"...what better proof of survival could be given by a deceased person than that of a message in his own handwriting, referring to events that happened after his death?" asks Doyle in The Case for Spirit Photography. The ghost hand writing examples on this page are found within the same book.
These photomicrographs are an analysis of the ghost handwriting compared with original penmanship by the same person before passing.
Second Photograph: Sometimes these mysterious handwritings from spirit were said to have appeared on the plates and around the subject, according to Arthur Conan Doyle.
In this particular example, two photographic plates were exposed of the man (Mr. Walker). Both of them contained the mysterious handwriting; but one of them had the same message in reverse. Doyle wrote that this was known in the field as "mirror writing."