Tribute to Holbein

Anamorphic Skull

The most notable and famous of Holbein's symbols in the work, is the distorted skull which is placed in the bottom center of the composition. The skull, rendered in anamorphic perspective, another invention of the Early Renaissance, is meant to be a visual puzzle as the viewer must approach the painting nearly from high on the right side, or low on the left side, to see the form as an accurate rendering of a human skull. While the skull is evidently intended as a vanitas or memento mori, it is unclear why Holbein gave it such prominence in this painting. One possibility is that this painting represents three levels: the heavens (as portrayed by the astrolabe and other objects on the upper shelf), the living world (as evidenced by books and a musical instrument on the lower shelf), and death (signified by the skull). It has also been hypothesised that the painting is meant to hang in a stairwell, so that persons walking up the stairs and passing the painting on their left would be startled by the appearance of the skull.  Artists often incorporated skulls as a reminder of mortality, or at the very least, death. 
 

Source: Wikipedia