators often come to locations where Marian apparitions have been reported "fully expecting some miraculous event", such as the 1988 Lubbock apparition of Mary in Texas, the Mother Cabrini Shrine near Denver, Colorado, in 1992, and Conyers, Georgia, in the early to mid-1990s.