Have you ever thought how often criminal trials hinge on the question of identification? The more one reflects on this point, the more one marvels at the really extraordinary powers of observation and memory, the most unlikely people seem to possess. For instance, a murder has been committed, and a passer-by has seen a man hurrying away from the scene. It's probably night time and he's had no more than a glimpse of somebody under a street lamp - yet he's able to give a detailed description.