In the 20th century, optical illusion skull postcards gained popularity in Europe. Known as Vanitas or Memento Mori, these cards made people reflect on mortality and human ambition. The metamorphic skull was a favorite illusion used in postcards, comics, ads, and covers since the 19th century. It has also been featured in satirical posters, magazine covers, CD covers, and movie posters. Modern artists continue to incorporate these illusions in their work.

h/t: vintag.es