Which artist wrote his notes in mirror writing?
Mirror writing is formed by writing in the direction that is the reverse of the natural way for a given language, such that the result is the mirror image of normal writing: it appears normal when it is reflected in a mirror. It is sometimes used as an extremely primitive form of cipher. A common modern usage of mirror writing can be found on the front of ambulances, where the word "AMBULANCE" is often written in very large mirrored text, so that drivers see the word the right way around in their rear-view mirror.
Some people are able to produce handwritten mirrored text. Notably, Leonardo da Vinci wrote most of his personal notes in this way. Mirror writing calligraphy was popular in the Ottoman Empire, where it often carried mystical associations.