Various 
The Veil Lifted / Modern Developments of Spirit Photography [PDF ebook] 

The term “spirit photographs” is generally used to describe photographs of psychic entities who cannot be seen by ordinary persons, but can be photographed by a medium, or with the help of a medium, and with the co-operation of these unseen entities. Such portraits are obtained both indoors and in the open air, with and without a background, by natural light and by artificial light; and, in the case of “materialised” forms at séances, which are visible to every person present, the portraits have sometimes been taken by light produced by the spirits, the exact nature of which is unknown.