"The Ponzo illusion is an optical illusion that was first demonstrated by the Italian psychologist Mario Ponzo (1882-1960) in 1913.
He suggested that the human mind judges an object's size based on its background.
He showed this by drawing two identical lines across a pair of converging lines, similar to railway tracks.
The upper line looks longer because we interpret the converging sides according to linear perspective as parallel lines receding into the distance.
In this context, we interpret the upper line as though it were farther away, so we see it as longer – a farther object would have to be longer than a nearer one for both to produce retinal images of the same size."